Injection Molding - Polymer Processing - Lecture Slides, Slides of Software Engineering

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ITEC 142

Injection Molding

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Chapter 11: Injection Molding

• Overview
  • Equipment
  • Material and product considerations
  • Operation and control of the process
  • Specialized injection molding processes

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Injection Molding Equipment

• Function
  • Injection
  • Molding
  • Clamping

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Injection Unit

• Purpose
  • Melt solid pellets to liquid form and then inject into mold
• Steps
  • Hopper- manual or pneumatic loaded. Can have a mixer, volumetric or gravimetric units to meter material.
  • Screw
    • Reciprocating screw
      • most common
      • similar to general purpose extrusion screw
      • much shorter than extrusion screws, L/D of 12:1 to 20:1 (E: 20:1 to 30:1)
      • compression ratios (diameter of feed to diameter of metering) are often 2:1 to 5: 1 which is lower than for extrusion.
      • lower compression ratio means less mechanical action and heating
      • Step 1: turns of the screw melts resin and collects it at end of screw
      • Step 2: the screw moves forward via a hydraulic mechanism
      • Step 3: retraction of screw
      • Step 4: part cooling and removal

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Injection Molding Ram Injection

• Ram injection
  • plunger type machine
  • used prior to the invention of the reciprocating screw
  • Step 1: resin melts via thermal heaters and collects in a pool called injection chamber
  • Step 2: resin pushed forward by action of plunger (ram or piston) driven by hydraulic system at the head of the machine. A torpedo or spreader is used in barrel to improve melting and mixing.
  • Step 3: resin flows into mold
  • Step 4: part cools and is ejected
• Ram injection advantages
  • less expensive
  • better for marbling of plastics
• Reciprocating screw advantages
  • more uniform melting
  • more uniform mixing
  • lower injection pressures
  • larger permissible part area
  • fewer stresses in part Docsity.com

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Injection Molding Ram Injection

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Injection Molding Terms

• Shot size- maximum weight of injection molding machine that

can be injected.

• Typical shot sized for injection molding machines
  • 0.7 ounces (20 g) to 700 ounces (20 kg)
• Rating system for injection molding machines is shot size
  • PS is the standard material since thermoplastics have varying densities
• Screw machines have a wider range of shot sizes than ram

injection machines

• Rule of thumb
  • reciprocating screw has a range of 1/200 of total size to max shot size
  • ram injection has a range of 1/5 of the total size to max shot size

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Injection Molding Molds

• The mold includes the shape of the part and is located between the

stationary and movable platens of the injection molding machine

• Key terms
  • sprue bushing - part of mold (cooled)
  • nozzle - end of injection (heated)
  • sprue channel - from bushing to runner
  • runners - feeds material from sprue to part
  • gate - mold area between runner and part
  • mold cavity - concave part of mold
  • mold core - convex part of mold
  • multi-cavity - more than one part in a cavity
  • ejectors - knock out pins
  • mold inserts - multiple cavities for same base
  • mold base - inserts used in same base
  • MUD base - Master Unit Die
  • draft angle - minimum angle from bottom to top of part
  • parting line - the split between core and cavity molds

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Runner System

• Runner size considerations
  • Although properly sizing a runner to a given part and mold design

has a tremendous pay-off, it is often overlooked since the basic

principles are not widely understood.

• Pros and cons of large runners
  • While large runners facilitate the flow of material at relatively low

pressure requirements, they

• require a longer cooling time, more material consumption and

scrap, and more clamping force.

• Pros and cons of small runners
  • Designing the smallest adequate runner system will maximize

efficiency in both raw material use and energy consumption in

molding. At the same time, however, runner size reduction is

constrained by the molding machine's injection pressure capability.Docsity.com

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Runner System

• Runner Balancing is an essential for a balanced filling

pattern with a reasonable pressure drop.

• Payoffs of good runner design
  • A runner system that has been designed correctly will:
    • Achieve the optimal number of cavities
    • Deliver melt to the cavities
    • Balance filling of multiple cavities
    • Balance filling of multi-gate cavities
    • Minimize scrap
    • Eject easily
    • Maximize efficiency in energy consumption
    • Control the filling/packing/cycle time.

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Gate System

• Several types of gates
  • rectangular simple gate
  • fan gate

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Clamping System

• Several types of clamping systems
  • rectangular simple gate
  • fan gate

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Ejector System

• Several types of ejector systems
  • ejector plate
  • ejector pins
  • mechanical plate
  • hydraulic pins

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Plastics Design for Injection Molding

• Part Design
  • The underlying principles behind part design, other than part functionality are
    • cooling of plastic from melt to glassy state
    • heat transfer from various sections
    • thermal shrinkage of the plastic parts
  • Heat transfer is best when the parts have the same thickness.
    • Inside portions of parts cool more slowly than the part surfaces
    • Center portion will shrink more than the surface