1、 The principle of gas assisted injection molding
The principle of gas assisted injection molding is relatively simple. During the injection filling process, a gas with lower pressure compared to the injection pressure is injected into the melt, usually several to several tens of megapascals. The gas pressure is used to achieve pressure maintaining and contraction. The principle of gas assisted injection molding. The principle of gas assisted injection molding is that gas enters the plastic melt from inside. During molding, accurately measured melt is first injected into the mold cavity, and then gas is injected into the melt through a special nozzle. Gas diffusion drives the melt to fill the mold cavity. After the filling of the mold is completed, the pressure of the gas in the melt remains unchanged or increases to maintain pressure and replenish the material. After cooling, the gas inside the plastic part can be removed and the mold can be demolded. Gas pressure forming method on the surface of plastic melt, also known as surface gas forming method. This method is a method of injecting high-pressure gas into the specially closed area of the bottom surface of the plastic injection molded product inside the mold, so that the surface of the injection molded product is free of shrinkage marks.

The characteristic of gas assisted injection molding is that this special closed area can also be referred to as the pressure zone, and each pressure zone is surrounded by sealing components connecting the finished product to prevent gas leakage. The cross-section of the sealing components can be rectangular or triangular, which can enhance the rigidity of the finished product. Of course, the surface gas molding method used by Dongguan Machiko Injection Molding Factory will leave obvious marks in the pressurized area, but it will never affect the surface of the injection molded products. The surface gas forming method is particularly suitable for the following situations:; When the shrinkage rate of plastic is very high; When the back of cylindrical objects in injection molded products is prone to dents; The process of melt filling the mold cavity is very long; When the finished product is thin-walled and irregular, and cannot be filled with air pockets or thick walls. In gas assisted injection molding, the quantification of melt is very important. If too much melt is injected, it will cause uneven wall thickness; On the contrary, if too little melt is injected, the gas will break through the melt and make the molding impossible.

2、 Classification and Process of Gas Assisted Injection Molding
The characteristics of gas assisted injection molding are that gas assisted injection molding can be achieved by adding a set of gas supply devices to existing injection machines. According to the usage abroad, gas assisted injection molding can be divided into standard molding method, melt reflux molding method, and movable core exit method.

（1） Standard forming method
(1) Standard molding method for gas injection from the injection machine nozzle. Standard molding method for gas injection from the injection machine nozzle. A portion of the melt is injected into the mold cavity by the injection machine material; Injecting gas from the injection machine nozzle to push the plastic melt into the mold cavity; Raise the gas pressure to achieve pressure maintaining and material replenishment; After maintaining pressure, remove gas and demould the plastic parts.
(2) Standard molding method for gas injection from the mold cavity. Standard molding method for gas injection from the mold cavity. The process is completely similar to the gas injection method introduced by Dongguan Machiko Injection Molding Factory above, except that the gas introduction point is different. The characteristic of the standard molding method is to fill the mold cavity with a quantitative plastic melt instead of filling the cavity, and the amount of plastic melt required needs to be determined through experiments.

（2） Melt Reflow Molding Method
Melt reflux molding method, which is characterized by gas assisted injection molding, first fills the mold cavity with plastic melt. Unlike the standard molding method, during gas injection, the excess melt flows back into the barrel of the injection machine.

（3） Active core exit method
The movable core withdrawal method fills the mold cavity with melt and maintains pressure; Injecting gas to move the core out of the cavity; Raise the pressure of the gas to achieve pressure maintaining and contraction; Exhaust gas releases the plastic part from the mold.

