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For example, an off-white color will mask defects better than gray or (even worse) black. Specific surface finishes can amplify or reduce the visibility of surface defects. The nature of a defect, such as sink-mark depth or width, may also increase the odds of visual detection. The color of objects around the part influence this effect as well. As a result, even the most sophisticated simulations may not be able to fully represent the end user's visual experience. The good news is that we're making progress. Some recent trends are pushing the envelope of visualization: • Exporting warped geometry as a CAD file (.sat or .step) for easier comparison with the original model; • Exporting sink-depth predictions from simulation tools to visualization tools (VRED or 3Ds Max); • High-quality rendering to more closely match re ality in terms of color, texture, and lighting (see Fig. 1). Meanwhile, manufacturers are moving ahead with new tech- niques to combat cosmetic defects, including more sophisticated thermal control of the mold, 3D texturing technologies, liquid silicone rubber (LSR) injection molding, and component integra- tion—such as multi-shot techniques. Nevertheless, engineers will always need reliable ways to identify and manage the risk of cosmetic issues. This, again, is where simulation comes in. FOUR KEY VARIABLES To overcome the challenges posed by cosmetic defects, engineers have four primary variables to explore. Depending on the scenario, simulation can help engineers consider ways to mitigate risks in the design phase or accelerate the troubleshooting process if defects show up in a prototype. 1. Process type: Anytime you have the ability to explore processes, it will change your approach to evalu- ating aesthetics. For example, gating thick to thin is a trusted, conventional approach. But gating into a large, thick area can cause jetting, depending on the type of gate (direct drop, tunnel, tab, or lap style). Manufacturing the same part with gas assist, coining or two-shot technologies could eliminate this source of surface blemishes. Another way to do this is to replace the pack-and-hold phase with microcellular foam growth, in which case it is ideal to gate into thin areas to control and contain bubble excitation. Of course, engineers could also vary the mold temperature to create a very resin-rich surface that improves finish quality. An example of this would be the use of rapid heating and cooling or induction heating (Fig. 2). Many of these process choices can be simulated, creating opportunities to see which one is most appropriate for ensuring the desired level of surface finish quality. 2. Material choice: Not all materials are created equally with respect to cos- metics. Simulation allows you to explore how material choices affect process- ability and surface defects. Understanding the properties of different materials can provide a plastic engineer with clues about how to design gates to avoid a defect like gate blush, for instance. Fillers, such as fibers or metal flakes, can also have a significant impact on the product's appearance, and simula- tion can be used to get a handle on this. Another area where simulation can be advantageous is in the study of weld lines. It is common for plastics engineers to examine the angle of the melt front as well as the temperature and Simulation allows you to explore how material choices affect processability and surface defects, like gate blush and roughness caused by fillers or reinforcements. QUESTIONS ABOUT INJECTION MOLDING SIMULATION? Visit the Tooling/Mold Simulation Zone. Simulation can predict the effect of process choices, such as induction heating and rapid heat/cool ("variotherm") molding on plastic part surfaces. FIG 2 @plastechmag 47 Plastics Technology M O L D I N G S I M U L AT I O N