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.