13-4 Plastic Molded Parts

7/29/2019 13-4 Plastic Molded Parts

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13-3 Powder Metallurgy-the process ofmaking 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


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13-4 Plastic Molded Parts

The design of molded parts involves several

factors not normally encountered with

machine-fabricated and assembled parts.


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Design Factors for Molded Parts

Shrinkage- defined as the difference between dimensions ofthe mold and corresponding dimensions of the molded part

Section thickness-where section thickness varies, areas withina molded part will solidify at different rates. The varying rates

causethus 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 intoareas subject to high stress levels, fatigue, or impact.


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Design Factors continued

Parting or flash line- flash generally forms at parting line &

varies in thickness between .002 and .016 inch

Fillets and Radii-easethe 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 .25 and 4 to

facilitate part removal


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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. Partswith external undercuts normally cannot be withdrawn from a

one-piece mold


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Assemblies: the design of molded parts thatare 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 isinserted.

External/internal threads can be molded into the part, but a methodof unscrewing the part or a split mold must be used which increasescost.

Inserts the molded part should be designed around theinsert


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Assemblies continued

Press & Shrink Fits- inserts may be secured by a pressfit or the plastic molding material may be assembled toa 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 encounteredwith 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 theboss against the expansion force exerted by tappingscrews


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Adhesive bonding-adhesives permit a strong, durablefastening

Ultrasonic bonding- parts transmit ultrasonic vibrationto small, hidden bonding areas, resulting in fast,perfect welds

Ultrasonic Stakinga stud molded into the plastic partprotrudes through a hole in the metal part. Thesurface of the stud is vibrated with a horn having high

amplitude and relatively small contact area. Thevibration causes the stud to melt and re-form into theconfiguration of the horn tip


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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


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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

Documents

13-4 Plastic Molded Parts