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  • Gas-assisted injection molding

Gas-assisted injection molding

Gas-assisted injection molding is a process in which high-pressure nitrogen gas is injected directly into the plastic being plasticized in the mold cavity through the main and auxiliary controllers (segmental pressure control system), causing the plastic part to expand internally and create a hollow, but still keeping the product and shape intact.


The gas-assisted injection molding process can be divided into injection period, inflation period, gas holding period, and demolding period. 

Injection phase: The plasticized plastic is filled into the mold cavity in a quantitative manner. The amount of plastic required is determined through experimentation to ensure that the gas does not break through the surface of the finished product during the nitrogen filling period and that an ideal volume of nitrogen is available. 

Filling period: The gas can be injected at different times during or after the injection, and the gas injection pressure must be higher than the injection pressure to achieve a hollow state.  

Gas pressure retention: When the product is filled with gas, the pressure of the gas in the hollow part of the product becomes the pressure retention pressure, which can greatly reduce the shrinkage and deformation rate of the product. 

Release period: After the cooling cycle is completed, the gas pressure in the mold drops to atmospheric pressure, and the finished product is ejected from the film cavity.


There are three different methods of gas-assisted injection molding in terms of technology.  

1. closed gas injection method (SEALED INJECTION GAS) is the direct injection of gas into the mold cavity so that the finished plastic hollow method. It does not need to use a live valve, just through a simple mold processing, the gas nozzle in the mold. On the same mold, single or multiple gas injection places are available.  

2. Method of gas injection from injection machine nozzle (IN⌒GAS NOZZLE): It is a special injection gas nozzle installed on the injection machine.

3. Surface gas molding method (EXTERNAL GAS MOLDING): It is a patented technology of CINPRES GAS INJECTION LTD in the UK, which is applicable to the production of cell phones, electronic notepads, and small home appliances, etc.


The advantages of external gas-assisted injection molding are 

1. Removal of depressions in the finished product   

2. Reduce warpage - In the conventional injection molding process, holding pressure can avoid the defects such as depression and shrinkage, but at the same time, it increases the internal stress of the finished product, causing warpage and deformation. However, external gas-assisted injection molding uses gas to pressurize the outside of the finished product to eliminate dents and shrinkage. Therefore, the internal stress of the finished product will not be increased and the warpage can be improved. 

3. Reduced clamping force - External gas-assisted injection molding uses gas to pressurize the surface of the finished product directly, eliminating the pressure loss during the conventional injection molding process when the holding pressure passes through the runners and gates to the surface of the finished product. Therefore, it is not necessary to increase the holding pressure to compensate for the energy loss, and the required clamping force can be reduced accordingly. 

4. Reduced cycle time - As the holding pressure process has been replaced, the cycle time is reduced.

Application Scope

The external gas-assisted injection can help solve problems that cannot be solved by conventional injection molding and products that are not suitable for internal gas-assisted injection.  

1. Smooth surface on one side of the product - gas is injected between the mold wall and the dissolved material to keep the backside of the product smooth and free of depression.  

2. No ribs and uneven thickness - even if the product does not have wide and thick ribs and uneven thickness, the surface of the product can be made smooth without dents by using this technology. 

3. No pores - Since the gas is not injected into the plastic, no pores will be formed inside the finished product.


The injection molding system is one of the more common production technologies used by plastic manufacturers in Hong Kong. Plastic injection molding technology has been developing rapidly in recent years, and one of the directions is to use a gas-assisted injection molding system to improve product quality and cost-effectiveness. However, the limitation of internal gas-assisted injection technology is that it cannot be applied to flat injection where there is only one visible plane and where there are pillars and changes in the cut surface.

Gas injection molding technology has evolved from traditional internal gas-assisted injection molding to external gas-assisted injection molding. The traditional internal injection system injects gas directly into the middle of the melt, while the external injection system selects a specific product surface and injects gas between the mold wall and the melt, so that the gas is directly pressurized on the cooled plastic surface, reducing the dissolution pattern and warpage caused by the hollow mold cavity.
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