DESIGNING PLASTIC PARTS Machinery's Handbook, 31st Edition
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Considerations and questions to ask in this phase of the project are: 1) What are the strength, structural stiffness, impact loading, exposure to chemicals, operating temperature range (and possible overheating and cooling), electrical factors, and other relevant material factors? 2) What certifications or government regulations are involved? 3) Must the design be approved by clients or in-house managers? 4) Has the marketing group determined the anticipated production run? 5) Input from the prospective mold builder and molder (often not the same) should be sought before the design is made final. 6) The cost for each part produced of the plastics to be used. Relative Plastics Costs: The list of some comparative material costs in Table 7 should serve to narrow materials choices, but current quantity quotations from various suppliers must be obtained. It is important to account for material density when calculating material costs, since the density of thermoplastics may vary significantly. For injection molding of a large production run, a crude estimate of part cost is double the material cost to account for amortized mold cost, as well as the molder’s processing costs. Table 7. Volumetric Costs of Plastics Type/Material Price (US $) Type/Material Price (US $) Per Pound Per Kilogram Per Pound Per Kilogram Thermoplastics PSU 5.50 12.13 ABS 1.24 2.73 PTFE 7.35 16.20 Acetal, POM 1.41 3.11 PVC 0.89 1.96 HDPE 0.70 1.54 TPU 2.06 4.54 LCP 6.40 14.11 Recycled Thermoplastics LDPE 0.78 1.72 HDPE 0.68 1.50 PA66, Nylon 1.64 3.62 Recycled LDPE 0.39 0.86 PC 1.77 3.90 Recycled PET 0.61 1.34 PEEK 45.00 99.21 Recycled PP 0.34 0.75 PEI 8.80 19.40 Thermoset PET, bottle 0.78 1.72 Epoxy 1.12 2.47 PMMA 1.28 2.82 Thermoset Melamine 1.55 3.42 Polyester 1.23 2.71 Thermoset Phenolic 0.80 1.76 PP 0.85 1.87 Thermoset Polyester 1.84 4.06 PPS, 30%GF 3.85 8.49 Thermoset Polyurethane 1.10 2.43 PS 1.10 2.43 Thermoset Urea 1.07 2.36 Based on 2018 market prices. Recycled material prices are for pelletized feed stock and vary with quality. Wall Thicknesses: Wall thickness throughout the part, referred to as the nominal wall thickness, is one of the most critical design parameters. Nominal dimensions and toler- ances should be calculated and decided before the mold is made, since modifications are costly. In general, wall thicknesses should be kept as thin as needed to meet the part’s de- sign requirements and as uniform as possible. Ideally, the flow of molten plastic should be arranged to flow through thicker sections into thinner ones rather than the reverse. This is accomplished—in consultation with the mold builder—by gating cavities in the thick sec - tions. The mold builder will help, too, with the decisions on how many cavities are needed, how to arrange them in the mold for optimal temperature control, the best runner layout and sizing, and what size mold and injection machine will be economical for the production de- sired. Most injection-molded plastics parts have wall thicknesses in the range from 0.02 to 0.25 inch (0.5 to 6 mm). For compounds containing half-inch (12.7 mm) and longer glass fi - bers, and for thermoplastic polyurethanes, the range shifts upward by a factor of 2 or more.
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