Plastic Milk Bottle Lids melted into cactus

Experiment #2: melt plastic with oven

One convection oven and a meat grinder, please!

After testing a heat gun to melt plastic – specifically milk bottle lids last week, I decided to take it to the next level for Plastics Experiment #2 by trying to shred and melt plastic with a cheap convection oven and a metal meat grinder.

New convection oven

The oven’s purpose is to melt the plastic better. I supposed I could have used my kitchen oven, but I was worried about contaminating it with plastic if I accidentally burned something. I bought the meat grinder to try to find a better way to cut up the plastic as my hands were tired from doing it with a pair of garden secateurs.

The Results?

After melting two buckets of HDPE bottle caps into many different types of metal and silicon moulds, I’ve learned a lot, but still don’t have a viable process or product.

The meat grinder was a waste of time as it just crushed the plastic rather than shredding it. I was ready to try various kitchen appliances next when I decided to first see if anyone else posted their results online. They all said NOT to waste your time and money.


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The meat grinder only crushed the plastic rather than cutting it.

I’m so glad I researched this first. From blenders to food processors and credit card shredders – nothing is strong enough to properly shred the plastic apparently.

So if I want to continue down this path, I have to either continue to cut the plastic by hand (UGH!), melt the lids as is (which is guaranteed to result in lots of air bubbles or buy a proper shredder. Out of all of the machines I need, an used shredder is the cheapest. So that may the right thing to do if I keep going.

Plastics Experiment #2:

Let’s go bigger!

Up to this point, I had a been playing around with cookie cutter sized moulds. With Recycled Plastics Experiment #2, it was time to try something bigger now that I had the oven. In the picture below, you can see that the square silicon baking tray failed to hold it’s form when it cooled down.

Failed test of melting HDPE in a silicon mould
Can I make a flower pot?

I was curious to see if I could make some sort of a flower pot using these two metal mixing bowls as a mould below. It came out better than I expected, but looking a bit like an ashtray instead. The unfortunate part was all the air bubbles in the none-pressed edges. I want to see if I can tidy it up with a friend’s mitre saw later.

Pressure Test

I decided to use pressure in this test by clamping two baking sheets together. The funky design was only because I ran out of the purple lids. This turned out pretty good even if a bit uneven. The pressure eliminated most of the bubbles from the sheet. Yay!

What’s creating the tiny bubbles?

In the final test, I wanted to know what was causing the tiny air bubbles in the cactus below. I tried cutting the plastic in different sizes, putting water drops into the mould and using a different colour lid (to ensure it wasn’t just an issue with the green colour lid).

When all of those tests still resulted in tiny bubbles, I cut up two of the little cacti I made earlier, and and remelted them into a metal pineapple mould resulting in no bubbles.

This makes me believe that it’s actually the silicon mould that is causing the bubbles, but I need to get another small silicon mould to double check this theory. It’s probably correct though given how much the large square mould had changed in shape once cooled.

Silicon versus metal mould experiment

Lessons Learned from Recycled Plastics Experiment #2:

  1. HDPE seems to melt best at 180 degrees Celsius (at least in my oven). However, if there’s any leftover milk, coffee, food etc residue, that will burn and usually turn into an ugly brown.
  2. Water and plastic don’t mix. I’d heard that before, but I thought it was because it was harder to cut the plastic in shredders. Instead, what I found was that wet plastic resulted in more bubbles in the end product especially in the bigger pieces.
  3. I don’t have to cut the lids for the larger moulds. They’re small enough to melt just fine. The main problem is that they seem to trap air bubbles as they melt because of their shape. So, if I care about a perfect shape, I still have to cut up the pieces, and the smaller the better to reduce the air bubbles it seems – unless it’s in a silicon mould which guarantees bubbles regardless.
  4. There’s no way to create a perfect flat surface with HDPE (#2) without using force to hold the form into shape while it’s cooling down. The prohibited cost of steel moulds is why I started down this pathway. So far, all the experiments are just confirming what I’ve already been told.
  5. Silicon moulds are by far the easiest when it comes to removing the form inside. In the large shallower pans, the plastic shrinks to make the removal easy too. However, it’s a fight for most other metal moulds.

Final thoughts?

Right now, I’m using these experiments to learn more about the properties of plastic, and to see if I can make something useful without having to spend a fortune on a traditional steel mould.

While, these plastics experiments have been a lot of fun in the process, I am also consciously noting the lessons learned for each test. I have learned heaps already, but I’m still a long way off a viable product. More to do.

Published by

Tammy Ven Dange

IT Consultant for the Not for Profit Sector | Host of "Executive with a Cause" Podcast

3 thoughts on “Experiment #2: melt plastic with oven”

  1. I melt my plastic lids in a hot press so they are indeed flat. I also can add layers to make it thicker. For some reason using a press cancels bubbles. Love what you’re doing keep up the great work.

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