Thesis-Rafael-Gaxiola-Cockburn-MSM

Vista previa del material en texto

Instituto Tecnológico y de Estudios Superiores de Monterrey
Monterrey Campus
School of Engineering and Sciences
Evaluation of the Reprocessability of Polypropylene by the Implementation of
Ultrasonic Micro Injection Molding
A Thesis presented by
Rafael Gaxiola Cockburn
Submitted to the
School of Engineering and Sciences
in partial fulfillment of the requirements for the degree of
Master of Science
in
Manufacturing Systems
Monterrey, Nuevo León, June, 2020
Instituto Tecnológico y de Estudios Superiores de Monterrey
Campus Monterrey
The committee members, hereby, recommend that the proposal presented by Rafael Gaxiola Cockburn
to be accepted to develop the thesis project as a partial requirement for the degree of Master of Science in
Manufacturing Systems.
Dr. Oscar Martı́nez Romero
Tecnológico de Monterrey
Principal Advisor
Dr. Alex Elı́as Zúñiga
Tecnológico de Monterrey
Co-Advisor
Dr. Daniel Olvera Trejo
Tecnológico de Monterrey
Committee Member
MSc. Cintya Soria Hernández
Tecnológico de Monterrey
Committee Member
Dr. Rubén Morales Menéndez
Dean of Graduate Studies
School of Engineering and Sciences
Monterrey, Nuevo León, June, 2020
I
Declaration of Authorship
I, Rafael Gaxiola Cockburn, declare that this dissertation titled, Evaluation of the
Reprocessability of Polypropylene by the Implementation of Ultrasonic Micro In-
jection Molding, and the work presented in it are of my own. I confirm that:
• This work was done wholly while in candidature for a research degree at this
University.
• Where any part of this thesis has previously been submitted for a degree or
any other qualification at this University or any other institution, this has been
clearly stated.
• Where I have consulted the published work of others, this is always clearly
attributed.
• Where I have quoted from the work of others, the source is always given.
With the exception of such quotations, this thesis is entirely my own work.
• I have acknowledged all main sources of help.
• Where the dissertation is based on work done by myself jointly with others, I
have made clear exactly what was done by others and what I have contributed
myself.
Rafael Gaxiola Cockburn
Monterrey, Nuevo León. June 16th, 2020.
@ 2020 by Rafael Gaxiola Cockburn.
All rights reserved
II
Dedication
To my beloved Griselda,
for helping me to overcome the obstacles in the pursuit of knowledge.
To my mother Beatriz,
my father Rafael,
and my brother Juan Pablo,
for sending me their unconditional support despite the distance.
”If you can make it through the night, there is a brighter day”.
III
Acknowledgements
I would like to acknowledge my advisor, Dr. Oscar Martı́nez, for giving me all of
his trust and be open to my proposals along this work. His patience and advices
helped me to keep the direction of this research.
I would like also to express my gratitude to Dr. Alex Elı́as, leader of the Nanotech-
nology for Devices Design research group, for his wise advices and for trusting in
my thesis proposal since the beginning of the Master’s program. He inspired me
to go beyond the common and put an extra effort to make my work disruptive.
My sincere thanks also to MSc. Cintya Soria for all of the technical support during
the experimental stage in the laboratories and her supervision during the writing
stage of the literature review.
I would like to acknowledge my lab mates from the MNT program and my col-
leagues from the MSM program, for all of the mutual support and the long collab-
orative hours. It was a pleasure to coincide and watch ourselves grow profession-
ally.
I am thankful to the people who helped me with their personal experiences and
recommendation letters when I was applying for my Master’s scholarship: Dr.
José Manuel Nieto, Dr. Adrián Rendón, Dr. Cecilia Ramı́rez, MSc. Guillermo
Kitazawa and MSc. Joaquı́n Acosta.
Finally, all of these would have never been possible without the sponsorship granted
by Tecnológico de Monterrey through the Research Group of Nanotechnology for
Devices Design, and by Consejo Nacional de Ciencia y Tecnologı́a de México
(Conacyt), Project Numbers 242269, 255837, 296176, National Lab in Additive
Manufacturing, 3D Digitizing and Computed Tomography (MADiT) LN299129
and FODECYT-296176. These institutions believed in my potential as a profes-
sional and researcher.
IV
Evaluation of the Reprocessability of
Polypropylene by the Implementation of
Ultrasonic Micro Injection Molding
By
Rafael Gaxiola Cockburn
Abstract
Polypropylene (PP) is one of the most consumed commodity thermoplastics world-
wide, thereby, it is critical to propose new alternatives for the recycling of its post-
industrial and post-consumer waste streams. This research evaluated methodically
the use of the novel Ultrasonic Micro Injection Molding (UMIM) technology,
to identify the changes in morphological, mechanical, thermal and rheological
properties, after the reprocessing of pure regrind material for several consecutive
cycles. Proper process parameters were obtained by a Design of Experiments,
achieving a reduction of micro defects, in addition to thermal stability and an en-
hancement of the mechanical properties of recycled PP (increase of 36% Young’s
modulus, 20% yield stress, 13% ultimate stress, 26% strain, 48% toughness). The
tests showed that PP was able to withstand up to five reprocessing cycles until
presenting the first signs of mechanical performance downgrading. A better un-
derstanding of the mechanochemical effects and degradation is provided by Differ-
ential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Fourier
Transform Infrared (FTIR) spectra and dynamic rheology. The results of this work
set UMIM in a more mature stage for its incorporation to the industry, while con-
tributing to the circular economy practice.
V
Contents
1 Introduction 2
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Problem Definition and Motivation . . . . . . . . . . . . . . . . . 5
1.3 State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.5 Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.6 Organization of document . . . . . . . . . . . . . . . . . . . . . 12
2 Theoretical Framework 13
2.1 Fundamentals of Polymers . . . . . . . . . . . . . . . . . . . . . 13
2.2 Molecular Weight . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.1 Number average molecular weight (Mn) . . . . . . . . . . 13
2.2.2 Weight average molecular weight (Mw) . . . . . . . . . . 14
2.2.3 Higher average molecular weight (Mz) . . . . . . . . . . 14
2.3 Polypropylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4 Conventional processing techniques for thermoplastics . . . . . . 16
2.4.1 Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.2 Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.5 Ultrasonic Micro Injection Molding (UMIM) . . . . . . . . . . . 19
2.6 Material reprocessability . . . . . . . . . . . . . . . . . . . . . . 21
2.6.1 Classification . . . . . . . . . . . . . . . . . . . . . . . . 21
2.6.2 Mechanical recycling . . . . . . . . . . . . . . . . . . . . 22
2.6.3 Direct grinding . . . . . . . . . . . . . . . . . . . . . . . 23
2.6.4 Indirect grinding . . . . . . . . . . . . . . . . . . . . . . 24
2.6.5 Material degradation . . . . . . . . . . . . . . . . . . . . 25
2.6.6 Polymer stabilizers . . . . . . . . . . . . . . . . . . . . . 27
2.7 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.8 Analysis of Variance (ANOVA) . . . . . . . . . . . . . . . . . . . 29
3 Materials and Methods 33
3.1 Material and specimen geometry . . . . . . . . . . . . . . . . . . 33
3.2 Ultrasonic Micro Injection Molding machine . . . . . . . . . . . 33
3.3 Experimental method . . . . . . . . . . . . . . . . . . . . . . . . 35
3.4 Characterization equipment .. . . . . . . . . . . . . . . . . . . . 39
VI
3.4.1 Scanning Electron Microscopy (SEM) . . . . . . . . . . . 39
3.4.2 Tensile Testing . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.3 Thermogravimetric Analysis (TGA) . . . . . . . . . . . . 39
3.4.4 Fourier Transformation Infrared Spectroscopy (FTIR) . . 40
3.4.5 Differential Scanning Calorimetry (DSC) . . . . . . . . . 40
3.4.6 Oscillatory rheology . . . . . . . . . . . . . . . . . . . . 40
4 Results and Discussion 43
4.1 Design of Experiments (DOE) . . . . . . . . . . . . . . . . . . . 43
4.2 Machine Performance . . . . . . . . . . . . . . . . . . . . . . . . 50
4.3 Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.4 Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . 54
4.5 Thermogravimetric Analysis (TGA) . . . . . . . . . . . . . . . . 58
4.6 Fourier Transform Infrarred (FTIR) . . . . . . . . . . . . . . . . 60
4.7 Differential Scanning Calorimetry (DSC) . . . . . . . . . . . . . 61
4.8 Rheological properties . . . . . . . . . . . . . . . . . . . . . . . 63
4.9 Molecular Entanglements . . . . . . . . . . . . . . . . . . . . . . 67
4.10 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5 Conclusions 77
5.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5.2 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
A Injection Molding Pressure Drop Calculation. 86
B Injection Molding Pressure Drop Simulation. 88
C Maxwell elements parameters for Wagner Constitutive model. 89
VII
List of Figures
1 Plastic waste generation by sector. . . . . . . . . . . . . . . . . . 3
2 Ultrasonic Micro Injection Molding (UMIM) scheme. . . . . . . . 4
3 Representation of the polypropylene semicrystalline structure. . . 16
4 Conventional Injection molding process and heating mechanism. . 18
5 UMIM process sequence. . . . . . . . . . . . . . . . . . . . . . . 19
6 UMIM heating mechanisms. . . . . . . . . . . . . . . . . . . . . 20
7 Grinding machine elements. . . . . . . . . . . . . . . . . . . . . 23
8 Grinder designs for direct grinding of plastics. . . . . . . . . . . . 24
9 Grinder designs for indirect grinding of plastics. . . . . . . . . . . 25
10 Chain scissions in polymer matrices . . . . . . . . . . . . . . . . 26
11 Degradation mechanism. . . . . . . . . . . . . . . . . . . . . . . 28
