Injection molding is a tried and true manufacturing method that has experienced small, incremental changes over the years. It used to be, if you had a product or part to be manufactured, your injection molding manufacturer would provide you with a list of polymers to choose from. Your design and engineering team would work together along with the injection molding team to select a resin that would suit your needs. A variety of factors would be considered including durability, cost, flexibility and cosmetics.
Ready for a secret? It still works like that.
With most manufacturers, you are going to need to select the resin for your part from a list of available options. However, modern facilities have moved into a more advanced method of providing an option not previously available on that list—custom engineered resins.