Lecture 07: Injection Moulding Technology

Injection Moulding Technology

Glossary of Terms

• Clamping Unit: Mounts the mould, providing force and movement.

• Daylight: Maximum distance between platens.

• Ejector: Ejects parts, activated by the clamping unit.

• Fixed Platen: Stationary face of the clamping unit.

• Hydraulic Clamp: Clamping unit using hydraulic cylinder.

• Injection Cylinder: Includes screw, nozzle, hopper, heaters.

• Injection Screw: Transports, mixes, and injects material.

• Injection Unit: Feeds, melts, and injects material.

• Mould (Tool): Contains the injection cavity.

• Moving Platen: Moves during mould closing; moving half of the mould is bolted on this platen.

• Plunger Unit: Injects and plasticizes material via heating chamber and plunger.

• Tie Bars: Link and align platens.

• Toggle Clamp: Clamping mechanism with toggle connected to moving platen.

• Reciprocating Screw: Plasticizes and injects material.

• Component (Part): Final moulded product

Injection Moulding Processes

• Clamping

• Injection

• Cooling

• Ejection

General Specifications

• Common machine: reciprocating screw

• Economical for large production quantities due to high mould cost.

• Typical cycle time: 10-30 seconds; can be longer.

• Mould may have multiple cavities.

Moulded Component

• Widely used for thermoplastics.

• Some thermosets and elastomers are injection moulded.

• Produces discrete components to net shape.

• Complex shapes are possible.

• Part size ranges from micrometers to automobile bumpers.

Injection Moulding Machine Components

• Injection Unit

• Clamping Unit

• Heaters

• Feed Hopper

• Cylinder for screw-ram

• Motor and gears

• Barrel

• Stationary & Movable Platens

• Reciprocating Screw

• Mould

• Tie Rods

• Nozzle

• Clamping Cylinder

• Non-return Valve

• Hydraulic Cylinder

Injection Moulding Machine - Principal Components

Injection Unit

• Plastication → Melting polymer

• Polymer melt delivery

• Injection (filling)

Clamping Unit

• Opens and closes mould each injection cycle

Injection Unit - Process

1. Plastication: High pressure and heater produce a homogeneous melt.

2. Delivery: Delivers the melt to the mould.

3. Injection: Actual injection of melt into the mould.

• Injection speeds are reproducible; slight changes cause variations.

Types of Injection Units

• Piston (Plunger):

  • Less common.

  • Used for elastomers (rubber).

  • Unsuitable for heat-sensitive material.

• Reciprocating Screw Piston:

  • Most common.

  • Used for thermoplastics, thermosets, and elastomers.

Reciprocating Screw Piston - Important Parameters

• L/D ratio (length to diameter):

  • Thermoplastics: minimum 20:1.

  • Thermosets and elastomers: approximately 14:1.

  • Extended plasticising: 24:1.

• Shot capacity: Amount of material injected per cycle.

• Plasticising rate: Maximum rate of polymer melt delivery.

• Plasticising capacity: Amount of material melted at a time.

Reciprocating Screw Piston - Parts

• Screw: Mixing, delivery, injection.

• Barrel: Houses the screw.

• Heaters: Plastication.

• Check Valve: Prevents backflow.

• Nozzle: Connects injection cylinder and mould tool.

Clamping Units - Functions

• Holds mould halves in alignment.

• Keeps mould closed during injection.

• Opens and closes mould.

Mould Closing Force and Clamping Force

• Closing Force: Force to physically shut the tool; low value (e.g., 5 kN).

• Clamping Force: Force to hold the tool shut during injection; higher than closing force.

• Prevent flash (swelling of mould).

  • Depends on:

    • Component size

    • Surface projected onto parting plane

    • Injection pressure

Setting Clamping Force

• Initial setting: 2.5 to 5 kN/cm$\frac{2}{}$$$\frac{2}{}$$

• Example: SAN material, 3.5 kN/cm$\frac{2}{}$$$\frac{2}{}$$, 90 cm$\frac{2}{}$$$\frac{2}{}$$ projected area, Clamping force = 315 kN

Injection Mould Tooling Basics

• Gate

• Sprue

• Runner

• Cavity

The mould

• Custom designed for the part.

