Thermoplastic Injection Molding

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Thermoplastic InjectionMolding

• An Overview

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Injection Molding

• Topics to be Covered
• Introduction
• Part Design
• Tooling Design
• Machine

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Plastics

• Thermoplastic
• Can be remelted to become viscous fluids and can
  then be cooled to a solid state to give the same
  properties as before melting.
• Thermosetting
• Cannot be remelted after formation

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Plastics

• Thermoplastic
• Most common. Can be fabricated by blow and
  injection molding, vacuum forming, extrusion and
  other methods.
• Thermosetting
• Can be injection molded or resin transfer molded
  (RTM). Can be laminated or bag molded or
  fabricated using lay-up methods.

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Why Injection Molding?

• High production rates
• Large volume production possible
• Low labor cost per unit
• Little or no post-processing required
• Many colors and finishes available
• Good for many complex shapes
• Good for small parts
• Minimal scrap
• Close tolerances can be maintained
• Insert molding possible

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Why Not Injection Molding?

• Machine costs are high
• Mold costs are high
• Process control issues
• Part design issues

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Part Design

• Constant Wall Thickness
• Nonuniform cooling leads to problems of voids,
  warpage and sink
• Radius Corners
• Improves plastic flow
• Reduce stress concentration
• Draft Angles
• Slight angle on walls perpendicular to parting
  line

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Part Design

• Shrinkage
• Amount part will reduce in size as it cools to
  room temperature
• Plastic dependent

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Tooling Design

• Mold Plates
• Ear Plates
• Ejector Pin Plates
• Mold Pins
• Sprue
• Runner
• Gate
• Vent

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Machine

• Process Cycle
• Polymer pellets fed into cylinder
• Plasticate in cylinder
• Inject polymer into mold
• Cool part
• Eject part

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Machine

• What happens in barrel?
• Conveying
• Compaction
• Melt film formation
• Melt pool formation
• Melting complete

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Machine

• Process Parameters
• Screw speed
• Injection speed
• Injection pressure
• Back pressure
• Shot size
• Barrel temperature
• Mold temperature
• Cooling time

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Machine

• Projected Area Press Capacity
• Includes mold cavity and runner system
• 2 to 2.5 tons per square inch of projected area

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