How to solve the problem of water graine of PVC injection molding?

March 27, 2019


Water pattern is a kind of silver grain, silver pattern of injection molding products, including surface bubbles and internal pores. The main cause of defects is the interference of gases (mainly water vapor, water ripple, decomposition gas, solvent gas, air). The specific reasons are as follows:
 
1. Machine side:
(1) The barrel, the screw wears or the rubber head and the apron have a dead angle of the material, which is decomposed by heat for a long time.

(2) If the heating system is out of control, causing the temperature to be too high and decomposed, check whether there are any problems with the heating elements such as thermocouples and heating coils. The screw is not designed properly, causing a solution or easy to bring in air.

 

2. Mold aspect:
(1) Poor exhaust.

(2) The frictional resistance of the runners, gates and cavities in the mold is large, causing local overheating and decomposition.

(3) Unbalanced distribution of gates and cavities, unreasonable cooling system will cause heat imbalance and local overheating or obstruction of air passage.

(4) The cooling passage leaks into the cavity.

 

3. Plastics:
(1) The plastic has a large humidity, and the proportion of added recycled materials is too large or contains harmful scraps (the scraps are easily decomposed), and the plastic should be sufficiently dried and the scraps should be eliminated.

(2) Moisture absorption from the atmosphere or moisture absorption from the colorant, and the coloring agent should also be dried. It is preferable to install a dryer on the machine.

(3) The amount of lubricant, stabilizer, etc. added to the plastic is excessive or unevenly mixed, or the plastic itself has a volatile solvent. Decomposition occurs when the mixed plastic is difficult to cope with heat.

(4) The plastic is contaminated and mixed with other plastics.
 

4. Processing:
(1) Set temperature, pressure, speed, back pressure, melt motor speed is too high to cause decomposition, or pressure, speed is too low, injection time, insufficient pressure, low back pressure, due to failure to obtain high pressure and density Insufficient gas cannot be melted and silver streaks appear. Appropriate temperature, pressure, speed and time should be set and multi-stage injection speed should be used.

(2) The back pressure is low and the rotation speed is fast, so that the air enters the barrel, and the melt enters the mold. When the period is too long, the melt is decomposed in the barrel due to excessive heat.

(3) Insufficient material, the feeding cushion is too large, the material temperature is too low or the mold temperature is too low, which affects the flow of the material and the molding pressure, and promotes the formation of bubbles.

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