Making Furniture From Fully Recycled Plastic

I made a plastic beams out of and HDPE and PP mix, extruded using recycled plastic. Assembled a chair using the beams.

1. Hardware: 1/8" Bolts, Washers, and Nuts (8 Bolts and Nuts, and 16 Washers)
2. 4x4" steel plate 1/4"(4)
3. Steel Square Tube 2 x 2" x 1/8 x 6'
4. 1/2 inch fitting
5. Various Tools and Plastic Extruder

When I decided to take this on for my long term project, there had already been work done. There were two existing molds, and one, the approximately 3” by 1” mold failed during a test extrusion due to the plastic cooling and solidifying, pressurizing the incoming plastic, which made the steel completely bulge out. Along with that, the results from the tests with that mold were suboptimal, so I decided we needed a new design that would allow the plastic to move fast and produce good stock, while still not cooling too fast or having inefficient pressure. We got a 6 foot long 1.75” by 1.75” steel tube, but I decided to first make a CAD model for the bench I wanted to make. My CAD file is attached right down here ⬇️

For the beam mold, I had to plasma cut a hole into two of the 4x4 inch 1/4" thick steel plates Next, I got the end plates for the beam mold, and drilled holes lining up with the plate welded onto the mold. every plate had to have holes for the hardware, so I used the drill press to drill the holes. For one end plate, I drilled a 1/2 inch hole in the middle for the 1/2 inch fitting, and welded on the fitting. To allow the extruder to seal to the mold. I had to learn how to weld to weld on the end plates. Overall, the build went well and I learned a lot.

For each beam, I used the extruder and followed these steps:

1. Heat extruder
2. Connect beam mold by threading into nozzle, leaving end plate off to allow air to escape.
3. Support the mold using a stand.
4. Fill the funnel with plastic mix: %70 PP %30 HDPE (I decided on this during planning)
5. Turn on the extruder with the green button.
6. Run the extruder for around an hour or untill step 7, refilling the funnel and not letting it run out.
7. Once the plastic has reached the end of the mold, bolt on the end plate to minimize excess air in mold.
8. Run the machine for around 10 more minutes to fill out and pressurize the mold.
9. Turn off the extruder and leave the mold on for at least a few hours to let it cool.
10. Unscrew the mold from extruder, breaking the plastic still connected in the nozzle of the extruder.
11. Leave the mold to cool for a day or so.
12. Once the mold has cooled, cut off excess plastic from mold and unbolt the hardware to take off the end plates.
13. Carefully slide the beam out of the mold. If you experience issues, you can find a cylinder that fits into the end of the mold, and pick up the mold and smack it down until the beam can slide out (sliding it out can be difficult, so you can have one person hold the mold and another pull the beam out)

After I had made around 10 beams, I decided I wanted to start assembly. I realized I would need a lot more beams to make a bench, so I decided to make a stool. This would allow my build to be much more compact, and would utilize the materials more effectively. I chose to keep a very similar design to the bench, but just to adjust the proportions. In the end I decided to make it 14” by 17.25” and 18.75” tall. This way, I would only need 17, 15, and 14” beam pieces. I cut them out on the miter saw and started to assemble them. I tried to screw them together normally, but I experienced some problems. First, when I attempted to screw in the middle of the beam, it would strip out due to defects from extruding. This was a major problem as I could not assemble the furniture according to my plan. I decided to pre-drill and put the screws closer to the edge of the stock, where there was not bubbles. This worked decently well, but I ran into a major problem. While the beams themself were very strong and did not break even under major pressure, the plastic was more brittle than expected. This was due to using around a 50/50 mix of HDPE and PP instead of a 30/70. I had a few ideas to solve this problem, including using glue, but I decided on replacing the broken sections with new ones, and putting the screw closer to the center, but not where it would strip out. This allowed me to assemble the top of my stool easily.

Overall, I enjoyed this project and it helped me learn a lot about plastic and manufacturing. Beaches are the frontline of the plastic pollution crisis, and by recycling trash at Morro Bay High School, this project helps directly reduce pollution in the ocean as my school is directly next to the beach. By doing this project, we gain visibility for Precious Plastics and recycling, educating people on waste. In the future, placing a bench we made of local trash right next to the beach, we can create a powerful visual reminder for everyone who visits to not only stop plastic and trash from going into the ocean, but also realize that they can have a personal environmental impact and remind them of the importance of recycling and reusing. While this was a good project, I would do some things differently if I repeated this project including:

1. Attempting to use plastic directly from the beach
2. Trying to find a way to clean material more effectively before use
3. Changing and testing different ratios of HDPE to PP
4. Shifting Furniture design to not rely as much on the screws going into the face of the stock
5. Prioritize PPE and respirators more often
6. Using smaller molds for more refined results due to better pressure
7. Cut the sections for furniture together to make sure all individual lengths are the same
8. Testing the beam's effectiveness before making lots of beams
9. Try doing some colored beams
10. Using brackets during or something other than just screws to make sure you get right angles

If you are trying to repeat this project, I hope you have learned something useful from my experience, or if you are just reading, I hope you know that you can have a positive environmental impact. P.S. it is also surprisingly satisfying to extrude plastic.

-Tanner Wade in Morro Bay High School's Engineering Department