How to identify types of plastics?

I want to do some plastic welding using hot air tool. I have rods for several types of plastic.
My problem is that the target item that I want to weld is not always marked
with an identifying number. Sometimes there is no marking at all.
Is there a test or visual clue to be able to identify a plastic as polyurethane or ABS or nylon or...?
In my experience, polyurethanes don't crack, whereas ABS and other "brittle" plastics are the usual "breakers".
What's a simple way to weed out plastics into their basic family groups?
Thanks, Dave
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DaveC wrote:

For your purposes, I'm not sure the plastic family is so significant. The properties of the plastic will be greatly affected by the filler. Most plastics are an engineered system of polymer, resin, coupling agents, and multiple fillers. There can also be other additives, such as opacifiers, for specific applications.
Some plastic families like polyurethane encompass a huge variety of molecular structures, with widely varying properties. A few like silicone have highly stereotyped properties.
I'd suggest developing your own tests for machinability and other parameters, and applying these tests to individual batches of plastic as a proprietary, specified incoming qualification test. That would be part of your value-added. You would develop expertise in judging the machinability of the plastics your customers wish to use.
Bear in mind that batch-to-batch variation in plastic formulated systems is suprisingly high. It isn't like reagent-grade sodium hydroxide, which is pretty much always the same thing.
The mine for a mineral filler may cut into a new stratum, and that will have downstream consequences for the plastics that use that filler. Competent vendors will blend fillers to attempt to hit a certain target, but you can't satisfy all target properties at once. If you want it to be very flowable before curing but high tensile strength after curing, you might have trouble doing both things simultaneously and even more trouble if other requirements are set.
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