12 Scaled ASTM D638 tensile specimen. . . . . . . . . . . . . . . . 33
13 Sonorus 1G UMIM machine. . . . . . . . . . . . . . . . . . . . . 34
14 Plunger position and injection velocity profile. . . . . . . . . . . . 36
15 Material preparation process. . . . . . . . . . . . . . . . . . . . . 37
16 Experimental methodology flowchart and response variables. . . . 38
17 Complete specimen before ejection. . . . . . . . . . . . . . . . . 43
18 Anderson-Darling test . . . . . . . . . . . . . . . . . . . . . . . . 46
19 Pareto plots for response variables . . . . . . . . . . . . . . . . . 49
20 Main Effects plot for response variables. . . . . . . . . . . . . . . 50
21 Process trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
22 SEM micrographs defects classification. . . . . . . . . . . . . . . 53
23 SEM micrographs morphologies comparison. . . . . . . . . . . . 54
24 Stress-strain curves for virgin and recycled PP. . . . . . . . . . . . 55
25 Continuous reprocessing specimens. . . . . . . . . . . . . . . . . 56
26 Stress-strain curves for continuous reprocessing. . . . . . . . . . . 57
27 Thermal degradation of PP by sections. . . . . . . . . . . . . . . 58
28 Thermal degradation of PP for continuous reprocessing. . . . . . . 59
29 FTIR spectra for continuous reprocessing. . . . . . . . . . . . . . 61
30 DSC melting peaks for continuous reprocessing. . . . . . . . . . . 62
31 DSC crystallization peaks for continuous reprocessing. . . . . . . 63
32 Shear viscosity for continuous reprocessing. . . . . . . . . . . . . 64
33 Crossover of Storage and Loss moduli. . . . . . . . . . . . . . . . 65
VIII
34 Storage and Loss moduli for continuous reprocessing. . . . . . . . 66
35 Phase shift angle (δ) and vector diagram. . . . . . . . . . . . . . . 67
36 Entanglement density. . . . . . . . . . . . . . . . . . . . . . . . . 68
37 Plateau moduli representation for continuous reprocessing. . . . . 70
38 Non-Newtonian shear viscosity fitting. . . . . . . . . . . . . . . . 86
39 Isothermal flow through profiles. . . . . . . . . . . . . . . . . . . 87
40 Final Pressure Simulation. . . . . . . . . . . . . . . . . . . . . . 88
List of Tables
1 State of the Art for UMIM. . . . . . . . . . . . . . . . . . . . . . 9
2 State of the Art for polymers recycling. . . . . . . . . . . . . . . 11
3 Classification of plastics recycling. . . . . . . . . . . . . . . . . . 22
4 Thermoplastics changes due to degradation. . . . . . . . . . . . . 25
5 Characterization techniques. . . . . . . . . . . . . . . . . . . . . 30
6 ANOVA results template table. . . . . . . . . . . . . . . . . . . . 32
7 Sonorus 1G specifications. . . . . . . . . . . . . . . . . . . . . . 34
8 Screening design. . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9 Screening combinations. . . . . . . . . . . . . . . . . . . . . . . 36
10 Results of Screening combinations. . . . . . . . . . . . . . . . . . 43
11 General Full Factorial Design. . . . . . . . . . . . . . . . . . . . 44
12 DOE response variables results. . . . . . . . . . . . . . . . . . . 45
13 ANOVA results. . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
14 Mechanical properties for different morphologies. . . . . . . . . . 55
15 Mechanical properties for continuous reprocessing. . . . . . . . . 57
16 Thermogravimetric analysis for continuous reprocessing. . . . . . 60
17 DSC for continuous reprocessing. . . . . . . . . . . . . . . . . . 62
18 Molecular weight data obtained with the rheological characterization. 65
19 Molecular entanglements data . . . . . . . . . . . . . . . . . . . 70
20 Maxwell elements parameters for Constitutive model . . . . . . . 89
IX
The following abbreviations and variables are used in this manuscript:
Acronym Description
UMIM Ultrasonic Micro Injection Molding
IM Injection molding
PP Polypropylene
DSC Differential scanning calorimetry
FTIR Fourier transform infrared spectroscopy
TGA Thermogravimetric analysis
XRD X-rays diffraction
GPC Gel permeation cromatography
SEM Scanning electron microscopy
DOE Design of Experiments
ANOVA Analysis of variance
SS Sum of squares
SST Total sum of squares
MS Mean square
DOF Degrees of freedom
PDI Polydispersity index
LVE-R Linear viscoelastic region
Variable Description Units
A Amplitude µm or %
V Injection velocity profile mm/s
MT Mold temperature ºC
F Injection force N
Mn Number average molecular weight g/mol
Mw Weight average molecular weight g/mol
Me Molecular weight between entanglements g/mol
σ Engineering stress Pa
∆H Enthalpy J/g
Xc Crystallinity %
G’ Storage modulus Pa
G” Loss modulus Pa
G∗ Complex modulus Pa
δ Phase shift angle º
G0N Plateau modulus Pa
η Shear viscosity Pa·s
η∗ Complex viscosity Pa·s
η0 Zero-shear viscosity Pa·s
γ̇ Shear rate s−1
ω Angular frequency rad·s−1
λ Characteristic time s−1
1
1 Introduction
1.1 Introduction
The invention of plastics has revolutionized our daily life, having presence in al-
most every place we visit or every product we buy. Its beginning dates back to
the last half of the 19th century, however, a more accelerated growth has occurred
since the 1940s. Statistics indicate that by 2015 the worldwide annual production
of plastics reached a value of more than 350 million tonnes annually [1]. From
these quantity, 171 million tonnes of wastes come from the industry of transporta-
tion, electric/electronics, and packing, as a demand mainly from the automotive,
aerospace, medical and packaging containers for food industries, which are big
consumers of injection molded parts. Figure 1 illustrates the plastic wastergen-
eration by sector. According to the Plastics Industry Association, in 2017, the
plastics industry was the 8th largest industry in the United States, and currently is
the source of 989 thousand direct jobs. By 2018 there was a growth in the total
plastics shipments of 6.9% equivalent to $432 billion sales [2]. Due to an increase
in the demand of electrical and electronics, as well as the automotive components
because of the Corporate Average Fuel Economy (CAFE) regulations, a growing
rate of almost 6% anually is expected in the injection molding industries until 2023
[3]. Thereby, the importance and impact of the injection molding industry itself
presents such a magnitude worth to be in constant monitoring.
Several plastics have followed a common path of development for a specific
purpose, but then, after finding more applications, they have been the cause of an
overproduction and commercialization that implicitly has involved the generation
of wastes and pollution to the environment at the industrial and consumer level.
By 2017, only 17 % of global plastics production was being recycled annually,
evidencing a huge gap in contrast to metals, that develop a recycling close to
70 % [4]. Such a difference between plastics and metals reprocessability relies
on the limitations and challenges that plastics recycling faces, starting from the
high cost of sorting-handling and the reduction of properties over the recycling
history of a resin. Because of the high production volumes and increasing costs of
some resins, saving projects are executed among manufacturers by blending virgin
resins with mechanically recycled plastic, typically regranulated. Thus, regrind is
the result of grinding or shredding a material that has been processed at least one
time before through extrusion or molding. This mechanical process, sometimes
2
Figure 1: Plastic waste generation by sector. Adapted from [1].
called granulation, milling or shredding, is the most common form of material
size reduction. From the plastics processing methods, injection molding is one of
the most utilized to produce a great variety of components in many sizes and batch
volumes and evidently the use of regrind by this method is a frequent task due to
the residues in terms of sprues and runners that are solidified every injection cycle.
Consequently, this opens the possibility of having a close-loop molding cycle if
the wastes are regranulated, blended with new resin, and fed back to the machine.
Within the world of high performance materials and composites, new man-
ufacturing methods are in constant development or evolution. A new alternative
process that implements ultrasonic vibration to melt a material has been devel-
oped to produce small sized products: that is, Ultrasonic Micro Injection Molding
(UMIM) [5]. This new manufacturing method is an analogous process of the tra-
ditional injection molding, with a few relevant differences. Figure 2 represents the
basic functioning of UMIM, where, the element called Sonotrode vibrates to ultra-
sound frequencies inside a cylindrical chamber, with direct contact to the plastic
pellets, transmitting mechanical oscillations until they dissipate into heat. As the
material melts, the plunger element moves forward, forcing the molten material to
flow into the cavity. According to Michaeli et al. [6] using ultrasound to plasticize
3
thermoplastics is an alternative to process small quantities of material to mold mi-
cro parts, reducing thermal degradation due to less residence time (3 to 6 s) in the
plasticizing chamber, and generating much less waste in big runners and sprues as
well. In addition, evidence of experiments with ultrasonic assisted extrusion has
found that ultrasound helps to improve miscibility of polymeric blends, as well as
to deliver a better dispersion of particles accompanied by an increase in viscosity
and mechanical strength for the fabrication of nanocomposites [7][8].
Figure 2: Ultrasonic Micro Injection Molding (UMIM) section view scheme. The pelletized material is
pushed upwards by the plunger as the sonotrode vibrates. As the material melts, it is injected into the cavity.
After a certain cooling time, the mold is opened and the ejector pins release the specimen.
In UMIM, the heating mechanism of UMIM is based in interfacial friction
and volumetric heating [6][9], and it is totally different to that from traditional
IM, arising the possibility of being an alternative method to process blends of re-
granulated materials, as well as composites and nanocomposites. Sanchez et al.
