Max Luu
18 January 2014
Printed Recycling: Finding a Recyclable Thermoplastic Best Suited for 3D Printing
Purpose:
The main purpose of this project is to find an alternative plastic recycling method by
using a 3D printer. The initial goal of this project is to build a filament extruder to create 3D
printer filament from scrap plastic. The secondary goal is to use this filament to find the
recyclable thermoplastic best suited for 3D printing as a function of warp percentage, as well as
tensile, compression, and flexural strength.
Background:
3D printing is currently one of the biggest rising technologies. As the market for 3D
printing increases, so will the impact of the thermoplastic materials used. Thermoplastic
polymers are plastics that can be melted and reformed into different shapes repeatedly without
losing their core strength properties.
Acrylonitrile butadiene-styrene (ABS) is the main thermoplastic polymer used in FDM
3D printing due to its high tensile strength, density, and elasticity.i Unfortunately, ABS also has
many downsides for printing, chief among them slight safety hazards and the environmental
impact. When heated, acrylonitrile butadiene-styrene releases styrene into the air. If the 3D
printer in question is not in a well-ventilated area, the styrene can cause itching in the throat and
skin and a “drunk” tired feeling, and at industrial levels (this is unlikely with 3D printing), can
cause the affected to lose consciousness.ii The other main problem with printing with ABS is that
it is partially petroleum and natural gas-based, and therefore is created (often for the sole purpose
of 3D printing) from rare materials, and our environment is being degraded in the process of
creating ABS.iii Thus, finding alternative recycled plastics for use in 3D printing is essential to
preserving our environment in the coming years as 3D printers become common household
appliances.
On most bottles and small plastic gadgets there is a plastic resin code used for recycling.
Most bottles, bottle caps, and other small-to-medium-sized rigid plastic pieces that are sold are
recyclable thermoplastics, meaning that there is a small code somewhere on them that indicates
how they can be recycled. These plastics coded 1-6 are PET, HDPE, PVC, LDPE, PP, and PS.
Polyethylene terephthalate, also known as PET or PETE, is the plastic used in plastic bottle
production. Like all of the recyclable plastics PET is a thermoplastic, meaning that it can easily
melt and reform. High-density polyethylene, also known as HDPE or PE-HD, is one of the most
common found household recycled thermoplastics, forming most plastic milk jugs, plastic bags,
and bottle caps. HDPE is also one of the biggest markets of plastics in the world, estimated to hit
a global market of $70 billion by 2019iv, and has already reached a net weight of 30 million
tons.v PVC is the only resin code plastic I will not be using. Vinyl chloride, the building block of
PVC resin, has been classified as a human carcinogen. vi Low-density polyethylene, also known
as LDPE or PE-LD, is a thermoplastic that is less used than its high-density counterpart. It is
mainly used in thin plastic bags as it has a low density and therefore is extremely elastic but still
holds together well. Polypropylene, known as PP, is used in a variety of things such as household
toys, food packaging films, and car bumpers. Polypropylene is used often for its low density,
high heat resistance, stretch-ability when thin, and high stiffness in thick sheets.vii Polystyrene,
known as PS, is likely the second-most used thermoplastic in everyday life, behind the two
polyethylenes, with uses like many plastic cups and computer and television housings. Due to all
of these figures, it is clear plastics are a large part of everyday life. Thus, it is essential a more
efficient recycling method be found to work towards saving the environment.
Methods:
To create plastic filament for my 3D printer from the recycled materials I used a machine
called a filament extruder. A filament extruder optimizes an auger drill bit to push plastic
through a barrel, heating the plastic and melting it as it goes along the barrel towards the end of
the barrel. As the plastic reaches the end of the barrel and pushes against the nozzle, the melted
plastic is extruded through a 1.75mm hole, creating plastic filament that fits in a 3D printer. For
reasons pertaining to cost, I built my own filament extruder.
To build the filament extruder, I purchased a 5/8-inch diameter auger drill bit, a 5/8-inch
inner diameter metal pipe, a K-type thermocouple, and a 5/8inch inner diameter heating band. I
also obtained a few scrap pieces of wood from my robotics lab. First I drilled a 7/8-inch hole (the
outer diameter of the metal pipe) in two pieces of 3x3x2 wood in the same place on both sides,
and then secured those pieces to a long and wide piece of scrap wood. Then I ground down the
metal pipe on a steel grinder so it had a hole in the middle where I could insert plastic pellets.
