Suitability of recycled HDPE for 3D printing filament Haruna Hamod

Suitability of recycled HDPE for 3D printing filament  Haruna Hamod
Suitability of recycled HDPE for 3D printing filament
Haruna Hamod
Degree Thesis: Plastics Technology
December 2014
ABSTRACT
Arcada University of Applied Science
Degree Programme:
Plastics Technology
Identification number:
12838
Author:
Haruna Hamod
Title:
Suitability of recycled HDPE for 3D printing filament
Supervisor (Arcada):
Valeria Poliakova
Examiner:
Mirja Andersson
Abstract:
3D printing technology has been popular lately and the most used filament is ABS and
PLA. There is a concern about how the filament should be made of recycled material,
therefore this research was aimed to find recycled HDPE specification and extrusion
parameters for 3D filament. Comparisons were made between ABS, PLA and recycled
HDPE in order to obtain these parameters. The mechanical properties of recycled HDPE
such as melt flow, tensile strength, young modulus and yield strain was observed to go
along the PLA parameters used as 3D filament. The result shows that recycled HDPE has
the potential properties as 3D filament with essential puling speed and specific cooling
method employed.
Number of pages:
53
Language:
English
Date of acceptance:
2
Table of Contents
1
INTRODUCTION........................................................................................................................... 9
1.1
BACKGROUND ................................................................................................................................... 9
1.2
RESEARCH QUESTION .......................................................................................................................... 9
1.3
AIMS AND OBJECTIVES ...................................................................................................................... 10
2
LITERATURE REVIEW ................................................................................................................. 10
2.1
3D PRINTING ................................................................................................................................... 10
2.2
TECHNOLOGY OVERVIEW .................................................................................................................. 11
2.3
DIFFERENT 3D PRINTING TECHNOLOGIES.............................................................................................. 11
2.3.1
Stereolithography ................................................................................................................ 11
2.3.2
Selective Laser Sintering ...................................................................................................... 12
2.3.3
Fused Deposition Modelling (FDM) / Fused Filament Fabrication (FFF) ............................. 12
2.4
MATERIALS FOR 3D PRINTING ............................................................................................................ 13
2.4.1
Plastics ................................................................................................................................. 13
2.4.2
Plastics classification ........................................................................................................... 13
2.4.2.1
2.4.2.2
2.4.2.3
2.4.3
2.4.4
2.4.5
2.4.6
2.4.6.1
2.4.6.2
2.4.6.3
2.4.6.4
2.4.7
2.4.7.1
2.4.7.2
2.4.7.3
2.4.7.4
2.4.7.5
Thermosets .....................................................................................................................................................13
Elastomers ......................................................................................................................................................14
Thermoplastics ...............................................................................................................................................14
ABS ....................................................................................................................................... 16
PLA ....................................................................................................................................... 18
HDPE .................................................................................................................................... 19
Filament Processing and testing .......................................................................................... 21
Recycling ......................................................................................................................................................... 21
MFI..................................................................................................................................................................24
Extrusion ......................................................................................................................................................... 25
Tensile testing.................................................................................................................................................27
Materials for 3D filament .................................................................................................... 28
ABS as 3D filament .........................................................................................................................................28
PLA as 3D filament ..........................................................................................................................................29
Earlier research concerning HDPE as a Filament ............................................................................................ 29
Diameter tolerance.........................................................................................................................................30
Filament roundness ........................................................................................................................................30
3
METHOD ................................................................................................................................... 32
3.1
RECYCLING ...................................................................................................................................... 32
3.2
MELT FLOW INDEX ........................................................................................................................... 37
3.3
INJECTION MOULDING....................................................................................................................... 38
3.4
TENSILE TESTING .............................................................................................................................. 41
3.5
FILAMENT EXTRUSION ....................................................................................................................... 42
4
RESULTS.................................................................................................................................... 44
4.1
TENSILE TESTING & MFI.................................................................................................................... 44
4.2
FILAMENT EXTRUSION OPTIMIZATION .................................................................................................. 45
5
DISCUSSION .............................................................................................................................. 46
3
5.1
CHANGES IN MECHANICAL PROPERTIES ................................................................................................ 46
5.1.1
Changes in melt index.......................................................................................................... 46
5.1.2
Changes in tensile strength ................................................................................................. 47
5.1.3
Changes in Young’s modulus ............................................................................................... 47
5.1.4
Strain ................................................................................................................................... 47
5.2
FILAMENT EXTRUSION ....................................................................................................................... 48
6
CONCLUSION ............................................................................................................................ 49
7
SUGGESTION FOR FURTHER WORK ............................................................................................ 51
8
REFERENCES.............................................................................................................................. 52
4
List of Figures
Figure 1 Schematic view of 3D printing [2] .................................................................................. 11
Figure 2 Ziegler-Natta polymerization of PP [6] .......................................................................... 13
Figure 3 Structural crosslinking of thermoset [6] ......................................................................... 14
Figure 4 Chemical structure of acrylonitrile [10] ......................................................................... 16
Figure 5 Structure view of styrene monomers [10] ....................................................................... 16
Figure 6 Basic Structure/property relationships of styrene polymers (pp.11) [11] ...................... 17
Figure 7 An overview of PLA production fermentation process (Cargill Dow LLC patented) [13]
........................................................................................................................................................ 18
Figure 8 Repeating unit chains of polyethylene [6] ....................................................................... 19
Figure 9 SPI identification codes [16] ........................................................................................... 22
Figure 10 Schematic view of an extruder (pp. 246) [5] ................................................................. 25
Figure 11 Example of tensile test specimen [19] ........................................................................... 28
Figure 12 Collection of HDPE plastics ......................................................................................... 32
Figure 13 Soaked HDPE plastic .................................................................................................... 33
Figure 14 Manual peeling of the labels ......................................................................................... 34
Figure 15 Similar washed plastic................................................................................................... 34
Figure 16 Mixed coloured washed plastic ..................................................................................... 35
Figure 17 Plastic shredder (Arcada plastic laboratory, June 2014) ............................................. 36
Figure 18 Mixed coloured flakes ................................................................................................... 36
Figure 19 Recycled HDPE pellets ................................................................................................. 37
Figure 20 Melt flow index plastomer (Arcada plastic laboratory, June 2014) ............................. 38
Figure 21 Virgin HDPE dog-bone piece........................................................................................ 40
Figure 22 Recycled HDPE dog-bone piece ................................................................................... 40
Figure 23 Dog-bone being tested with testometric tensile testing machine (Arcada plastic
laboratory, June 2014) ................................................................................................................... 41
Figure 24 Manual pulling of filament ............................................................................................ 42
Figure 25 An electric pulling device .............................................................................................. 43
Figure 26 Filament extrusion (Arcada plastic laboratory, Oct 2014) ........................................... 43
Figure 27 Recycled HDPE filament ............................................................................................... 46
5
List of Tables