[10] proved that UMIM is a novel processing technique to produce composites. In
their investigation they were able to produce UHMWPE/graphite composites with
enhanced mechanical properties and thermal stability. Planellas et al. [5] demon-
strated by electron microscopy observations and XRD experimentation that ultra-
sound delivers uniform exfoliated structures with high oriented clay nanosheets
in the direction of the polymer flow. Additionally, Negre et al. [11] investigated
the processing parameters and their outcomes for polypropylene, finding a rela-
4
tion between injection velocity and the porosity obtained. Grabalosa et al. [12]
developed a deep analysis of the processing conditions of polyamide, pointing the
sensitivity of the cavity filling, homogeneity and mechanical properties of the fi-
nal part. This processing conditions research sets the outlook of UMIM in a more
explored stage of this novel technology.
It is worth remarking that no study of the application of regrind for UMIM
has been published yet, therefore, the viability of reprocessing plastics through
this method remains unknown, but less degradation could be expected due to the
absence of repeated shear stress that the reciprocating screw of traditional IM pro-
duces. Also, ultrasound is being used as a way to disperse fillers for nanocompos-
ites, thereby, UMIM is able to develop a good performance for mixing substances
and could be a manufacturing method substitute for the fabrication of micro com-
ponents in the automotive, aerospace and medical industries.
This thesis proposes the implementation of Ultrasonic Micro Injection Mold-
ing to test the reprocessability of polypropylene (PP), one of the most used com-
modity thermoplastics. The research procedure will follow a path of continuous
closed-loop of molding-grinding, performed for 100% regrind specimens, the ob-
jective of this stage is to monitor how the properties of the material vary after each
reprocessing cycle under ultrasonic plasticizing, aiming to develop a good perfor-
mance and finding thereby possible applications for plastic microcomponents with
superior behaviors and dimensional excellence. The solution plan includes a De-
sign of Experiments (DOE) to perform tests, analyze the effect of each parameter
in the molding process, and determine the values to use. An attrition mill is used
to grind molded specimens, sprues and runners.
1.2 Problem Definition and Motivation
Currently, the number of end-of-life products has increased the availability of post-
consumer plastics [13], as well as the in-factory streams generated by the manu-
facturers themselves, which opens a new area of opportunity for companies to
recycle plastics in order to reduce costs and environmental impact. Furthermore,
the lack of legislations in the past have lead to the cheap and simple practice of
landfill with residues, which is causing a serious environmental threat and concern
to society.
In comparison to conventional injection molding, the novel UMIM process
presents ephemeral residence times for polymers processing, providing a promis-
5
ing outlook to reduce wastes and to employ recycled resins in a close-loop and
from external sources as well, while opening the possibility of processing new
polymer blends to enhance chemical and mechanical properties, complying with
the requirement of reliable micro components with assurance of dimensional ac-
curacy andenhanced performance. However, by the time of publication of this
thesis, there is no study that explores the actual degradation effects of reprocessing
repeatedly thermoplastics with UMIM yet. There are no novel methods for ma-
terial properties enhancement explored under the effects on UMIM either. Even
though the recycling of plastics through regrind combination with virgin material
has been a solution to part of the waste streams managements within the molding
industry, there is much to develop yet. The fact that, for traditional IM, previ-
ous research has pointed that the properties of polymers tend to diminish as they
are reprocessed has limited molders only to reutilize small percentages of regrind,
however, it does not mean that recycled plastics can not be enhanced. In fact, re-
search for UMIM has achieved results that suggest an enhancement of mechanical
performance [14][15].
A motivation for this technology would be oriented to present a better un-
derstanding of the process and internal phenomena, to motivate a migration and
place UMIM one step closer of commercial applications, as a solution to reduce
environmental impact, convert to sustainability, and achieve cost reductions in raw
material of up to 50% [16].
1.3 State of the Art
The use of ultrasound as an alternative for injection molding has had a boom since
the year 2010. The current research for UMIM has focused mainly on the direct
effect of molding parameters in the molded parts. Michaeli et al. [6] studied the
effects of the process parameters on the shot weight of PP and polyoxymethylene
(POM) by doing a DOE. In their study, they found that the compression force and
the amplitude of the sonotrode have small effects on the micro parts weight, how-
ever, they also found some defects on the morphology, presented as flow marks
in the outer layer of the specimens. Negre et al. [11] analyzed the effects of
PP pellets humidity, sonotrode velocity, and mold temperature in the molding of
polypropylene micro parts. The results showed that dried pellets weight less and
they present less porosity, but the porosity of the part intensifies as the section is
closer to the injection point. They also found that for higher sonotrode veloci-
6
ties, the porosity increases, and non-completed parts appear. Better results were
performed for velocities below 7 mm/s, and with the application of increasing ve-
locity ramps. Pre-heating of the mold delivered less porosity in this experiment
too. Grabalosa et al. [12] developed an energy model for the interaction of the
mechanical waves and the pellets, they also performed tests, and found no accu-
rate relation between the molding parameters and the temperatures reached by the
polymers. Results also demonstrated that for higher values of parameters, filling
rate, dimensions, homogeneity and yield strength improved, but in reality, is not
as simple as just increasing everything to maximum because it would lead to ma-
terial degradation. Finally, although there was some polymer degradation, they
concluded that ultrasonic vibration did not generate it. They associated the weight
variations in the experiment to chain scissions as consequence of shear stress. Sac-
ristán et al. [17] developed a study of the effects of ultrasonic vibration on micro
molding for polylactide, regarding to the heating mechanism for ultrasonic tech-
nology in order to understand what is happening inside the polymer. According to
them, by applying ultrasound, the viscosity of the material is reduced significantly,
because of a physical and chemical effect. The physical effect refers to vibration
that alters the melt flow and reduces the elastic tensile strains, as well as molecu-
lar chain entanglement, while, the chemical effect implies nucleation growth and
formation of cavities. Sánchez et al. [10] studied the processing of Ultra high
molecular weight polyethylene (UHMWPE) / graphite composites. Graphite con-
tents of 1 wt%, 5 wt%, and 7 wt% were mechanically pre-mixed with UHMWPE
powder, and pressed at 135 ºC. Later, the samples were cut into irregularly pieces
and processed through UMIM to fabricate tensile specimens. Taguchi statistical
method was applied to optimize the moulding parameters and to maximize the
tensile strength. The tensile strength was increased in 8.8% for 1 wt% graphite
samples and all composites presented an increase in the tensile modulus. Graphite
caused a decrease of crystallinity, and oxidative degradation and chain scission
were found. Dı́az et al. [18] investigated the dispersion of micro and nano silica
particles in a biodegradable polyester matrix derived from 1,9-nonanodiol and aze-
laic acid. Minimal degradation and good dispersion of the particles were detected.
Furthermore, thermal stability was slightly improved. The overall crystallization
rate of the composite of was greater than that determined for the polymer matrix.
The previously discussed research is summarized in Table 1. As said earlier, no
study of recycling under UMIM has been published yet, but the evidence suggests
7
that less degradation is expected.
The experimentation of reground resins with virgin materials for traditional
injection molding can be taken as a reference framework. The combination of
reground resins with virgin materials for traditional injection molding has been
studied in the past. McLauchlin and Savage [19] studied the reprocessability of
polyether ether ketone (PEEK). The study compared the effect of repeated use
cycles against the properties of virgin material. Through tensile testing, X-Ray
diffraction (XRD) and differential scanning calorimetry (DSC), they evaluated the
mechanical properties, thermal behavior and crystallinity. Results showed that
pure regrind PEEK can be reprocessed up to three cycles without presenting drops
in tensile strength. Sobotova et al. [20] studied of the impact of regrind on me-
chanical properties of polybutylene terephthalate (PBT) with 30 % glass fiber and
traditional IM. Tensile, Charpy impact, and hardness tests were performed under
20, 40, 70 and 100 % of regrind. They found that the impact toughness reduced
up to 24 %, thus, the material became more fragile and prone to cracking. Wagner
et al. [21] developed a case of study for post-consumer ABS mechanical recy-
cling, they reground plastic housings of TVs, purified them in a water with NaCl
solution, and molded tensile specimens and thin wall boxes. With tensile and im-
pact test, the results showed that ABS can be reused and develop better properties
than commercial recycled resins in the market, however, they found some impuri-
ties and aesthetic defects. Spicker et al. [22] investigated the influence of multi-
ple reprocessing cycles on the rheological, thermal and mechanical properties of
polypropylene manufacturing scrap. The degradation mechanisms were separated
to investigate the thermo mechanical, thermal and thermal-oxidative degradation
of polypropylene. Melt flow rate (MFR), dynamic rheology, gel permeation chro-
matography (GPC), DSC, tensile test, color analysis, and Fourier transform in-
frared spectroscopy (FTIR) were part of their methodology. An increase in the
melt flow rate and a decrease in the viscosity and the mass average molecular
weight were found. Gall et al. [23] presented a case of study for the validation
of circular plastics economy from informal pickers sources. The materials tested
were post-consumer PE and PP regrind from containers, boxes and bottles. A
cleaning process by hot washing was evaluated before the injection molding. The
characterization was performed with FTIR, TGA, MFR. It was concluded that
post-consumer plastic wastes from informal sources can be processed into ma-
terials that are comparable to recyclates obtained from advanced formal systems,
8
Ta
bl
e
1:
St
at
e
of
th
e
A
rt
fo
rU
M
IM
.
A
pp
ro
ac
h
M
at
er
ia
l
C
ha
ra
ct
er
iz
at
io
n
C
on
tr
ib
ut
io
ns
R
ef
er
en
ce
D
ir
ec
t
ef
fe
ct
of
m
ol
di
ng
pa
ra
met
er
s
in
th
e
m
ol
de
d
pa
rt
s.
