It is important for part and mold designers not only to understand design principles, but also to understand the fundamentals of plastics. The choice of a plastic, or polymer, has a strong impact on the outcome of a molded part.
Polymers are categorized by processing methods and temperature behavior. The major groups are thermoplastics, thermosets, and elastomers. There are also compounds made up of polymer blends, co-polymers, composites, and thermoplastic elastomers.
The most common group of polymers are thermoplastics. These are polymers that melt by heating and solidify by cooling. This behavior is different from Thermoset materials.
Thermoset materials, generally rigid polymers, are injected at a low temperature and then cured with heat inside the mold. This creates an internal chemical reaction that forces the molecular chains to cross-link and form a solid molecular structure. The molecular chains create a chemical bond that cannot be re-melted. Watch the video.
Within the category of thermoplastics there are thousands of different base materials and grades. Although it’s virtually impossible to fully understand every available material, having a solid understanding of basic polymer characteristics is imperative to successful part and mold design. This group is further broken down into two main categories, amorphous and semi-crystalline.
As materials flow, their molecular chains align. When they stop flowing, the molecular chains return to their natural coil-type structures. When polymers go back to their natural relaxed molecular state the molded part shows very little molded-in stress and therefore, less potential part warpage and higher dimensional stability.
Amorphous materials have a wide process range and can be molded with fewer challenges resulting in molded parts that are more dimensionally stable.
Semi-crystalline materials have a narrower material processing temperature range. These materials are opaque, have a good resistance to chemicals, and lower moisture absorption. However, they have higher shrinkages. This is important for process technicians to understand when molding semi-crystalline materials.
Having a general understanding of the basic properties and structures of polymers will help part and mold designers, as well as processors, make choices and decisions that will optimize the final part.
If you are interested in learning more about polymer materials, check out the full demo lesson.