Gas-assisted injection molding can be divided into short shot and full shot.
The short shot method is shown in picture 1, it is suitable for thick-walled plastic parts with low mold filling resistance, especially rod-shaped parts, which can save a lot of raw material, we should inject part of resin into the cavity and core (usually only fills 50% volume of caity and core) during the short shot, and immediately inject gas into the center of the resin, push the gas and obtain a hollow plastic part, For thin-walled plastic parts with high mold filling resistance, it’s better to use full shot molding.
For short shots, the percentage and prolong time of plastic melt fills the cavity and core before the gas injected are the main factors to control the length of the gate channel, in addition, the further shrinkage of the plastic will make the channel continue to be longer, if too much plastic is injected at early stage, it will cause the gas flow length is not enough, but if too little plastic is injected, the gas will quickly penetrate the plastic flow front and cause waste.
2. Sub-cavity method (full shot)
The secondary cavity molding method, it is also called the overflow well method, it is a kind of full shot process (as shown in picture 2). the secondary cavity method requires a secondary cavity that can connect with the mold cavity in outside, the secondary cavity follows the main cavity, the valve needle between the cavities is opened or closed by the hydraulic cylinder or air cylinder, first, close the sub cavity, inject plastic melt into the cavity until the cavity and core is full, then open the sub cavity, and inject gas into the cavity, the penetration of gas pressure causes the excess melt to flow into the secondary cavity, when the gas penetrates to a certain extent, the secondary cavity is closed, and the gas pressure continues to increase to maintain pressure and feed the melt in the cavity with core side, and finally open the mold to eject products.