There’s a trick in figuring out how a plastic item has been made, and that’s by looking at the number of those items being created. Some plastic goods can be made at home, where as others involve the investment of extensive equipment. The manufacturing processes that require low investments are often craft based, and tend to be slower than high investment manufacture.
A good example of this is injection moulding. This is a process that is only economically viable if a very high output is being produced. An injection moulding machine can convert plastic granules to a safety helmet in 40 seconds, that is 2160 in 24 hours. That is a lot of helmets in a short space of time, an amount that simply can’t be produced in the same time-frame with craft based methods.
Casting, fabrication and rotational moulding processes are more craft based. As a result these processes cost less to set up, but are slower in the production phase. Excluding plastic bags, there are far more plastic goods manufactured using injection moulding than any other process.