RePoly : a Machine to Recycle Polythelene Plastic Bottles Into 3d Printable Filament

When we dream of a better world, we inherently assume that today’s world falls short when it comes to building a sustainable future.

In this Instructable, we contribute to this vision of a better future by tackling one of today’s major environmental problems, Plastics. Not through some complex new technology, but with a relatively simple tool and an upcycling mindset centered around sustainability.

We present RePoly, a machine we designed to reduce plastic waste at both the individual and community level with minimal effort. RePoly converts plastic bottles into R-PET filament for 3D printing.

Existing solutions for similar projects usually require advanced electronics knowledge and programming skills. Our machine removes that barrier completely. It needs no electronics expertise and no coding experience whatsoever, because it uses standalone PID controller units paired with a simple user interface to manage all its internal components.

Another distinction from common recycling focused machines is the use of MS to make RePoly machine.

We could have built the machine entirely from 3d printed plastic components, but since plastic waste is the root cause of the problem we are solving, we chose a durable, long-lasting material that will stand the test of time and is easily recyclable at the end of its useful life.

RePoly uses the following components, I have also attached a BOM of the same in pdf format with this Instructable.

1. 12V 20A SMPS (240W)
2. REC-100 Digital PID Temperature controller
3. Fotek Solid State Relay, 3-32V DC Input- 0-200V DC Output
4. 12V Johnson DC Metal Geared Motor 12 RPM.
5. 12V Single Motor Speed Controller (2A)
6. Heater Element (Salvaged off old 3d printer)
7. K Type Thermocouple : Temperature Sensor

Instead of limiting the user to a specific kind of recycled output, we wanted to make a machine which will work as just a simple tool in the recycling process. for example, many injection mold recycling machines convert recycled plastics into Hair Combs, Spectacle frames, etc, these products even though useful, limit the scope of the recycling endeavor effectively turning into another piece of waste pretty soon.

So, we designed RePoly to be able produce a filament called as R-PET, this allows RePoly to aid existing 3D printers in making any product according to user choice while also recycling plastic.

Another important constraint in the design process was durability of the machine. We wanted a machine which would actually last in workshop environments and would not become a maintenance nightmare, this was important to create consistent filament output. This became a major reason is fabricating the machine in Mild Steel.

So, the RePoly Machine consists of two ends, a PET strips end and a filament output end. Users are supposed to load the PET strips end with, you guessed it, strips of PET plastic, these are then fed through the center extrusion stage by the motor attached at the other end of the device.

How does this convert PET strips into usable filament ?

So, that's where Pulltrusion comes in, a temperature controlled hotend sits at the center of the RePoly Machine. As the strip of PET cut at a specific width using our PET cutter passes through the hotend, the constant tension applied on it using the motor and the heat generated in the operation converts the PET strip into round filament.

The thickness or diameter of the PET filament is controlled by controlling the temperature of the hotend and the speed of rotation of the motor, hence we tried tweaking both of these parameters to generate a filament of our desired size.

Just uploading the source files for a project does not make it Open Source, we believe it is necessary to actively design a solution to be easily reproducible for it to work as an actually open source project. That is exactly why we used a NO Code solution to control the RePoly machine. This is done by using an Industrial P.I.D controller and a P.W.M motor speed controller, both of which are available across the world and are very affordable, costing $10 and $3 respectively.

Its not that we don't like coding, its just better to limit the steps involved in making the device, ensuring easy replicability.

After getting a basic idea of the design, its constraints and the many features that we wanted to include, we started with some sketching, this was a rather slow process and we ended up on various design variants. Then we did a feasibility review to select the most usable design based on various constraints.

Off the 4 design variants this design was finalized to be designed in CAD.

Now, it was time to design using a CAD Software, we first recreated dummy versions of the various components to be used in this machine and imported those to fit in proper positions for effective pull-trusion.

We have attached the design files here within the instructable as well as the Github repository.

We have used Fusion360 to design RePoly machine, you can use any CAD software of your choice.

As mentioned before, the RePoly Machine is completely open source, you will find all the design files as well as instructions on how to make one attached in this instructable and the following github repository.

We encourage all of you to contribute to this project on the github repository and provide feedback in the comments section.

The machine is made using MS (Mild Steel) and weighs around 6kgs, a reason for the relatively higher weight of the machine is because we chose a 2mm thickness panel, you can significantly reduce on the weight while maintaining structural integrity by making your RePoly machine with a 1.2 mm MS panel.

After designing the machine, we sent it to fabrication at a local fabrication shop.

We need to mount a hotend in the center of the machine, this hotend heats the PET plastic bottle strips into filament of desired diameter. Ideally we want our filament to be between 1.75mm-1.8mm for easy 3d printing. We used an L bracket for this purpose with 2x 2mm holes on either sides to mount the aluminum heat block.

