Every rotational molder has experienced scrap parts due to pin holes in the parting lines, leaks near inserts, and excessive parting-line flash. One member survey, by the Association of Rotational Molders (ARM), reported pin holes (blow holes and vacuum holes) to be the number-one defect in parts produced by ARM members. Parting line holes are commonly called pin or blow holes but are often holes created by vacuum. It is important to determine if the holes are blow or vacuum holes in order to correct the problem. A vacuum hole is created by air sucking in through the parting line into the mold during cooling, usually leaving a bubble or funnel-shaped hole on the inside of the part. A hole created from pressure build-up inside the mold escaping through the parting line, during the heating cycle, may blow out through the molten plastic through the mold parting line, creating a blow hole without an internal bubble.
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