In the process of mold filling, the interference of gas often causes silver streaks or tiny bubbles on the surface of products or bubbles in the thick wall of products. The main source of these gases is that the raw materials contain excessive moisture or volatile substances or lubricants, or the degradation of plastics due to high material temperature and long heating time.
1, Equipment
The nozzle hole is too small, the material is salivating or drawing at the nozzle, the barrel or nozzle has obstacles or burrs, and the friction heat is generated when the high-speed material flow passes through to decompose the material.
2, Mold
(1) Because of the design defects, such as: gate position is not good, gate is too small, gate arrangement of multi gate parts is asymmetric, flow channel is small, mold cooling system is unreasonable and the mold temperature difference is too large, the molten material flow in the mold cavity is not continuous, and the air channel is blocked.
(2) The mold parting surface is lack of necessary exhaust port or the exhaust port is insufficient, blocked and in poor position, and there is no processing gap such as insert and thimble to exhaust, so that the air in the mold cavity cannot leave at the same time when the plastic enters
(3) The surface roughness of the die is poor and the friction resistance is large, which causes local over hot spots and decomposes the passing plastic.
3, Technology
(1) If the material temperature is too high, resulting in decomposition, too high barrel temperature or heating maladjustment, the barrel temperature shall be reduced step by step. If the temperature of the feeding section is too high, part of the plastic will melt prematurely and fill the screw groove, and the air cannot be discharged from the feeding port.
(2) The injection pressure is small and the holding time is short, so that the melt and the surface of the cavity are not closely adhered.
(3) The injection speed is too fast, which causes the molten plastic to decompose under the action of large shear and generates decomposition gas; the injection speed is too slow to fill the mold cavity in time, resulting in insufficient density of the product surface and craze.
(4) The flow and forming pressure of the melted material will be affected by insufficient material, too large feeding buffer, too low material temperature or too low mold temperature, which will produce bubbles.
(5) To reduce craze with multi-stage injection: medium speed injection to fill the runner → slow filling the gate → fast injection → low pressure and slow filling the mold, so that the gas in the mold can be eliminated in each section in time.
(6) When the screw is pre molded, the back pressure is too low and the rotation speed is too high, which makes the screw return too fast, and the air is easy to push to the front end of the barrel together with the material.
4, Raw materials
(1) When a large amount of powder is mixed in the raw materials with different plastics or granular materials, the air is easily entrained during melting, and sometimes craze occurs. When the raw materials are polluted or contain harmful chips, the raw materials are easy to be heated and decomposed.
(2) The particle structure of the recycled material is loose, and the air stored in the micropore is large; the regeneration times of the recycled material are too many or the proportion with the new material is too high (generally less than 20%)
(3) The raw materials contain volatile solvents or liquid auxiliaries in the raw materials, such as white oil, lubricant silicone oil, dibutyl plasticizer, stabilizer, antistatic agent, etc., which are used too much or mixed unevenly to enter the mold cavity in the accumulation state and form craze.
(4) The plastic is not dried or hygroscopic from the atmosphere. The material should be fully dried and a drying hopper should be used.
(5) Some brands of plastics can not bear higher temperature or longer heating time. Especially when there is trace water, catalytic cracking reaction may occur. For this kind of plastics, it is necessary to add external lubricants such as stearic acid and its salts (50g / 10kg material), so as to reduce the processing temperature as much as possible.
5, Product design
The wall thickness is too thick, and the cooling speed in the surface and inside is different. The size of the main flow channel, the shunt channel and the gate should be appropriately increased during the mold manufacturing.
