Picture of 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. 
taur5613 months ago

This method is fantastic . I have successfully melted plastic bags and colored plastic containers . They mix well in the softened state and bond as one piece under pressure which also expresses the oil at the same time . I recommend this for anyone experimenting with plastic molding or making of single parts . Wish I could show you guys the colors achieved by blending clear plastic bags with other colors . I think this would make really cool teens fashion accessories .

Oil is entirely unnecessary. Baking the plastic in an oven is already used in Africa to make sandals out of all types of plastic. They use a large sun reflector to heat a metal funnel and to some degree, all metal above it attached. Once the plastic heats enough, it runs into the form which is I think coated with powder to prevent any gelling issues with the plastic. This can be done using a toaster oven with a hole drilled in the base of it I suppose. You might install a valve inside the oven which you can turn from a distance with a long rod leaving the oven from the side so you don't burn your hand or have to open the oven to release the liquid once heated to melting point.

The oil provides even heat conduction to all the plastic, making it easier to get a consistent melt.

mfoster (author)  mikefromspace1 year ago
I tried directly heating the HDPE in a toaster in forms in the hope that it would slump into the forms. It has been my experience that the HDPE does not flow easily when melted. I can't imagine that one could get it hot enough to flow by gravity alone without it catching fire or browning from hot spots. I think that most industrial processes involving HPDE use pressurized extrusion. My tests in the toaster oven produced unacceptable results.
Ahh yes, but this is because the air is not hot enough and the metal is too hot. A convection oven would yield far different results, as would the very slow rise in heat to about 270F; since the heating coils will heat the plate up to 400F in 2 minutes even if you're set to 200F.
I did this with a convection oven in a pyrex bowl ontop a metal plate and she made a wonderful little ufo shaped blob.
mfoster (author)  mikefromspace1 year ago

True. Today I set my little convection toaster on 400F, sliced up a milk jug and put it in a pyrex measuring cup. It took a little while to get it all to melt, but it slumped into a blob without and browning. I though that it might be hard to get it out, but when I cooled the container with cold water the shrinkage pulled it away from the sides. It still has some air pockets in it, but this certainly is a cleaner process. You should write up your process and submit a new instructable. Thanks for the insight.

Dark Solar mfoster11 months ago

tried the the toaster oven & pyrex trick with with fair success. managed to eliminate almost all air bubbles (also made some very nice foam a few times) by placing hot dish on rack in vacuum-modded pressure cooker and pulling partial decompression, then returning to oven for a few minutes.

arvevans7 months ago

Press the blob between a pair of boards, using parchment paper to keep it from sticking to the boards. Add small metal nuts or wooden blocks as spacers between the boards to control thickness. Woodworking clamps hold the boards together while your blob cools. HDPE contracts as it cools, expect that to happen.

mfoster (author) 10 months ago

1/8 inch should be stiff enough and you cold cut it with a utility knife. I'm not sure how you would press out sheets. I think that the commercial process for thin sheet forming is rolling. I don't think that you could accurately press 1/8 sheet in a mold.

vgm10610 months ago

I am debating with thickness of HDPE that I need to get. What is the minimum thickness should I get so that HDPE sheet is rigid (Eg. like a writing pad as opposed to milk jug quality)? Will it be thin enough to cut and snap with a utility knife? I just don't have the space for power tools.

hdpe melting point is exactly 266F degrees (130 celcius). 10 to 15 degrees beyond this should be all you need.

great instructable!
is it resistant cause i want to make a slingshot out of HDPE would work ??
mfoster (author)  The-Red-Knight0051 year ago
I'm not sure what you mean by 'resistant', however I don't think that it would be suitable for any part of a slingshot. The oil in the process will leave small voids that will weaken the plastic.
ok thank you
This is a great project and I appreciate you sharing it. I am considering this as a source of raw materials for the production of rehabilitation equipment in developing nations. Do you (or any one else) have any idea if it would work just as well to use motor oil instead of food oil. Oil in the plastic is not a problem as it will simply make the plastic parts self-lubricating. Thanks again.
mfoster (author)  josh.swinehart1 year ago
I would bet that motor oil would work just as well. The trick is to have a good press to form the blocks and squeeze out as much oil as possible.
I was thinking about a press similar to this one.  I could press blocks and then shape/machine them or I may be able to press the plastic into molds to pre-shape individual parts. I have some ideas about forming prosthetic feet and knees that I may get to try out in the next few weeks. I am looking around for parts and things now.
bruce50001 year ago
can anyone tell me a way to color these ( hdpe )?
bruce50001 year ago
mfoster? im working on make some of you,r ( HDPE) now checking around for (delrin)
these is what im makeing would i be beter with (delrin) for aknife sharpening jig ?
thank,s keep these good ideas comeing
bruce50001 year ago
can you tell me about how it will hold up to wear like if the plastic is rubbed aginst wood
mfoster (author)  bruce50001 year ago
It depends on the pressure. I think that it would be fine for a drawer slide, but it is probably much too soft for a bearing. Delrin or one of the other hard plastics would be more suitable.
rimar20001 year ago
Very useful info, thanks for sharing it. I suppose you can use this procedure with bottle caps too, they are an interesting source of plastic.

Did you try with PET?
mfoster (author)  rimar20001 year ago
I have not tried it with PET. What I did find is that colored plastic, HDPE from laundry detergent bottles, and other colored HDPE did not not really melt or blend well. Probably due to the additives.
great instructable! i see that there's another one here that explains how to make carving wax often used in CNC work. apparently it's nothing more than paraffin wax plus as much as 25% HDPE, only much more expensive. melting them together works really great -- just substitute the melted the candle wax for the oil.
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 (author)  Jack Daniels2 years ago
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.
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
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 (author)  chuckmeister2 years ago
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.
gmurray22 years ago
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 (author)  gmurray22 years ago
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.
daschick2 years ago
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 (author)  daschick2 years ago
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.
rallen713662 years ago
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 (author)  rallen713662 years ago
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.
yomero2 years ago
could this work with PET?
mfoster (author)  yomero2 years ago
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 mfoster2 years ago
cool, thanks!
scoochmaroo4 years ago
This would also be great broken out into a step-by-step Instructable!
SWV17874 years ago
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