Causes and solutions for the formation of trapped gas in injection molds
In the injection molding industry, gas distress is one of the most frequently encountered injection molding defects. Trapped gas can easily directly cause injection molding defects such as material shortage, scorching, gas streaks, and surface fogging, and indirectly cause injection molding defects such as shrinkage marks and exposed glass fibers. According to the location of trapped gas, it can be divided into three types: poor exhaust gas on the parting surface, trapped gas at the end of the material flow at the deep rib position, and encapsulated air.
Trapped gas refers to the phenomenon that the gas in the mold cavity cannot be discharged from the injection mold in time or cannot be discharged from the injection mold in the process of filling the mold cavity, preventing the rubber from continuing to fill, resulting in injection defects such as lack of material and scorching. Under normal circumstances, trapped gas occurs in the process of filling the mold cavity, and does not occur in the process of packing and shrinking. There are two sources of gas in the mold cavity: one is the air in the mold cavity, and the other is the decomposition gas and a small amount of water vapor produced by the rubber itself. From the definition, it can be seen that solving the problem of injection molding gas is to solve the problem of how to reduce the decomposition gas and how to exhaust the gas in the mold cavity out of the mold cavity.
Causes of injection molding defects caused by trapped gas:
If the gas not discharged in the mold cavity is regarded as an ideal gas, and the gas is analyzed in the process of filling and pressurizing, due to the short injection time, it can be approximated as an adiabatic process, according to the laws of thermodynamics and the ideal gas equation of state PV=nRT, then T2> T1, P2 > P1, if P2 is large enough to be equal to the pressure at which the melt arrives, the filling stops causing a lack of material. If T2 rises to the decomposition temperature of the material, it will cause the material to become trapped and scorched. At the same time, the combustion and decomposition of polymer materials is an exothermic reaction, which will also produce a large amount of gas, and the temperature is higher, forming a vicious circle. Air streaks are a slight manifestation of material shortage, and surface haze is also a slight manifestation of material decomposition. As for the injection molding defects such as shrinkage marks and exposed glass fibers caused by trapped gas, they are caused by slowing down the injection speed in order to ensure exhaust during the process debugging process.
Solutions to the trapping:
1. The end of the material flow at the deep rib position is added with exhaust inserts, inlay needles or breathable steel.
2. If it is difficult to open the exhaust at the end of the pipeline, you can try to open the exhaust as much as possible in other easy to add exhaust positions, and use low-pressure and low-speed injection at the end of the glue filling when the process is adjusted to meet customer requirements
3. Sometimes the product design engineer will deliberately make an inner arc at the position of the deep rib and the trapped air to achieve the purpose of product beauty.
The parting surface itself is a place where it is easy to open an exhaust slot, but the gas is not completely discharged from the parting surface, causing the trapped gas to burn. This is mainly caused by the insufficient overall exhaust of the mold, and a large amount of gas is compressed to a certain position at the same time.
Categories
For more details, please visit our other website：Injection Mold