PP
an
d
PO
M
O
pt
ic
al
m
ic
ro
sc
op
y,
D
es
cr
ip
tio
n
of
th
e
in
te
rf
ac
ia
l
fr
ic
tio
n
an
d
vo
lu
m
et
ri
c
he
at
in
g
m
ec
ha
ni
sm
s.
Fl
ow
de
ff
ec
ts
.
M
ic
ha
el
ie
ta
l.
(2
01
1)
[6
]
E
ff
ec
ts
of
ul
tr
as
on
ic
vi
-
br
at
io
n.
PL
A
SE
M
,
SE
C
,
FT
IR
,
T
G
A
,
D
SC
,T
en
si
le
.
D
im
in
is
hm
en
to
f
de
gr
ad
at
io
n
of
PL
A
by
pa
ra
m
et
er
s
op
tim
iz
a-
tio
n.
H
ig
h
ef
fe
ct
of
am
pl
itu
de
in
de
gr
ad
at
io
n.
Sa
cr
is
tá
n
et
al
.(
20
13
)[
17
]
M
et
ho
d
to
di
sp
er
se
na
n-
oc
la
ys
in
po
ly
m
er
m
at
ri
-
ce
s.
PL
A
an
d
PB
S
FT
IR
,
SE
M
,
W
A
X
D
,
T
G
A
,D
SC
,T
en
si
le
.
Fi
rs
t
re
se
ar
ch
of
pr
od
uc
in
g
na
no
co
m
po
si
te
s
w
ith
U
M
IM
.
D
ev
el
op
m
en
t
of
hi
gh
ly
or
ie
nt
ed
cl
ay
na
no
sh
ee
ts
.
Pl
an
el
la
s
et
al
.(
20
14
)[
5]
E
ff
ec
t
of
hu
m
id
ity
,
in
je
c-
tio
n
sp
ee
d,
an
d
m
ol
d
te
m
-
pe
ra
tu
re
.
PP
St
er
eo
m
ic
ro
sc
op
y
Q
ua
nt
ifi
ca
tio
n
an
d
re
du
ct
io
n
of
po
ro
si
ty
by
dr
yi
ng
th
e
m
at
er
ia
l
an
d
us
in
g
lo
w
sp
ee
ds
.
N
eg
re
et
al
.(
20
15
)[
11
]
D
is
pe
rs
io
n
of
fu
nc
tio
n-
al
iz
ed
si
lic
a
m
ic
ro
an
d
na
no
pa
rt
ic
le
s
by
U
ltr
a-
so
ni
c
M
ic
ro
-M
ol
di
ng
.
Po
ly
no
n-
am
et
hy
le
ne
A
ze
la
te
T
E
M
,
SE
M
,
G
PC
,
FT
IR
,
X
PS
,D
SC
.
D
es
ir
ed
di
sp
er
si
on
of
pa
rt
ic
le
s
an
d
im
pr
ov
ed
th
er
m
al
st
ab
ili
ty
.
M
in
im
al
de
gr
ad
at
io
n
de
te
ct
ed
.
D
ı́a
z
et
al
.(
20
15
)[
18
]
In
flu
en
ce
of
pr
oc
es
si
ng
co
nd
iti
on
s
on
m
an
uf
ac
-
tu
ri
ng
po
ly
am
id
e
pa
rt
su
s-
in
g
A
N
O
VA
.
Po
ly
am
id
e
FT
IR
,G
PC
,S
W
A
X
S,
O
p-
tic
al
m
ic
ro
sc
op
y.
U
se
of
en
er
gy
m
od
el
.
E
vi
-
de
nc
e
of
ch
ai
n
al
ig
nm
en
t
an
d
de
gr
ad
at
io
n
re
fle
ct
ed
in
th
e
M
W
.
N
ec
es
si
ty
of
th
re
sh
ol
d
le
ve
ls
to
im
pr
ov
e
di
m
en
si
on
s
an
d
ho
m
o-
ge
ne
ity
.
G
ra
ba
lo
sa
et
al
.(
20
16
)[
12
]
Pr
oc
es
si
ng
of
U
H
M
W
PE
us
in
g
ul
tr
as
on
ic
vi
br
at
io
n
en
er
gy
.A
N
O
VA
an
al
ys
is
.
U
H
M
W
PE
SE
M
,
FT
IR
,
Te
ns
ile
,
T
G
A
,
G
PC
,
O
pt
ic
al
m
ic
ro
sc
op
y.
M
od
e
of
ac
tiv
at
io
n.
Ir
re
gu
la
r
sh
ap
e
de
liv
er
ed
be
st
re
su
lts
fo
r
U
H
M
W
PE
.D
ec
re
as
e
of
M
W
fo
r
hi
gh
M
W
po
ly
m
er
s.
In
cr
ea
se
d
cr
ys
ta
lli
ni
ty
.
Sá
nc
he
z
et
al
.(
20
17
)[
10
]
9
however, thermal and processability variations are limited to non demanding prod-
ucts. Pietroluongo et al. [24] used radiator parts made of PA66 reinforced with
35.7 % of short glass fibers from an end-of-life vehicle. They remolded the ground
material two consecutive times to compare with the reference material through ten-
sile and rheological testing, DSC, TGA, and XRD. They highlight is that despite
the degradation and fibers shortening, the material preserves mechanical accept-
able mechanical performance for other automotive applications. Even after the
two remolding cycles, PA66 increased its deformation capacity in 40%, but the
tensile strength was reduced in 11%. An oscillation in the crystallinity degree was
found, probably influenced by the thermo-mechanical degradation of the matrix.
The information of the investigations described above is summarized in Table 2.
1.4 Objectives
This research project has an overall objective related to the process and product.
The objective is to demonstrate that UMIM is a reliable manufacturing method
for the production of micro components made of thermoplastic recycled resins,
delivering stable mechanical, thermal and chemical properties.
The particular goals are listed below:
• To demonstrate that UMIM is a potential manufacturing method for micro
components that employ recycled materials.
• To guarantee morphological and mechanical excellence for specimens, through
a Design of Experiments.
• To track the behavior of the rheological and molecular weight properties and
strength for recycled material under UMIM, as a complementary criteria to
decide if the material complies with an application.
• To overcome the main challenges of plastic recycling contamination and degra-
dation.
• To determine which are the most significant molding parameters when treat-
ing with recycled material in UMIM.
10
Ta
bl
e
2:
St
at
e
of
th
e
A
rt
fo
rp
ol
ym
er
s
re
cy
cl
in
g.
A
pp
ro
ac
h
M
at
er
ia
l
A
dd
iti
on
al
tr
ea
tm
en
ts
C
ha
ra
ct
er
iz
at
io
n
R
es
ul
ts
R
ef
er
en
ce
Po
ly
m
er
ic
bl
en
ds
w
ith
20
/8
0,
40
/6
0,
70
/3
0
an
d
10
0/
0
PB
T
D
ri
ed
at
12
0
ºC
fo
r3
.5
h.
Te
ns
ile
.
Sh
or
e
ha
rd
ne
ss
.
C
ha
rp
y
im
pa
ct
.
Im
pa
ct
to
ug
hn
es
s
de
cr
ea
se
d
24
%
w
ith
10
0%
re
gr
in
d.
M
at
er
ia
l
be
-
ca
m
e
fr
ag
ile
.
Te
ns
ile
st
re
ng
th
an
d
ha
rd
ne
ss
m
ai
nt
ai
ne
d.
So
ba
to
va
et
al
.
(2
01
3)
[2
0]
M
ec
ha
ni
ca
lly
sh
re
d-
de
d
pl
as
tic
.
In
je
ct
io
n
m
ol
di
ng
of
sp
ec
im
en
s
w
ith
25
%
,
40
%
,
50
%
,
60
%
,7
5%
re
gr
in
d.
PE
E
K
N
on
e.
Te
ns
ile
.
X
R
D
.
D
SC
.
3
re
pr
oc
es
si
ng
cy
cl
es
w
ith
ou
t
lo
ss
of
pr
op
er
tie
s.
D
ro
ps
in
el
on
ga
tio
n
at
br
ea
k
af
te
r3
cy
cl
es
.C
ry
st
al
lin
ity
m
ai
nt
ai
ne
d.
Te
ns
ile
st
re
ng
th
m
ai
n-
ta
in
ed
.
M
cL
au
ch
lin
an
d
Sa
va
ge
(2
01
4)
[1
9]
U
til
is
at
io
n
of
PE
T
G
sc
ra
p
as
a
m
at
ri
x
fo
r
m
an
uf
ac
tu
ri
ng
of
a
co
m
po
si
te
re
in
fo
rc
ed
w
ith
PE
T
fib
re
s.
PE
T
G
an
d
PE
T
Se
pa
ra
tio
n.
R
es
to
ra
-
tio
n
of
m
ol
ec
ul
ar
m
as
s
th
ro
ug
h
ch
ai
n
ex
te
nd
er
(3
w
t
%
Jo
nc
ry
l)
.
R
em
ov
al
of
pr
in
tr
es
id
ue
s.
M
FR
.
N
ot
ch
im
-
pa
ct
st
re
ng
th
.L
ig
ht
m
ic
ro
sc
op
y.
D
SC
.
Te
ns
ile
.
G
oo
d
co
m
pa
tib
ili
ty
fib
er
s/
r-
PE
T
G
m
at
ri
x.
In
cr
ea
se
d
im
pa
ct
an
d
fle
x-
ur
al
(5
%
)
st
re
ng
th
of
co
m
po
si
te
.
