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13-3 Powder Metallurgy-the process of making parts by compressing

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13-3 Powder Metallurgy-the process of making parts by compressing & sintering various metallic and nonmetallic powders into shape. Briquetting machines- used to ... – PowerPoint PPT presentation

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Title: 13-3 Powder Metallurgy-the process of making parts by compressing


1
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

2
13-4 Plastic Molded Parts
  • The design of molded parts involves several
    factors not normally encountered with
    machine-fabricated and assembled parts.

3
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 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 into areas subject to high
    stress levels, fatigue, or impact.

4
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 .25
    and 4 to facilitate part removal

5
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

6
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

7
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

8
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

9
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

10
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
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