Alicyn Rhoades, associate professor of engineering at Penn State Behrend, working on a flash differential scanning calorimeter (Flash DSC). (Credit: Robb Frederick/Penn State Behrend)

Researchers are getting a better basic understanding of how plastics cool from a liquid to solid shape in injection molding. They hope to understand how these polymers crystallize during flow in order to put this information into injection molding software.

The rate of heat transfer, either into or out of a polymer, makes an incredible difference in how a polymer behaves once it cools down. The goal was to achieve the levels of heat transfer relevant to injection molding if she used a device known as a flash differential scanning calorimeter, or Flash DSC. The machine heats small amounts of polymers up thousands of degrees in a fraction of a second.

The bulk of the experiments mimic what happens during injection molding. They take a sample of material in solid form, heat it to just above its melting point, apply a force to the material to simulate how it flows into an injection mold and then, once the sample has been allowed to cool, they study it.

They model what happens to individual polymer molecules as they are subjected to the forces of injection molding. Computer simulations are run that solve physics equations for each molecule, indicating where each molecule is likely to move in the next moment and what force it might apply to nearby molecules. This gives a picture of how molecules are likely to be oriented once they cool into a static shape.

Their initial experiments have shown other mysteries taking place within liquid plastics subjected to flow, in addition to the somewhat spontaneous rice grains and shish kebabs. For example, if a polymer is melted only briefly, it seems to “remember” its molecular orientation as a solid and recrystallizes into that much more quickly than if it stayed in liquid form for longer.

Each new experiment the team takes on is yet another attempt to peek under the hood of polymers crystallization, and their goal is to use this new knowledge to update injection molding software. This could save companies hundreds of thousands of dollars, in addition to ensuring the durability of their products.