COMPRESSION AND TRANSFER MOLDING

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COMPRESSION AND TRANSFER MOLDING
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Compression Molding

• The process of molding a material in a confined
  shape by applying pressure and usually heat.
• Almost exclusively for thermoset materials
• Used to produce mainly electrical products

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CompressionMolding
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Background

• One of the oldest known molding processes.
• Widely used to produce articles from
  thermosetting and thermoplastic materials.
• The plastic material is placed in a mold cavity
  and formed by heat pressure.
• Most compression-molding equipment is typically
  sold by the press or platen rating.
• A force of 2900psi is usually required for
  moldings up to 1inch (25 mm) thick. An added
  725psi should be provided for each 1inch (25 mm)
  increase.

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In terms of processing, the past begins with the
manufacturing process known as compression
molding

The very first plastic products included items
such as, buttons, brush and mirror handles that
were compression molded.
All were relatively small and had relatively
simple geometries.
Hand made compression mold for a hand held mirror
(c.1870).
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The items shown below were compression molded
from a shellac based thermoplastic molding
compound known as Florence compound. Its
inventor, Alfred Critchlow, founded the Pro
Corporation, Florence MA in 1847. Pro is said to
be the very first plastics molding company.
Daguerreotype case (c.1856).
Hand held mirror frames (c.1866).
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Over the years, compression molding (and transfer
molding) press capacities (size) increase so that
larger thermoplastic and thermoset plastic parts,
such as this radio housing, could be molded.
Trimming flash from Ekco radio cabinets (c.1934).
Large scale compression molding press (c.1935).
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PROCESS
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Compression Molding a Composite
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Stages in Compression molding cycle

• The various stages of the compression molding
  cycle time can be represented as a function of
  the force required to close the mold at a
  constant rate. In the plastication stage (tlttf)
  the force increases rapidly as the polymer feed
  is compressed and heated. The second stage flow
  commences, once the yield stress of the elastomer
  is exceeded. tc is the point at which the mold
  fills and compression of the melt occurs.
  Ideally, to aid mold filling the majority of
  chemical reaction should take place after tc.

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Transfer Molding

• A process of forming articles by fusing a plastic
  material in a chamber then forcing the whole mass
  into a hot mold to solidify.
• Used to make products such as electrical wall
  receptacles and circuit breakers

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T R A N S F E R
M O L D I N G
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T R A N S F E R
M O L D I N G
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• Injection Compression Process
• CPI Process

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MATERIALS

• Thermoplastic
• Thermoset
• Composites

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PROCESS VARIABLES
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Variables

• Amount of charge
• Molding pressure
• Closing speed
• Mold temperature
• Charge temperature
• Cycle time

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Process Window

• There is an optimum process window for the
  filling stage, which can be represented by a
  Moulding Area Diagram (MAD).

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EQUIPMENT
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C O M P R E S S I O N
M O L D I N G
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Press calculations

• Press capacity
• Required force to press a part

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MOLD
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• Mold material
• Mold closure types

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TROBLE SHOOTING
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FLOW SIMULATION
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Advantages and Disadvantages

• ADVANTAGES
• Little waste (no gates, sprues, or runners in
  many molds)
• Lower tooling cost than injection molding
• Good surface finish
• Less damage to fibers
• Process may be automated or hand-operated
• Material flow is short, less chance of disturbing
  inserts, causing product stress, and/or eroding
  molds.
• DISADVANTAGES
• High initial capital investment
• Labor intensive
• Secondary operations maybe required
• Long molding cycles may be needed.