Transcript 13-4 Plastic Molded Parts

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
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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with external undercuts normally cannot be withdrawn from a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…