Originally Posted by Koldkut
I think Koldkut got it right. Plastics will melt, and if you join them together while melted and let them cool, they are "glued" together by their own material.
For example, I used to make double-length plastic worms before they appeared in stores, by nipping off the ends of two worms with a razor blade, then holding the ends over a lighter or candle until they melt and start to burn, then put them together, blow out the flame (if any) and hold them for about 20 seconds until they cool while fused together.
In other words, no glue. It works.