Table 1 Common characteristics of amorphous crystalline (pp. 4-5) [5]...................................... 15
Table 2 Material properties of PLA ............................................................................................... 19
Table 3 Characteristics of different PE grades (pp. 19) [15] ........................................................ 20
Table 4 Common engineering properties of HDPE (pp. 31-34) [15] ............................................ 20
Table 5 PLA, ABS and HDPE comparison .................................................................................... 31
Table 6 Optimized set up parameters for the injection moulding .................................................. 39
Table 7 Comparison table for 3D filament .................................................................................... 44
Table 8 Optimization table of recycled HDPE (Filament Die) ...................................................... 45
Table 9 Recycled HDPE filament size parameters ........................................................................ 50
6
Abbreviation
ABS: Acrylonitrile butadiene styrene
HDPE: High density poly ethylene
rHDPE: Recycled high density polyethylene
MFI: Melt flow index
PLA: Polylactide or poly (lactic acid)
SLA: Stereolithography
SLS. Selective laser sintering
FDM: Fused deposition modelling
FFF: Fused filament fabrication
RepRap: Self replicating rapid prototyper
7
Foreword
All praises and adoration is due to almighty Allah for making it possible for me to finish this
thesis throughout the ups and down, I say Alhamdulillah!
Firstly, I would like to thank Arcada University of Applied Science for given the opportunity to
study in an international atmosphere and most especially to the lecturers at the department of
energy and materials technology for sharing their academic knowledge and life experience during
my studies.
My profound gratitude and appreciation goes to my supervisor Valeria Poliakova and examiner
Mirja Andersson for their enormous support and guidance for the success of this thesis.
Secondly, to my parent who has been given me words of encouragement and prayers all the time,
I pray may you eat the fruit of your labour.
Last but not the least, to friends and fellow students who has in one way or the order contribute to
my knowledge, I say thank you!
8
1 Introduction
1.1 Background
Plastic usage has become an integral part of society with growth in population and technological
development, making plastics production increasing more than expected for its ability to replace
metal, wood, paper and glass in variety of engineering application. The tremendous uses range
from domestic and industrial application, which can be found and seen in products like credit
cards, computers, calculators, milk jugs, shampoo bottles, detergent plastic bottles, cosmetics,
toys etc. However, the usage of this plastic has caused some substantial environmental burden on
both land and water pollution as plastics tends to decompose very slowly and taking up landfill
and seashores.
A significant concern has been raised and seen over the years with the rate at which plastics are
used and disposed within communities of different countries occupying landfills and the
advocacy of more ecological environment has been highlighted in many ways and recycling is
part of them. If recycling is done more often, then plastics waste that ends up in the landfills will
be less and will reduced the need for new raw material while serving as a resources to
manufacture new product.
1.2 Research question
3D printing technology nowadays has emerged to be known for different application and used in
many areas but concern is raised on how the filament should be more eco-friendly and
reproduced from recycled material. The necessities for 3D filament and printed products to be
more sustainable influence this research. The engineering properties of ABS and PLA that makes
it suitable for filament would be examined and analysed to obtain relevant information and be
compared to recycled HDPE. These comparisons will enable proper parameter optimization of
the recycled HDPE that will be suitable to produce the filament.
9
1.3 Aims and Objectives
The aims and objectives of this thesis are:
1. Recycling HDPE to produce filaments for 3D printer
2. Finding the specifications and parameters used for 3D printers filaments.
3. Comparing and evaluating the engineering properties of ABS, PLA and recycled HDPE.
4. Determining the properties suitable for 3D printer filament from recycled HDPE.
5. Using the filament to print a product.
2 Literature review
This chapter of literature review is categorized in two parts; the first part gives some important
explanation of those plastics involved with 3D printer filament (types, processes, applications and
uses). The second part review extrusion, recycling, machining processes, 3D printing and earlier
research done concerning HDPE as a filament.
2.1 3D printing
3D printing inception could be traced back to the late 70s, when the inkjet printer was invented.
In 1984, the co-founder of 3D systems Charles Hull created the first 3D printer, the process of
rapid prototyping that enables 3D object to be created from digital data. Further down the lane,
Hull obtained the patent for stereo-lithography (SLA) that was used to established 3D systems
and developed the STL file format which would complete the electronics transmit file for 3D
printing objects form computer aided design (CAD) software. The stero-lithography apparatus
continued to be developed until the first commercial 3D printer was made available to the general
public, SLA-250. This technological research has been engaged and applied in limited areas of
industries such as the medical and engineering, the medical applications are production of
implants and prosthetics. Although it shows a tremendous potential in others application as the
future unfolds and series of research and experiment is done. [1, 2]
10
Figure 1 Schematic view of 3D printing [2]
2.2 Technology Overview
3D printing also referred to as additive manufacturing (AM) or rapid prototyping (RP) is defined
as the process of joining making three dimensional solid objects from a digital model, i.e. it is a
prototyping process whereby a real object is created from a 3D design. The objects are achieved
during the process by creating a lay down successive layers of material. It is considered distinct
from the conventional machining technique that typically depends on the removal of material by
milling, drilling, boring, cutting etc. Various technologies and manufacturing methods are used in
3D printing few common ones are briefly explained below.
2.3 Different 3D printing technologies
2.3.1 Stereolithography
SLA is the original and the oldest 3D printing process technology developed by Hull in 1984. It
works by converting liquid resin into solid materials with high-intensity light, which is often
ultraviolet (UV) laser. This UV laser beam traces the slices of an object on the surface of this
11
liquid causing the thin layer to be hardened. Once the UV laser beam has drawn a 2D path along
the surface, the freshly polymerized model layer is lowered into the surrounding resin bath to be
followed by another fresh surface curing and joining until complete object is printed. [3]
2.3.2 Selective Laser Sintering
Selective laser sintering (SLS) is a rapid prototyping technique with powder based manufacturing
similar to SLA except for the UV beam replaced by lasers and the resin replaced by powdered
bed. The powder has the attractive feature of being self-supporting for the generated product.
This process involves a layer of plastic powder spreads over the machine base area uniformly and
the object is built by using a laser to selectively fuse together successive fine plastic powder into
layers. Upon completion, the remains of the unsintered powder will eventually be removed for
reuse purpose. This technology is one of the most economical methods and its tolerant in terms of
deigns guidelines. [3]
2.3.3 Fused Deposition Modelling (FDM) / Fused Filament Fabrication (FFF)
It is an additive manufacturing technology where by thermoplastic material that has a melting
point below 3000C are used. This melted thermoplastic is extruded from a temperature-controlled
nozzle to produce the layers of object to be created at a certain high degree of accuracy. FDM is
most commonly used process that works with thermoplastics material such as ABS, PLA, and
HDPE etc. In this process, the 3D object designed with CAD software is imported as an STL file
to the 3D printing software that would enable temperature controlled and the thermoplastic
material layer by layer. The material changes from solid to semi-liquid state during the extrusion
process to form layers upon layers. Each new layer will stack on top and fused with the previous
layer when the material is harden almost immediately. [4]
Above all, the major working principle of all 3D printing are similar since almost all of the
technology need 3D design (e.g. CAD or CAM) at first, which is a digital representation of an
object before the client printer control software send the instruction to the printer and provides a
real-time interface functions and settings.
12
2.4 Materials for 3D printing
2.4.1 Plastics
Plastics today continue to be an exciting material to use because of their diverse characteristics
and usefulness in all works of life. There are numerous areas being developed yearly to utilise the
properties of plastic and new processing technologies are emerging to exploit and take advantage
of their easy manufacture process into all types of end product. They are produced from
polymers; the polymers are formed from a repetitive long chain of methane or ethane molecules.
This joining process of these molecules is best known as polymerization.
Polymerization is a chemical reaction process by which monomers are joined together in order to
form long molecular chains called polymers. An example of such polymerization is
polypropylene; Polypropylene is made by joining several structural molecule unit of propylene
into large monomers by polymerization to form polypropylene. [5]
Figure 2 Ziegler-Natta polymerization of PP [6]
2.4.2 Plastics classification
Plastics are often classified as follows:
2.4.2.1 Thermosets
Thermosetting polymers are usually made from low molecular weight such that when they are
heated they become very high cross-linked structure, thereby forming an infusible and insoluble
product. The polymeric process is irreversible when it’s cured through heat or catalyst.
13
Thermoset has two-phase formation; phase one is the formation of the long-chain molecules
while the second phase is the cross-linking of long chain molecules that usually takes place when
heat and pressure are applied. In the thermoset material, the highly cross-linked molecules
produces by strong chemical bonds enhance the high mechanical and physical strength of the
materials when compared to that of thermoplastic. [5] Figure 3 shows the structure view of
thermoset and their crosslinking.
Figure 3 Structural crosslinking of thermoset [6]
2.4.2.2 Elastomers
Elastomers materials are conventionally member of rubber family that consists of a long chainlike molecule similar to thermoplastic. The molecules are joined together by chemical bonds to
assume a bit of cross-linked structure. It is a group of material with different properties that has
excellent processing behaviour. Their characteristic enables it to be shaped within its design
temperature range, possessing elastomeric behaviour without cross-linking during production. Its
glass transition is below the room temperature that makes is soft and rubbery. Its young modulus
is relatively low while the yield strength is high when compared to other polymers [7].
2.4.2.3 Thermoplastics
Thermoplastic material consists of long chain-like molecules that are linked together by
intermolecular interactions force. As the material is heated up to a high temperature passing the
14
glass transition point the intermolecular forces are weakened that makes it becomes soft and
flexible and the material becomes solid as it cools down. [5]
Thermoplastics can take two different types subdivision structures within thermoplastic group
depending on the degree of intermolecular interactions that occurs between the polymer chains.

