The following guidelines should be considered:
Resins Used
Depending on the emissions or gas residue given off during the molding process and flash point of the resin, the molder will have to evaluate whether to use 7- or 20-micron pore size. Resins such as ABS, polypropylene, soft type PVC, polyethylene, acrylic, polyurethane,

and styrene work very well with a 7-micron (.0003") pore size. For low viscosity or talc-filled resins, 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 will work, but only until the corrosive gasses close the pores. This can still be a feasible method if disposable inserts are used. The 20-micron pore size (.0008”) will vent about 20 percent more gas than the 7 micron pore size.

 Surface Finish Requirements
Since most applications can be addressed using inserted pieces on the core side, an EDM finish is acceptable and will vent better than any other surface finish. Should EDM be impractical, after EDM a stoned surface will provide the next best venting surface. The 7-micron pore size provides the best finish. It is important to remember that if the venting steel is used on the cavity side of a mold, the gloss level will be severely reduced (from #9 to #3 with PP), so matching should be considered.

 Cleaning, Maintenance and Resin Removal
There are two times cleaning must be performed to gas permeable mold steel: (1)when the tool shop has prepared the insert or cavity. The material is 25 percent air by volume and the cutting fluids will displace the air, thus filling and clogging the pores. After the permeability has been restored by EDM, the venting surface and steel should be professionally cleaned, due to the dangerous nature of the cleaning process; and (2) after the insert or cavity has been in operation and film, a thin layer of mold release agent, resin residue, shop oil or any other contaminant has partially or totally blocked the air flow.                     

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