Plastic injection molding     Mold Making   Injection Molded Plastics      When attachment points and points of alignment are needed, bosses are added. They too should be no thicker than the nominal wall and can be strengthened by gussets to counter lateral forces. In bosses for self-tapping screws, configure inside diameter and wall thickness to avoid excessive hoop stress. When molding-in a tapered pipe thread, avoid a wedging action on the boss and, if possible, use the male rather than female thread. Stress that accumulates in sharp corners as a plastic shrinks after molding can cause failure under high load or impact. To avoid this, use generous radii on corners where ribs, bosses, sidewalls and other features connect. Inside corner radii (fillets) vary with stress. In stress-free areas, radii can be as small as 0.005 in., while those where loads are high should have radii of over 0.02 in. The radius of an outside corner should equal that of the inside corner plus the wall thickness.

      Cores add openings in parts to reduce weight or provide mounting points. They are created by pins that project into the mold cavity. The minimum core size depends on how well its pin can withstand the force of flowing plastic. Pins for blind holes are supported at one side of the mold. Their depth should be less than three times their diameter or minimum cross-section (or less than 2 times, if the minimal dimension is under 0.25 in.). Cores that extend to other parts of the mold can be twice as long as blind cores, because they are supported at both ends. Mold release parts should be designed so mold tools open and eject the part easily. This is most often done by tapering ribs, bosses and other elements. Taper (or draft) should be at least 0.5 deg. per side, although 1.5 to 3 deg. per side is more common. Draft also depends on mold surface finish, and increases at least 1 deg. per side for each 0.001-in. depth of texture. A 1 deg. draft yields a 0.017-in. taper per inch of length.

     Design parts to keep the mold as simple as possible, because complex parts may need expensive tooling with involved mechanical movements, such as side action, cams, pullers and collapsing cores, to enable release. This means considering where parting lines, undercuts and other features fall in the mold. In some cases, special mold surface treatments or a mold release spray may be needed.

    In general, approach the design of plastic parts from a system standpoint. Choices made in the material selected, design of the overall part and its elements and the assembly methods chosen influence each other. As the design process progresses from concept to production, these choices should be reevaluated repeatedly.
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