Gas-assisted injection molding is to inject high-pressure nitrogen directly into the plasticized plastic in the mold cavity through the main and auxiliary controllers (segmented pressure control system), so that the inside of the plastic part expands and becomes hollow, but the product and appearance are still intact.

Principle

The principle of gas-assisted injection molding. It can be explained through the process of gas-assisted injection molding and the technical methods of gas-assisted injection molding.

1. The process of gas-assisted injection molding is:

1. Injection period: Fill the mold cavity with a certain amount of plasticized plastic. The required amount of plastic must be found out through experiments to ensure that during the nitrogen filling period, the gas will not break the surface of the finished product and that there will be an ideal nitrogen filling volume.

2. Inflation period: Gas can be injected at different times during or after injection. The pressure of gas injection must be greater than the injection pressure to achieve a hollow state of the product.

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

Demolding period: With the completion of the cooling cycle, the gas pressure of the mold drops to atmospheric pressure, and the finished product is ejected from the film cavity.

2. Three different technical methods of gas-assisted injection molding:

1. Sealed gas injection method (SEALED INJECTION GAS) It is a method of directly injecting gas into the mold cavity to make the plastic finished product hollow. It does not require the use of a live valve, but simply installs the gas nozzle in the mold through simple mold processing. On the same mold, a single or multiple places for injecting gas can be used.

2. Method of air intake from the injection machine nozzle (IN-GAS NOZZLE): It is to install a special injection nozzle on the injection molding machine.

3. Surface gas molding method (EXTERNAL GAS MOLDING): This is a patented technology of CINPRES GAS INJECTION LTD in the UK, suitable for the production of mobile phones, electronic notebooks and small household appliances.

Features

Advantages of external gas-assisted injection molding:

1. Eliminate the dents of the finished product.

2. Reduce warping. In the traditional injection molding process, pressure holding can avoid defects such as dents and shrinkage of the finished product, but at the same time it increases the stress inside the finished product, causing warping. 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 increase, and the warping phenomenon can be improved.

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

4. Reduce cycle time. Since the holding pressure procedure has been replaced, the injection cycle time is also shortened.

5. External gas-assisted injection molding can help solve the problems that traditional injection molding cannot eliminate and the finished products that internal gas-assisted injection molding is not suitable for. The following three items are the characteristics of the finished products that generally adopt this technology:

a. One side of the finished product is a smooth surface. The gas is injected between the mold wall and the melt to keep the back of the finished product smooth and free of depressions.

b. The finished product has no ribs and uneven thickness. Even if there are no wider and thicker ribs and uneven thickness products, this technology can be used to make the surface of the finished product smooth and without depressions.

c. No pores. Because the gas is not injected into the plastic, no pores will be formed inside the finished product.

Development

The injection molding system is one of the more commonly used production technologies by Hong Kong plastic manufacturers. Plastic injection molding technology is developing rapidly, and one of the development directions is to use gas-assisted injection molding systems to improve product quality and cost-effectiveness. However, the limitation of internal gas-assisted injection molding technology is that it cannot be applied to injection molding of planes with only one visible plane and with pillars and cross-section changes.

The improvement of gas injection molding technology has developed from traditional internal gas-assisted injection molding to external gas-assisted injection molding. The traditional internal injection molding system directly injects gas into the middle of the melt, while the external injection molding 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 melt lines produced when the plastic is cooled and the warping caused by the hollow mold cavity.

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