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

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Injection Molding Molds

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Introduction

• Background
  • Concept is simple
    • Melt plastic, flow into mold and take part shape, cool, demold
  • Injection molding makes parts in discrete (discontinuous) process
  • More injection molding machines used for plastic processing

than any other equipment

• Almost all thermoplastic and some thermosets materials can be

injection molded

• Process is automated and highly repeatable parts
• Injection molding parts are finished with little post molding

operations

• Very complex parts can be made
• Machines are expensive
• Molds are expensive, usually P-20 steel

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Runner System

• Several types of runners
  • single part runner
  • multiple part runner
    • symmetrical runner
    • non-symetrical runner
  • runner-less designs with hot manifolds

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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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Hot Runner System

• The ideal injection molding system delivers molded parts of uniform

density, and free from all runners, flash, and gate stubs.

• To achieve this, a hot runner system, in contrast to a cold runner system,

is employed. The material in the hot runners is maintained in a molten

state and is not ejected with the molded part. Hot runner systems are also

referred to as hot-manifold systems, or runnerless molding.

FIGURE 1. Hot runner system types: (a) the insulated hot runner, (b) the internally heated hot-runner system, and (c) the externally heated hot-runner system Docsity.com

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Gate System

• Several types of gates
  • 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

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Injection Molding Operations

• Cycle Time · Injection Pressure

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Injection Pressure Equations

• Equations
  • Based on a simplification of classic fluid mechanics theory
  • P is the injection pressure and n is a material constant (the power-law

coefficient), which typically ranges from 0.15 to 0.36 (with 0.3 being a

good approximation) for a variety of polymer melts.

• Circular channel flow
  • The melt flow in the sprue, runner, and cylindrical gates
• Strip channel flow
  • Such as melt flow in a thin cavity

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Injection Molding Thermal Process

• Temperature History in part

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Injection Molding Operations

• Fountain Effect Flow
  • Hot resin flow from the middle of the flow channel to the walls and cools

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Viscosity and Temperature and Shear Rate

• Effects of temperature and pressure
  • Since the mobility of polymer molecular chains decreases with decreasing temperature, the flow resistance of polymer melt also greatly depends on the temperature. The melt viscosity decreases with increasing shear rate and temperature due to the disentanglement and alignment of the molecules and enhanced mobility of polymer molecules, respectively. In addition, the melt viscosity also depends on the pressure. The higher the pressure, the more viscous the melt becomes.
• Shear rate: velocity divided by distance.

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Cavities

• The number of cavities depends on the available production time,

product quantity required, machine shot size and plasticizing capacities, shape and size of the moldings, and mold costs.

• Number of cavities
  • Product Quantity: If the dimensional tolerance of the part is not

very critical and a large number of moldings is required.

• Machine shot capacity: Number of cavities = S / W