Environmental Engineering Reference
In-Depth Information
an.automated.version.of.the.blacksmith's.craft.of.pounding.horseshoes.out.
of. rods. of. heated. steel.. At. some. point,. however,. forging,. stamping,. and.
other.mechanical.deformation.methods.become.too.unwieldy.a.technique.
to.obtain.highly.complex,.intricate.shapes..Thus,.processes.that.rely.on.luid.
low,.such.as.casting.and.injection.molding,.are.used.to.fabricate.the.many.
intricate.parts.in.a.model.car.kit.or.in.a.medical.device..Other.techniques.
that.rely.on.a.“writing”-type,.layer-by-layer.process.also.provide.increased.
control.for.three-dimensional.structures.
Dimensional tolerance and surface inish
..The.importance.of.the.dimen-
sional. precision. and. the. surface. inish. affects. the. type. of. manufacturing.
process. selected.. For. example,. vacuum. forming,. which. uses. a. rigid. tool.
on. one. side. and. a. lexible. surface. on. the. other,. is. a. much. cheaper,. lower.
force,.and.more.forgiving.process.than.forming.with.a.pair.of.matched.die.
molds,. but. the. produced. part. can. have. much. greater. thickness. variations.
and. surface. roughness.. Products. such. as. automotive. body. panels. require.
a. “Class. A”. surface. inish. that. displays. no. scratches,. dimples,. wrinkles,.
or. other. defects. that. would. detract. from. the. high-luster,. polished. appear-
ance..These.panels,.however,.only.require.such.a.inish.on.one.side.of.the.
part.—.for.example,.no.one.looks.at.the.underside.of.the.hood..Cast.parts.
typically. have. poor. surface. inish. and. dimensional. tolerance. because. of.
the.shrinkage.and.porosity.that.occurs.as.the.molten.metal.cools..Polymers.
also.tend.to.shrink.signiicantly.upon.cooling;.thus,.many.parts.requiring.
strict.dimensional.control.utilize.illed.polymers.—.that.is,.polymers.mixed.
with.short.chopped.ibers.or.other.illers.—.to.reduce.shrinkage,.moisture.
absorption,.and.creep.
Number of parts
..The.anticipated.volume.of.parts.and.desire.for.lexibility.
in. design. play. an. important. role. in. process. selection.. Expensive. tooling,.
costing.on.the.order.of.tens.of.thousands.of.dollars.and.up,.is.only.practical.
if.the.cost.can.be.spread.over.many.parts..In.contrast,.customizable.prod-
ucts. must. rely. on. easily. modiied. processes. such. as. machining. and. rapid.
prototyping..Another.example.can.be.found.within.the.many.variations.on.
the.casting.process.—.sand.casting,.lost.wax.or.investment.casting,.die.cast-
ing,.centrifugal.casting,.etc..The.irst.two.variations.involve.destroying.the.
mold.for.each.part,.whereas.the.latter.two.variations.utilize.reusable.molds..
Reusable. molds. are. fabricated. from. much. more. expensive. materials. and.
only.become.economical.for.the.production.of.a.large.number.of.parts.(or.
fewer.but.more.expensive.parts).
Material
.. In. the. ield. of. materials. engineering,. a. common. description. of.
the.interrelation.of.multiple.factors.is.the.structure-property-processing.tri-
angle.(Figure.3.4)..The.design.low.does.not.have.a.single.starting.point,.as.
each.node.affects.the.other.two..For.example,.the.rate.at.which.a.polymer.is.
extruded.and.cools.affects.its.crystallinity.(structure),.which.then.affects.its.
stiffness.and.strength.(property)..A.material.that.is.brittle.(property).would.
not.be.suitable.for.forging.(processing).
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