Amorphous structure: This type of structure is responsible for the elastic properties of the
thermoplastic materials, as the polymer chain adopt a bundled structure disorderly or a
random structure.

Crystal structure: The polymer chains acquire an orderly compacted structure directly
responsible for mechanical properties of resistance to stress or loads and the temperature
of the thermoplastics materials. [8]
Table 1 shows the essential characteristic and common features properties of amorphous and
crystalline structures.
Table 1 Common characteristics of amorphous crystalline (pp. 4-5) [5]
Amorphous
 Mostly transparent
 It has low shrinkage because of the
random arrangement of molecules
produces little volume change


Poor fatigue and wear resistance
because of random structure of
molecules
Low chemical resistance; this is a
result of the same random
arrangement of molecules which are
more open that enables chemicals to
penetrate deep into the material and
destroy many of the secondary
bonds
Crystalline
 Usually opaque
 High shrinkage: this is because as
the material solidifies the molecules
in the polymer are tightly packed to
a high aligned structure
 Good fatigue and wear resistance
because of the uniform structure

High chemical resistance; this is
because the tightly packed structure
that prevent chemical attack deep
within the material
An example of standard thermoplastic which are widely referred to simply as ‘‘plastics’’ include
Polypropylene (PP), Polyvinyl Chloride (PVC) Polycarbonate (PC), Polystyrene (PS), Polyamide
15
(PA), poly-methyl methacrylate (PMMA), Polyethylene (PE), Polylactide Acid (PLA),
Acrylonitrile-butadiene-styrene (ABS) etc. These materials have different properties and are used
in various applications but for the relevancy of this research ABS, PLA and HDPE are explained
as follows.
2.4.3 ABS
Acrylonitrile-butadiene-styrene is a thermoplastic family of ‘‘terpolymers.’’ It is the combination
of three different monomers Acrylonitrile, Butadiene, and Styrene to form a single polymer. The
involvement of these materials contributes to the outstanding impact strength and high
mechanical strength that makes it an opaque and stiff thermoplastic polymer suitable for tough
consumer products. It has broad processing properties strong and durable at low temperatures
with good heat and chemical resistance.
The following figure is the structures of the forming monomers and the representation of the
property mix and arrangement of SAN and ABS [9].
Figure 4 Chemical structure of acrylonitrile [10]
Figure 5 Structure view of styrene monomers [10]
16
Figure 6 Basic Structure/property relationships of styrene polymers (pp.11) [11]
The good balance of properties between the monomers has two phases of polymer blend. The
first copolymer phase is styrene-acrylonitrile copolymer (SAN) that gives the material its rigidity,
hardness and heat resistance while the toughness is as a result of polybutadiene rubber particles
that is evenly distributed in SAN matrix that gives the second phase blend. [11]
The combination of polyacrylonitrile, polybutadiene and polystyrene to form a single polymer
gives it high impact strength, rigidity and hardness within the temperature range of -40°C to
110°C when compared to other engineering plastics. It has a mechanical behaviour of yielding
plastically rather than tearing and the failures are ductile. ABS has a good long-term load
carrying ability with stresses above their tensile strength, but modulus of elasticity and hardness
are higher. Its relatively low creep nature and high heat resistance makes it a good dimensionally
stability and electrical insulation material properties. ABS plastics are used in wide range of
application in industry nowadays. Its uses among many others manufacturing methods are
injection moulding, blow moulding, extrusion and as a filament for 3D printing products. [11, 12]
17
2.4.4 PLA
PLA is a term referred to as poly (lactic acid) or simply polylactide biodegradable thermoplastic
made from lactic acid. It is categories among the biopolymers because of its renewability and
degradability to nature, and has made it more interesting for wide range commodity applications.
The lactic acid is versatile due to its application in the food, textile, pharmaceutical and chemical
industries. They are naturally organic acid that can be produce in various ways of chemical
synthesis or fermentation. [13, 14]
This fermentation could be derived from carbohydrate sources such as corn, wheat or sugarcane.
It is a low cost process that combines significant environmental economics benefits of
synthesizing lactide and PLA in melt instead of solutions that enable the production of workable
biodegradable commodity polymer made from renewable resources. Below is an illustration
overview of PLA production by fermentation process developed and patented by Cargill Dow
LLC.
Figure 7 An overview of PLA production fermentation process (Cargill Dow LLC patented) [13]
Poly (lactic acid) material properties are good in comparison with many other bio-based
materials. These properties depend on the processing temperature, component isomers, molecular
weight, crystallinity, annealing time and glass transition (Tg). PLA is high strength and high
modulus thermoplastic, which can be processed by most conventional plastic processes like
injection moulding, extrusion, blow moulding and even used as 3D printing filament.
18
Table 2 below list some PLA material properties related to this research following the ISO
standard testing conditions [14].
Table 2 Material properties of PLA
Physical Properties
Nature Works PLA
Melt flow rate (g/10 min)
2.4 – 4.3
Density (g/cm3)
1.25
Mechanical properties
Tensile strength at yield (MPa)
53
Elongation at yield (%)
10-100
Flexural modulus (MPa)
350–450
Thermal properties
HDT (°C)
40–45, 135
Melting point (°C)
120–170
GTT (°C)
55–56
2.4.5 HDPE
Polyethylene is a thermoplastic material available in different forms and grades for various
applications. Being the most important and dominant polymer that covers the largest part of
plastic family, the polymer chain and molecular structure is critically important for the polymer
synthesis preparation. Polyethylene is vinyl polymer that makes up the largest family of polymer.
They are made from the monomer ethylene and are identified to how the chains are formed. The
structure repeating unit can be seen in figure 8.
Figure 8 Repeating unit chains of polyethylene [6]
19
The degree of how highly dense these materials are depends on crystallinity which in turns
