Injection mold venting is needed on all injection molds.

Some require more venting than others due to part design and also the type of plastic being used. Injection mold venting is needed due to the in-rushing of plastic during the injection molding phase of the cycle. After the mold is closed and ready to be injected with plastic, the cavity is filled with air.  As the plastic is injected, the pressure is so great and fast it tries to push the air out of the cavity.  If this does not happen, the plastic is not able to flow and make a complete part which results in a scrap part. A vent must be put in the mold at the trouble area which is found where the part is not filled or else a noticeable burn mark is seen.

Injection mold venting consists of a groove that is most commonly .0005 to .0015 deep. The depth is always noted in the material data sheet and must be put in at that depth. The easiest and fastest way to vent a mold is at the parting line. The vent is ground to the recommended depth within .015 of the cavity, from that point; the vent is ground to a depth of .015/.020 all the way out the mold into the atmosphere. Vents are always put directly across from the gate.

More complicated injection mold vents are needed at the bottom of ribs or deep cavities. This is very difficult to achieve if the cavity and ribs are cut in the solid insert.  One way to add a vent is the use of a vent pin, but this is sometimes not an option due to the small space involved. This problem can be eliminated during the design phase of the mold when inserts can be put in the mold and allow for easy venting options. An example of this is having an insert in the center of a rib so a vent can be installed. Knowing where to use inserts comes with years of injection molding and injection mold building experience.

Another way to vent a mold is the use of a mold vacuum. This is a very expensive option; however, sometimes is the only option. This is achieved by sealing all parts of the mold cavity by the use of an O-ring.  Once the mold is closed, all of the air is drawn out of the cavity before or at the same time the injection phase of the injection molding cycle is started.  With no air present in the cavity, a perfect condition has been created for trouble-free filling of the cavity.

Toth Mold/Die Inc. is a small shop that specializes in custom injection molding and injection mold building. We offer a variety of services from product design to final part production. We also build, sample and debug molds for other molding companies.