Recycling plastic bottle caps

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Yesterday, we did a workshop with my students and I am so proud to present you that we can recycle the bottle caps with the easily way. The first thing we did is to cut the PET bottle caps into pieces. We can use the hand tool such as scissors for metal cutting. Somehow, you can use the jigsaw band in the machine shop to cut it into two pieces first. It can reduce the time of cutting.


Then we create a metal mold by cutting and using the arc welding machine. It looks like box that the top opens for laying the plastic pieces over. From the Youtube demonstration, we need two clamps that can hold the box and compress the plastic when it melt. At the beginning, we used the baking paper that used for bread. The paper resists the high temperature. You can see the picture below when we place the paper into the oven.


The plastic will melt down at the temperature 140 celsius with the length of 10 mins. When it melted down, we can use the leather grove to move and shape it. It looks like a gum. The thing that we did wrong is to put into the mold later because it is difficult to shape it when the plastic started to form. As a result, we put the metal mold into the oven directly.


We try different kind of grinding method as well. We put the cap into the grinding machine and the result looks like the powder. One thing that we realise is that the size of plastic scrap does not need to be powder. The appropriate size like a pieces of small gravel is ok. After the plastic cap melt down into the metal mold. We press the top cover of the metal box and use the hammer to compress it down to every conner of the metal mold. Then we take it out from the mold. It is quite stiff but not difficult to take it out because we use the vegetable oil and paint it into the metal mold. The final result looks like this.


The top part may not smooth like the bottom of the mold since it was cover. We may need to close the top and put into the oven next time. We did a good job on making a piece of plastic. We asked students to check whether it is still floating on the water or not. Since we know that HDPE plastic can float on the water. We expect that it could float. The assumption is correct. It floats on the water. We asked students to think more about the mold that has a logo of sponsor and combining the 3D printing component into the design. The application of this materials can be designed as toys, knife handle and others. The next process is to cut this plastic with the jigsaw band. You can this method to be an exercise for STEM high school.

Caution: when you melt down the plastic, you should do it in the open space where the flow of air coming. It is quite stink a little bit. 🙂