The problem with plastics (polymers, really) is that if even 1% off collected polymer of type A (say PE) is off type B (say ABS), then their melt wont mix. If

The problem with plastics (polymers, really) is that if even 1% off collected polymer of type A (say PE) is off type B (say ABS), then their melt wont mix. If the mix doesn't melt you get spinal decomposition, as microstructure that looks like lamella. It is very weak compared to plastic A or B. Therefore you cannot simply melt plastic back together unless you have an almost 100% pure feedstock. But melting things together is the easiest and cost effective way to recycle anything really. Metals aren't as finicky and they are relatively easy ways to separate them in a dumb fashion (big old electromagnet). With plastics you can just break down and make (relatively bad) feedstock or burn it for energy (actually a good solution). Burning plastic, however, has the risk of burning chlorinated or fluoridated plastic and that really sucks. Polymers are amazing, but they also suck.