The main structural body of the machine is metal fabricated using mild steel, however parts which were too complex to metal fabricate or needing additive manufacturing as the only suitable method of making were 3d printed. This includes the following components.

1. PET Strips Spool
2. Spool holder for PET strips
3. Pull-trusion spool motor attachment
4. Pull-trusion Spool

We have tried to salvage as many components as possible from existing devices, for example the Heating element and temperature sensor in this project has been used from an existing old 3d printer which was just lying around.

I have attached the wiring diagram here, Mains power enters through the white wall switch on the far left and connects via red and black wires to the input terminals of the large metal Power Supply (PSU) on the bottom left. The PSU distributes low-voltage DC power (red = positive, black = negative) through multiple wires to all the control components.

The PID temperature controller (REX-C100, top center) is powered directly from the PSU. Its thermocouple input receives the temperature signal from the silver probe mounted on the hotend via red and black/purple wires. The PID’s control output is wired to the input side of the Solid State Relay (SSR-10 DA). The SSR’s output then switches power to the hotend’s heater cartridge through the red wires connected to the gold-colored hotend block.

Independently, the PWM motor speed controller (small green PCB with black knob in the lower center) is also powered from the same PSU via red and black wires. Its output terminals connect directly to the 12V DC geared motor (bottom right) using red and black wires, allowing the motor speed (and therefore filament pull rate) to be adjusted manually with the potentiometer knob.

All negative/ground connections are tied together through the black wires, creating a common ground across the PSU, PID, SSR, PWM board, and motor. The overall layout uses thick red lines as a power distribution bus and keeps the temperature control loop (PID → SSR → hotend heater + thermocouple feedback) completely separate from the motor speed control loop (PWM → DC motor). This results in a simple, standalone, no-code wiring scheme.

If you decide to use the RePoly machine in your local makerspace, this will be a fun activity for all community members. Before feeding PET strips into the machine, we need to process them.

The steps for this are pretty simple and more importantly fun, just make sure to take proper care while using tools in your workshop.

Step. 1. Find PET Bottles

Step 2. Clean the bottle, remove any labels and residue adhesive

Step 3. Insert about 50ml water inside the bottle and close the lid shut, tight.

Step 4. Wear a pair of gloves and start heating the PET bottle from its bottom, where the water currently sits.

Step 5. As you heat the bottle, water evaporates resulting into slow removal of any creases along the bottle.

Step 6. Slowly open the lid and insert the bottle in a water tub (To avoid shrinkage as air inside contracts)

Step 7. Get the PET bottle stripper (3d print files attached with this instructable)

Step 8. (Wear protective gloves, mandatory !) Strip the PET bottle into a single strip of around 18mm.

Step 9. We are done, just roll the PET strips tight on the PET machine's 1st. Spool.

For this step, you do not need to switch on the machine, keep the machine powered OFF and follow the step.

After loading up our PET strip end of the machine, we will make a slanted cut on the PET strip to insert the PET strip inside our machine. Then slowly pull out the filament from the other side and knot it tight through the hole made inside our pull-trusion spool.

Once you are sure of loading the PET strip nicely in the machine, you can power it up !

Once you switch on the machine you should see the PID controller show some values on its numeric display. If you find any errors, make sure to follow through the Wiring Diagram step mentioned before in this Instructable.

There are 4 buttons on the PID controller, Set, Back, Up and Down, with two numeric displays, PV and SV. PV stands for percieved value and SV stands for Set Value.

PV = Actual value from the temperature sensor

SV = The value you want the device to achieve.

The PID controller will use Proportional, Integral and Derivative correction methods to attain the SV and maintain in throughout the operation. You need not worry about it, but if you want to understand how it works, here is a great explanation video : https://youtu.be/qKy98Cbcltw?si=YM7y35w6-CnUlF_m

Set the SV value to around 140 using the 4 available buttons and observe as the PV climbs to 140 deg C. Make sure to check if the PV value maintains itself at 140 deg C.

At this point, you can insert the hotend cover on the device and start to slowly turn the motor speed controller.

The motor speed controller does not provide any feedback on its rpm and hence it can be tweaked by eyeballing the output from our extruder.

You can use a vernier caliper to measure size of the filament output. Ideally an RPM of 8 with a SV of 140 deg C, produces consistent 1.75mm-1.8mm filament.

You can fiddle with the RPM and SV parameters to tune the filament size output to the desired size, this allows for intuitive understanding of how Pull-trusion works without the needs of complex coding or supporting infrastructure, hence making it ideal for maker lab environments even in communities with very scarce STEM infrastructure.

After making some filament using this method we attached individual strands of filament using a SunLu Filament joiner, and finally printed some interesting objects on the machine.

We also 3d printed an instructables robot in the transparent PET output, hope you like it. The prints from the R-PET filament will always remind its user of what these objects are made from, recycled PET bottles.

We believe building a better future starts with a genuine attempt to fix the mistakes of today. Thank You.