How to Deal with the Silver Streaks and Bubbles of the Injection Molded Plastic Shell?

If the plastic is disturbed by the gas during the filling process, then the surface of the plastic shell will have silver stripes, tiny bubbles or bubbles formed in the wall of the plastic shell. The main sources of these gases are: the raw materials contain moisture or volatile substances, and the lubricant is excessive. The cause of this problem may also be that the material temperature is too high, the plastic is heated for a long time, and the degradation occurs and the degradation gas is generated.

1. Equipment

If the nozzle hole is too small, the material will be salivated or drawn at the nozzle; there are obstacles or burrs at the barrel or nozzle, and frictional heat will be generated when the high-speed material flows to decompose the material.

2. Mold

(1) Some molds have some defects in the design, such as: poor gate position, too small gate, multiple gates, asymmetric gate distribution of the injection molded plastic components, and small runners. The unreasonable mold cooling system makes the mold temperature difference too large, etc., which causes the melt flow in the mold cavity to be discontinuous, which blocks the air passage.

(2) The mold parting surface lacks necessary venting holes or the venting holes are insufficient, blocked, poorly positioned, and there is no processing gap exhaust such as inserts and thimble. These will cause the air in the cavity to not leave at the same time when the plastic enters.

(3) The roughness of the mold surface is poor, and the frictional resistance is large, causing local overheating and decomposing the passing plastic.

3. Process

(1) The material temperature is too high to cause decomposition. When the barrel temperature is too high or the heating is out of adjustment, the barrel temperature should be reduced step by step. Too high temperature in the feeding section will cause a part of the plastic to 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 pressure holding time is short, so that the melt does not adhere to the surface of the cavity.

(3) The injection speed is too fast to cause the molten plastic to be decomposed by large shearing, and decomposition gas is generated; the injection speed is too slow to fill the cavity in time, resulting in insufficient density on the surface of the plastic shell 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, and generate bubbles.

(5) Use multi-stage injection to reduce silver streaks: medium-speed injection 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, which makes the screw return too fast, and the air is easily pushed to the front of the barrel along with the material.

4. Raw materials

(1) Mixing dissimilar plastics into the raw materials or mixing a large amount of powder into the pellets will easily entrain air when melting, and sometimes silver streaks will appear. When the raw material is contaminated or contains harmful scraps, the raw material is 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 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 liquid additives in the raw materials, such as dye auxiliary white oil, lubricant silicone oil, plasticizer dibutyl ester, stabilizer, antistatic agent, etc. Excessive or uneven mixing, It enters the cavity in the state of accumulation, forming a silver streak.

(4) When the plastic has not been dried, or the plastic absorbs moisture from the atmosphere, 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.

5. The design of the plastic shell

The outer shell is too thick, resulting in different cooling rates between the surface and the inside. The size of the main runner, runner and gate should be appropriately enlarged during mold manufacturing.