13-3 Powder Metallurgy -the process of making parts by compressing & sintering various metallic and nonmetallic powders into shape Briquetting machines- used to compress the powders into shape Powder metallurgy is most applicable to the production of cylindrical, rectangular, or irregular shapes that do not have large variations in cross-sectional dimensions. Examples- splines, gear teeth, axial holes, counterbores, straight knurls, serrations, slots, keyseats

13-4 Plastic Molded Parts The design of molded parts involves several factors not normally encountered with machine-fabricated and assembled parts.

Design Factors for Molded Parts Shrinkage- defined as the difference between dimensions of the mold and corresponding dimensions of the molded part Section thickness-where section thickness varies, areas within a molded part will solidify at different rates. The varying rates cause…thus uniform section thickness is important – Irregular shrinkage – Sink marks – Additional strain – warpage Gates – location should be anticipated during design & located in the heaviest section of the part. Avoid gating into areas subject to high stress levels, fatigue, or impact.

Design Factors continued… Parting or flash line- flash generally forms at parting line & varies in thickness between.002 and.016 inch Fillets and Radii-ease the flow of plastic within the mold & facilitate ejection of the part & distribute stress evenly Molded holes- avoid holes anything but perpendicular to flash line & through holes are more accurate & economical Internal & External draft- varies between.25⁰ and 4⁰ to facilitate part removal

Design factors continued… Threads-external/internal threads can be easily molded by means of loose-piece inserts and rotating core pins. External threads can be formed by placing the cavity so the threads are formed in the mold pattern Ribs & bosses- – ribs increase rigidity without increasing wall thickness – Bosses reinforce small, stressed areas, providing sufficient strength for assembly with inserts or screws Undercuts – parts with undercuts should be avoided. Parts with external undercuts normally cannot be withdrawn from a one-piece mold

Assemblies : the design of molded parts that are to be assembled involves factors different from those normally encountered with metal Holes & Threads- – holes need to be spaced no less than a diameter in thickness apart (three times the OD for threaded holes). – Drilled holes are more accurate even though they require a second operation. – Tapped holes should be countersunk to avoid chipping when the tap is inserted. – External/internal threads can be molded into the part, but a method of unscrewing the part or a split mold must be used which increases cost. Inserts – the molded part should be designed around the insert

Assemblies continued… Press & Shrink Fits- inserts may be secured by a press fit or the plastic molding material may be assembled to a larger part by a shrink fit Heat Forming & Heat Sealing- most thermoplastics can be reformed by the application of heat & pressure Mechanical Fastening- molded parts must have sufficient strength to withstand stresses encountered with fasteners Rivets- conventional rivets can be used with plastics Boss Caps- a boss cap is a cup-shaped metal ring that is pressed onto the boss. It is designed to reinforce the boss against the expansion force exerted by tapping screws

Assemblies continued… Adhesive bonding-adhesives permit a strong, durable fastening Ultrasonic bonding- parts transmit ultrasonic vibration to small, hidden bonding areas, resulting in fast, perfect welds Ultrasonic Staking –a stud molded into the plastic part protrudes through a hole in the metal part. The surface of the stud is vibrated with a horn having high amplitude and relatively small contact area. The vibration causes the stud to melt and re-form into the configuration of the horn tip

Assemblies continued… Friction or Spin welding-the faces to be joined are pressed together while one part is spun & the other is held fixed. Frictional heat produces a molten zone that becomes a weld when spinning stops

Drawings 1.Can the part be removed from the mold? 2.Is the location of the flash line consistent with design requirements? 3.Is the section thickness consistent? 4.Has the material been correctly specified? 5.More on page 386…