20
%
re
du
ce
d
te
ns
ile
st
re
ng
th
an
d
4X
r-
M
FR
.
Fr
an
ci
sz
cz
ak
et
al
.
(2
01
8)
[2
5]
M
ec
ha
ni
ca
lly
sh
re
d-
de
d
po
st
-c
on
su
m
er
pl
as
tic
.
In
je
ct
io
n
m
ol
di
ng
of
sp
ec
im
en
s
an
d
th
in
-w
al
lb
ox
es
.
A
B
S
D
en
si
ty
se
pa
ra
tio
n
at
w
at
er
(fl
oa
tin
g
se
pa
ra
te
d)
an
d
w
at
er
w
ith
N
aC
l
(s
in
ki
ng
se
pa
ra
te
d)
.
D
ri
ed
at
80
ºC
fo
r4
h.
Te
ns
ile
.
C
ha
rp
y
im
pa
ct
.
M
i-
cr
os
co
py
.X
R
D
.
Sl
ig
ht
re
du
ct
io
n
of
Y
ou
ng
’s
m
od
u-
lu
s.
Te
ns
ile
st
re
ng
th
re
du
ce
d.
W
ag
ne
re
ta
l.
(2
01
8)
[2
6]
C
om
bi
na
tio
n
of
70
%
re
gr
in
d
w
ith
30
%
vi
r-
gi
n
m
at
er
ia
l.
PP
R
ep
el
le
tiz
in
g
w
ith
tw
in
sc
re
w
ex
tr
ud
er
(1
80
to
22
0
ºC
pr
ofi
le
at
15
0
R
PM
).
M
FR
.
Te
ns
ile
.
G
PC
.
E
xt
en
si
on
al
vi
sc
os
ity
.
FT
IR
.
C
ol
or
an
al
ys
is
.
D
ou
bl
in
g
of
M
FR
.
D
ar
ke
ni
ng
of
m
at
er
ia
l
co
lo
r.
M
ol
ec
ul
ar
w
ei
gh
t
re
du
ce
d
in
20
%
.
Sp
ic
ke
re
ta
l.
(2
01
9)
[2
2]
M
ec
ha
ni
ca
lly
sh
re
de
d
fu
ll
re
gr
in
d
fr
om
po
st
-
co
ns
um
er
so
ur
ce
s
PE
an
d
PP
W
as
hi
ng
an
d
se
pa
ra
tio
n.
R
ep
el
le
tiz
in
g
w
ith
tw
in
sc
re
w
ex
tr
ud
er
(1
95
to
20
0
ºC
pr
ofi
le
at
40
0
R
PM
).
FT
IR
.
T
G
A
.
D
SC
.
M
FR
.
C
ha
rp
y
im
-
pa
ct
.T
en
si
le
.
Po
ly
ol
efi
n
cr
os
s-
co
nt
am
in
at
io
n.
N
o
ch
an
ge
in
m
el
tin
g
te
m
pe
ra
tu
re
.
M
or
e
pr
on
ou
nc
ed
lo
ss
of
m
as
s.
V
is
-
co
si
ty
in
cr
ea
se
d
du
e
to
co
nt
am
i-
na
nt
s.
H
ig
he
rs
tr
ai
n
fo
rh
ot
-w
as
he
d
sp
ec
im
en
s.
G
al
let
al
.
(2
02
0)
[2
3]
M
ec
ha
ni
ca
l
re
cy
cl
in
g
of
E
O
L
au
to
m
ot
iv
e
co
m
po
si
te
co
m
po
ne
nt
PA
66
w
ith
gl
as
s
fib
er
s
W
as
hi
ng
an
d
se
pa
ra
tio
n.
R
em
ol
de
d
tw
o
tim
es
.
Te
ns
ile
.
R
he
ol
og
-
ic
al
te
st
in
g.
M
i-
cr
os
co
py
.
X
R
D
.
D
SC
.T
G
A
.
In
cr
ea
se
d
de
fo
rm
at
io
n
in
40
%
.T
en
-
si
le
st
re
ng
th
re
du
ce
d
in
11
%
.O
sc
il-
la
tio
n
of
cr
ys
ta
lli
ni
ty
up
on
re
m
ol
d-
in
g.
Pi
et
ro
lu
on
go
et
al
.
(2
02
0)
[2
4]
11
1.5 Hypothesis
Ultrasonic Micro Injection Molding is a reliable technology that modifies the
molecular order of polypropylene to produce mechanically enhanced recycled mi-
cro components with preserved functionality and quality.
The research questions to be answered are:
• How reliable is to reprocess polypropylene with UMIM?
• What are the changes induced in polypropylene upon consecutive reprocess-
ing by UMIM?
• What is the degradation mechanism of viscoelastic plastizicing with ultra-
sound?
1.6 Organization of document
The organization of this document is as follows: In Chapter 1, an introduction, the
objectives and the problems that this thesis research aim to solve are mentioned, as
well as the hypothesis with some research questions that are going to be answered
when the investigation is completed. A review of previous work is included along
the text. Chapter 2 includes the theoretical fundamentals of the research. Chapter
3 describes the steps that form the methodology of the thesis. Chapter 4 includes
all the results and their discussion. Finally, the conclusions, contributions and
future work are included in Chapter 5.
12
2 Theoretical Framework
2.1 Fundamentals of Polymers
Polymers are often classified as macromolecules due to their large composition
of successively repeat units, thereby, smaller molecules from which polymers are
synthesized are called monomers [27]. One of the simplest representations of
polymers is the carbon-chain structure, where the repetition of carbons form the
backbone of the chain, and other atoms, radicals, or molecules may be attached to
all the remaining bonds. A homopolymer is a chain that is formed of a same repeat
unit. On the other hand, a copolymer is constituted of two or more different repeat
units. From the point of view of chemical bonding, the functionality of a monomer
is defined as the number of bonds it is able to form with other monomers, there-
fore, a monomer is bifunctional when it may react to form two covalent bonds with
other monomers resulting in a structure of two dimensions. By this means, a tri-
functional monomer will have three bonds and a three-dimensional structure. The
molecular weight (molar mass) of a polymer is the result of summing the weight
of the atoms that form the molecules. Practically, it indicates an average length of
the polymer chains of a bulk. In reality, not all the chains that constitute a polymer
have the same length, thereby, the molecular weights are expressed as averages.
The molecular weight is of high importance because properties like viscosity and
number of entanglements are inherently dependent of it. In the next subsections,
the most common ways to calculate the molecular weight are presented.
2.2 Molecular Weight
The molecular weight of a polymer can be calculated in many manners, however,
one should determine that specific way depending on the desired properties to
study or the one the investigation is interested the most.
2.2.1 Number average molecular weight (Mn)
Properties like relative lowering, elevation of boiling point, depression of freezing
point, and osmotic pressure [28], are only sensitive to the number of molecules,
therefore, the number average molecular weight should be selected, which implies
the total weight of the polymer divided by the number of molecules. In other
words, it is a simple mean value. M0 can be denoted as the individual molecular
13
weight of each monomer that is in a chain. In the end, the total weight of the chain
will be the sum of the product of the number of molecules Ni that have a same
weight Mi:
TW =
∞∑
i=1
NiMi (1)
Then,
Mn =
∑∞
i=1NiMi∑∞
i=1Ni
(2)
But the number fraction is
Xi =
Ni∑∞
i=1Ni
(3)
Finally,
Mn =
∞∑
i=1
XiMi (4)
2.2.2 Weight average molecular weight (Mw)
Light scattering is a property that depends on the weight of each molecule in a
polymer chain [28]. Thus, the weight average molecular weight is employed.
Here, the mass of every molecule is considered in the formula. Based on Mn, the
number of molecules Ni is substituted by NiMi, giving:
Mn =
∑∞
i=1NiM
2
i∑∞
i=1NiMi
(5)
But the weight fraction is
Wi =
NiMi∑∞
i=1NiMi
(6)
Finally,
Mw =
∞∑
i=1
WiMi (7)
2.2.3 Higher average molecular weight (Mz)
Mz is nothing but the second moment of the function of molecular weight. By
the same procedure the weight average molecular weight function can be obtained
14
from the number average molecular weight by substituting NiMi in Ni, which
represents the first moment of the function, it is able to substitute NiM 2i to obtain
M2 = Mz. Mz relates to the amount of high molecular weight chains and it is
used to compare several Mz values to determine which has more entanglements.
Mz =
∑∞
i=1WiM
2
i∑∞
i=1WiMi
(8)
2.3 Polypropylene
Polypropylene (PP) is a semi crystalline thermoplastic polymer that is obtained
from the chain-growth catalyst polymerization of propylene, giving the chemical
formula (C3H6)n. PP is one of the most used materials in the plastic industry,
because it can be treated as a plastic or fiber [29], a structure representation is
included in Figure 3. PP is a versatile tough material that exhibits considerable
stiffness, good chemical and fatigue resistance, and that is obtained for a low price
[30]. Its density is between 0.8555 g/cm3 for the most amorphous structure and
0.946 g/cm3 for the most crystalline. In the same way, the melting temperatures
are between 130 ºC and 171 ºC. The molecules of PP are composed of a main
chain of carbons from which methil groups CH3− are bonded in any side of the
chain. PP can be found as a homopolymer (pure PP), and as copolymer (two or
more monomers, usually combined with blocks of PE). For the homopolymer,
depending on the orientation of the methil groups, there can be three types of PP:
• Isotactic: It presents an homogeneous distribution of the methil groups, all of
them oriented to the same side. This type is the most used for plastic injection
components.
