From sunglasses frames to high-precision optical network connections, plastics are everywhere. More than 85 billion pounds of polymer products are manufactured each year in North America, most by a process known as injection molding, says John P. Coulter, professor of mechanical engineering and mechanics.
Coulter and Ph.D. candidate Qi Li are expanding the scientific understanding of how molten polymers flow through intricate networks of tubes, or “runners,” into the molds for individual parts. Along with other Lehigh participants, such as undergraduate student Lauren Walker, they are collaborating with Beaumont Technologies Inc. (BTI), a company based in Erie, Pa.
The project, which is funded by the National Science Foundation, spans materials science, fluid dynamics and thermodynamics. The researchers are developing techniques to manipulate polymer flows so that parts have consistent quality, which can afford U.S. companies a competitive advantage.
Injection molding forces a molten polymer into a steel mold at high temperature and pressure. The polymer winds through a series of twists and turns to fill as many as 64 product cavities simultaneously, Coulter says. The connecting runners range from 1/8 inch in diameter to 1/2 inch or larger.
Conventional wisdom in the industry says that the runners don’t affect the end result as long as they are “naturally” balanced, meaning the paths to each cavity have the same geometry and maintain similar temperatures. But Coulter’s research with BTI shows that assumption is incorrect.
Inside the steel mold’s machined runners, the flowing polymer melt experiences shear forces induced by the channel walls. Higher shear rates in the outside regions of these flow paths cause the molecules to disentangle so “the material is thinner and flows faster,” Coulter says. This shear thinning is known as non-Newtonian shear thinning. Compounding this is the significant localized frictional heating that is developed in these same high sheared outer regions of the flow channel. As flow is laminar, these hotter high sheared lower viscosity materials stay in the outer laminates and do not mix with the material in the inner regions of the channel.
“When you come to a branch in the runner the thinned material in the outer regions of the feed runner follow their position and and flow along the inside edge of the branching runner and the low sheared cooler material in the center of the feed runner end up on the outside edge of the branching runner. The result is different melt conditions are now flowing down the branching runner. If you are making 32 products in a mold you have a lot of intersections, and every time you reach one you have a different resulting flow and shear history distribution,” Coulter says. Cavities fill at different melt conditions and at different rates depending on the mixture of thin and thick polymer flowing to them. In some cases, not all the cavities fill completely.
After discovering this phenomenon, John Beaumont, a professor at the Plastics Engineering Technology program at The Behrend College at Pennsylvania State University in Erie, patented a technique called melt rotation. The technique provide a means of controlling the position of these high and low sheared laminates to strategically place them to eliminate cavity to cavity variations in multi-cavity molds. He started Beaumont Technologies Inc. (BTI), which licenses the technology worldwide to plastics producers.
Melt rotation doesn’t eliminate the problem, Coulter says, but it uses an understanding of the shear imbalance to modify runner junctions to rotate or “flip” the polymer flow by specific angles in order to achieve a desired result.
Coulter and Beaumont began collaborating a decade ago to try to achieve a better understanding of what happens inside injection molds. In their current NSF project, they are particularly interested in improving the filling of cavities with material of identical composition and properties. This technology is designed to work with nearly any mold during the injection molding process where temperatures can exceed 700F, injection pressure over 40,000 psi and clamp tonnages of upwards of 7,000 tons.
The Lehigh team has developed a technique to create injection molds with sturdy “windows” of polymethyl methacrylate (PMMA) that are optically transparent and can withstand real-world molding conditions. By placing high-speed video equipment and optically polarizing films inside the molds, researchers can observe temperature differences and molecular orientation in the molten polymer in real time. This enables them to judge the effectiveness of melt rotation techniques in specific applications. “We have no more mysteries about what is happening inside a steel mold,” Coulter says. “And if you can see what is happening, you can do something to fix it.”
The Lehigh technique enables researchers to compare different melt rotation techniques. BTI has also introduced adjustable melt rotation devices that can be changed as needed. The joint research project, says Coulter, “is exploring the effects of melt rotation on the properties of the polymer, and not just on the filling of cavities, for the first time.”
Coulter believes the project will change long-held misconceptions in the industry. “Historically the focus was on understanding what happened inside the cavities,” Coulter says. The assumption that runners are straight pipes, and that intersections don’t matter, he adds, is baked into the computer simulation software that is used in the field.
“If you use simulation codes based on that thinking, you can get beautiful color graphics showing the cavities all filling simultaneously. Then you build the mold and it doesn’t work. “Even today, there are those who question whether the runners really matter,” Coulter says. His team is providing enhanced scientific analysis of the phenomena that occur in injection molding and evidence that the runners affect the properties of the finished products.
Both Lehigh and BTI have benefited from the collaboration, says Coulter. “It really makes a difference for the students involved at all levels to connect with and, if possible, spend time in industry. Industry is often focused on making earnings and putting out fires, so connecting to the thorough but traditionally slower-moving university research environment is not always easy.
“This kind of partnership allows us to accomplish what often doesn’t get accomplished.”
Headquartered in Erie, Pa., Beaumont Technologies is the pioneer and world leader of in-mold rheological control technologies for plastics injection molding. The inventor of MeltFlipper® and other technologies, Beaumont provides a broad range of plastics engineering services, including molding simulation, industry education courses, material characterization, and process/design development consultancy. Thanks to Lehigh University for your collaboration and use of this story.