How to solve the surface fog of injection molding products?

August 4, 2017

There is a flaw in the injection molding, that is, the surface will be fogged. So, how to make the best out of gas without generating fog phenomenon? Master machine skills correctly is very effective in practical work.

Phenomenon

The surface of white fog, from the visual perspective, it appears white, so the white product is not clear, the black light is most obvious, PP material products, surrounding surface shows the large and small white markings, like clouds of symptoms, here classified as white fog.  

Theory

According to the theoretical basis of the above obtained is: there is a gap between the plastic and plastic material, the screw shear was into the barrel in the shear process, the material will occur inside a physical change from the glass state into high elastic state, from high elastic state to viscous flow, this process the change may be only a few seconds (tens of seconds, according to the machine or product size) in such a short period of time to change a complex process, injection molding defects was also produced, the gap between the inner compressed plastic melt to produce gas, if the plastic is failed to be discharged to the external cylinder gas there, gas is wrapped in a melt is injected into the mold cavity, whether the mold can be discharged smoothly on the exhaust of the mold depends on whether the good is produced fog trapped gas or charred phenomenon.  

Obviously, the fog is a phenomenon of trapped gas, many engaged in injection molding technology personnel mistakenly believe that only the trapped gas is burnt, or trapped gas to vent open, this is not scientific, because the trapped gas is burned too serious, gas at high pressure, partial combustion under high speed is not compressed to release the combustion temperature can instantly reach 600-800 degrees, which belongs to the highest level of trapped gas.  

 

Solution

How to get the best gas discharge and no fog phenomenon?

 

Mold

The exhaust is not completely rely on the mold to solve, for the characteristics of the PP material, the temperature range is very wide, the flow of superior performance, if the exhaust groove die too big or not in the right place, not right exhaust, but also prone to flash bad, so the right exhaust is used.  

 

Process

Mastering the correct tuning technique is very effective in actual work.  

 

1) such as reducing the screw rotation speed, can make the melt temperature uniformity, the air entrainment to small, such as screw rotation speed is too fast, the opportunity to air entrainment; melt by shear force, friction heat leads to uneven melt temperature, melt decomposition of gas.  

 

2) improve the mold temperature, can be conducive to high pressure, slow filling to win time, if the mold temperature is too low, the product surface is easy to freeze early, slow filling can not be achieved, may lead to lack of glue.  

 

3) reduce the injection speed, the faster, the faster the gas compression, the higher the temperature. The slower the rate, the slower the gas compression, the more conducive to the discharge of gas.  

 

4) the clamping force, clamping force is bigger, the cavity surface exhaust effect is poorer, the clamping force is smaller, the cavity surface exhaust effect is better.

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