• Syndiotactic: It is less cristallyne than isotactic PP. The methils alternate their
orientation, which make it more elastic than isotactic, but also less resistant.
• Atactic: It is the least cristallyne of the homopolymer PP, because the methil
groups are in disorder. Its properties make it sticky, therefore, it is used for
adhesive applications.
From the injection molding perspective, PP has a wide spectrum of applica-
tions, from packaging and food containers, to medical consumables and devices,
automotive connectors and housings for several types of sensors.
15
Figure 3: Representation of the polypropylene semicrystalline structure.
2.4 Conventional processing techniques for thermoplastics
2.4.1 Extrusion
Extrusion is a fundamental process for other manufacturing techniques that are ex-
plained below. In general, the sequence of extrusion starts by heating and melting
a polymer, then, it must be pumped through a shaping unit, which is particularly
a die with a desired geometry and dimensions that give the final product after its
solidification. Most of the extruders are conformed by one or two Archimedean
screws (Single Screw and Twin Screw Extruder, respectively) that are able to rotate
within a heated barrel. Pelletized or granulated material is fed through a hopper
that is mounted over a gap between the screw and the barrel. The screw diameter
goes from small to big in the flow direction, in order to reduce the depth of the
conveyingchannel as the material melts. The screw can be divided in three sec-
tion: conveying zone, melting zone, and metering zone. In the conveying zone,
the material is compacted, then, in the melting zone the heating mechanism starts
to act by creating a thin film of molten material due to contact with the barrel walls
(heater) and the intense frictional shearing forces caused by the screw motion pro-
duce a viscous dissipation. In the metering zone, the molten material finally is
directed to the die, to be extruded in the desired shape [31].
• Extrusion Blow Molding: The molten polymer is extruded through an annular
die to form a tube or parison that is enclosed by a mold with the desired
shape. The parison is inflated by injecting air to the inside, until it expands
and adopts the shape of the mold to later solidify. Finally, the mold is opened
16
and the blown molded part is released. This process can be continuous or
intermittent depending on the polymer properties.
• Extrusion Blown Film: The polymer is extruded through an annular die (tubu-
lar mandrel) to form a parison to which air is blown from below the mandrel
to form a hollow balloon-like film. The film is pulled axially by a set of guide
rollers while it is cooled by a perpendicular stream of air. Finally, the film
is also pinched to cut it into a defined number of products. The thickness
of the film can be adjusted by moving the mandrel. Typical thickness goes
from 0.005 to 0.25 mm, with die diameters from 10 to 120 cm. The bubble
diameter with respect to the die usually goes from 1.2 to 4 times bigger.
• Extrusion Fiber Spinning: The extruder is accompanied by a gear pump that
forces the molten polymer to flow through a spinneret with many small holes
(like a shower). The thin fibers are pulled by a series of godets while they are
cooled by a perpendicular stream of air.
• Extrusion Cast film: The molten polymer is extruded through a flat die to
create a film. After the die, this film enters a water-cooled chromed roll where
its temperature is lowered to solidify it. It is then passed through a series
of rollers where the edges are trimmed, and other treatments (like Corona
treatment) are applied in case it is necessary. Finally, the film is winded in
rolls.
2.4.2 Molding
• Injection Blow Molding: Injection blow molding is the other way to blow
mold products, but is not the same as extrusion blow molding. Here, as a first
step, the molten material is injected to a mold where it takes a preform, then
this preform is taken to a blow mold where air is injected until the parison
acquires the final shape. This method offers more dimensional control than
extrusion blow molding and it’s preferred for products with small L/D ratio.
• Compression Molding: The polymer is placed over heated platens where it is
compressed by a hydraulic press. It is mainly used in high content of fibers
reinforced polymers.
• Rotational Molding: Plastic powder is placed inside a mold that is later closed
and heated while it is rotated in two axis of rotation. As the polymer melts,
17
the rotational motion makes the plastic to stick to the walls of the mold. When
the mold is cooled, the product solidifies forming a hollow part. This method
requires less investment due to simplicity, however, the cycle times are way
higher than injection molding.
• Injection Molding: This process starts by melting the polymer in a similar
way as in the extrusion process. The melting mechanism for a semi crys-
talline polymer is simplified and illustrated in Figure 4: Thus, it can be sum-
marized as a result of the heat transfer from the wall of the barrel to the plastic
pellets, and the heat generated by the viscous dissipation [32] of the melt film
within the pellets and its surrounding surfaces, mainly as a consequence of
the shearing and stretching during the screw motion. It is worth remarking
that, the difference for injection molding with the extrusion process is that the
screw is reciprocating, meaning that as it turns it is moving back. When the
desired shot sized is molten at the end of the barrel, the screw is rammed for-
ward to inject the material at high pressure to a mold. The melt flows through
a sprue and later moves through runners that connect with a gate to the cavity
of the desired shape. There must be a holding stage where the machine packs
the polymer until it cools down and the product solidifies to be ejected after
the mold opens. The resins used in this process have melt flow indexes from
1 to 100 g/10 min.
Figure 4: Conventional Injection molding process and heating mechanism. (a) The chains start to disentan-
gle as heat is transferred and shear is applied. (b) The chains are disentangled and start to flow.
18
2.5 Ultrasonic Micro Injection Molding (UMIM)
The pursuit of miniaturization of devices has brought the development of new
manufacturing processes. Ultrasonic Micro Injection Molding is a fast, and low-
cost new alternative process that implements ultrasonic vibration to melt a resin
and inject it into a mold [5]. This new manufacturing method has several similar-
ities with ultrasonic welding, which has been used as a starting point to identify
the phenomena behind heating mechanisms [33]. It is as well an analogue pro-
cess of the conventional injection molding, with some relevant differences. In this
case, instead of dosing and melting more material than needed per shot, just the
exact amount required for one shot is placed inside the chamber. The heating in
the chamber is not external or by the motion of a screw anymore, now, an ele-
ment called Sonotrode [34], vibrates uniaxially to ultrasound frequencies, usually
between 20 and 40 kHz [35], inside the chamber with direct contact to the plas-
tic pellets, transmitting these mechanical oscillations until they dissipate into heat
and the material melts. The amplitudes handled by sonotrodes under ultrasonic
frequencies are commonly bounded between 12 and 100 µm [36].
Figure 5: UMIM process sequence. (a) Sonotrode activation and plunger motion. (b) Material melting and
injection. (c) Injection completed and start of holding. (d) Complete specimen after ejection.
Figure 5 presents the plasticizing, injection and holding stages of the cycle: (a)
the sonotrode starts vibrating as the plunger ascends, reducing the free space and
compressing the pellets. Then, in (b) the material starts to melt and it is injected
19
gradually through a runner. Finally, (c) shows the material completely injected
into the cavity as a result of the pressure exerted by the plunger, at this point the
holding stage is performed before the mold is opened to release the piece (d).
The heating mechanism of UMIM is based in interfacial friction and volumet-
ric heating [6], which are illustrated in Figure 6. It has been proven that interfacial
friction is a transient process that happens in very short periods of time (less than
0.5 s) and has a strong effect at the beginning of the plasticizing [37]. Interfacial
friction is determined by the relative movement (Vrel) between granulates, which
depends on the respective kinetic states ( ~V1 and ~V2), also, the surface area (AS) and
friction coefficient (µ) have influence, as shown in Figure 6 (a). The volumetric
heating, also classified as viscoelastic heating, is caused by the internal damping
properties of the material.
Figure 6: Heating mechanisms. (a) Heating due to interfacial friction. (b) Volumetric heating.
The sinusoidal movement of the sonotrode loads and unloads the granulates as
a fluctuating stress. By definition, the viscoelastic effect is a property of materials
to have both viscous and elastic behaviors (storage and loss modulus) when they
are deformed, in other words they have resistance to shear flow and linear strain,
and also the ability to return to original state after being stressed. The viscoelas-
tic heating mechanism can be modeled through the generalized Maxwell model
[38][39]. The storage modulus is proportional to the storedenergy (elasticity),
while the loss modulus is proportional to the dissipation of energy as heat (vis-
cosity), in a deformation cycle. Although in reality unit cells are deformed in a
combined direction, for terms of practical modelling, the stress in a unit cell is
often represented as a uniaxial normal stress dependant of time, which is the si-
20
nusoidal function corresponding to the sonotrode in Figure 6 (b), with the same
amplitude and frequency. A hysteresis loop if formed by the loading and unload-
ing curves, and it is known that the area of the bounded region is the work done
by the sonotrode for the internal friction in one cycle. Then, the heat generation
of each unit cell per time is [38]:
Q = f
∮
σ(t)d�(t) = fσ0�0ω
∫ 2π/ω
0
sin(ωt)cos(ωt− δ)dt = fπσ0�0sin(δ) (9)
where f is the vibration frequency, σ is the amplitude of the stress, � is the strain,
ω is the angular sonotrode frequency and δ is the lag angle of strain.
The complex modulus of a polymer is:
G∗ =
σ0
�0
(cos(δ) + isin(δ)) = G′ + iG” (10)
where G’ is the storage modulus and G” the loss modulus.
Substituting Equation 10 in Equation 9, it is found that viscoelastic heating
depends on the ultrasonic frequency, amplitude, and the loss modulus of the ma-
terial:
Q = fπ�20G” (11)
2.6 Material reprocessability
2.6.1 Classification
The use of regrind can be categorized in two groups: post-industrial (pre-consumer)
and post-consumer. Post-industrial plastics are those that were already processed
through a manufacturing method but they never left the place they were treated,
in this category the closed loop cycles are included, where the recycled plastic
comes from the same process it is later used in. Closed loop regrind comes from
discarded materials that are placed in a grinder immediately after they come out
from the process machine (molder, extruder, press). If there are any impurities
they can be removed directly or easily, and normally the polymer is stable enough
to be exposed to high-temperature processes [40]. These elements include sprues,
runners and scrap pieces. Post-industrial plastics are easier to treat since they
present less contamination, have less time degradation, and the exact composition
and quality of resin is known as well.
