External Gas Moulding (EGM)

EXTERNAL GAS MOULDING: is based on

Injecting gas to form a micro-thin layer of gas between one surface of the plastic and the adjacent mould cavity surface.

Using the gas to apply pressure to the plastic whilst cooling, forcing it against the opposite mould cavity surface to improve replication of that surface. 

EGM APPLICATIONS:

Flat panels for office equipment, Computer enclosures, Furniture, i.e. tabletops, Automotive panels. 

Domestic appliances - e.g. fridges

'SHORT SHOT' METHOD:

Process Sequence

1. Mould closes and a precise controlled volume of plastic is injected into the mould cavity.

2. After a selected time delay, gas is injected to fill the cavity with plastic and gas.

3. There is a further penetration of gas as a gas expands to compensate for volumetric shrinkage 

of the plastic as it cools.

4. A uniform gas pressure is applied throughout the plastic.

5. Gas is exhausted to atmosphere or for recovery before mould opening.

6. Plastic refill in the moulding machine can commence as soon as the plastic feed point has solidified, 

or after the nozzle valve is closed.

Typical Applications:

The 'Short Shot' method is used for thick section mouldings, typically handles and tubular components. Care must be taken to avoid 'hesitation' marks at extremity of plastic fill before plastic recommences flow, urged forward by the gas injection.

Advantages:

Substantial cost reductions resulting from:

1. Reduction in moulded plastic weights, and therefore cost of material.

2. Reduction in moulding time cycles, and therefore cost of production.

3. Reduced in-mould pressures, and therefore less wear on moulds.

'FULL SHOT' METHOD:

After mould closing thge plastic is injected to fill or nearly fill the mould cavity, but the plastic is not 

packed by the moulding machine.

2. After a selected time delay, first phase gas is injected.

3. Second phase gas penetration to compensate for volumetric shrinkage of the plastic as it cools.

4. A uniform gas pressure is applied throughout the plastic.

5. Gas is exhausted to atmosphere or for recovery before mould opens.

6. Plastic refill commences after the nozzle valve is closed or after the plastic feed gate has solidified.

Typical Applications:

The 'Full Shot' method is normally applicable for components in which there are thick and thin sections. The gas flows into the path of least resistance in the thicker sections where the plastic interior is still in a molten state.

Advantages:

1. The use of the gas as a means of transmitting pressure uniformly throughout the moulding.

2. Elimination of sink marks.

3. Avoidance of plastic packing from the moulding machine.

4. Reduced in-mould pressures by up to 70% and therefore reduced press lock forces enabling larger 

Mouldings on smaller machines.

5. Reduced power consumption.

6. Reduced moulded in stress, and therefore improved dimensional stability with no distortion.