Female form material types

Undercuts and reentrant shapes are possible in many designs. They require movable or collapsible mold members, but with small undercuts they can often be sprung from a female mold while the formed product is still warm. This type of action works best when the plastic has some flexibility, as do the TPEs, or the material is very thin. Guidelines for the maximum amounts of undercutting that can be stripped from a mold are as follows 0.04 in. (0.1 cm) for acrylics, PCs and other rigid plastics 0.060 in. (0.15 cm) for PEs, ABSs, and PAs 0.100 in. (0.25 cm) for flexible plastics such as the PVCs. 

Insect chemosterilants may act in several ways. They may cause the insects to fail to produce ova or sperm antimetabolites, when they are also chemosterilants, act in this way. Compounds that cause the death of sperm and ova after they have been produced would also be considered chemosterilants, but I do not know of any compounds of this type which are being considered for use in insect control. A third type of action, and the one in which we are most interested at the present time, is that shown by the radiomimetic compounds. These compounds apparently injure the chromatin, or genetic material, in the sperm and ova so severely that, although they remain alive and the sperm retain full motility, the zygotes, if formed, do not complete development into mature progeny. This type of action is desired because the males sterilized in this manner compete readily with normal males for the available females and transfer motile sperm to the spermathecae of the females, with the result that the mating requirements of the females are satisfied to the same extent as in a mating with a normal male. 

The positive type mold closure has a long shearing surface between the male and female segments of the mold. This surface is formed when the mold segments are in close proximity to each other. The gap between the male and female segments is maintained closely, thus no flash is formed. The amount of material charged into the mold is carefully controlled because excess material would prevent mold closure. Insufficient charge results in a short shot, that is, an incomplete part will form. 

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