21
On the other hand, post-consumer plastics are those that were part of a prod-
uct and ended its service life, also known as End of Life (EOL). This group could
be subdivided in short life [40], for example packing elements, disposables, bot-
tles and bags; and used goods like cars, televisions, cellphones, computers. Thus,
post-consumer recycling means to gather plastics from any processes or source,
and give them a new application. Post-consumer plastics imply more work be-
cause they may be contaminated with other materials and may be degraded by any
means, also they could present different quality, consequently the cost of recy-
cling increases. Table 3 displays the classification of plastics recycling, according
to ASTM and ISO standards.
Table 3: Classification of plastics recycling.
ASTM D5033 ISO15270 Equivalent
Primary Mechanical Closed loop
Secondary Mechanical Downgrading
Tertiary Chemical Feedstock
Quaternary Energy recovery Valorization
2.6.2 Mechanical recycling
The action of grinding, sometimes also called milling, takes place inside machines,
as illustrated in Figure 7, that typically have a feeding unit, where material is
disposed; a shredding unit, usually composed of rotating blades and knives that
impact and cut the material; a classification screen with holes where the granulated
material passes when it reaches a certain size; and a collection element where the
regrind is evacuated through gravity or by vacuum [41][42].
The milling action produces stresses in the particles being ground, hence, the
most common are compression, attritions, shear and impact [43]. When the im-
pacting force of the blades is enough, the trapped particles are compacted to form
irregularly shaped macro particles. As deceleration occurs, shear and attritions
stresses during the impact take place as a radial displacement of powder particles.
It should be considered that there is resistance to the displacement due to friction
between particles and colliding surfaces, also the morphology of the final regrind
might depend mainly on the ductility or brittleness of the milled powders. When
brittle materials are exposed to high impact forces, they are subjected to continu-
ous fracture [44].
22
Figure 7: Grinding machine elements.
Grinders or mills can be divided in Direct milling and Indirect milling. Direct
grinders are more industrial oriented because they can handle greater volumes,
while indirect grinders are more laboratory oriented for smaller volumes and more
size reduction.
2.6.3 Direct grinding
For direct milling there are blades, cutters, impellers or rollers that are in contact
and transmit directly kinetic energy to the material, the most relevant designs are
included in Figure 8. Attrition mills have a vertical cylinder chamber where an
impeller is mounted on a concentric shaft, as the shaft rotates at high speeds, the
blades of the impeller impact and shear the material, generating spherical granu-
lates. Pan mills consist of a stationary and a moving pan that have similar struc-
tures and are placed together, being each of them divided by parallel lines. The
materials move in a spiral way during pulverization. The equipment squeezes and
splits the materials in between in a three-dimensional way. Pan mills have been
used to pulverize both brittle and elastic polymer materials, achieving even ul-
trafine size scale powders [45]. Roll mills are two same dimensional horizontal
rollers that rotate in opposite directions, inducing a high shear stress. Sometimes
the distance between the rollers is reduced as the milling goes on, to produce
smaller particles. Helical staggered mills consist of one horizontal roller that con-
tains helical blades on its surface. The roller rotates and drives the material through
23
a chamber that usually contains fixed blades to increase the pulverization of the
plastic. Inside the chamber there is a screen that filters the particles that already
reached the desired size. The N staggered blades is basically the same concept
than the helical staggered grinder but in this case the blades are parallel to the
orientation of the roller.
Figure 8: Grinder designs for direct grinding of plastics.
2.6.4 Indirect grinding
In indirect milling, the motion is applied to the housing of the mill and eventually
is transferred to the material by friction inside a chamber [43]. The function-
ing principle of indirect milling relies either on centrifugal or gravitational forces.
These methods are used mainly for laboratory purposes for powders particle reduc-
tion, the principal designs are the tumbler ball mill, vibratory mill, and planetary
ball mill. The tumbler ball mill is a horizontal cylinder filled partially with steel
balls that on rotation fall and hit the particles, thus, a greater cylinder diameter will
enhance the milling, however, there will be a speed limit to avoid the steel balls
and material to stick to the wall due to centripetal acceleration. Vibratory mills
consist of a shaking cylinder that oscillates back and forth at high frequencies.
The rate of milling depends on the mass inside, the frequency and the amplitude,
usually small amplitudes and high frequencies deliver the best results. In the plan-
etary mill, multiple cylinders are mounted on a slot over a rotating disc, which
causes an orbital movement of the chambers. The centrifugal force generates the
collision ans transfer of kinetic energy between the material and the walls of the
chamber.
24
Figure 9: Grinder designs for indirect grinding of plastics.
2.6.5 Material degradation
Regardless of the origin of the recycled plastic, some of the concerns or limitations
of employing regrind in the plastic industries are the polymer degradation and itseffects. Degradation is associated with a decrease of the mechanical properties due
to the repeated and long exposure of the material to temperatures above its melting
point, due to chains scissions during the grinding process and also due to shearing
during processing. Some of the changes induced in thermoplastics during grinding
and processing are presented in Table 4. Chain scission results in a reduction of the
average molecular weight, which decreases the overall viscosity since the chains
might have less entanglements and flow easier. The heat exposure could cause
crosslinking, which results in a more rigid structure by bridging adjacent polymer
chains, showing up as a more viscous or gelated bulk. Inevitably, the stabilizer
consumption will happen after a couple remolding cycles, making the polymer
more susceptible to any type of degradation every time. The previous issue is
also related to structural irregularities which make polymer resins more sensible
to photochemical ageing leading to discolorations.
Table 4: Thermoplastics changes due to degradation. Adapted from [46].
Processing changes Effects
Chain scissions Viscosity decrease and embrittlement
Branching (crosslinking) Increase of viscosity and gelation
Stibilizer consumption Decrease of stability
Structural irregularities Color changes and photochemical ageing
A practical challenge of molding and grinding is to preserve the molecular
weight of the polymers in use. Polymer degradation occurs when the covalent
25
bonds of molecular chains break, causing a shortening, as seen in Figure 10, which
in consequence reduces the molecular weight. It is important to consider that
degradation can occur prior to the injection process, when the material is in molten
state; during injection; and during grinding if it is going to be reused.
Figure 10: Chain scissions in polymer matrices as stress is applied.
A desired preparation step in molding processes that should be performed
also for regrind is the drying of the material, since if a high level of moisture is
present in it, a chemical reaction might be triggered while the granulates are in
the injection unit [47]. In hydrolyzable plastics such as PA, PBT and TPU, water
reacts with the backbones of the molecules and break them. For the case of PP and
hydrophilic plastics such as PPS, ABS and POM, moisture only causes cosmetic
defects because water reacts with pendant groups instead of the backbones [48].
Mechanical grinding induces a complex combination of shearing, extension,
fracture and cold-welding of polymer particles that conducts to chain scission or
hydrogen abstraction and generation of free radicals as well, it also causes a de-
crease in the packing density crystallinity [49]. As said earlier, chain scission
would be accompanied by a reduction in molecular weight, and that free radicals
may promote interchain reactions and chemical crosslinking. The chains form
crystals, thus milling has a severe impact on molecular alignment and structure
modification in some thermoplastic polymers [50].
In polymers, mechanochemical degradation is a set of changes in physical
properties as a consequence of chemical reactions triggered by mechanical energy,
such as fractures and deformations. These change of polymers might be either
bond scissions or cross-links. From a strict point of view, application of mechani-
cal forces is considered to be related to macroscopic effects, while chemical reac-
26
tions are microscopic or molecular phenomena. Free radicals are generated by the
mechanical energy, and since these radicals have an unpaired electron, they are un-
stable chemical species, hence, a series of chemical reactions can be initiated by a
free radical [51]. Primarily the causes of mechanical degradation of polymers are,
of course, input of mechanical energy but the degradation processes themselves
are a result of chemical reactions, such as main chain scissions and crosslinking in
polymer matrices. Even though there are different types of polymer degradation,
such as photo, radiation, thermal, and mechanical, from grinding and molding
processes attention should be focused in the last two, considering chemical bond
scission reactions in macromolecules. The shearing motions of each molecule un-
der strong stress are independent of each other and a crack caused by the strong
force may grow in a solid by ruptures of the van der Waals bonds between the
particular molecules on which the stress is concentrated.
A macroscopic fracture is in essence the simultaneous and incorporated mo-
tion of each monomer due to large shearing displacement of a single polymer
molecule. Such cooperative motion caused by the external force is resisted by the
interactions with adjacent polymers. In a polymeric chain, the sum of interaction
energies is the result of the number of monomers (n) times the activation energy
(E), if this result is larger than the bond energy of a C-C bond (Ec) a crack may
break the bond and a pair of mechanoradicals is formed by main chain scission
[52]. The critical number ne is determined by
ncE = Ec (12)
Since any free radical is unstable, the mechanoradicals can initiate a series of
reactions. A single mechanoradical induces many main chain scissions. This is
a type of self-degradation caused by the input of mechanical energy. If the chain
reactions occur in a crystalline part, a group of the broken chains formed by a
single end radical forms a kind of void in the crystallite [53] [49] [54].
