The modified engineering plastics are subject to gas interference during the filling process, and often appear on the surface of the product as silver streaks or tiny bubbles or bubbles in the thick walls of the product. The source of these gases is mainly that the modified engineering plastic raw materials contain water or volatile substances or lubricants excessively, or the temperature of the materials is too high, and the plastics are degraded by the heat for a long time to generate degradation gas.
First, the equipment aspect
The nozzle hole is too small, the material flows or draws at the nozzle, the barrel or the nozzle has obstacles or burrs, and the high-speed material generates frictional heat to cause the material to decompose.
Second, the mold aspect
1. Due to design defects such as: poor gate location, too small gate, asymmetrical gate placement, small runner, and unreasonable mold cooling system, the mold temperature is too different. The material flows discontinuously in the cavity, blocking the passage of the air.
2. The mold parting surface lacks the necessary venting holes or insufficient venting holes, blockage, poor position, and no machining gaps such as inserts and thimbles, so that the air in the cavity cannot enter the plastic at the same time. Leave.
3. The surface roughness of the mold is poor, and the frictional resistance is large, causing local hot spots to decompose the passing plastic.
Third, the process aspect
1. The material temperature is too high, causing decomposition, the barrel temperature is too high or the heating is out of adjustment. The temperature of the barrel should be reduced step by step. The temperature of the feeding section is too high, so that some of the plastic melts prematurely and fills the groove, and the air cannot be discharged from the feeding port.
2, the injection pressure is small, the holding time is short, so that the surface of the melt and the cavity are not closely attached.
3, the injection speed is too fast, so that the molten plastic is decomposed by large shearing to produce decomposition gas; the injection speed is too slow, and the cavity cannot be filled in time to cause the surface density of the product to be insufficient to produce silver streaks.
4. Insufficient material, too large feeding cushion, too low material temperature or too low mold temperature will affect the flow and molding pressure of the melt and generate bubbles.
5. Use multiple injections to reduce the silver grain: medium-speed injection filling flow channel → slow filling filling gate → rapid injection → low pressure slow speed to fill the mold, so that the gas in the mold can be eliminated in time.
6. When the screw is pre-molded, the back pressure is too low and the rotation speed is too high, so that the screw retracts too fast and the air is easily pushed to the front end of the barrel together with the material.
Fourth, raw materials
1. The raw materials are mixed with dissimilar plastics or pellets and mixed with a large amount of powder. When melted, it is easy to entrain air, sometimes silver streaks. When the raw materials are contaminated or contain harmful scraps, the raw materials are easily decomposed by heat.
2. The granular structure of the recycled material is loose, and the amount of air stored in the micropores is large; the regeneration amount of the recycled material is too much or the ratio with the new material is too high.
3, the raw materials contain volatile solvents or liquid additives in the raw materials such as dyeing agent white oil, lubricant silicone oil, plasticizer dibutyl ester and stabilizers, antistatic agents, etc., too much or uneven mixing, to product The state enters the cavity to form a silver streak.
4. The plastic is not dried or absorbed from the atmosphere, and the raw material should be sufficiently dried and a dry hopper should be used.
5. Some grades of plastics cannot withstand higher temperatures or longer heating times, especially when containing trace amounts of water. Catalytic cracking reactions may occur. For this type of plastics, it is necessary to add external lubricants such as stearic acid and its salts. Class to minimize its processing temperature.
Five, product design
The wall thickness is too thick and the cooling rate is different. The size of the main flow channel, the runner and the gate should be appropriately increased during the mold manufacturing.
Source micro injection