3、 Characteristics of Gas Assisted Injection Molding
The characteristics of gas assisted injection molding and injection molding are that gas assisted injection molding technology can be used for various thermoplastic injection molding products, such as television or audio casings, various plastic products in cars, showers, kitchenware, household appliances, daily necessities, and toys. Compared with traditional injection molding methods, gas assisted injection molding has the following characteristics:
(1) Capable of forming plastic parts with uneven wall thickness and complex three-dimensional hollow plastic parts.
(2) The gas forms a continuous airflow channel from the gate to the flow end, without pressure loss, and can achieve low-pressure injection molding. From this, plastic parts with low residual stress can be obtained, with small warping deformation and stable size.
(3) Due to the auxiliary filling effect of air flow, the injection molding performance of plastic parts is improved. Therefore, using gas assisted injection is beneficial for forming thin-walled plastic parts, reducing the weight of the plastic parts, and saving raw materials.
(4) Due to the reduction of the forming pressure in the mold cavity, the loss of the mold is reduced, and the working life of the mold is improved. Due to the lower injection molding pressure, larger plastic parts can be molded and processed on injection molding machines with lower locking forces.
(5) Shorten the production cycle of plastic parts and save production time.

Gas assisted injection molding has the following drawbacks:
(1) The need to add gas supply devices and inflation nozzles has increased the cost of the equipment.
(2) When using gas assisted injection molding technology, there are certain requirements for the accuracy and control system of the injection machine.
(3) The surface of plastic parts injected with gas and not injected with gas will produce different glosses.

4、 Cycle of gas assisted injection molding
The cycle of gas assisted injection can be divided into four stages: plastic injection period, inflation period, gas holding period, and demolding period.
(1) During the plastic injection period, the injection machine injects a certain amount of plasticized plastic melt into the mold cavity. The required amount of plastic should be determined through experiments to ensure that the gas does not break the surface of the finished product during inflation and can have an ideal inflation volume.
(2) During the inflation period, gas can be injected at different times during the middle or late stages of injection. The pressure of gas injection must be greater than the injection pressure to achieve the hollow state of the injection molded product.
(3) During the gas holding period, when the interior of the finished product is filled with gas, the pressure exerted by the gas on the hollow part of the injection molded product is called the holding pressure. The holding pressure can greatly reduce the shrinkage and deformation rate of the finished product.
(4) During the demolding period, as the cooling cycle is completed, the pressure of the gas inside the mold decreases to atmospheric pressure, and the plastic injection molded products can be pushed out of the mold cavity.

5、 Gas assisted injection system
The gas assisted injection system can use a 32-bit microprocessor, combined with RISC-BASED input and output interfaces, and control the segmented pressure of the gas in a closed-loop manner, making the pressure increase of the gas during injection faster and more accurate. This system complies with European CE, ULC, and American UL standards. The gas assisted injection system mainly consists of a nitrogen production machine and a nitrogen recovery system.
The working principle of the gas assisted injection system. After connecting compressed air to the nitrogen production machine, the purity of the produced nitrogen is over 98%. The nitrogen from the nitrogen production machine enters the low-pressure storage tank, with a storage capacity ranging from 220L to 490L, with a pressure of 1MPa. The low-pressure nitrogen enters the E.D.C. booster through an electric control valve and filter, and the low-pressure nitrogen is pressurized to 35MPa by the E.D.C. booster. The high-pressure nitrogen enters the high-pressure storage tank through the filter, with a storage capacity ranging from 10L to 37L. Then high-pressure nitrogen directly enters the gas assisted main system, which controls the pressure and time of nitrogen injected into the mold.
The nitrogen inside the mold enters the low-pressure storage tank through the recovery pipeline and filter, and the recovered nitrogen is filtered before being used. The injection molding machine for gas assisted injection molding must be equipped with a bow nozzle and an electronic scale for screw travel. The former's function is to prevent high-pressure gas from entering the screw of the injection machine, while the latter's function is to transmit a triggering signal to the gas auxiliary main system, thereby injecting high-pressure gas into the mold cavity.

6、 Application examples of gas assisted injection molding
The characteristics of gas assisted injection molding and injection molding. Dongguan Machiko Injection Molding Factory provides an example of the practical application of gas assisted injection molding for TV casing. It adopts both gas injection from the inside of the plastic melt and surface gas molding methods. At point A, gas was injected from the inside of the plastic melt to form the internal pores at the boundary between the two walls of the plastic part as shown in the A-A and B-B profiles; The surface gas molding method was used at point B, where gas enters the pressurized area of the sealing element on the mold and is used to shape the groove on the outside of the plastic part shown in the C-C profile.