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pressure to identify regions where melt fronts are separated by an obstacle (the part's geometry or multi-gates) and then recom- bine. These dynamics affect both the strength of the part and its cosmetics. With simulation, you can try the options available to you and let the software iterate to help you choose the best material, gate location(s), and geometry sooner. 3. Part geometry: Simulation excels at the classic DFM (design for manufacturing) approach, giving engineers a reliable way to examine the impact of thick- ness variation, such as rib-to- surface ratios, thick-to-thin ratios, and flow phenomena. A quick check of drafts and undercuts can also be useful when evaluating mold cost, manufacturability, texture depths, and the potential for ejection distortion (such as scuff marks). Flow quality and thickness go hand in hand. The cardinal rule for balanced fill is unifor- mity—whether of melt pressure, temperature, velocity, or volume/ thickness—which is impor- tant for cosmetics too. Flow hesitation effects are often visible and are very thickness dependent. Injection molding simula- tion can quickly help identify problems using contour plots to identify flow-related defects, as well as using flow-front velocity, shear stress at the wall, temperature at the flow front, and other parameters calculated by the software. Simulation software can also ensure that a mold fills easily and uniformly without significant temperature variations, and also help you predict jetting effects, weld-line positions, and air traps. 4. Mold design: Mold design is an extensive area to explore, in particular gate design and blush. Gate area and thickness transitions matter a great deal for cosmetics, and shear rate is a key result to focus on. Simulation offers a proven way to see whether, for example, changing a circular gate to a more rectan- gular shape can reduce shear rates. Simulation also makes it easy to find the best gate locations and flow rate, then size the gates to minimize the shear rate (Fig. 3). In addition, simulation can help engineers look at new ways to avoid surface defects, such as rapid heating and cooling (aka "variotherm" molding). Simulations can show a transient or dynamic thermal analysis of the mold and the interface of the plastic and tool steel. Induction heating is another approach that can be modeled to optimize temperature distri- bution without affecting cycle time. Another choice engineers have to make is the type of runner system and gate (cold gate, hot tip, or valve gate). Simulation can capture the mechanics of opening and closing, such as the valve-pin opening speed, as well as create a detailed flow- front profile. This profile identifies poten- tial hesitations that may change flow-front speed and the material melt properties, which have a significant effect on the imprint of the texture and surface finish. A CASE STUDY A quick case study of an actual part can illustrate how all of this complexity comes together to affect the part's cos- metic quality—and how simulation can help find a solution. In this case, the part was a TP elas- tomer molded with a two-shot process. It was cosmetically challenging due to the Flow hesitation effects are often visible and are very thickness dependent. Injection molding simulation can quickly help identify problems using contour plots to identify flow-related defects. Case history: Two-shot overmolding with TPE was successful until the program changed the material and colorant, creating a defect in the bottom of the part. FIG 4 Simulation makes it easy to find the best gate locations and ideal flow rate, then size the gates to minimize shear, which can affect surface appearance. FIG 3 Before Sizing After Sizing 3176.1 (1/s) 1.388E + 05 (1/s) 35776.(1/s) 14109.(1/s) 3913.8(1/s) 3039.8 (1/s) 5277.8 (1/s) 8143.7 (1/s) 13524.(1/s) 24910.(1/s) 3176.2 (1/s) 48 APRIL 2017 Plastics Technology PTonline.com T ips & Technique s