DESIGNING PLASTIC PARTS Machinery's Handbook, 31st Edition
594
Draft Angle α
Parallel Draft
Depth of Draw, D
Dimensional Difference, d
Fig. 19. Defining Dimensions Related to Draft and Draft Angle Where minimal variation in wall thickness is needed to produce sidewalls that are perpendicular to the direction of draw, the mold can be designed with parallel draft, as indicated at the left in Fig. 19 . The amount of draft also depends on the surface finish of the mold walls. Any texturing of the surface will increase the draft requirement by at least 1 degree per side for every 0.001 inch (0.025 mm) of texture depth. Most plastics parts are so designed that they can be ejected parallel with the direction of mold parting. Complex parts with undercuts may require mold designs with cavity- forming projections that must move at an angle to the direction of opening. Between these two extremes lie such items as “windows,” or simple openings in the side of a molding, which can be produced by the normal interaction of the two main parts of the mold. Fillets, Radii, and Undercuts: Sharp corners are always to be avoided in injection-molded part designs because they represent points of stress concentration. Sharp corners in metal parts often are less important because the stresses are low compared with the strength of the material or because local yielding redistributes the loads. Sharp inside corners are particularly to be avoided in moldings because severe molded-in stresses are generated as the material shrinks onto the mold corner. Sharp corners also cause poor material flow patterns, reduced mechanical properties, and increased tool wear. Therefore, inside corner radii should be equal to half the nominal wall thickness (as shown in Fig. 17 for a 1 mm rib thickness) with a minimum of 0.020 inch (0.51 mm) for parts subject to stress, and 0.005 inch (0.127 mm) radius for stress-free parts. Outside corner radius should equal the inside corner radius plus the wall thickness. With an inside radius of half the wall thickness, a stress concentration of 1.5 is a reason able assumption, and for radii down to 0.1 times the wall thickness, a stress concentration of 3 is likely. More information on stress concentrations is found in Working Stress on page 212 . A suitable value for q in Equation (8) on page 212, for plastics materials, is 1. That is, the actual stress-concentration factor equals the theoretical value. Impact Resistance: The impact resistance of a plastics part is directly related to its ability to absorb mechanical energy without fracture or deformation, and this ability depends on the material properties and the part geometry. Increasing wall thickness may improve impact resistance but may also hurt impact resistance by making the part too stiff so that it is unable to deflect and distribute the force. Validation is required in critical applications. Melt Flow in the Mold.— The designer of plastics products needs a basic appreciation of what goes on during injection molding and the special properties of plastics that affect molding. Some of these, such as thermal properties, have been previously described. It is the flow properties of molten plastics that dominate mold filling. Foremost are the very high viscosities of thermoplastic melts—thousands to hundreds of thousands times that of room-temperature water—requiring pressures ( P ) of 5,000–30,000 psi (35–206 MPa) to move the melt quickly from the injection cylinder through the mold runners and into the cavities. Second is the strong dependence of viscosity on temperature, so the transfer to the chilled mold must be nearly instantaneous to obtain fully filled cavities and accurately dimensioned products. Because of the high viscosities and rapid flow, frictional heat is dissipated within the melt, helping to keep it warm during its travel. Another property,
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