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  • How to solve the silver streaks and bubbles of injection products?

    Discussion in 'Plastic moulding' started by MIA WANG, Nov 3, 2021.

    1. MIA WANG

      MIA WANG Member

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      Plastics are disturbed by gas during the filling process, and silver stripes or tiny bubbles often appear on the surface of the product or bubbles are formed in the thick wall of the product. The main source of these gases is that the raw materials contain moisture or volatile substances or excessive lubricants, or it may be that the material is too hot and the plastic has been heated for a long time and degrades to produce degradation gas.
      Equipment: the nozzle hole is too small, the material is drooling or drawing at the nozzle, the barrel or nozzle has obstacles or burrs, and the high-speed material flow generates frictional heat to decompose the material.
      Mould aspect:
      (1) Due to design defects, such as: poor gate position, too small gate, asymmetric gate arrangement of multi-gate parts, small runners, unreasonable mold cooling system, so the mold temperature difference is too large, etc. The melt flow is discontinuous in the mold cavity, which blocks the air passage.
      (2) The mold parting surface lacks the necessary vents or the vents are insufficient, blocked, poorly positioned, and there are no processing gaps such as inserts and thimbles to exhaust, causing the air in the cavity to be unable to enter the plastic when the plastic enters. Leave at the same time.
      (3) The surface roughness of the mold is poor, and the frictional resistance is large, causing local hot spots and decomposing the passing plastic.
      Process:
      (1) The material temperature is too high, causing decomposition, the barrel temperature is too high or the heating is out of adjustment, 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 prematurely and fills the screw groove, and the air cannot be discharged from the feeding port .
      (2) The injection molding pressure is small and the pressure holding time is short, so that the melt does not adhere to the surface of the cavity.
      (3) The injection molding speed is too fast, so that the molten plastic is decomposed by large shearing, and decomposition gas is generated; the injection molding speed is too slow, and the cavity cannot be filled in time, resulting in insufficient surface density of the product to produce silver streaks.
      (4) Insufficient amount of material, too large feeding cushion, too low material temperature or too low mold temperature will affect the flow and molding pressure of the melt, resulting in bubbles.
      (5) Use multi-stage injection molding to reduce the silver streak: medium-speed injection molding fills the runner → slow-speed fills the gate → fast injection → low-pressure and slow-speed injection to fill the mold, so that the gas in the mold can be removed in time in each section.
      (6) When the screw is pre-molded, the back pressure is too low and the speed is too high, so that the screw returns too fast, and the air is easily pushed to the front of the barrel along with the material.
      Raw materials:
      (1) When the raw material is mixed with dissimilar plastics or a large amount of powder is mixed with pellets, it is easy to entrain air when melting, and sometimes silver streaks appear. When the raw materials are contaminated or contain harmful scraps, the raw materials are easily decomposed by heat.
      (2) The structure of the recycled material is loose, and the amount of air stored in the micropores is large; the number of regenerations of the recycled material is too much or the ratio with the new material is too high (generally less than 20%)
      (3) The raw materials contain volatile solvents or the liquid additives in the raw materials such as white oil, lubricant silicone oil, plasticizer dibutyl ester, stabilizer, antistatic agent, etc. are used too much or mixed unevenly. The accumulated state enters the cavity, forming a silver streak.
      (4) The plastic has not been dried or absorbed moisture from the atmosphere, so the raw materials should be fully dried and a drying hopper should be used.
      (5) Some brands of plastics cannot withstand higher temperatures or longer heating times, especially when they contain trace amounts of moisture, catalytic cracking reactions may occur. For this type of plastic, it is necessary to consider adding external lubricants such as stearic acid and its salts (up to 50g per 10kg of material) to minimize its processing temperature.
      Product design aspects: the wall thickness is too thick, and the cooling rate is different between the surface and the inside. The size of the main runner, runner and gate should be appropriately enlarged during mold manufacturing.

      #Injectionmoldingchina #injectionmolds #Injectionmoldingparts
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