MoldMaking Technology

AUG 2015

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Mold Material 36 MoldMaking Technology August 2015 By Tom Schade Stop fighting venting problems with mold retrofits and prevent them by designing the original mold with an enhanced venting steel. The Potential of Enhanced Venting Materials flows merge: failure of the resin to fuse sufficiently due to a temperature drop from traveling over a long distance or residual air at the resin's emerging point in the cavity. The permeability of enhanced venting materials placed near potential gas traps reduces back pressure and improves the flow rate, which reduces cooling of the resin. These materials also reduce the presence of residual gas in the cavity by giving it a way to escape. Shrinkage. The presence of an air or gas layer between the mold surface and the resin is one cause of material shrinkage. Enhanced venting materials prevent the formation of this layer, allowing for close contact between the mold surface and the resin. Short shots. Insufficient injection pressure, low resin tem- perature and trapped gas can cause incomplete filling of a mold Images courtesy of International Mold Steel. O ne of the advantages of being in the office on a Saturday is the quiet. After finishing the projects you came in to address, you can kick back, relax and let your mind wander. One Saturday a few weeks ago, I reached over to my bookcase and picked up a 1993 Toyota speaker grill (see Figure 1). It is a beautifully molded part with perfect fill and finish, and a fine mesh across the face—a technically revolutionary piece. One look at the back of the part told me it was filled with one sub-gate (see Figure 2). In the U.S at the time, we were trying to pack out this tye of speaker grill with 12-drop sys- tems, which resulted in poor fill and a lot of stress in the piece. In Japan, moldmakers were using a mold material developed to enhance venting. This was a steel manufactured with intercon- necting pores so the gas could pass through the seemingly solid piece of metal. To make this steel, powder metal was combined with metal fibers for added strength, cold-pressed into master blocks measuring 215 by 300 by 650 mm, sintered and heat- treated to 35 HRC. It was available with average pore diameter of either 7 or 20 microns; porosity averaged 25 percent of the mass of the block. Other mate- rials available at the time ranged from porous ceramics to sintered porous vent buttons. Twenty years ago, I was convinced that when molders and mold designers under- stood the potential of these enhanced vent- ing materials to eliminate trapped gas issues they would revolutionize design and molding processes. A few of the problems these vent- ing materials can solve include: Weld lines. There are two primary reasons for the occurrence of weld lines where resin Using an enhanced venting material allows the fll of a fne mesh speaker grill with one sub-gate. FIGURE 2 This 1993 Toyota speaker grill was vented with a core made with an enhanced venting material. FIGURE 1

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