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Reduces cycle time. Because of the reduction in back pressure within the mold, the plastic fills the cavity faster, thus allowing for a reduction in temperatures, which shortens cooling and cycle times. Eliminates short shots. A short shot is another condition caused by too low of an injection pressure or trapped gases in pocket areas of the cavity. This results in the part not being completely filled out. A porous metal reduces back pressure, therefore, less injection pressure is needed. And because it vents trapped gases, both causes of short shots are eliminated. Enhances part appearance. By using a porous metal in the mold, webbed, ribbed and other difficult-to-fill, thin-walled designs are greatly enhanced and cosmetically defined. Molding highly detailed, thin-walled and aesthetic parts is easily accomplished with the reduced back pressure and added venting of this material. Porosity is the key to this material's effectiveness. Reduces gloss. When using a porous metal as a cavity in an injection mold, the air that would otherwise get trapped between the cavity and the resin and cause a gloss to show on the part is instead allowed to escape through the pores, thus leaving a dull matte finish. This often eliminates the need for costly secondary spray-painting operations. Simplifies the tool. When back pressures, injection pressures and cycle times are lowered, fewer drops are needed to ensure proper filling of the cavity. Lowering the number of drops needed simplifies the design while saving tool costs. Aids in part ejection. In many situations, an air poppet is needed to help break the vacuum from the core in a molded part. Inserting a porous metal into the core with an air blow setup helps break the vacuum seal and aids the ejector pins in doing their job. Design Guidelines While porous metal is ideal for core pins and plugs, there are some practical guidelines to consider when determining its suitability for various other applications. Resin type. Depending on the emissions or gas residue given off during the molding process, the molder will have to evaluate whether to use a 7- or 20-micron pore size. Resins such as ABS, polypropylene, soft-type PVC, polyethylene, acrylic, polyurethane and styrene Visit our Mold Components Zone for work very well with more information about mold bases, pins, a 7-micron (0.0003ejectors, lifters, bushings, guides and inch) pore size. alignment devices. For low-viscosity Go to moldmakingtechnology.com/zones or talc-filled resins, for a complete list. it may be necessary to have an automated system reverse the airflow after each shot to purge the impurities from the pores. Rigid PVC resins, phenolics and natural rubber resins will work, but only until the corrosive gases close the pores. This can still be a feasible method if disposable inserts are used. The 20-micron (0.0008-inch) pore size will vent about 25 percent more gas than the 7-micron pore size, so, where extreme venting is needed, a 20-micron pore size is recommended. The 20-micron grade was developed to be used with a milled finish. When milled properly, this process leaves a pore that is partially open (about 40 to 50 percent). Rigid PVC, clear polycarbonate, liquid silicone and foaming urethanes all are questionable resins to use with this material and should be avoided. LEARN MORE Proper venting. Experts recommend that at least 10 percent of the cavity area be made of a porous sintered metal to ensure proper venting. While this is not always possible, it is important to remember that the more square inches of venting that are used in the cavity area, the lower the back pressure will be in the cavity. However, if the distance that air has to travel through the material, and ultimately to the exhaust line, is excessive, then venting capacity will be compromised. Thus, the shorter the distance through the metal to the exhaust line, the better. Due to the larger surface area, the insert will not require cleaning quite as often as a smaller piece. If a porous metal is used as a core or cavity half instead of as an insert, there may not be a need for parting line vents. moldmakingtechnology.com 25