Craze: long, needle like silvery white frost like fine lines on the surface of injection molded parts. The opening direction is along the direction of material flow. In the place where the plastic part is not fully filled, the front end of the fluid is rough, which is called Craze (silvery grain).
The plastic melt is interfered by gas in the filling process, and silver streaks or tiny bubbles often appear on the surface of the products or form bubbles in the thick wall of the products. The sources of these gases are mainly the moisture or volatile substances in the raw materials or excessive lubricant, or the decomposition gas is produced due to the degradation of the plastics after a long time of heating due to the high material temperature.
raw material
When the raw material is polluted or there is too much dust, the raw material is easy to decompose by heating
Too much nozzle material (recycled material) is added, the regeneration times of nozzle material are too much or the proportion of nozzle material is too high (generally it should be less than 20%)
The additives in the material decompose to form craze
Too much moisture in the material can not be fully dried, resulting in craze
Mould
Due to the defects in the mold design (such as: poor gate position, too small gate, asymmetric gate layout, small runner, unreasonable cooling system of the mold), the temperature difference of the mold is too large, which leads to the discontinuous flow of molten material in the mold cavity, blocking the air channel, leading to craze
The corner position of the die is too sharp, and the shearing of the material flow is too large to cause crazing
The mold parting surface lacks the necessary vent hole or the exhaust duct is insufficient, the position is not good, and there is no processing gap such as insert and thimble to exhaust, resulting in the air in the mold cavity can not be eliminated, resulting in craze in the parts
The surface of the mould is too rough and the friction resistance is large, which causes the local temperature of the mold to be too high, which causes the plastic passing through to decompose. Result in craze on the surface of parts
Mold oil leakage, water leakage, air leakage caused parts surface craze
equipment
If the nozzle hole is too small, the material salivates or draws wire at the nozzle, and there are obstacles in the barrel or nozzle, friction heat will be generated when the high-speed material flow passes by, which will cause the material to decompose
There is a dead angle of material flow when the barrel or screw is worn or the rubber head and rubber ring pass through, which will decompose due to long-term heating
Heating system out of control, resulting in high temperature and plastic decomposition
Improper screw design, easy to bring in air
technology
The material temperature is too high, causing decomposition
If the barrel temperature is too high or the heating is out of balance, the barrel temperature should be reduced step by step
The temperature of the feeding section is too high, so that part of the plastic melts too early to fill the screw groove, and the air cannot be discharged from the feeding port
If the injection speed is too fast, the molten plastic will decompose under the action of large shear and produce decomposition gas
The injection speed is too slow to fill the cavity in time, resulting in insufficient surface density and crazing
The flow and molding pressure of the molten material will be affected by insufficient material quantity, too large feeding buffer, too low material temperature or too low mold temperature, resulting in bubbles
Too low back pressure and high speed of screw pre molding make screw return too fast, and air is easy to push to the front end of barrel along with material
Material Science
Dry the raw materials before injection molding
Choose raw materials with high stability
Control the proportion of nozzle material less than 20%
Avoid impurities entering the raw materials
mould design
Increase the size of sprue, runner and gate
Design sufficient exhaust position
Sharp corners should be avoided in the mould and the surface of the mould should be smooth as possible
Repair the mold regularly to avoid oil leakage and water leakage
Molding process
Choose the right injection molding machine,
Choose the right screw,
Increase the back pressure of injection molding machine,
Adjust the corresponding parameters to the best range,
Improving the exhaust system
On site investigation and guidance
Check whether there is obvious phenomenon of water and moisture in the material. You can visually observe whether the outer package has excessive water and moisture, and when you open the material package, you can feel whether the material is wet or not.
Check the feeding process, ensure that the feeding process is clean and avoid mixing of different materials; mainly check whether the hopper is clean.
Check the drying equipment, check whether the heating temperature of the drying equipment is normal (there is a thermometer at the bottom of the material barrel), whether the air blowing in and out is smooth, and whether the air outlet has obvious air outlet feeling by hand.
Check whether the temperature of the nozzle electric heating ring and the barrel is normal. The strip can be used to mark at the nozzle. If the strip melts, the temperature is normal. If the strip does not react, the temperature is abnormal. For the barrel temperature, check the temperature control panel, and the actual temperature cannot deviate from the set value by more than 20 ℃.
Check the key conditions affecting crazing in the injection molding process, reduce the loosening position as far as possible, increase the back pressure appropriately, and adjust the injection speed to a suitable range.