depends on the molecular weight and what polymer branching structure it has. HDPE has as
excellent mechanical properties like high compressive tensile strength, high stiffness and melting
point, and greater crystallinity that make it outperform other polyethylene. However, there are
other polyethylene that differs in densities, hyper-branched, crystallinities and categories
according to their molecular weight and chain structure, listed in the table below [15]:
Table 3 Characteristics of different PE grades (pp. 19) [15]
PE
LDPE
Density
Degree of Crystallinity
Number of branches (per 1000
(g/cm³)
(%)
carbon atoms)
0.910 – 0.925
20– 30 (methyl); 3– 5 (n-
40 -50
butyl)
LLDPE 0.910– 0.925
-
-
MDPE
0.926– 0.940
4– 6
HDPE
0.942– 0.965
4 (Phillips); 5– 7 (Ziegler)
70 – 90
Generally, HDPE is considered exceptional because of its impact strength relative to other
thermoplastics (one of the best impact resistance thermoplastic) and has an excellent
machinability. It is somewhat hard and more opaque with range flexibility depending on
production process and it can withstand rather higher temperatures. The processing temperature
window for HDPE for example is, melt temperature (160-250°C) and an expected shrinkage of
1,5 - 3 %. It is used for products like rods, trays, detergent bottles, cosmetic containers, domestic
water pipes, food boxes, used in automobile parts, industrial applications and the list goes on. The
most common engineering properties of HDPE can be seen in table 4. [15]
Table 4 Common engineering properties of HDPE (pp. 31-34) [15]
Property
Unit Value
Density
0.942– 0.965 (g/cm³)
Melting point
120 - 135°C
Tensile Strength
20 - 40MPa
Strain at break
100 – 1000 (150%)
20
Tensile modulus
413- 1241MPa
Elastic modulus
0.2 – 1.2 (GPa)
Glass transition temperature
110°C
Coefficient of Thermal expansion
100 - 120×10-6 m/m oC
Thermal conductivity
0.38- 0.51W/mK
Notched Impact strength (charpy)
2 -12 kJ/m²
Resistance
Above 100°C
Crystallinity
Greater than >90% (high crystalline)
Flexibility
More rigid
2.4.6 Filament Processing and testing
The processes involved in filament manufacturing and testing are described as follows.
2.4.6.1 Recycling
Plastics can be found in almost all domestic and industrial products which given more concern
and raise questions on how to recycled and manage the waste. Recycling today has been
beneficiary to us human and the environment at least to some extent yet a lot still need to be done
to improve on. Plastic recycling or reprocessing plastic is the process by which waste or used
plastic are reprocessed and developed into useful new products or processed to the same products
or processed to completely different products.
The first step towards making recycling possible is by identification of the plastics, which was
developed by the society of plastic industry (SPI). The identification methods consist of three
arrow triangular shape with a number indicating the type of resin/polymer used in the plastic.
Often the plastic is abbreviated at the bottom triangular shape sign shown in figure 9. There are
other practical ways of recognizing a plastic such as burning tests, observation test and floatation
test. An example of floatation test is separation; it involves separation of different plastic from
the other by soaking them in water, the ones with less density like polypropylene will flow up
while others sinks down. It is the basic one that is simple and easier to use. Some certain plastics
are also recognized by the burning test according to their flame colour but experienced or expert
21
engineers often make this observation. Nevertheless, in the recycling stream today, plastics are
more recognized and separated according their resin codes with technologies such as optical
cameras and different sensors [16].
Figure 9 SPI identification codes [16]
Mechanical recycling
Mechanical recycling process involves reprocessing of plastic residues (post-consumer products)
into new, the same or different products entirely. The practical main goal of mechanical recycling
is to make new products by limiting the use of new raw material through a mechanical process.
It is regarded as the simplest and straightforward method of recycling. The steps generally consist
of the following: collection, separation, grinding or shredding, washing &cleaning, drying,
pelletizing, manufacturing and reprocessing by suitable method of the desired product usually
injection moulding or extrusion. The process are shortly explained below which varies from
material to material and sorted according to their resin code before further processing.

Collection
Generally, collection stage is often done to gather all kind of plastics into single place for
further treatment. The collection is of the plastic waste in a large-scale industry are
managed efficiently by categorization. They are classified into two groups, post-consumer
22
plastic and post-industrial plastic waste depending upon the sources of plastic waste. Postconsumer plastics are recycled to another consumable product of the same type or
different product such as PET bottles to food grade materials while the post-industrial
plastic waste are recycled for industrial application like chairs, tables used in household
appliances and automotive.

Separation and sorting
After the materials are collected to a certain place, the separation phase is done to sort and
separate the plastic according to resin identification codes. As the quality of the resin
partly relies on the sorting process, it is important the sorting of the plastic is done
efficiently to avoid different plastic mixtures along the way. Improper sorting could also
affect the quality of the resin produced, so attention should be paid when sorting. In the
large-scale industry several methods and technology are used for sorting.

Shredding
Shredding process involves grinding the collected, sorted plastic material into smaller
pieces called flakes. The flakes are then put forward for further processing in an extruder
to produce pellets.

Washing
Washing is usually done to remove contaminant from the plastic like dust, grease, labels,
oil etc. They are washed with surfactants (detergents) or sodium hydroxide (Na0H)
solution. The washing could either be done before the plastics are shredded into flakes or
washed afterwards, depending on post-processing requirements and level of
contamination.

Drying
The drying phase simply requires enough heat so that all moisture is left out of the plastic
and it is done using a drying machine at recommended drying temperature of the material.

