Plastics Technology

APR 2017

Plastics Technology - Dedicated to improving Plastics Processing.

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dispersion of the colorant. However, initial runs were perfect. Then the program changed both the material and the colorant, creating a defect in the bottom of the part (Fig. 4). Overlaying various analyses—including pathlines, flow-front progression, and single contour—within the simulation software helped engineers visualize the flow velocity and identify the hesitation near the rim that was causing the flow-front speed to vary and resulted in a visual defect (Fig. 5). This example highlights how the decisions we make as engineers can have unpredictable effects on surface quality. In this case, the hesitation was present in both situations. But one material masked it, while the other material exaggerated it. The part volume in this case was very small, which made it hard to control the molding process. Some process changes did alleviate the defect to a degree, but the underlying cause of the hesitation was part geometry—specifically a thickness variation ABOUT THE AUTHOR: Brian Pelley, Moldflow Technical Specialist with Autodesk, is a plastics engineer with more than 20 years' experience in simulation, CFD, and FEA with large OEMs and custom manufacturers. His areas of expertise include analysis, support, training, consulting, and sales. Contact: brian.pelley@autodesk.com; autodesk.com. Gate area and thickness transitions matter a great deal for cosmetics, and shear rate is a key simula- tion result to focus on. that resulted in the hesitation and the ensuing racetrack effect. Once the root cause of an issue was identified, it quickly became apparent what to do next. All plastics engineers face similar challenges when it comes to optimizing surface-finish quality, but there is a huge array of options available to resolve problems before they become too expensive to fix. However, the resulting tradeoffs are not always easy to quantify. Judging what is acceptable is often subjective and application-dependent. The underlying causes of surface-finish defects, however, are not subjective. Simulation provides a wide range of analytic tools engineers can use to examine them and understand how choices in process, material, geometry, and mold design might change the look of the final product. Simulation technology, such as Autodesk Moldflow, can provide insight quickly, enabling engineers to consider more options within a tight schedule and reach a satisfactory answer sooner. PLASTIC PURGINGS www.Vecoplanllc.com 5708 Uwharrie road • archdale, nc 27263 (336) 790-5752 BALED FILM AINERS Built To Run: 24 Hrs. a Day, 7 Days a Week, 365 Days a Year UNDER THE MOST EXTREME CONDITIONS! GRAPHIC FILM PLASTIC PALLETS KEVLAR HELMETS W I T H PRODUCT DEFECTS M O L D I N G S I M U L AT I O N

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