1, The reason why the trapped gas of PP and PVC injection molded parts is difficult to solve by adjusting the machine
When the injection molded parts have trapped gas problems, they usually appear to be filled with dissatisfaction, or scorched, whitish, and have coarse gas lines. Many trapped gas problems can generally be solved by adjusting the machine, and eventually the trapped gas position becomes a small weld mark or an insignificant dot
In the actual production, if the injection molded parts of PP material have the problem of trapped gas, if no effective exhaust measures are taken, it is difficult to solve it by adjusting the machine. Either the filling is insufficient or it is burnt or whitish
According to the principle of ‘dead ends’ and ‘returns’ of trapped gas, to completely solve the problem of trapped gas, the trapped gas must be dissolved into the melt. ABS and other hard plastics have the functions of moisture absorption and air suction. The gas can be dissolved into the melt by the action of pressure, and the defects can be solved. However, this function of PP material is quite poor, so the water content in the raw materials is generally very small, usually can be directly used for production without baking material, so it is unable to eliminate trapped gas. This is the reason why the problem of PP material is difficult to solve by adjusting the machine.
In addition to PP materials, the trapped gas problem of PVC materials is also relatively difficult to solve, but it is different from PP materials. PVC materials are caused by relatively easy scorching. When air is pressed into the melt, the temperature of the gas will increase with the increase of air pressure, so that the easily scorched PVC material will be easily scorched or whitish, so the problem of trapped gas is also more difficult to solve.
2, Solutions for PVC injection molded parts whitish due to trapped gas
Due to the cause of easy scorching, the problem of trapped gas during PVC injection is generally difficult to solve completely. If there is a problem of trapped gas during injection molding, from time to time there will be a large number of plastic parts that are whitened due to trapped gas
Under normal circumstances, the injection parts that have been blushed can only be treated as waste products, but after experiments, the method can be solved by soaking in hot water.
This method is very simple, as long as the blanched PVC parts are immersed in boiling water for about 1 minute, the problem of whitish injection parts may be automatically eliminated
It should be noted that the longer immersion time is beneficial to the problem of whitening, but the injection molded parts will be deformed. Therefore, be careful when immersing, control the time, and do not leave it unattended
3, Strengthening the mold exhaust will greatly help to improve the difficulty of filling thin-walled parts
The wall thickness of the injection molded part is too thin, the heat of the melt is dissipated too quickly, and the pressure obtained by the melt flowing in the front will become very low, so it is difficult to fill the thin wall part itself. At this time, if the exhaust of the mold is not smooth, the air in the cavity will be compressed, causing the air pressure to rise, forming a reverse resistance to melt filling, which increases the difficulty of filling, and melt filling becomes more difficult.
Therefore, strengthening the exhaust capacity of the mold can greatly reduce the resistance caused by the air pressure, which will greatly help to solve the problem of filling the thin-walled parts.
4, Analysis of the cause of trapped gas when the melt temperature is too high
During injection molding, if the melt temperature is too high, the gas generated by thermal decomposition will greatly increase, and the gas to be removed by the mold will increase, which is likely to cause trapped gas problems.
If there is a dead corner where the exhaust is not smooth, when the plastic melt is filled into this place, the gas cannot be eliminated and it is wrapped by the melt, thus forming a so-called dead corner trapped gas problem, which will increase the difficulty of solving the problem, and sometimes even To no avail.
Therefore, when solving the problem of trapped gas, you should first check whether the melt temperature is too high, and see if there is a large amount of smoke generated when the hole is shot. If there is a lot of smoke, the temperature is too high.
Similarly, if the mold temperature is too high, it will also cause difficulties in solving the problem of trapped air. Because the mold temperature is too high, the air temperature in the mold will rise, and the air pressure will also rise, thereby increasing the resistance of the melt to advance and the content of trapped gas. If the heat dissipation at the location where the trapped air is generated and the mold temperature is further increased, then the problem of dead corner or trapped trapped air at that location is more difficult to solve.
Although solving the problem of dead ends or trapped gas traps sometimes requires raising the temperature of some melts and molds, it cannot be raised too high, otherwise it would be counterproductive.
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