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Hot Runners 24 MoldMaking Technology —— AUGUST 2016 By Damiano Balzi A Different Approach to MOLD VENTING P roper mold venting is essential for producing quality parts. During the molding process, the air contained in the mold needs a way to escape, otherwise the melt will compress and trap that air in the cavity. A combination of melt pressure and high temperature will then ignite the oxygen (called the "diesel effect"), causing burnings (carbonization), gloss marks and stress cracks in the finished plastic part. Poor venting, along with the diesel effect, can also cause the mold to wear on the mold parting lines, resulting in flash (excess plastic) on the injected part. Reducing injection speed will allow more time for the air to escape from the cavity through the vents, but this leads to other problems, such as short shots (incomplete parts). This happens because, as injection speed slows, the temperature of the plastic decreases. The lower the temperature of the plastic, the higher the melt viscosity, causing the plastic to begin to harden before the mold is completely full and therefore pro- ducing incomplete parts. Also, when semi-crystalline polymers are being injection- molded, it is necessary to apply pack pressure (increased injection pressure) to the material after filling is complete. This helps the polymer chains to rearrange while freezing and form partly ordered regions, guaranteeing the finished part's mechanical properties. Once the material reaches the glass-transition temperature, the movement of molecular chains is frozen. Packing is possible as long as a center-core flow path of melted material can transfer the packing pressure from the injection point to the entire injected part. The longer the fill- ing time, the cooler the material, which can result in poor packing. This poor packing causes warpage of the part and less conformity with cavity design and texture. Knit lines and other aesthetic characteristics of the part are strictly related to the plastic's temperature during molding, so efficient filling time is essential for achieving proper welding lines and quality parts. VIDEO Access video at end of article. Improper cavity filling also can trap air between two plas- tic flow fronts. Software analyses that can predict the critical spots at which air traps are likely to occur can help ensure proper placement of mold venting. Standard mold venting involves machining pathways between the mold's mechanical elements through which air can escape. Typical vent locations may include, for example, between the parting lines, pins or ejector pins, or sliding elements such as sliders and cams. When parting lines and sliding elements don't allow for enough venting, the mold cavity can be divided into inserts to increase the number of possible venting areas. Material viscosity limits the recommended size of vents that should be used in a mold. The lower the viscosity, the smaller the recommended vent. In fact, industry standards for vent Alternative venting valves can help overcome standard mold venting limitations and improve mold performance. Dynamic (as opposed to static) venting valves can be used externally to and inside the mold cavity. They are designed to allow air and resin gases to rapidly escape from the cavity. The surface of a venting channel typically is a few square millimeters—just large enough to overcome the geometrical limits of standard venting grooves.