Machinery's Handbook, 31st Edition
592 DESIGNING PLASTIC PARTS Other ways to improve section properties include use of top-hat and corrugated sections, crowning or doming of some areas, and reinforcement with metal or other inserts that are placed in the mold before it is closed. To keep molded parts uniform in wall thickness, cores or projections may be provided in the mold to prevent a space being filled with mold ing material. Blind holes can be cored by pins that are supported on only one side of the mold and through holes by pins that pass through both sides. The length-to-width ratio should be kept as low as possible to prevent bending or breakage under the high pressures used in the injection molding process. Agency approvals for resistance to flammability or heat, electrical properties, or other characteristics are usually based on specific wall thicknesses. These restrictions some times necessitate thicker walls than are required for structural strength purposes. When determining wall thickness, design engineers must also have some knowledge of mold design and should consider the ability of plastics to flow into the narrow mold channels. The degree and rate of flow depend on temperature and pressure to some extent but vary for different materials, as shown in Fig. 18 . Spiral flow tests provide much more accurate information about the achievable flow lengths for a given wall thickness than melt flow indices (MFI), since they are performed with shear rates similar to those of molded parts. However, spiral flow tests tend to overestimate flow lengths relative to real applications, because they calculate the maximum effect of the moving melt’s heat convection relative to radial flow fronts that often occur in mold filling.
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Fig. 18. Typical Spiral-Flow Curves for (1) Nylon 6 / 6, (2) Polyester Thermoplastics PBT, Liquid-Crystal-Glass-Reinforced, and Polyphenylene-Sulfide-Glass-Reinforced, (3) Acetal Copolymer, and (4) PBT-Glass-Reinforced Plastics Materials Stresses and Deflections in Load-Bearing Parts.— If the plastic part is to carry loads, load-bearing areas should be analyzed for stress and deflection according to the princi - ples presented earlier. If stress or deflection is found to be too high, solutions include using ribs or contours to increase section modulus; using a higher-strength or higher-modulus (i.e., fiber-reinforced) material; or increasing the wall thickness if it is not already ex - cessive. Where space allows, adding or thickening ribs can increase structural integrity without making walls thicker. Formulas given in Beam Calculations starting on page 256, which presume elastic behavior, can be used if care is taken to use moduli and strengths related to anticipated service times and temperatures. Example: Consider a simple case of a beam with rectangular cross section b wide and h deep, of length l centrally loaded and freely supported at its ends. This is an example of Case 2 on page 257 with a beam cross section depicted generically in the first diagram of Moments of Inertia, Section Moduli, and Radii of Gyration on page 241. Applicable formulas for the moment of inertia I of the beam cross section, the section modulus Z , and the maximum tensile stress s beneath the central load are Z I bh Z Wl bh Wl 6 4 4 6 2 2 σ = = = I bh 12 3 = = ( h ⁄ 2)
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