This unique question was submitted by one of our more active bloggers…
What would be the difference in injection molding old fashioned rubber-like materials and the newer polymers like PBT?
Although I have only had a few opportunities to process natural rubbers, it is an interesting experience.
Processing PBT – Polybutylene Terephthalate, PBT, is a semi-crystalline polymer which is heated above the Melting Temperature (Tm) before processing. In this state, it flows very easily and is usually melted using a straight or reverse barrel temperature profile. Because the polymer is processes above the melting temperature, it flows relatively easily. Most amorphous synthetic polymers such ABS or Styrene, do not have a melting temperature, but are processed above the Glass Transition Temperature (Tg) which is a softening temperature present in all polymers.
Processing Natural Rubbers – Natural Rubber, or Polyisoprene, is an amorphous polymer which is often processed at a temperature near or below it’s Glass Transition Temperature. This makes it more difficult to mold, resulting in a behavior similar to the molding of many PVC materials. As with PVC, you tend to use a forward temperature profile, and require a special screw configuration, often with a high flow check ring… or no check ring at all.
The molding of natural rubbers can be a messy business and there are many variations of these materials depending on the degree of polymerization and molecular weight distribution.