Environmental Engineering Reference
In-Depth Information
in. less. time. than. at. atmospheric. pressure.. This. pressure. delays. the. sepa-
ration. of. the. part. from. the. mold. wall. due. to. shrinkage. during. the. cooling.
phase,.aiding.cooling.of.the.part..The.main.drawback.to.this.is.the.danger.to.
the.operator.of.explosion.of.a.pressurized.part..This.has.prevented.adoption.
of.mold.pressurization.on.a.large.scale.by.rotomolding.manufacturers.
4.9.7
Mold Release Agents
A. good. mold. release. agent. (MRA). will. allow. the. material. to. be. removed.
quickly. and. effectively.. Mold. releases. can. reduce. cycle. times,. defects,. and.
browning.of.the.inished.product..There.are.a.number.of.mold.release.types.
available;.they.can.be.categorized.as.follows:
•. Sacriicial.coatings:.The.coating.of.MRA.has.to.be.applied.each.time.
because. most. of. the. MRA. comes. off. on. the. molded. part. when. it.
releases.from.the.tool..Silicones.are.typical.MRA.compounds.in.this.
category.
•. Semipermanent.coatings:.The.coating,.if.applied.correctly,.will.last.
for.a.number.of.releases.before.requiring.reapplication.or.touching.
up..This.type.of.coating.is.most.prevalent.in.today's.rotational.mold-
ing.industry..The.active.chemistry.involved.in.these.coatings.is.typi-
cally.a.polysiloxane.
•. Permanent.coatings:.Most.often.some.form.of.PTFE.(polytetraluoro-
ethylene).coating,.which.is.applied.to.the.mold..Permanent.coatings.
avoid.the.need.for.operator.application,.but.may.become.damaged.
by.misuse.
4.9.8
Materials
More.than.80.percent.of.all.the.material.used.is.from.the.polyethylene.fam-
ily:.cross-linked.polyethylene.(PE),.linear.low-density.polyethylene.(LLDPE),.
and.high-density.polyethylene.(HDPE)..Other.compounds.are.glass.and.car-
bon.iber,.PVC.plastisols,.nylons,.and.polypropylene.and.ceramics..The.order.
of.materials.most.commonly.used.by.industry.is:
•. Polyethylene
•. Polypropylene
•. Polyvinyl.chloride
•. Nylon
•. Polycarbonate
These. materials. are. also. occasionally. used. (not. necessarily. in. order. of.
most.used):
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