MoldMaking Technology

OCT 2017

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moldmakingtechnology.com 15 Material selection impacts stress, and each type of material has a recommended shear rate limit. TABLE 1 the runner system or within the part geometry, especially around sharp corners. The more times a material changes direction, the more shear is put into the material. Higher shear rates will increase the temperature of the molten mate- rial flowing through the mold. If the shearing is high enough, the temperatures could exceed recommended limits, causing the polymer chain, the additive or both to break. Once this breakage occurs, the part becomes weaker and likely fails functional testing. 3. Design core-out features correctly. Core-outs are fea- tures like through holes, blind holes or any element around which the material must flow. Visualize a rock in a river. The water must flow around that rock and reconnect on the other side. In molding terms, the rock is the core-out, the river shape is the part geometry and the water is the flowing plastic. Core-outs can come in all shapes, sizes and locations within the mold cavity. As material flows around the core-out and tries to shrink during the cooling phases, the metal that forms these holes prevents the material from returning to its natural, relaxed state. When the material is oriented and stretched during fill- ing, it is no longer in a straight line. It must bend to the shape of the core-out. When the material cools, it tends to relax, but the metal core-out in the mold will not allow that to occur. The polymer chain is stuck in a stretched-out, bent shape. Remember that every time the material flows around a feature, there is an increased chance of stress. Keep in mind also that mold designers and molders cannot overcome what is inherent to a specific geometry. To correctly design core- out features, avoid corners or edges, and place gating in a proper location. This way, the material can flow in a parallel direction, not a perpendicular direction, to the feature. Material Selection Another variable that design engineers can control to impact stress is material selection. Each material has a recommended shear rate limit (see Table 1), and if that recommendation is violated, excessive shear heating and potential shear stress will result (see Table 2). The shear rate limits of the polymer are not the only concern. Consider also the shear rates of the various additives (color, ultraviolet (UV) stabilizer, heat stabilizers and lubricants) that are compounded to make the polymer. And, since most additives do not come with processing recommendations, it's difficult to understand the limitations. This can result in additive degradation before the polymer chains are broken. Once that occurs, the molded part's expected performance A consistent flow rate through the water channels is critical for an even part temperature at ejection. This ASTM tensile test bar is used to determine material properties. FIGURE 1 Material Neat Filled ABS 50,000 25,000 PA 60,000 30,000 PC 40,000 20,000 PP 100,000 50,000 PVC (Rigid) 20,000 10,000 This chart shows the rates of PSI and MPa for different types of material that experience excessive shear heating and shear stress. TABLE 2 Material PSI MPa ABS 43.5 0.30 PA 72.5 0.50 PC 72.5 0.50 PP 36.3 0.25 PVC (Rigid) 29.0 0.20

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