• Replaced with a new mould for the next part when production run is finished

• Types:

  • Two-plate mold

  • Three-plate mold

  • Hot-runner mold

Two-Plate Mold Components

• Stationary & Movable Platens

• Molded Part (Cavity)

• Runner

• Nozzle

• Sprue

• Gate

• Ejector Pins

• Sprue Puller

• Water Channels

• Parting Line

• Ejector Housing

• Ejector Plate

• Ejector Pin Plate

Hot-Runner Mould

• Eliminates solidification via heaters.

• Plastic remains molten in sprue and runner channels.

• Advantage: saves material.

Three-Plate Mold

• Separates parts from sprue and runner.

• Advantages:

  • Automatic operation

  • Runner and parts disconnect and drop separately

Injection Moulding Cycle

1. Clamping: Closes and maintains force.

2. Injection: Inject molten plastic.

3. Packing: Fills and pressurizes the mould.

4. Cooling: cools before ejection.

5. Ejection: Eject the part.

Injection Moulding Cycle - Detailed

1. Clamping:

   • Moves the moving half and closes the mould

   • Maintains necessary force to keep the mould closed while it is being filled with material.

   • Closure and clamping time is influenced by the size of the injection molding machine

2. Injection:

   • Once the polymer is melted (plastication) and the correct quantity of molten plastic is measured in the screw (dosing), The correct dose is injected into the mould via the injection nozzles

   • The injection time varies depending on the type and size of the mould and the polymer

3. Packing:

   • Cavity is filled completely with molten plastic and then subjected to pressure

   • Essential for high-quality parts.

   • Contributes to gate seal.

4. Cooling:

   • Critical for dimensional stability and structural integrity.

5. Ejection:

   • Part is ejected by ejector pins after ejection temperature reached

Cycle Time

• Cooling time constitutes over 50% of cycle time, up to 85% if inefficient.

Pressure

• Pressure increases during injection, is maintained during packing, then reduces as the part cools.

• Mould opens and releases part when pressure is completely removed.

Temperature

• Component is at melt temperature initially.

• Cooling starts during packing.

• Ejection temperature is manually set to ensure safe release without deformation.

• Ejection Temperature

  • It is the temperature at which the centre of the component is solidified.

  • The ejection temperature is determined by material properties

Shrinkage

• Polymers have high thermal expansion coefficients.

• Amorphous thermoplastics: 0.5% - 1% shrinkage.

• Semi-crystalline materials: 1.5% - 5% shrinkage.

Compensation for Shrinkage

• Cavity dimensions must be larger:
  $$Dc = Dp + DpS + DpS^2$$

• $D_c$$$D_c$$ = dimension of cavity

• $D_p$$$D_p$$ = dimension of part

• $S$$$S$$ = shrinkage factor

Other Factors Affecting Shrinkage

• Fillers reduce shrinkage.

• Higher injection pressure reduces shrinkage.

• Longer packing time reduces shrinkage.

• Higher moulding temperature reduces shrinkage.

Process Parameters

• Temperature: barrel zones, tool, die zone

• Pressures: injection max, hold

• Times: injection, hold, tool opening

• Shot size: screw travel

Common Defects

• Short Shots: Incomplete filling.

• Sink Marks: Surface depressions due to uneven cooling.

• Burning: Discoloration due to excessive temperature.

• Flash: Excess material beyond parting line.

• Warping: Deformation due to non-uniform cooling.

Short Shots - Root Causes and Remedies

1. Injection pressure is too low

   • Increase the injection pressure and/or injection time

2. Low Melt Temperature

   • Increase the melt temperature

   • Increase the mould temperature

3. Small Gates or Runners

   • Increase the size of gates or runners

   • Increase the number of gate locations

Sink Marks - Root Causes and Remedies

• Root cause: Non-uniform cooling

• Adjust Injection Molding Parameters:

  • Increase Injection Speed

  • Increase Injection Pressure

• Optimize Mold Design:

  • Placing gates in thicker sections

  • Optimize the mould cooling system

• Modify Part Design:

  • Uniform Wall Thickness

  • Rib Design

Novel Development

• Gas assisted injection moulding

• Conformal Cooling

• Additive Manufactured Tooling

• Micro injection moulding

Gas Assisted Injection Moulding

• Uses compressed gas to hollow out sections.

Conformal Cooling

• Efficient and uniform cooling using complex cooling channels.

• Advantages:

  • Reduced cycle time

  • Reduced Warping and Distortion

  • Energy Efficiency

Additive Manufactured Tooling

• Creates complex moulds with conformal cooling channels

Micro Injection Moulding

• Produces small, precise parts for medical, electronics, telecommunications, and microfluidics