2.6.6 Polymer stabilizers
Some polymers can easily form reactive free radicals during grinding or when
molten [55], therefore, they are compounded by incorporating a special class of
additives called stabilizers, which can be naturally sourced or synthetically pro-
duced materials, with the objective of avoiding undesired degradation. Hence,
27
Figure 11: Degradation mechanism.
stabilizers are another option when treating with recycled thermoplastics. These
additives aim to prevent further damage in the polymers by inhibiting or retard-
ing the degradation mechanisms as they are reprocessed. Stabilizers like hindered
phenol are effective in polyolefins, polyamides, polycarbonates, and polyesters
during processing at high temperature and for long-term use under ambient con-
ditions [56]. Some stabilizers, also called radical scavengers, work by reacting
rapidly to the available free radicals, to produce another less active free radical
and diminish the degradation process [57].
2.7 Characterization
In the molding industries, the incorporation of regrind tends to follow a practical
application that depends on simple criteria like the melt flow index (MFI or MFR).
However, to fully validate any plastic material, a deeper and more scientific char-
acterization procedure for recycled thermoplastics could provide a more adequate
implementation of recycled resins percentages and hence, optimize resources and
savings while decreasing environmental impact. Some relevant characterization
techniques for polymers are summarized in Table 5. These techniques describe
the thermal, chemical, mechanical, rheological and morphological properties. The
DSC measures the difference of heat between the sample and a reference by ex-
ecuting a temperature sequence. The enthalpies of fusion and crystallization are
28
obtained from the endoterms and exoterms, respectively. In the end, the melting
temperature, glass transition temperature and crystallinity are properties obtained
by DSC. In GPC the sample is dissolved, usually in etrahydrofuran, to create a
solution. Then, the solution is pumped into porous columns to separate the sample
by molecular weight. Finally, a UV detector measures the quantity of polymer
diffused in a specific time. The polydispersity and different average molecular
weights are obtained by GPC. TGA measures the mass of a sample over time as
temperature is changed, by this, degradation temperatures can be obtained. FTIR
emits infrared radiaton through a sample where themolecules will absorb it and
convert it into vibrational energy. The chemical nature and functional groups are
identified by FTIR. XRD is used to study the crystal structure of crystalline and
semi crystalline materials. An X-Ray beam is directed towards the sample while a
detector is rotated to measure the diffraction angles, the basic principles of XRD
are derived from Bragg’s law. Phases, lattice parameters, crystallinity and compo-
sitions can be determined by XRD. The capillary rheometry (MFR) is performed
by depositing material inside a heated capillary and then placing weights on the
top of the instrument to force the material to flow through a die. The flow rate
is determined by measuring the mass that flows from an orifice in certain time
intervals. Generally the measure is give in g / 10 min. The oscillatory rheometry
is performed with a dynamic rheometer equipped with different plate geometries
(parallel plates, cone-plate, concentric cylinders). If the sample is a solid, it is
molten by heating one of the plates, then the upper geometry imposes a deforma-
tion by oscillatory motion. Complex viscosity, shear viscosity, storage and loss
moduli, among other rheological properties are obtained by these type of rheome-
ters. The tensile testing is performed in universal testing machines by placing a
tensile specimen between two load cells, then, the sample is strained uniaxially in
order to measure the force, from which the stress and several mechanical moduli
can be obtained.
2.8 Analysis of Variance (ANOVA)
In multi-factor models, the Analysis of Variance (ANOVA) is used to identify
those significant factors over a response variable. Hence, the response variable is
dependent and the factors are independent variables. The commonly procedure is
the execution of a DOE where the values (levels) are set for each of the factors and
then the response variable is measured. The model for the ANOVA can be stated
29
Table 5: Characterization techniques.
Technique Functioning principle Properties measured
Differential Scanning Calorime-
try (DSC)
Difference of heat (energy) be-
tween sample and a reference
measured as a function of tem-
perature [58].
Melting temperature (Tm).
Glass transition temperature
(Tg). Degree of crystallinity
(Xc). Heat of fusion.
Gel Permeation Chromatogra-
phy (GPC)
Separation of sample into its
constituent parts based on
molecular size, by dissolving it
in a mobile phase and passing it
through a porous column [59].
Dispersity (D). Number average
molecular weight (Mn). Weight
average molecular weight (Mw).
Size average molecular weight
(Mz).
Thermogravimetric Analysis
(TGA)
The mass of a sample is mea-
sured over time as temperature
changes [26].
Mass (m). Degradation temper-
ature.
Fourier Transform Infrared
Spectroscopy (FTIR)
Measure how well a sample ab-
sorbs or emits light at different
wavelengths [60].
Functional groups. Chemi-
cal bonds. Chemical structure.
Supramolecular inteactions.
X-Ray Diffraction (XRD) X-rays produce a monochro-
matic radiation that is directed to
a sample, generating reflection
patterns [61].
Phase identification. Lattice
parameters. Crystallographic
structure. Grain size. Compo-
sition. Macrostresses.
Capillary Rheometry (Melt flow
rate)
Measure of the mass of polymer
that flows from an orifice under
a given weight at a certain tem-
perature in ten minutes (ASTM
D1238 / ISO 1183) [62].
Melt flow index (MFI)
Oscillatory Rheometry Homogeneous regime of defor-
mation with strictly controlled
kinematic and dynamic charac-
teristics. A maintained regime of
flow for unlimited period of time
[63].
Shear viscosity (η). Storage and
Loss modulus; complex modu-
lus. Relaxation modulus.
Tensile Testing A controlled strain of a spec-
imen is executed in a uniaxial
direction and the stress is mea-
sured.
Normal stress. Young’s modu-
lus. Ultimate stress. Elongation
at break. Toughness.
30
in two mathematically equivalent ways [64]. The interest of this thesis focuses in a
multi-factor model, therefore, the statements are presented for a two-way ANOVA,
instead of a single-way. The concepts of the two-way are the same for N factors.
A cell in ANOVA is each combination of factors and levels, usually addressed with
a subscript i for the level of factor 1, j for the level of factor 2, and the subscript k
for the kth observation within the (i, j) cell [64].
The first model, displayed in Equation 13, considers a response (Yijk) formed
by a mean for each cell (µij) and an error term (Eijk).
Yijk = µij + Eijk (13)
The ANOVA calculates estimates for each cell mean which are the predicted
values of the model (Equation 14). Equation 15 gives the residuals by taking the
differences between the response variable and the estimated means.
Ŷijk = µ̂ij (14)
Rijk = Yijk − µ̂ij (15)
The second model separates the response into an overall mean, factor effects (α̂i
and β̂j, which represent the effects of the ith level of the first factor and the jth
level of the second factor), the effect of interaction (γij) between two factors, and
an error term [65].
Yijk = µ+ αi + βj + γij + Eijk (16)
The ANOVA provides estimates of the overall mean and the factor effects. The
predicted values (Equation 17) and the residuals (Equation 18) are:
Ŷijk = µ̂+ α̂i + β̂j (17)
Rijk = Yijk − µ̂− α̂i − β̂j (18)
The second model is selected for this document since it makes the factor effect
more explicit.
The variance is decomposed into sums of squares:
31
• Sum of squares for each factor (SS).
SS(A) = rl
l∑
i=1
(Yix − Y )2 (19)
where r is the number of replicates, l is the number of levels, Yix is the mean
of the ith level, Y is the overall mean.
• Total sum of squares (SST).
SST = SS(A) + SS(B) + SS(AB) + SSE (20)
• Residual sum of squares (SSE).
• Mean Square (MS)
MSTerm =
SSTerm
DOFTerm
(21)
The ANOVA will quantify the variance in the data (SST) due to the factor
effects (SS) and random error (SSE) [65]. Nowadays, because of the complexity
of some cases, ANOVAs are performed using software to execute the extensive
calculations, in the end most of the information is summarized in formats like the
one in Table 6, where the formulas for Degrees of Freedom (DOF) are shown, in
addition to the Mean Squares.
Table 6: ANOVA results Table. Adapted from [65].
Source SS DOF MS F
Factor A SS(A) (a-1) MS(A)=SS(A)/(a-1) MS(A)/(SSE/(N-ab))
Factor B SS(B) (b-1) MS(B)=SS(B)/(b-1) MS(B)/(SSE/(N-ab))
2-way interaction AB SS(AB) (a-1)(b-1) MS(AB)=SS(AB)/((a-1)(b-1)) MS(AB)/(SSE/(N-ab))
Error SSE (N-ab) SSE/(N-ab)
Total (corrected) (N-1)
32
3 Materials and Methods
3.1 Material and specimen geometry
The material studied during this research is isotactic polypropylene (Axlene 12
Homopolymer) for general injection purposes with a density of 0.9 g/cm3 and
MFI of 12 g/10 min, purchased from Indelpro (Mexico). The pellet particles have a
semi-spherical shape with an approximate size of 5 mm. A dog-bone shaped micro
injection mold cavity was used to produce specimens for all the characterization
experiments. The geometry was obtained from the ASTM D638 Standard for
tensile properties of plastic specimens, adapted by a 1:5 scale reduction, displayed
in Figure 12.
Figure 12: Scaled ASTM D638 tensile specimen.
3.2 Ultrasonic Micro Injection Molding machine
For this investigation, the ultrasonic micro injection molding machine Sonorus 1G,
from Ultrasion (Spain) was used to produce micro tensile specimens. This equip-
ment, shown in Figure 13 (a), can be considered as the first commercial machine of
its type, since the ones used in previous research were experimental setups of the
different components, inspired in ultrasonic welding devices. In Figure 13 (b), the
main components are displayed: An ultrasonic unit, which includes the ultrasound
generator and transducer, a sonotrode, a mold (upper and lower), a plunger, and
for this case a dosing unit and pick and place for automation purposes. There is
33
a control panel (Human-Machine Interface) where