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Plastic Parts Design Kevin Schneider, Autodesk MA32-2 .
Plastics -“Polymers” Polymers Poly(many) Mer(parts): A large molecule made up of one or more repeating units(mers) linked together by covalentchemical bonds bonds. Example: polyethylene or poly(ethylene)
n = number of monomers reacting >> 1
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Effect of Molecular Weight on the Properties of Polyethylene
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Outline Fundamentals Concept Models and Layouts Splitting Technical Modeling The Assembly Process
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Design Example File naming Wall thickness Fillets, Draft and Ribs Design For Manufacturing
Uneven wall thickness present challenges to the plastic molder manufacturer. Designing your part with uniform walls and cross section will simplify manufacturing and costing. At wall intersection or "tees" sinking will occur. Thick walls cool slower and greater shrinking will occur. Thin walls cool faster as thus, less shrinkage.
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Ways to deal with this "shrikage“
a) b) c) d) e) f) g)
Unwanted sink Disguise (texture) Core out top Core out bottom Foaming agent (structfoam) Gas assist molding Spread sink over more area
Fillets, Draft and Ribs • Your rules may be different… • • • •
Fillets should be min ¼ X Wall thickness High stress parts, ¾ x Wall thickness Draft min of 1 deg. deg Finish affects draft Ribs should be 1/2 to 2/3 of the nominal wall thickness and less than 3 times thickness in height height. Taper of 1 deg. is typical. Note: excess thickness promotes shrinkage. shrinkage Excess rib height combined with taper will produce thin sections requiring extra e at tthe e mold. od fill ttime 9
Mechanical fasteners are point fasteners. Localized regions of potentially high stress. Holes >>> stress concentration and weld line formation. Thermal expansion mismatch. mismatch Additional pieces / parts. G k t to Gasket t achieve hi a fluid fl id or gas tight ti ht seal. l
Fastener Advantages • Operable (or reversible) joints or permanent assembly. • An effective method for joining most thermoplastic & thermosetting parts (except very ) flexible items). • Join parts produced in similar or dissimilar materials. • Available in a variety of sizes and materials. • The joining practices are very conventional. • Metal “fastener’s” properties are independent of temp., te p, • The assembly strength is achieved quickly. 35
Material sources: David O. Kazmer, P.E., Ph.D. Department of Plastics Engineering U i University it off Massachusetts M h tt Lowell L ll One University Avenue