Venting Molds in Rotational Molding

The purpose of a vent tube is to allow air to move in and out of the mold during the molding process. A clean vent tube is essential in both the oven and cooling cycles. During the oven cycle the vent tubes relieve the mold of pressure build up. During the cooling cycle they relieve the mold of vacuum. A clogged vent tube will likely cause either blow holes, vacuum holes, excessive flash, shape or dimensional issues in the final part. A vent tube consists of the tube itself as well as packing material placed inside the tube. Vents are typically made of Teflon tubing and packing is preferably either insulation or scouring pad. The industry doesn’t have an exact standard for vent ID sizing, however 0.5 to 0.75 inches per cubic yard of mold volume is commonly used. Keep in mind that packing material will reduce air flow. Most often over venting is better than under venting.

Depth – How far a vent tube is inserted into the mold is very important. If the tube is pushed too far in, melted plastic will accumulate around the end. This usually creates a knot or a raised spot underneath the vent tube on the inside surface of the part. If not pushed in far enough the powder will flow into the vent causing it to clog easily. As a general rule for molding parts there should be at least 4” to 6” between the inside end of the vent tube and the surface of the other half of the mold. If this is not possible then the vent needs to reach the halfway point between both inner wall surfaces of the mold.

At Entec we are here to help and assist you with properly training and coaching your team in mold venting. We believe that proper venting should be one of the critical components in your efforts to reduce scrap and second quality parts. For a more detailed discussion please feel free to reach out and let us know how we can help!

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