Pelletizing or granulation
23
Once the plastic flakes gotten from shredding are properly dried, they are process in an
extruder to obtain pellets. The processing is done according to the material data sheet of
the specific material. If for examples polyethylene (PE) is recycled and to be process to
obtain pellets the temperature zones of the extruder would range between 190 - 200°C.
2.4.6.2 MFI
Melt flow index often shorten as (MFI), is a common measurement that is used to characterize
thermoplastic polymers. It measures how easily or poorly a thermoplastic polymer is able to flow,
by defining the mass of the polymer in grams per time period i.e. the output rate (flow) of the
polymers in grams that occurs during a certain period of time. The result of this output flow rate
gives an indication of how the thermoplastics will be easy to injection mould, hard to injection
and easy or hard to extrude [17].
Melt flow index is an assessment of the molecular weight of plastic and is an inverse measure of
the melt viscosity. I.e. the higher the melt index value, the lower is its viscosity and therefore, the
lower the average molecular weight of the polymer. In terms of the mechanical strength, the
lower the melt flow rate results the higher the mechanical strength and the higher melt flow
results the lower the mechanical strength. This strength values could be observed in tensile stress,
young’s modulus of the material among others. During testing, it is important to note the weight
load and temperature of the polymer because different weight load would produce different MFI
results. [18].
The testing of MFI is according to ISO 1133 standard or ASTM D1238 standard depending on
the choice to follow. It is practically done by applying certain kilogram of load via a piston at a
melting temperature of the polymer and the polymer flow out through an orifice die while been
measured at time interval. It is typically expressed in terms of grams of polymer that flows out
per minute period (g/min). An example of this test for polyethylene (PE); a weight load of 2,16kg
is applied onto a piston at temperature of 190°C when pushes down, the plastic melt flow out
through a die and cut to be measured and calculated in g/10min. However, some polymers types
often report melt index at different conditions, some polymers are measure at higher weight and
temperatures and even different orifice die diameter. For example, polypropylene and ABS with
weight load of 2,16kg melting at 230°C
24
2.4.6.3 Extrusion
Extrusion process is considered as one of the most used processing techniques in the plastic
industry. It is a method by which raw material is forced through a die in a continuous process to
form the desired shape die called extrudate (the product). The manufacturing products are mainly
used to produces different cross-sectional components such as profiles, pipes, films, window
frames, wire insulations, plastic sheets, filaments etc.
The following are the major components of an extruder:

Hopper

Screw

Die

Barrel

Heater bands

Cooling

Pulling device

Cutting device
Figure 10 Schematic view of an extruder (pp. 246) [5]
25
Hopper
The hopper of an extruder is where the thermoplastic material is been feed through, either by
gravity or regulatory device. They can be of different shape and size but the most commonly use
shape is of the cone type.
Screw
The screw is one of the most important component of the extruder and its design crucially for
proper mixing and processability of the polymers. It consists of three main geometrical zonesections feed zone, compression zone and metering zone.

Feed zone/section: This section is the deepest part of the screw where the pellets are
introduced. The function of this zone is to preheat and convey the material forward to the
subsequent zones. This continues at least as long as the material is solid and below the
melting point.

Compression zone/section: Compression zone or transition zone as it may be called is
where the melting of the pellets takes place. The depth of the screw gradually becomes
shallower so as to compact the plastic towards the barrel and transcends from the feed
depth to the metering depth.

Metering zone/section: The metering zone of the screw is where the melting of plastic is
completed and ready pumping through the die. The depth here is constant and the supply
material is uniform temperature and pressure. This zone is also referred to as pumping
zone. (pp. 246 – 247) [5]
Die
The die is design and manufacture according to specific size of an extruder and the desired shape
of the final product to be produced. Many die designs are available, depending on the extrusion
process and the specification of the die is essential such that the final product would be produced
accurately. There are several types of die that work corresponding to the job at hand:

Lace die: they are used for producing pellets.
26

Sheet die: to produce sheet shape products.

Tube die: to produce tubes, profiles, hollow shapes etc.
The practical concept of extrusion process goes by feeding thermoplastic raw material (pellets or
granules) into the hopper by gravity or regulatory device; this granules is fed through the feed
throat on the screw feed zone, the rotation screw convey the plastic granules forward to the
melting zone where the granules is melt through friction with the rotating screw and heating from
the heater bands, afterwards the granules is pushed to the metering zone of the screw, this is
where the homogenized material is brought to the desired process temperature then the molten
plastic will pass through the die to take the desired shape. After forming the orifice shape of the
die the profile will be channelled for cooling, during this cooling the plastic solidifies and take its
final shape. The cooling could be done either by water-cooling or air-cooling among several other
types.
2.4.6.4 Tensile testing
Tensile testing is the mechanical properties testing commonly used to obtain various data such as
tensile strength, stress, yield, elasticity, toughness, strain, elongation and so on, for different
material (metals, plastics, composites etc.). The test is used to determine the amount of force
required to break a material. It measures the displacement and extent at which the material is
been stretch before reaching to that breaking point. The core aspect of this test is to determine the
strength and deformation of a material, as the force applied increase so does the elongation at the
gage length until it fails. The strength of a material is often the main concern before its selection
for any engineering applications, which are measured on how much it can deform before it
fractures. The obtained data is used to specify the material, to design parts to withstand
application force and as a quality control check of materials. [19]
27
Figure 11 Example of tensile test specimen [19]
Tensile test piece also referred to as dog-bone are used for the test shown in figure11. It has wide
shoulder for the gripping of the machine. The gage section plays an important role of the
specimen for which its cross-sectional area are subjected to stretching until it fails or breaks when
forces are applied to the shoulder. However, the gage length should be large enough and greater
than its diameter because the stress state will be more complex than simple tension if reduced
gage length is present. The test provides a stress-strain curve diagram that gives information
about the mechanical properties of the material tested and is done according to ISO standards.
2.4.7 Materials for 3D filament
2.4.7.1 ABS as 3D filament
The thermoplastic properties of ABS is explored and found suitable as a filament because of its
durability, mild flexibility, high strength, high glass transition temperature, impact resistance and
very good heat resistance. Printing with ABS normally operates with hot end nozzle and bed at a
recommended temperature. It is approximately printed with temperature between 230 – 2560C
and bed temperature of 80 to 1100C depending of the printer. For ABS adhesion on print bed,
different recommendations are made to use polyimide tape (Kapton tape or PET tape) and ABS
juice. The ABS juice is basically mixture of ABS and acetone. The polyimide tape or ABS juice
is applied on the print bed to get the first layer of prints to stick. The 3D printer nozzle diameter,
layer height and printing speed settings are taken to consideration to obtain a substantial printed
part.
28
2.4.7.2 PLA as 3D filament
PLA is considered to be the easiest material to work with during printing. It is a tough
biodegradable thermoplastic with a little brittleness as it cools down. They are normally extruded
between 160 – 2200C and print at high speeds. A hot printing bed is not necessarily mandatory
but warming the bed up with a temperature of about 600C could beneficial to the printed object.
On the other hand, cooling fan is recommended to be installed and pointed at the extruder in
order to speed up the cooling process because PLA is quite slow to cool down. They are used in
medical application and food packages. Its application in the medical world is surgical implants,
as it possesses the ability to degrade into lactic acid in the body system.
2.4.7.3 Earlier research concerning HDPE as a Filament
Specific information about recycled HDPE for 3D printer filament is difficult to find because the
processing and quality varies, but some of the studies about HDPE as 3D printing filament has
been done and few of them will be reviewed.
Christian Baechler analysed the testing procedures of diametrical consistency needed to ensure
HDPE filament could feed consistently into the 3D printer. During the extrusion the following
observation were made known.

