This is an easy and inexpensive method to embed small objects in HDPE. HDPE is a type of plastic that is recyclable and is readily available. Using this method you can make useful knobs for bolts, embed bearings or just make something pretty. Lets get started!
Step 1: Prepare Your HDPE
Look for the HDPE symbol on plastic containers that you use and keep them aside when empty. When you're ready, clean your containers in hot water and soap. Make sure you remove any labels or caps (if not HDPE). Then cut into small squares. The more colors you have the better your end result is going to be and the smaller your squares are the easier it will be to melt.
Step 2: Prepare Your Caulking Gun Tube
Keep your empty caulking gun tubes when you're finished with them, as they will be used to make your mold. You will need an 8mm threaded rod that's roughly bigger than the tube. You need to push out the plastic plunger inside the tube by pushing the threaded rod through the opening the material comes out of. You can use a floor to push the rod against to help you. Keep the plastic plunger and ensure that any left over material is cleaned out. Empty silicone tubes work the best for me.
Step 3: Prepare Your Inset
Find something you want to embed in the plastic. It will need to have a hole where the threaded rod can be secured to it. I used a top old an old hard drive disk, but you could use a bearing, washer, trinket or even just use a 8mm bolt. Place nut on the end of the threaded rod and then slide the item you want next to the nut. The next part is a little tricky. From the inside of the tube feed the threaded rod through the opening keeping the nut and object to embed on the inside. Once done, secure the fitting by tightening a nut on the other side. Now you're ready to cast your mold!
Step 4: Melt Your HDPE
There are lots of resources online on how to melt HDPE but this is the method I use. You will need a old metal container, some wax paper (for baking) and oven gloves. Cut the wax paper to fit the container and ensure the correct side is on top. Place cut HDPE on the wax paper and put into the oven at 392 degrees Fahrenheit (200 degrees Centigrade). After about 20mins you should see the plastic melting a bit. Put on your oven gloves and carefully take the container out the oven. Remember its very hot so be careful with the next step. Fold over the wax paper over and over until you have a square piece of plastic. Try push down on it to compress all into a single lump. Pop back into oven for another 10minutes, then take out and start pulling and folding the plastic. Each time place back in when it starts getting hard. When you're happy with the result roll it into a ball that will fit the caulking gun tube and place back in the oven for 15mins.
Step 5: Casting Your HDPE
While you wait for the ball of HDPE to soften prepare your caulking gun. Place the tube with the object you want to embed in the caulking gun and have the plastic plunger ready. The quicker you are able to compress the HDPE the better end result you will have. Ensure you have the oven gloves on and take the soften ball out the oven and place in the tube, put the plastic plunger in afterwards and quickly compress down. Try put as much force as possible into this step. Keep ratcheting the handle to make sure the plastic fills around the embedded object. Then while holding down the lever place an elastic band around it to keep it compressed. Leave for a few minutes and then try compress again. The threaded rod should be hot and the top end of the tube should be slightly bulged. Then leave to cool.
Step 6: Finished Item
Once cooled down for about an hour, release the compression and take out the tube. Screw off the securing nut and then push the threaded rod onto the floor to slide out the finished product. You should be able to screw out the threaded rod which you can use again for the next cast. The end product should need little finishing off now. Hope you had fun!