International Mold Steel The potential of enhanced venting materials
USA - Many moulders are fighting venting problems that would not exist if they had designed the original mould using an enhanced venting steel. Tom Schade explains how the use of porous metal inserts and no additional venting can eliminate chronic flow lines or virtually eliminate material spray.
One 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. It is a beautifully moulded 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 US at the time, we were trying to pack out this type of speaker grill with 12-drop systems, which resulted in poor fill and a lot of stress in the piece. In Japan, mouldmakers were using a mould material developed to enhance venting. This was a steel manufactured with interconnecting 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 materials available at the time ranged from porous ceramics to sintered porous vent buttons.
Twenty years ago, I was convinced that when moulders and mould designers understood the potential of these enhanced venting materials to eliminate trapped gas issues they would revolutionise design and moulding processes. A few of the problems these venting materials can solve include:
Weld lines. There are two primary reasons for the occurrence of weld lines where resin 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.
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