It was observe that variance of the extrusion rate should be avoided such that the extruder
itself is automated with constant material feed rate that will enhance successful filament
consistency. In other words, there should be constant rate of extrusion.

Filament consistency should be measured along the way at an interval rate such that
optimization would be improved.

The filament was successfully extruded at an average rate of 90 mm/min and used to print
parts.

During printing, the addition of a heated print bed is recommended to limit the effects of
thermal warping vulnerability on printed parts. [20]
29
Andreas Bastian described HDPE filament printing process and changes in his research. In an
attempt to print with HDPE filament, several changes were made to the temperatures and printing
speed in other for obtain suitable outcome. The hot bed of the printer was optimized such that
HDPE would adhere and the extrusion temperature. After several temperature and printing speed
changes, the data used were; a print speed of 5mm/s, a bed temperature of 700C and an extrusion
temperature of 2300C [21].
The RepRap community also gave some recommendation according to an experiment run that
involve HDPE as filament. It shows that the filament size and extruding temperature corresponds
by indicating viscosity changes, as the extruding temperature get higher so does the filament size
diameter decreases. In regards to the printing, a foam-core board was discovered to be compatible
with HDPE as a print bed material. [22]
2.4.7.4 Diameter tolerance
In normal circumstances, filament should maintain constant diameter across the entire spools, but
due to manufacturing processes, there is always tolerances allowed depending on the size of
filament been produced. For examples a 1.75mm filaments diameter allow tolerances of ±0.3mm.
Then again, careful attention should also be paid during extrusion because inconsistence
extrusion also leads to inconsistence diameter. (Inconsistence extrusion = inconsistence
diameter).
2.4.7.5 Filament roundness
The regularity of the filament during production is of important across the entire length of the
spool. This is because the filaments undergo some sort of compression upon making contact with
wheels due to gripping and rotation winding.
Over all, the importance of high quality filament in 3D printing is to ensure uniform filament
diameter (consistency) during production process and ability to optimize this diameter at a
bending angle to rap around the spool that will be used to suit the 3D printer available. The most
commons standard filaments are 1.75mm and 3mm.
30
The data in the following table are some of the recommended processing parameters specification
during filament production that could also be optimized to suit available extruder and 3D printer.
The comparisons are made between ABS and PLA that would be used in processing HDPE and
rHDPE to produce the filament. These numbers ranges between different grade and standard
testing results. [10, 13, 14]
Table 5 PLA, ABS and HDPE comparison
Properties
PLA
ABS
HDPE
Melt flow rate
2.4 – 4.3
22- 48
4–8
50 - 55
30 - 52
20 - 40
Strain at yield (%)
10 - 100
3 – 75
>100
Young’s modulus
3500
1700 – 2800
200 – 1200
120 - 170
200 - 230
120 - 190
50- 60
100
80 - 110
160 – 220
210- 230
130 - 190
55–56
105
110
Crystallinity
~37
N/A
Cooling time
Long
Medium
(g/10 min)
Tensile strength
(MP)
(MPa)
Melting
temperature (0C.)
Glass transition
Temperature
(GTT) (0C.)
Extruding
Temperature (oC)
Glass transition
Temperature GTT
(°C)
Medium
31
3 Method
In this chapter, the research methods used are explained. The methods can be divided into three
different part; processing the material, testing the material, filament production and 3D printing.
The material processing part is categorised as recycling where the plastic shredder, granulation
process and injection moulding are used. The testing part was carried out using melt flow index
plastometer and testometric tensile testing machine. The third part involves using extruder for the
filament production while 3D printer was used to print the filament gotten from the production.
3.1 Recycling
Collection of material
Generally all sort of plastic are gathered into a single place for further treatment but in this study
only HDPE is gathered, since it’s the only material involved in recycling process. Different
HDPE containers were collected in a plastic bag shown in figure 12.Most these collected material
are from domestic products such as; used shampoo bottles, detergent containers, cleaning agent
bottles, milk jars to mention few.
Figure 12 Collection of HDPE plastics
32
Sorting
The collected plastics were sorted and separated into two categories: (1) similar plastic of the
same colour and (2) different coloured plastic together. This categorisation was performed to
ensure proper process and avoid contaminant along the way for further post-processing.
Washing and separation
The plastics was soaked in 60°C of water for approximately 5 hours in order to remove the labels
and the glue while washing at the same time. The labels were removed manually with hand by
peeling. Afterwards, they are separated into similar plastic of the same colour and the different
colour plastics are put together. The labels could have been removed alternatively by chemical
solution but this choice is made since it peeled off easily and easy process to avoid chemical
contamination or extensive washing post-processing.
Figure 13 Soaked HDPE plastic
33
Figure 14 Manual peeling of the labels
Figure 15 Similar washed plastic
34
Figure 16 Mixed coloured washed plastic
Shredding and Drying
The plastic shredder was used to shred the HDPE material into recyclable material. Similar
plastic was shredded together and the mixed plastic together to obtain the flakes that will be used
produce pellets in extrusion. Some of the big plastic was unable to fit in the shredded and was cut
with hacksaw machine to reduce its size to enable the fitting.
The shredder works by throwing the plastic through a hopper into the shredder that cuts the
plastic into recyclable material (flakes). The flakes are collected through a 5mm filter located at
the bottom of the shredder to get an approximate uniform flake size.
After getting the plastic flakes from the shredder, the flakes are dried in flexible modular drying
unit. This is done in order to remove extra moisture from within the plastic flakes. If not done,
extra moisture may affect the material when being processed. In this case the moisture could be
noticed as runny plastic melt or air bubbles within the plastic melt.
35
Figure 17 Plastic shredder (Arcada plastic laboratory, June 2014)
Granulation process
The granulation process was when the recycled HDPE flakes are produced to pellets (granules)
using the extruder. This was done in order to get a uniform size pellets that would enhance the
output flow of the molten plastic coming out of the extruder and used for the filament production.
Figure 17 shows the mixed coloured flakes that were shredded together and figure 18 shows the
produced pellet gotten from extrusion.
Figure 18 Mixed coloured flakes
36
Figure 19 Recycled HDPE pellets
3.2 Melt flow index
The phase of this experiment is to perform MFI test for the materials involved. This is done with
melt flow index plastometer. As mentioned in the literature study, melt flow index (MFI) is used
to determine how easily or poorly material is able to flow and it’s measured in grams per
minute’s period (g/min). The test is done according ISO 1133 standard using Mitaten MFI
extrusion plastometer. The materials involved are HDPE and rHDPE and were tested according
to ISO 1133 standard.
Since the part of this research objective is to find filament specification and compared to
