As Ralph Colby peers at the microscope image in front of him, he thinks he can make them out — “shish kebabs,” as polymer scientists call them. Nobody knows for sure what they are, but these shapes that appear at seemingly unpredictable times when certain plastics cool have a big impact on the overall properties of plastics. It’s not a big deal when a plastic fork breaks, but if a bearing cage on an airplane were to break, it could put people in danger.
Colby has partnered with two other Penn State researchers to get a better basic understanding of how plastics cool from a liquid to solid shape in injection molding. Their work, which involves some new techniques, is already helping industry partners.
“Ultimately, we’re hoping to come away from our research with a better basic understanding of how these polymers crystallize during flow, and also the knowledge to put this information into injection molding software,” said Colby, professor of materials science and engineering.
From pellet to product
Most plastics are produced by injection molding, a process through which small pellets of plastic are melted, forced into a mold of a shape and quickly cooled. In melted form, polymers are like a bowl of spaghetti, with individual molecules being a disarray of noodles. As they cool down, they start to form a structure and this process is known as crystallization. The way crystals form can affect the strength, durability and other properties of the material.
The goal of injection molding is to get polymers to orient themselves and crystallize in a specific fashion. It’s not as simple as just heating and cooling the material; rather, it requires the right amount of pressure and temperature to make individual molecules play nicely with one another and get into the right order.
Injection molding is complicated enough that it requires the use of software to control different parameters of the machine throughout the process. That software is based on data from decades ago that sorely needs to be updated, said Alicyn Rhoades, associate professor of engineering at Penn State Behrend, which has a renowned plastics engineering technology program.
“Since the 1950s, polymer engineers have been designing manufacturing processes like injection molding with baseline data that was generated with plastics changing 10 degrees per minute — but in manufacturing, polymers are subject to cooling at a rate of anywhere from 10 to 1,000 degrees per second,” she said.
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. It’s similar to cooking, said Rhoades. Putting cake batter in the oven makes a far different product than plopping it on the griddle.
Rhoades knew she would able 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.
Rhoades began discussing the issue with General Motors Company, and the research concept immediately struck a chord with their polymer engineers. In 2013, GM made a gift to Penn State so that Rhoades could purchase a Flash DSC.
“Groups around the world use the Flash DSC to study glass or for pharmaceutical research, but we are the first to be using it for plastics engineering,” she said.
Rhoades travels to the University Park campus often for her research to make use of the Materials Characterization Laboratory, part of the Materials Research Institute. The lab is designed to characterize or quantify the properties of, different materials.
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