Gas Assisted Injection Moulding

Gas-assist injection molding is a process that utilizes an inert gas (normally nitrogen) to create one or more hollow channels within an injection-molded plastic part. At the end of the filling stage, the gas (N²) is injected into the still liquid core of the molding. From there, the gas follows the path of the least resistance and replaces the thick molten sections with gas-filled channels. Next, gas pressure packs the plastic against the mold cavity surface, compensating for volumetric shrinkage until the part solidifies. Finally, the gas is vented to atmosphere or recycled.

Gas-assist injection molding has been around for well over two decades and many people have had concerns over patent, rights and royalty fees. Within the past few years, some of the original patents have expired. And now, gas-assist injection molding is widely practiced. Design engineers and processors alike are discovering that this technology is an attractive option for certain applications and offers many benefits. It is the responsibility of the manufacturer to ascertain that their practice or technology is not covered by current patents.

Variants of the Process
The main two applications of Gas Assisted Moulding are to either inject the gas into the component cavity (internal gas injection), or to use the gas on the outside surface, but still within the mould cavity, to consolidate the component (external gas injection).

Internal Gas Injection - Most widely used process

Benefits of Internal Gas Injection Moulding:

• Substantial cost reductions resulting from:
  Reduction in molded plastic weights, and therefore cost of material.
  Reduction in molding time cycles, and therefore cost of production.
  Reduced in-mold pressures, and therefore less wear on molds.
• The use of the gas as a means of transmitting pressure uniformly throughout the molding.
• Elimination of sink marks.
• Avoidance of plastic packing from the molding machine.
• Reduced in-mold pressures by up to 70%, and therefore reduced press lock forces enabling larger moldings on smaller machines.
• Reduced power consumption.
• Reduced molded in stress, and therfore improved dimensional stability with no distortion.

External Gas Injection - used for enhanced surface definition

Benefits of External Gas Injection Moulding:

• Can eliminate sink marks.
• Virtually eliminates moulded-in stress and therefore distortion.
• Improves dimensional stability.
• Applies pressure more efficiently, and therefore less pressure is required:
  - reducing lock forces or machine size.
  - reducing wear on moulds.
  - reducing power consumption.
• More design freedom:
  - thicker ribs with reduced wall thicknesses.
  - multi-rib components.
  - flat PP and PE products.

A number of variants of gas use are incorporated into the Internal gas injection process:

• Full Shot Internal Gas Assisted Moulding
• Short Shot Internal Gas Assisted Moulding
• Plastic Expulsion Process ∦ PEP
• Moving Core Gas Assisted Moulding
• Gas Cool for Internal and External Gas Moulding

Each variant has its uses and benefits.

Why Gas Injection Moulding?
Techniques have been developed whereby inert gas nitrogen is injected into the still molten plastic in the mould cavity. Acting from within the component shape, the gas inflates the component and counteracts the effects of the material shrinkage. The effect is to keep an internal pressure on the material until it solidifies and skins at the mould cavity surface. This is independent of any gate freezing.

Raw Materials
Most thermoplastics can benefit from the use of gas assisted moulding including Polypropylene (PP), ABS, HIPS, Polycarbonate (PC), PPC and Nylon (including glass filled grades).

Tooling
Machined steel. Must be specially designed with mold flow analysis to enhance the hollowing-out of thick areas

Cost
Tooling costs are generally high. Part prices generally higher than with injection molding.

Advantages

• Material savings (weight, cost) for thick-walled parts up to 40%
  The combined benefits of not packing a moulding are less material is used. By not having to pack the material, and in thicker components the resultant hollow core, can save as much as up to 40% on the material used.
• Reduced Cycle times by 50% or more when compared to standard injection molding of thick-walled parts
  Another major benefit is the reduction in machine cycle times that can be achieved. With no molten core to solidify, the material in the mould cavity solidifies quicker thus enabling the component to be ejected sooner.
• Smooth surface in comparison with structural foam
  External gas injection provides an enhanced surface definition of the component.
• Lower clamp forces
• Improved holding pressure effect
• High flexural stiffness and torsional rigidity
• Low internal stress level and low warpage for thick and thin wall combinations (uniform shrinkage and pressure)
• Reduction of sink marks
• Design freedom
• Fewer weld lines due to fewer injection points
• Longer flow lengths or lower number of injection points required for large thin-walled molded parts because gas channels act as flow leaders

Disadvantages

Special care must be taken in designing parts. High cost of tooling and mold flow analysis.

Applications
Most plastic injection moulded components can benefit from the use of gas assisted moulding. Applications from consumer goods to automotive parts benefit from the process. The typical are: Toys, auto parts & anything with thick areas.

External Gas Assisted Moulding Applications:

• Flat panels for office equipment.
• Computer enclosures.
• Furniture, i.e. tabletops.
• Automotive panels.
• Domestic appliances - e.g. fridges.