Two plate mold is the easiest plastic injection mold structure and has many advantages. It's consist with A side and B side 2 main parts, when it is load in ejection machine to do plastic injection molding, A side is fixed and held still, B side is movable. See below figure to get the details of the 2 plate mold.
Three plate mold
3 plate mold is also named as small gate mold, comparing with 2 plate mold, one of the differences is additional injection mold plate is required between top clamp plate and cavity plate, which called runner plate, it allows the runner scrap and plastic molded parts released separately, see below figure of three plate mold structure.
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