Environmental Engineering Reference
In-Depth Information
mold.may.be.thicker.than.the.others..After.the.mold.is.rotated.and.all.the.
surfaces.are.suficiently.coated.with.the.melt-low,.the.rotation.stops.and.
the.melt-low.is.allowed.to.pool.at.the.bottom.of.the.mold.cavity.
Wall. thickness. is. important. for. corner. radii. as. well.. Large. outside. radii.
are.preferable.to.small.radii..Large.inside.radii.are.also.preferable.to.small.
inside.radii..This.allows.for.a.more.even.low.of.material.and.a.more.even.
wall.thickness..However,.it.is.to.be.noted.that.an.outside.corner.is.generally.
stronger.than.a.inside.corner.
4.9.10
Process: Advantages, Limitations, and Material Requirements
Rotational.molding.offers.design.advantages.over.other.molding.processes..
With.proper.design,.parts.assembled.from.several.pieces.can.be.molded.as.
one. part,. eliminating. high. fabrication. costs.. The. process. also. has. inherent.
design.strengths,.such.as.consistent.wall.thickness.and.strong.outside.cor-
ners. that. are. virtually. stress. free.. For. additional. strength,. reinforcing. ribs.
can.be.designed.into.the.part..Along.with.being.designed.into.the.part,.they.
can.be.added.to.the.mold.
The. ability. to. add. preinished. pieces. to. the. mold. alone. is. a. large. advan-
tage..Metal.threads,.internal.pipes.and.structures,.and.even.different.colored.
plastics.can.all.be.added.to.the.mold.prior.to.the.addition.of.plastic.pellets..
However,.care.must.be.taken.to.ensure.that.minimal.shrinkage.while.cool-
ing.will.not.damage.the.part..This.shrinking.allows.for.mild.undercuts.and.
negates.the.need.for.ejection.mechanisms.(in.most.pieces).
Another.advantage.lies.in.the.molds.themselves..Since.they.require.less.
tooling,. they. can. be. manufactured. and. put. into. production. much. more.
quickly. than. other. molding. processes.. This. is. especially. true. for. com-
plex.parts,.which.may.require.large.amounts.of.tooling.for.other.molding.
processes.
Rotational. molding. is. also. the. desired. process. for. short. runs. and. rush.
deliveries..The.molds.can.be.swapped.quickly.or.different.colors.can.be.used.
without.purging.the.mold..With.other.processes,.purging.may.be.required.
to.swap.colors.
Due. to. the. uniform. thicknesses. achieved,. large. stretched. sections. are.
nonexistent,. which. makes. large. thin. panels. possible. (although. warping.
may.occur)..Also,.there.is.little.low.of.plastic.(stretching),.but.rather.a.plac-
ing. of. the. material. within. the. part.. These. thin. walls. also. limit. cost. and.
production.time.
Another.cost-limiting.factor.is.the.amount.of.material.wasted.in.produc-
tion.. There. are. no. sprues. or. runners. (as. in. injection. molding),. no. off-cuts.
(thermoforming),.or.pinch.off.scrap.(blow.molding)..What.material.is.wasted,.
through.scrap.or.failed.part.testing,.can.usually.be.recycled.
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