optimized HDPE to rHDPE material properties. ExxonMobil™ HDPE HMA 014 grade was used
as virgin HDPE in order to compare the values to the recycled one. The choice of this material is
due to two factors. The first reason was the material fairly good mechanical properties and
possibility to process with extrusion and this grade is specifically recommended for extrusion.
The second factor is the availability of these materials in Arcada plastic laboratory. The materials
are always preheated for 5min at 1900C temperature before applying load of 2.16kg onto the
piston that will pushes down the molten plastic through a die. After performing recycling process
on the HDPE materials collected to get the rHDPE pellets, MFI test is also performed at 190°C
melting temperature with the same load as virgin HDPE. Each material test was run 4times
consecutively to obtain average figures. Figure 20 shows the extrusion plastometer used to
perform the melt flow test.
37
The data was obtained by setting a cutting interval of 30 seconds on the plastometer. In other
words, for every 30 second the plastometer is extruding a cutter is set to cut the molten plastic
and weight afterwards. The following calculation method is used to obtain the data.
Recycled HDPE weighs 0,18g in 30 seconds, In 1 min equals 0,36g and in 10 min will equals
3,6g/10min.
Figure 20 Melt flow index plastomer (Arcada plastic laboratory, June 2014)
3.3 Injection moulding
Injection moulding took place in other to produce the dog-bones to be tested with tensile testing
machine. Input values such as nozzles, cylinders, hopper temperatures, clamping force, cooling
time, injection speed, injection times was used according to the moulding parameters of
ExxonMobil™ HDPE HMA 014 grades. Although some of these values were optimized in other
to produce dog-bone piece that suit the purpose of this research. This is due to the fact that the
values on material data sheet are used for many different types of process and it is not said to be
specific a certain product. After several optimization runs of the dog-bones the parameters shown
38
in table 6 were the input values for the virgin HDPE and the same value were used to produce the
recycled HDPE. Engel 90 CC was the injection moulding machine model used for the
production.
Table 6 Optimized set up parameters for the injection moulding
Parameters
Value
Unit
Hopper
50
0
C
Cylinder 2
190
0
C
Cylinder 3
190
0
C
Cylinder 4
180
0
C
Nozzle
200
0
C
Mould temperature
43
0
C
Clamping force
300
kN
Cooling time
15
s
Injection speed
50
m/s
Injection time
4
s
Holding pressure
140
Bar
Holding time
5
s
Plasticizing stroke
25
mm
Temperatures
Injection
Holding pressure
The Virgin HDPE and recycled HDPE dog-bone produced from injection moulding are shown in
figure 21 and 22 respectively.
39
Figure 21 Virgin HDPE dog-bone piece
Figure 22 Recycled HDPE dog-bone piece
40
3.4 Tensile testing
In order to be able to test and compare results, the dog-bone produced from injection moulding
machine were tested. This was done according to the explanation in chapter 2.4.6.4. It was done
to obtain mechanical data of the dog-bone such as tensile strength, engineering stress and strain,
young modulus, deformation etc. The virgin HDPE and recycled HDPE were tested with
testometric tensile testing machine. The tests were conducted at a load speed of 51mm/min
following ISO 1133 standard. The amount of dog-bones tested was set to be seven in order to be
as accurate as possible by obtaining an average value and test specimen dimension are 120 x 12.8
x 3.1 mm.
The following figure shows the material been tested and the result obtained are listed in chapter
4.
Figure 23 Dog-bone being tested with testometric tensile testing machine (Arcada plastic laboratory,
June 2014)
41
3.5 Filament extrusion
The filament was produced using KFM Eco Ex model extruder that as six temperature zones. The
first two zones function as feed zone heating temperature, the second two zones is the
compression zone heating while the last two zone heats the pumping zone and the die. During
the extrusion, several cooling methods were performed to obtain the filament size such as cold-air
gun, water-bath, and table blower. An electric spooling device serve a puller for the filament
during production while the filament is rap around a plastic profile tube.
Prior getting the desire size of the filament several optimizations was done. The optimization
production is listed in the figure below and the parameters obtained are detailed in the result
chapter. Figure 23 shows the manual pulling of the filament to the pulling device before been
automated. After several pulling distance between the extruder and the puller, it was observed
that the appropriate pulling and distance between the extruder the puller is one meter.
Figure 24 Manual pulling of filament
42
Figure 25 An electric pulling device
Figure 26 Filament extrusion (Arcada plastic laboratory, Oct 2014)
43
4 Results
4.1 Tensile testing & MFI
Table 7 Comparison table for 3D filament
Tests
PLA
ABS
Tested HDPE
Tested
(Mean Value)
rHDPE
(Mean Value)
Melt index
2.4 – 4.3
22- 48
3,37
2,85
50 - 55
30 - 52
25,45
25,59
3500
1700 – 2800
463,35
428,38
10 - 100
3 – 75
16,12
16,12
120 - 190
200 - 230
190
190
160 – 220
210- 230
190
160 - 190
(g/10min)
Tensile
Strength (MPa)
Young’s
modulus (MPa)
Strain at Yield
(%)
Melting
temperature
(0C)
Extruding
Temperature
(oC)
44
4.2 Filament extrusion optimization
Table 8 Optimization table of recycled HDPE (Filament Die)
Test runs
Extrusion
Extrusion
Cooling
temperatures
speed (rpm) method
zones (0C)
Pulling
Filament
device
size (mm)
voltage (V)
1
175
20
Cold air gun
9,5
1,86±0,01
2
175
15
Cold air gun/
8,4
1,72±0,01
8,6
1,6±0,01
8
1,78±0,02
8,1
1,76±0,02
8,4
1,75+0,02
table blower
3
175
15
Room
temperature
4
175
15
Room
temperature
5
175
15
Room
temperature
6
175
15
Room
temperature
45
Figure 27 Recycled HDPE filament
5 Discussion
To determine the mechanical properties of recycled HDPE to suit 3D filament and the variability
in these properties, this investigation looked at the relationship between melt flow rates, tensile
strength of recycled HDPE compared to ABS and PLA and produced the filament. Table 7 and 8
shows the obtained data and optimization respectively
5.1 Changes in mechanical properties
5.1.1 Changes in melt index
The melt flow rates of recycled HDPE and virgin HDPE were studied to compare to ABS or
PLA. The tested value of virgin HDPE gives 3, 37 g/10min and recycled HDPE gives 2,85
g/10min. Their difference of 0,52 g/10min indicates a decrease of 15%. This decrease in value of
46
the recycled HDPE can be evaluated as result of degradation of the molecular chain that hinder
the melt flow to become lesser. Comparing 2,85g/10min melt flow of recycled HDPE to PLA, it
can observed that the flow rate only falls between the range of 2,4 – 4,3 g/10min and this
signifies the possibility of recycled HDPE to be used as 3D filament.
5.1.2 Changes in tensile strength
Recycled HDPE gives an average value of 25,59MPa and the virgin HDPE gives an average
value of 25,45MPa among the seven run tested. The value of tensile strength of the recycled
HDPE is observed to increase slightly. This increase is however not significant given 0,55% with
respect to the virgin HDPE. Comparing the recycled value HDPE to ABS or PLA tensile
strength, it is observed that there is enormous value gap between both of them. This difference
could also be established with different grade of materials used for specific processing methods.
As a result of this, the recycled HDPE used for this analysis may have contained different plastic
grade that are produced with different methods. It is also recalled that almost 70% of the gathered
HDPE plastics are bottles, therefore this could be that the plastic grades involve are meant to be
produced by injection mould blowing.
