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Accounting For Pressure Dependent And Elongational Viscosities To Improve Injection Pressure Predictions In Mold Filling Analysis


The increasing demand for shorter product development timelines and more robust plastic part designs has made injection-molding simulation a critical tool for plastic part and mold designers. One of the most common objectives of injection-molding simulation is determining the pressure requirement to manufacture the part for a given resin and set of process parameters. The reliability of injection pressure prediction is dependent on many factors including accurately modeling the part and mold geometries, and predicting the material behavior during this dynamic process. While advancements have been made with regards to improving the ability to properly represent the mold and plastic part geometries, the ability to adequately model the molten polymer behavior remains a difficult task. Of particular importance is the ability to properly characterize the material viscosity during the injection molding process. The ability to account for the effects of pressure and elongation deformation on the material viscosity is critical for providing reliable injection pressure predictions. This paper will present the results of an experimental validation study in which the effect of accounting for the pressure dependence and elongation deformation on the material viscosity influences the injection pressure predictions.


Injection molding is used to efficiently mass produce high quality, tight tolerance parts. It is a pressure-driven process in which the hot molten polymer is injected from the injection unit into a cooler mold cavity to form the desired geometry. The amount of pressure required to form the part is dependent on many factors including: the part wall thickness, injection speed, melt temperature, and flow length. The industry trend of light-weighting plastic components and thinning the part wall thickness result in higher injection pressures required to fill the part, and are commonly approaching the injection pressure limit for the injection unit. Additionally, the compressed product development timeline restricts a designer’s ability to redesign a plastic part if the original design cannot be manufactured as a result of excessive injection pressures.

In response to these design pressures, injection-molding simulation has become a prevalent tool to help virtually validate plastic part designs before any physical parts are manufactured. From the author’s experience, one of the common objectives of injection-molding simulation is to determine the pressure required to fill the mold. The ability to accurately predict the pressure required to fill the mold can help designers and molders make more informed decisions regarding the runner design, minimum clamp force requirements, and overall dimensional stability of the plastic part. While it is desired to accurately predict the maximum injection pressure, the ability to properly model the complex relationship between the variables that influence this pressure prediction often force analyst to remain conservative with their results.

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Erik Foltz, M.S.

Erik Foltz is a Senior Managing Engineer and Partner at The Madison Group. Erik received his M.S. Degree from the Polymer Engineering Center at the University of Wisconsin – Madison. His specialties include plastic part design verification, process optimization and troubleshooting for injection and compression molding, and plastics failure. He has experience with thermoplastic, thermoset, elastomeric, and composite materials. Erik is also a Certified Professional Moldflow® Consultant.