Plastic Injection Moulding

The following AVI illustrates the fundamentals of Injection moulding. The video clearly shows the four main functions of an injection moulding machine: Clamping, injection, cooling and finally ejection - of course there are several vital functions happening during these main functions so much so that all modern machines have an on-board  computer.

A mould is bolted into the Clamping section of the machine. The machine closes the mould, then applies a large force to “Lock” the mould closed. Inside the closed mould is a cavity that is the exact shape of the plastic part.

The Injection section of the machine has a hopper to hold plastic pellets; a barrel with heater bands to liquefy the plastic pellets; a feed screw to move the pellets forward in the barrel; a check valve to force the liquid plastic into the mould, and a nozzle to seal the injection section to the mould. The liquefied plastic is forced into the cavity of the mold with high pressure.

Once the liquid plastic has been injected into the mould, the machine goes into the Cooling phase. The liquid plastic must cool enough to turn solid so it takes on the shape of the cavity and stays that way. While the cooling take place, the screw will rotate, bringing in more pellets for the next part.

When the part is ready to be removed from the mould, the clamp will open, and the part will be removed from one half of the mould. Then the part will be Ejected from the other half of the mould, and a new cycle will start.

Gas Assisted Moulding G.A.M

Gas-assisted injection moulding is a significant manufacturing advancement, it is a low-pressure process utilising nitrogen gas to apply uniform pressure throughout the moulded plastic part. By displacing molten plastic from thicker sections of the part toward areas in the cavity that are last to fill, nitrogen gas pressure creates channels within the part. Through the gas channels, pressure is transmitted evenly across the part; eliminating warpage, sink marks and internal stress. As a result, clamp tonnage, cycle time and part weight is significantly reduced, while increasing the strength and rigidity of the part.

Thin wall parts with heavy ribs, bosses and gussets are formed to high standards of flatness without sink marks or long cycle times. Long shapes are produced without multiple drops or hot runner systems, eliminating weld lines and notably lowering tooling costs. Multiple parts with complex design and differing wall thickness are moulded as a single part without defect. Clamp tonnage requirements are dramatically reduced in most gas assist applications. There are essentially two methods:

External G.A.M

Gas is injected between one surface of the plastic and the core. High pressure via nitrogen gas is then evenly exerted onto the part whilst it cools, forcing it against the cavity to improve detail and surface duplication.

 

Benefits:

• Virtually eliminates moulded-in stress and therefore distortion.
• Improves dimensional stability.
• Applies pressure more efficiently, and therefore less pressure is required:
  - reducing clamp forces or machine size.
  - reducing wear on moulds.
  - reducing power consumption.
• More design freedom:
  - thicker ribs with reduced wall thicknesses.
  - lighter and more ridged components
  - multi-rib components.

Internal G.A.M

This technique not only reduces the volume of material required for each component but, since hollow  parts require less time to cool, shortens cycle time as well. Moreover, the pressurised gas maintains the "packing" pressure on the shot (provided only by the screw or ram in conventional injection) after the injection gate closes, helping to prevent warp by maintaining compaction until the part is fully solidified. Further, the process  reduces moulded-in stress, sink marks, internal stratification and other process-related problems. 

Benefits:

• Uniform pressure throughout the moulding
• Eliminates sink marks
• As the packing is done by the gas, usually only 1 plastic injection gate is required - resulting in considerable mould cost savings.
• Virtually eliminates moulded-in stress and therefore distortion.
• Improves dimensional stability.
• Applies pressure more efficiently, and therefore less pressure is required:
  - Lower clamp tonnage or machine size.
  - reducing wear on moulds.
  - reducing power consumption.
• More design freedom:
  - lighter and more ridged components
  - multi-rib components.
• Reduced material usage

TCI New Zealand (1995) Ltd.

January 2012