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Mold Components By Bob Salhaney For Better Mold Venting, Selecting the right material for specific mold components can help prevent a number of costly issues. A s part specifications and complexities continue to rise, proper mold venting is increasingly critical. Insufficient venting can lead to burning, shrinking and other costly issues (not to mention longer cycle times). Selecting the right material for certain mold components can go a long way toward preventing these problems. For example, a porous sintered metal, such as one that is composed of 25 percent air by volume1, can vent gas without the need for a separate venting component. As a result, core pins and plugs made with this material help reduce mold size, cost and complexity. You can use this material to help your customers achieve better part cosmetics, while also saving time and money. A porous metal eliminates trapped gas in the mold. It is heat-treated to 30 to 40 HRC with porosity around 20 to 30 percent by volume. A system of interconnected pores with an average diameter of 7 micron (0.0003 inch) or 20 micron (0.0008 inch) is dispersed throughout the material. This porosity is the key to the material's effectiveness. When used as a core pin material, the metal eliminates trapped gas problems that occur in inadequately vented areas within the mold. It is often difficult to provide adequate venting in these hard-to-mold areas. Traditional methods of venting, such as parting-line vents and vent plugs and pins, often do not provide sufficient surface area to accommodate the large volumes of gases that can be generated. This material provides a location-specific method of venting gas in a targeted area. Since it is 25 percent air by volume, 1/4 of the surface area becomes a vent. The larger the surface area of the piece installed, the greater the venting capacity. Material Benefits This method of venting delivers numerous benefits, including reducing injection pressure as well as substantially reducing 24 MoldMaking Technology December 2013 Images courtesy of DME Company. Start with the Metal scrap and reject rates. Utilizing a porous sintered metal for core pins and plugs also enhances part production in the following ways: Prevents burning and shrinkage. Burning is a condition caused by compressed gases trapped by the flow of molten resin in a cavity pocket. By using a porous metal, gases are permitted to evacuate through the steel to the Core pins and plugs made with porous, sintered outside atmosphere, thus eliminating metal help reduce mold the burning condition. Additionally, size, cost and complexity. air bubbles trapped between the resin and mold steel surface can cause shrinkage or sink that shows as a ripple in the otherwise straight plastic surface. Using a porous metal on the trapped-air side of the cavity will eliminate shrink in most cases. Prevents flow and knit lines. Minimizing or eliminating flow and knit lines is an additional benefit of using this material. Knit lines occur at points where resin flows converge after molding around an obstruction or protrusion within the mold, usually away from the gate area. There are two primary reasons for this occurrence: 1. The failure of resin to sufficiently fuse due to the temperature drop it experiences after flowing over long distances. 2. The presence of residual air at the resin flow convergence point at the cavity obstruction, prohibiting the proper fusion of the flows. The permeability of a porous metal prevents defects that arise from residual gases normally trapped inside the mold cavity. Also, using this material reduces back pressure and improves the flow rate, enabling the resin flows to merge while still hot.