5.1.3 Changes in Young’s modulus
Young’s modulus value of the tested virgin HDPE as average value of 446,37MPa and the
recycled one gives an average value of 428,38MPa indicating the decrease of 17,99MPa, which is
4,03% from the original virgin material. It can be established that the rigidity of the recycled
HDPE decreased slightly and can be linked with ABS that has mild flexibility that makes rHDPE
a probable material for filament.
5.1.4 Strain
The yield strain value of both the virgin HDPE and recycled HDPE indicates an average value of
16,12% and there was no difference on the average value. This strain value can be established
with PLA strain data indicating high probability of recycled HDPE for filament.
47
All of these tested value depends on each other in one way or the other, that is to say, melt flow
results indicates a consistent decrease of 15% and young’s modulus decrease of 4% while the
tensile strength indicates an increase value of 0,5%. The noticeable increase and decrease during
the testing goes with explanation in chapter 2.5, that low melt flow index value results in higher
mechanical strength and higher melt flow index value results in lower mechanical strength. For
instance, the recycled HDPE is less viscous when compared to the virgin. In terms of mechanical
properties, it can be observed that the recycled HDPE shows a decrease value in melt flow rate
and an increase value in tensile strength that proves the literature findings mentioned above. The
slight tendency changes of young’s modulus between the recycled and virgin material can be
established to be the rigidity of the recyclate content, grades difference and possible degradation.
5.2 Filament extrusion
During the extrusion experiment two type of die was attempted to produce the filament. Lace die
was first attempted to produce the filament with water-bath cooling but the diameter of the die
(6mm) was too big to optimize to the desired filament size and the filament shrinks too fast and
became tangled as soon as it touches the water. Therefore, water-bath cooling was out of the
question as a cooling method.
The filament die of 2mm was first tested with virgin HDPE at the same cooling method and
pulling voltage to obtain some rough data to be optimized. More than seven trials were made but
the only reasonable data was recorded and it served as a preliminary for the recycled HDPE
material. The preliminary set data is indicated as the first test run in table10. When the recycled
HDPE was in used, the extrusion temperature remained the same all thorough while other
parameters were modified.
During the second and third production, the extrusion speed and the pulling device voltage was
reduced by 25% and 60% respectively of the virgin material production (20 rpm and 21 V) then
the filament size got close to the wanted size and cooling method was also changed to a room
temperature.
Further on the production, the room temperature cooling method was observed to be enough for
cooling at one-meter distance to the pulling device. This gives the molten plastic an approximate
48
70% cooling before been pulled by the pulling device. Then, it finally cools down after passing
through the puller. The top and bottom wheel of the puller was adjusted to ensure the filament
formed the required circularity shape that would fit the 3D printer nozzle available in the lab. At
this stage and up to the final production, changes were only made to the pulling device voltage by
reducing it with percentages.
The filament size measurement was done with venire calliper at an interval of 5meters with a
tolerance of +0,02 in order to be as accurate as possible while producing them. The tolerance of
+0,02 is allowed according to previous type filament production and the available 3D printer also
gives a room of 0,3 tolerance (1,75mm+0,3 tolerance).
6 Conclusion
This thesis work covers recycling process of HDPE, finding 3D filament specification and
production parameters. Comparison of ABS, PLA and HDPE was done to see the probable
relationship between the materials and it is observed that recycled HDPE data goes along with
PLA data with respect to melt flow, yield strain melt temperature and extruding temperature.
Tensile strength and young’s modulus only differs from the test performed and it is neither close
to ABS nor PLA. It can be established that recycled HDPE is suitable as 3D filament because of
the data relationship to PLA.
As can be seen, changes in mechanical properties were monitored through recycling by
measuring flow rate and tensile properties. Increases in tensile strength of the recycled material
and decrease in melt flow rate were obtained. Comparing these engineering properties, the result
shows that recycled HDPE is less viscous than the virgin HDPE. This amount to the prove of
lower melt flow rate results to higher the mechanical strength and vice versa while the tensile
strain remains even on both virgin HDPE and recycled HDPE.
The findings in the literature review concerning 3D filament production were built upon and
viable corrections were made during filament production. Inconsistency in extrusion rate was
avoided by ensuring constant feed of material is available that eventually improves the constant
rate of extrusion.
49
Based on the data obtained during filament production, the puller speed is seen to be a great
influence during the optimization simply because the voltage is been reduced at several
percentage before getting the desired size. The percentage reduction basically improves the
filament sized up to 60% from the started 21V. The extruding temperature of recycled HDPE also
contributes to the optimization process by reduction. During the optimization, the temperature
was reduced by 7,9% to enhance the procedure. It can be concluded that the higher the extruding
temperature of rHDPE the more viscous the molten plastic becomes therefore reducing the
temperature by percentage (7,9% of 190) turns out to balance the required viscosity.
Eventually, the parameter specifications of obtained for recycled HDPE filaments are:
Table 9 Recycled HDPE filament size parameters
Extrusion
Extrusion
Cooling
Pulling device
Filament size
temperatures
speed (rpm)
method
voltage (V)
(mm)
15
Room
8,4
1,75+0,02
zones (0C)
175
temperature
The results clearly shows that the pulling speed of filament is essential and have a big impact
during the optimization with drastic improvement of 60% close to the 1,6 mm filament size.
Distance between the die and the pulling device and room temperature cooling method also play
a significant role to obtain the filament size diameter. During the filament production, another
important part is filament consistency and diameter circularity in order to obtain desire 3D
filament size. It can be concluded that recycled HDPE was found suitable as filament with
respect to the data obtained and in relation with PLA data.
On a final note, 3D printing with the produced filament was unable completed due unavailability
of form-core board that is compatible with HDPE which will serve as a printing bed but the
filament was tested with the printer available (MINIFACTORY 3D-PRINTER) in the lab to
observe the diameter tolerances, circularity and ability to print. The circularity and diameter
tolerance was discovered to match the printer.
50
7 Suggestion for further work
Investigation could be done on recycled HDPE to improve the shrinkage. One example would be
adding additives and finding the relationship with cooling method.
Another investigation could be about why recycled HDPE tangles during cooling with water that
makes water-bath cooling not suitable before the filament size is optimized to desire diameter.
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8 References
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