Manufacturing Technologies for Turbine Power Blades - Hybrid Anisotropic Materials for Wind Power Turbine Blades - page 113

4.9.9.2 Designing for Rotational Molding

Another.consideration.is.in.the.draft.angles..These.are.required.to.remove.
the.piece.from.the.mold..On.the.outside.walls,.a.draft.angle.of.1°.may.work.
(assuming.no.rough.surface.or.holes)..On.inside.walls,.such.as.the.inside.of.
a.boat.hull,.a.draft.angle.of.5°.may.be.required..This.is.due.to.shrinkage.and.
possible.part.warping.
Another. consideration. is. of. structural. support. ribs.. While. solid. ribs.
may. be. desirable. and. achievable. in. injection. molding. and. other. pro-
cesses,.a.hollow.rib.is.the.best.solution.in.rotational.molding..A.solid.rib.
may.be.achieved.through.inserting.a.inished.piece.in.the.mold,.but.this.
adds.cost.
Rotational. molding. excels. at. producing. hollow. parts.. However,. care.
must.be.taken.when.this.is.done..When.the.depth.of.the.recess.is.greater.
than. the. width,. there. may. be. problems. with. even. heating. and. cooling..
Additionally,. enough. room. must. be. left. between. the. parallel. walls. to.
allow.for.the.melt-low.to.properly.move.throughout.the.mold..Otherwise.
webbing.may.occur..A.desirable.parallel.wall.scenario.would.have.a.gap.at.
least.three.times.the.nominal.wall.thickness,.with.ive.times.the.nominal.
wall. thickness. being. optimal.. Sharp. corners. for. parallel. walls. also. must.
be.considered..With.angles.of.less.than.45°.bridging,.webbing.and.voids.
may.occur.

4.9.9.3 Material Limitations and Considerations

Another.consideration.is.the.melt-low.of.materials..Certain.materials,.such.
as. nylon,. will. require. larger. radii. than. other. materials.. Additionally,. the.
stiffness.of.the.set.material.may.be.a.factor..More.structural.and.strengthen-
ing.measures.may.be.required.when.a.limsy.material.is.used.

4.9.9.4 Wall Thickness

One. beneit. of. rotational. molding. is. the. ability. to. experiment,. particularly.
with. wall. thicknesses.. Cost. is. entirely. dependent. on. wall. thickness,. with.
thicker.walls.being.costlier.and.more.time.consuming.to.produce..While.the.
wall.thickness.can.be.nearly.any.thickness,.designers.must.remember.that.
the.thicker.the.wall,.the.more.material.and.time.will.be.required,.increasing.
costs..In.some.cases,.the.plastics.may.signiicantly.degrade.due.to.extended.
periods.at.high.temperatures..Also,.different.materials.have.different.ther-
mal.conductivity,.meaning.they.require.different.times.in.the.heating.cham-
ber.and.cooling.chamber..Ideally,.the.part.will.be.tested.to.use.the.minimum.
thickness. required. for. the. application.. This. minimum. will. then. be. estab-
lished.as.a.nominal.thickness.
For.the.designer,.while.variable.thicknesses.are.possible,.a.process.called.
stop rotation .is.required..This.process.is.limited.in.that.only.one.side.of.the.