[Jewelry making - Article 00129] - Re: [Orchid] Vent holes in rubber molds

[Jwryrubber]

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    I like to open a discussion of mold venting by first telling folks
    that air is very, very skinny. That is to say it will slither out of
    the tiniest opening without trouble. This means that there is
    absolutely no need to drill holes, burn slots, cut v-shaped channels
    or cut vents at the entry sprue. A vent at entry does nothing. 

    The vents must be slit at the far end of the pattern. That is where
    the air gets pushed and trapped. Most of the air will escape between
    the mold halves ahead of the wax, hence the comment from someone
    that many molds don't need vents. 

    Vents are cheap. Throw a few into your mold. They do not shorten the
    life of the mold. Flex the mold and slice several slits leading to
    the outside of the mold. Start the slice in the corner of the model,
    or anywhere that may trap air. In mold rubber, especially silicones,
    smooth slit surfaces tend to adhere to each other, almost like
    tackiness. It is not tackiness, but that requires another discussion
    and the science of what is going on there is not important right
    now. Because the sides of the slits want to adhere to each other and
    block air flow, they should be lightly dusted to take this tendency
    away. The dusting is not for the purpose of keeping the slits
    slightly open. It is too eliminate the tendency to adhere. 

    As for getting crispness of lettering, there are only two mold
    rubbers which provide that today. Call me for our 4X ZeroShrink and
    our LSLT-160. These are very dense products. Your mold will feel
    like a rectangular hockey puck, but you will get true zero shrink in
    one case and ten minute cure times in both cases. You will get
    beautiful, crisp replication of lettering and other difficult
    details. These metal mold like silicone rubbers still provide high
    tear resistance and customary ease of cutting and nice flexibility. 

Regards,
Bill Mull

Zero-D Products, Inc.
precision engineered materials solutions
http://www.zerodproducts.com
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