DESIGNING PLASTIC PARTS FOR ASSEMBLY Machinery's Handbook, 31st Edition
601
Poor Designs
Preferred Designs
Gap Between Added Bosses Prevents Excessive Bending of Housing as Bosses Touch and Go into Compression
Potential High Bending Stress as Bolt is Tightened
Pan Head Screw
Plastics Part
Countersunk Screw
Metal Component
Alternative Design Uses Recess to Avoid Wedging Action
Potential High Stress Due to Wedging Action of Screw Head
Round-head Screw
Shoulder Screw
Plastics Part
Metal Component Fig. 25. Examples of Bad and Good Designs in Assembling Plastics with Metal Fasteners Thread-cutting and -forming screws are widely used with plastics parts. Information on standard self-threading screws is found in SELF-THREADING SCREWS starting on page 1833. Thread-forming screws must be used carefully with high-modulus, low-creep plastics, as high hoop stresses can be generated during insertion. Screws with multiple lobes and screws with alternating low and high threads have excellent holding power in plastics. Molded holes must have sufficient depth to prevent bottoming, and boss walls must be thick enough to resist stresses. As a general rule of thumb, the outside diameter of the boss should be double the major diameter of the screw. Gussets and ribs can be used to increase the torsional strength of bosses. Hollow aluminum or other metal rivets are often used in plastics assembly, as are stamped sheet metal components, especially push-on or -in designs. Molded plastics fas teners are also frequently used. Machining Plastics.— Plastics can be molded into complex shapes and so do not usually need to be machined. However, machining is sometimes more cost-effective than making a complex tool, especially when requirements are for prototype development, low-volume production, undercuts, angular holes, or other openings that are difficult to produce in a mold. Special methods for development of prototypes are discussed on page 611. All
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