Making Blocks out of HDPE milk jugs

  This is a brief explanation of the process that I have used to reuse one gallon HDPE ( recycling #2) milk jugs. There are plenty of FAQs documenting HDPE sheet welding for fabric and vacuum forming sheet stock, but I couldn't find anything on forming solid blocks. I was looking for an inexpensive material to use for test blocks that I could put on the CNC machine that would not abuse the router bits.

The process is quite simple. Cut up the bottles, stew them in really hot canola oil on the stove in a pot, and then take the hot plastic and press it into a form to cool. Please note that melting some plastics can release toxic fumes. I have had no issues with HDPE, however I am not a chemist and cannot comment on the health risks of this process.

The pieces of cut up bottle can be any size that fit in the pot. I washed the bottles and removed the labels before cutting. Pouring a small amount of near boiling water into the bottle and swishing it around will help release the glue on the labels. Otherwise the labels tend to tear.

Most any cooking oil will work, and you don't need too much. I just put a couple of inches in the bottom of the pot. Canola oil is inexpensive and has a smoke point of over 400 F (204 C). The melting point of HDPE is in the mid 200 F range but the "extrusion" temperature range is 350 - 500 F. I uses oil heated on my stove top to a little over 350. Just make sure that the bottle pieces are dry or the water will cause oil to pop out of the pot. I used a candy thermometer to keep track of the temperature.

When heated the plastic will not flow. At best I get a really hot clear gooey ball. When melted, as you can see in the images, the plastic is clear. I have found that the colored HDPE used in detergent bottles and other containers does not work as well as the translucent milk jugs.

The form that I used applies pressure from all sides. This is important because it will squeeze out most of the bubbles of oil that are inside the blob of hot plastic and give a better starting point for milling the blocks. I drop the blob in the middle of the form and then press the top on with the two C clamps. Then I insert the two end blocks and clamp them together. The material holds heat for a long time. I left the block in the form for 15 min. and when I took it out it was still to hot to hold. The form I used was plywood that I had lightly oiled with the canola oil. The residual oil in the pot will get really thick when cool from what I assumed was some of the dissolved HDPE, however it does clear up when heated and can be reused. I'm not sure how many times you can reuse the oil. I assume that this type of remelting does weaken the material over time, but the small blocks that I made cannot be bent by hand at all. I have also shown a block of plastic that I formed in a muffin tin and cut in half to show the consistency of the reformed material.

Once the blocks were cool and washed I ran them through a joiner and a planer to clean up the dimensions. The blocks machine easily with all woodworking tools and can be carved with a knife without difficulty. The HDPE has a very waxy feel to it.  It will weep a little oil over time because of tiny fissures in the plastic that hold small amounts off fluid.

I also tested a block on the CNC router and had excellent results. 

Comments

[Jack Daniels]

why not use one of those silicone bread molds that are rated for 600F or higher cut the bottles up into small bits and bake them in the oven at 350F wouldn't that give you the same result and no messy oil to deal with?

[mfoster]

I had tried to do this in a metal bread mold with really poor results. The problem seems to be that the heating elements in the stove radiate a much higher heat to get the oven up to temperature. The result is that the bits on the top get burned and discolored while the lower parts don't really melt at all. Using either method the plastic never really pools or settles into shape by itself. It requires mechanical pressure to form the blocks.

[Jack Daniels]

do a modifies pie crust method fill one pan with the plastic (again use silicone) put another stacked on top this will prevent direct exposer and fill the top one with dried beans that will give you the mechanical pressure

[chuckmeister]

How slick were the blocks? I am thinking about making athletick flooring this way? Any way you can think of to make blocks less slick if yours were slick?

[mfoster]

sorry that I missed your question. Better late than never. The blocks are pretty slick. There is no way that I could find that would fully express the oil. When the blocks were cut up when they were cool there were some, not many, very small oil bubbles inside. The blocks are slightly slicker than the original plastic.

[gmurray2]

I just tried this today with colored HDPE from a new, never used one gallon gas canister because I wanted to see if the color would stay in the plastic. I might not have gotten the canola oil hot enough because the pieces never did really soften, but what puzzles me is that the they did expand to over twice their original thickness and stayed that way after cooled. Has anyone else seen this or have an explaination?

[mfoster]

I did a couple of test with old laundry detergent bottles and I had really bad results. I know that the oil was hot enough because I was using some translucent milk bottle is the same batch and it melted just fine. I suspect that there are some other additives in the colored plastics to make them more suitable for caustic or corrosive applications. Thanks for the note, hopefully it will save someone else a wasted batch.

[daschick]

Thanks, this is great! How many bottles did you use for that block and what were the approx. dimensions?
I'll start saving my bottles today (we go through 3-5 a week).

[mfoster]

The block was originally 3/4" x 3" x 5". I ran it across a jointer to smooth the sides so the block pictured is 5/8". I cut up about a dozen 1 gallon milk jugs throughout this project. I recommend running some hot water over the jugs just before you cut them as it will make them much easier to cut. The block pictured is probably 3 or 4 jugs worth.

[rallen71366]

I've heard of doing this with some of those throw away plastic shopping bags from W*lmart, and other general merchandise stores. Have you tried those? It would finally be a good re-use of those nasty things.

[mfoster]

I tried the shopping bags. You need a really huge pile of bags to make a usable block. Think suitcase full of bags. One of the side effects of the process is that tiny bubbles of oil will get stuck in between the bits of plastic and the more layers the harder it is to get them out. More layers made more mess.

[yomero]

could this work with PET?

[mfoster]

I really don't know. HDPE is really easy to work with. PET I think requires higher temperature and might not react well to the oil. I have seen instructions on extruding PET into injection molds, but it is a more complicated process.

[yomero]

cool, thanks!

[scoochmaroo]

This would also be great broken out into a step-by-step Instructable!

[SWV1787]

I like this I think I might have to try an make some hard plastic casting molds this way, or even making a hard protective case for smallish electronics. nice instructable and photos