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material will reach. However, this tactic is particularly critical
for LSR molds, because the need for tight fits between com-
ponents in these tools presents fewer options for venting. For
instance, straight-walled ejector pin holes can't double as vents
in LSR tooling, because the material would flash into the gap
surrounding the pin. Only a tapered shutoff will do.
• Fewer moving parts. Ejector pins aren't the only common
mold component missing from most LSR tools. These molds
don't usually incorporate slides, lifters or other moving actions
either. That's because undercuts simply aren't as much of a
concern. As an example, Finnie cites a protective face mask
that can literally be peeled off the core without risk of dam-
age—the material will simply stretch and snap back into place.
• Challenging ejection scenarios. Although the material's
high elasticity can simplify mold designs, LSR presents its own
challenges for part ejection. With plastic, the right mold design
can ensure the material will essentially wrap itself onto a spe-
cific portion of the geometry as it cools and shrinks. However,
that's not the case with LSR, which expands as it cures. "We
have to go to extra lengths to ensure that parts will be where
we want them to be when the mold opens," Finnie says.
Consider the baby bottle nipple pictured on page 40. The
large flange at the bottom prevents the nipple from falling into
LSR requires creativity when it comes to removing parts from the mold.
This particular tool employs pop-up ejection.