After that I drilled a hole in the metal pipe the diameter of a thermocouple. I then put the metal
barrel in the wood slot and attached collars on both sides so it wouldn’t move. I attached the
heating band and the thermocouple, and then wired the thermocouple to a thermocouple-reading
chip, which was attached to my main circuit that consisted of a seven-segment display, AVR ISP
(programmable Arduino AVR chip), and opto-isolator to give voltage to the heating band. With
the help of a friend I made a program to monitor the temperature of the metal barrel of the
extruder using the thermocouple and adjust the heat of the heating band based on that. I also
attached a cap to the end of the barrel where the bit would be pushing plastic, and drilled a
1.75mm hole in the middle of the cap. I then ran the auger bit while heating the barrel with a
drill, and fed plastic in through the slot in the barrel. The plastic was extruded from the top of a
table and as gravity brought down the extruded plastic it cooled and curled up into a coil.
After creating the 5 different spools of plastic (one for each type of resin code plastic
except PVC), I used the filaments to 3D print small pieces to use for testing on my FlashForge
Creator X Pro. First, I designed three pieces using computer-aided design (CAD) on my
computer that could be used for the three types of tests I was going to conduct: tensile strength,
compression strength, and flexural strength. I then printed one of each piece in each blend ratio
and conducted their respective tests, carefully recording the data.
To conduct the first test, tensile strength, I 3D modeled a piece that I could use to hang a
bucket and a number of weights to put strain on the length of the piece. The piece consisted of a
rectangular prism with the dimensions 4 inches x 1 inch x .15 inches. It had two holes on it for
attachment purposes: one on one side of the length and one on the other. I then attached the top
of the piece with the top hole to a reinforced steel truss in a building with spectra cable, a
polyethylene woven cable rated to withstand over 1000 pounds of force. I then attached a bucket
to the other side of the piece, making sure to take down the masses of the bucket and cable to use
in the measurements. I started adding weights in five-pound increments, waiting 10 seconds
between adding weight until the piece cleanly broke along its length, recording the breaking
weight of the weights, bucket, and cable. I conducted this test for all 5 different materials.
For the second test, compression strength, I 3D modeled a piece that I could use to put
weights on top of to test the strength of the piece when compressed along its height. The piece
consisted of a large cylindrical barrel with .03-inch wide walls. The inner radius was 1.5 inches,
outer radius was 1.53 inches, and the height was 1 inch. I placed the piece flat on a table and
placed a flat square metal plate on top of the cylinder and placed a bucket on top of that,
carefully measuring the masses of these two pieces to include them in the measurements. I
started adding weights in five-pound increments, waiting 10 seconds between adding weight
until the piece cleanly broke along its height, recording the breaking weight of the weights,
bucket, and plate. I conducted this test for all 5 different materials.
For the third and final test, flexural strength, I 3D modeled a piece that I could use to test
the strength along the height by putting strain perpendicular to the length of the piece and the
width of the piece and parallel to the height. The piece was a rectangular prism. The height was
.15 inches, the length was four inches, and the width was medium sized. I placed the piece
between two tables that were three inches apart from each other so that the length and width
were parallel to the flat faces of the tables. I hung a bucket off of the exact middle of the piece
with spectra cable, measuring the mass of the weights to include in the measurements. I started
adding weights in five-pound increments, waiting 10 seconds between adding weight until the
piece cleanly broke along its length, recording the total breaking weight of the weights, bucket,
and cable. I conducted this test for all 5 different materials.
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i
ii
http://plastics.ides.com/generics/1/c/t/acrylonitrile-butadiene-styrene-abs-properties-processing
http://www.plasticsmadesimple.com/DataSheets/ABS_MSDS_GP.pdf
iii
http://www.plasticseurope.org/what-is-plastic/types-of-plastics-11148/engineeringplastics/abs.aspx
iv
http://www.plasticsnews.com/article/20130312/NEWS/130319979/global-hdpe-market-toreach-70-billion-by-2019
v
http://www.dynalabcorp.com/technical_info_hd_polyethylene.asp
vi
http://www.grrn.org/page/pvc-recycling-killer-public-health-menace
vii
http://www.essentialchemicalindustry.org/polymers/polypropene.html