Source:  ETMM The Website

“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.”

Read More: The Potential of Enhanced Venting Materials by Tom Schade