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Sink marks. Weld lines. Splay. Blush. Scuff marks. Everyone involved with developing molded components should know this list very well (and it's a really long list). The unavoidable truth is that cosmetic factors signifi- cantly affect the perceived quality of a plastic part, whether they influence its functionality or not. Many molded parts only serve an aesthetic purpose, making cosmetic defects an even bigger issue. In both cases, the fundamental challenge is that the prevalence and severity of cosmetic defects are difficult to assess using intuition and experience alone. That's where computer-aided simulation technology comes in. Injection molders are generally familiar with use of such software to predict and optimize mold filling and cooling. But its role in anticipating, troubleshooting, and resolving or—better yet—preventing surface defects is less familiar. In this article, we will show how a combination of good design practices and simu- lation technology, like Autodesk Moldflow, can help engineers deal with cosmetic defects more effectively. CLARIFYING THE CHALLENGES One of the reasons why surface finish is so problematic is simply the number of decisions involved. The matrix of choices with respect to part geometry, materials, processes, mold design, and mold textures—and how the consequences of each choice affect other decisions—makes cosmetic defects difficult to visualize until you see the molded part. Depending on your role, you may not always have control over these decisions. For example, you may find that the most straightforward way to solve a cosmetic issue would be using a different material color, mold texture, or process type, but that choice may not be available given brand constraints, the project's budget, or the schedule. A third challenge is the nature of cosmetic defects. They are primarily visual phenomena that are hard to quantify and difficult to simulate. Surface textures and colors are not easy to charac- terize with data in the same way as mold-filling conditions (such as flow-front temperature, shear, or velocity). Defects may be minimized or exaggerated depending on the lighting intensity, direction, source spectrum, viewing time, angle, and distance. By Brian Pelley Autodesk It's not just for successful mold filling and cooling. Simulation can help predict and overcome cosmetic defects in molded parts. Use Molding Simulation To Avoid Surface-Finish Defects Newer simulation software features high-quality rendering that more closely matches visual reality in terms of color, texture, and lighting—allowing users a better assessment of the cosmetic effects of material, tooling, and process choices. FIG 1 46 APRIL 2017 Plastics Technology PTonline.com Tips and Techniques