Update
Complete success.
Over the past week or so I collected HDPE number 2 bottles and as I was watching TV I just kept cutting the bottles into smaller smaller pieces. Note double check the recycle number, I found a few bottles that looked like HDPE but were poly polypropylene number 5 more on this later.
My first mold I filled up my small loaf pan lined with parchment paper with the multicolored pieces and baked it at 350 to 400 until its sort of looked "plasticy" aka the plastic looked melted in texture. I then added more as it also collapsed as it melted. They never became truly liquid more like bubble gum so I had to push down on the mass with a block covered in more Parchment paper.
After an hour or so I took it out and clamped the block on top.
For the black I used another approach. I had read that HDPE for bottles is formulated differently than HDPE thats injection molded because bottles are blow molded. So the injection formulation will melt to a more liquid state.
At work I get long rolls of vinyl in boxes with black HDPE plastic ends that support the rolls in the box and I have pounds of them. They are injection molded so I made one mold with just that plastic.
For this one I didn't cut it up so much and I layered the plastic in batches letting each layer mostly melt before adding the next which helped with any bubbles and I think sped up the melting. I didn't have to clamp it after it came out of the toaster oven and as each layer melted it was much more fluid and almost all the bubbles floated to the top where I could easily pop them.
After both of them were cool I then carefully trimmed them on my table saw. Be careful they are slippery buggers.
Now as for the poly polypropylene number 5. It melted just as the HDPE did and I got a nice block of red from a few cat litter buckets. The problem was it doesn't cut as nicely as the HDPE I think its has a lower melting point and it gummed up on the table saw.
As for smell there was nothing except the smell of what ever was in the bottles. No mater how much I washed them out I still smelled the gel dish washing and shampoo and the fragrance from the kitty litter.
So now have more than enough material to mill into a special tube in slimline mold I'm making. Also material for other jigs or non stick bushings.
Next time I may try to mold my recesses into the plastic so I don't have to cut them out of a block.
This video is the one I got most of my info from. Beware after watching these videos you maybe tempted to try to make a HDPE blank. I wish resin could be moved around like what he does.
https://www.youtube.com/watch?v=V4zRTDQukRM&t=181s
