The reasons why gas marks or spray marks are easily generated at the gate
During the plastic injection molding process, air marks or jet marks at the gate are common defects that not only affect the product’s appearance but can also reduce its mechanical properties. These defects are often closely related to a variety of factors, including injection molding process parameters, mold design, and the properties of the plastic raw material, requiring analysis and troubleshooting from multiple perspectives.
Improper injection molding process parameter settings are one of the main causes of gate air marks or shot marks. When the injection speed is too fast, the plastic melt will generate severe turbulence when entering the gate, entraining a large amount of air into the mold cavity. This air cannot be discharged in time, forming air marks near the gate. At the same time, excessively fast injection speeds will also subject the melt to strong shear at the gate, causing local temperature increases, triggering plastic decomposition, and producing decomposition gases, further exacerbating the air mark phenomenon. In addition, if the melt temperature is too low, the plastic’s fluidity is poor, and it is easy to stagnate due to poor flow when passing through the gate, forming shot marks; while if the melt temperature is too high, it will cause the plastic to overheat and decompose, also resulting in streaks at the gate.
Improper mold gate design can also have a significant impact on the formation of air marks or shot marks. A gate that is too small is a common problem. A gate that is too small will subject the melt to excessive shear force when passing through, causing the melt temperature to rise sharply. At the same time, it increases the flow resistance of the melt, making it difficult for air to be discharged, thus forming air marks or shot marks near the gate. An unreasonable gate position can also cause defects. If the gate is facing the cavity wall or core, the melt will directly impact the mold surface after entering the cavity, generating backflow and eddy currents, entraining air to form air marks. In addition, a high surface roughness at the mold gate will increase the flow resistance of the melt, causing irregular flow of the melt at the gate and forming shot marks.
The characteristics of the plastic raw materials and improper handling are also factors that cannot be ignored. Excessive moisture or volatiles in the plastic raw materials will evaporate during the injection molding process, generating gases that accumulate near the gate and form air streaks. For example, in highly hygroscopic plastics like polyamide (PA), if not fully dried, the moisture will convert to water vapor at high temperatures. This, when mixed with the melt, can create noticeable bubbles and air streaks at the gate. Furthermore, impurities or incompatible additives in the plastic raw materials can create localized flow obstructions during melt flow, leading to melt turbulence at the gate and the formation of jet streaks.
A poor mold exhaust system is a key factor leading to gate air marks. If the air in the mold cavity and runner cannot be exhausted in a timely manner, it will be compressed during the melt filling process, forming high-pressure gas. When the gas pressure exceeds the melt pressure, air marks will be generated near the gate. Poor exhaust is usually caused by unreasonable exhaust groove design, such as improper exhaust groove location, insufficient depth or insufficient width, which causes air to accumulate in the dead corners of the cavity or near the gate. In addition, the mold’s parting surface, the clearance between the ejector pin and the template is too small, which will also affect the exhaust effect, making it impossible for the air to be discharged smoothly and exacerbating the air mark phenomenon.
Performance and debugging issues with the injection molding machine can also cause gate air marks or shot marks. Severely worn screws or poorly sealed check rings can cause the melt to backflow during the plasticization and injection process, increasing the air content in the melt and forming air marks at the gate. At the same time, if the nozzle temperature of the injection molding machine is too low, the melt’s fluidity will deteriorate due to a drop in temperature before entering the gate, causing the melt to stagnate at the gate and form shot marks. Furthermore, improper back pressure settings on the injection molding machine—too low a back pressure prevents the air in the melt from being effectively expelled, while excessive back pressure can overheat the melt and cause plastic decomposition, both of which can lead to defects at the gate.
