Lecture 08: Compression, Transfer & Blow Moulding

Compression Moulding

• Used with thermosetting plastics in granule, putty, preform, or viscous form.
• Composite manufacturing process for high production volume components.
• Typically done at 5–10°C above the polymer's glass transition temperature.
• Process cycle:
  • Mould temperature: 180°C, Mould pressure: 700 kPa, Curing time: 3 min.
  • Material placed in heated mould cavity, mould is closed, pressure applied.
  • Heat and pressure maintained until material is cured.
• Applications: Electric plugs, sockets, housings, pot handles, dinner plates.
• Moulds are simpler than injection moulds.
• Generally limited to simpler part geometries.
• Cycle:
  • Mould Preheating/Preparation: Cleaning, applying release agent, heating to increase viscosity.
  • Charge Preparation: Unpacking, cleaning, cutting, sizing, weighing, heating.
  • Charge Loading: Placing charge on lower mould part.
  • Compression: Applying relative linear motion.
  • Curing: Hardening/setting of compressed charge.
  • Cooling: Ensuring ideal temperature and mechanical properties.
  • Ejection: Releasing the product (manual or automated).
• Compression parameters:
  • Temperature: Reduces viscosity.
  • Pressure: Higher for denser charge (e.g., 2 - 14 MPa for carbon fiber reinforced polymers).
  • Time: Gradual compression with or without withholding time.
• Curing types:
  • Condensation type: Uses tin catalyst.
  • Addition type: Uses platinum catalyst.
• Cooling Purposes:
  • Ensuring Ideal mould temperature.
  • Ensuring component develop desired mechanical and thermal properties.

Transfer Moulding

• Thermoset charge loaded into a chamber, heated, and pressure applied to force it into mould cavity.
• Types:
  • Pot transfer moulding: Charge injected from a "pot" through a vertical sprue channel.
  • Plunger transfer moulding: Plunger injects charge from a heated well through channels.
• Scrap is produced in each cycle and cannot be recovered.
• Capable of moulding more intricate shapes than compression moulding but less intricate than injection moulding.
• Suited for moulding with inserts.

Product Design Guidelines

• Thick cross sections waste material and increase cooling time.
• Reinforcing ribs increase stiffness without excessive wall thickness.
• Avoid sharp corners to prevent surface defects and stress concentrations.
• A moulded part should be designed with a draft on its sides to facilitate removal from mould.

Blow Moulding

• Forming a hollow object by inflating a thermoplastic molten tube (parison) in a mould cavity.
• Two steps:
  • Fabrication of parison by extrusion or injection moulding.
  • Inflation of tube to final shape using air pressure.
• Products: one-piece hollow plastic parts with thin walls (e.g., bottles).
• High production quantities.
• Three main processes:
  • Extrusion blow moulding (largest).
  • Stretch blow moulding.
  • Injection blow moulding.

Extrusion Blow Moulding

• Parison is extruded, pinched, sealed, and inflated.

Stretch Blow Moulding

• Mechanical assistance stretches the part in longitudinal and radial directions, improving mechanical properties (biaxial orientation).

Injection Blow Moulding

• Parison is injection moulded around a blowing rod and then inflated.

Materials and Products

• Limited to thermoplastics like HDPE, PP, PVC, and PET.
• Products: disposable containers, shipping drums, storage tanks, gasoline tanks, toys, boat hulls.