Silver Streaks Definition
When plastics are disturbed by gas during the mold filling process, silver streaks 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. It may also be that the material temperature is too high and the plastics are heated for a long time to degrade and produce degradation gases.
Reason
Equipment
The nozzle hole is too small, the material is salivating or drawing at the nozzle, there are obstacles or burrs in the barrel or nozzle, and frictional heat is generated when the high-speed material flow passes by, causing the material to decompose.
Mold Aspect
- Due to design defects, such as: poor gate location, too small gate, asymmetric gate arrangement of multi-gate parts, small flow channels, unreasonable mold cooling system, large mold temperature difference, etc. This causes the molten material to flow discontinuously in the mold cavity and blocks the air passage.
- The mold parting surface lacks necessary exhaust holes or the exhaust holes are insufficient, blocked, or poorly positioned, and there are no processing gaps such as inserts and ejectors for exhaust, resulting in the air in the cavity not being able to pass through when the plastic enters. Leave at the same time.
- The mold surface roughness is poor and the friction resistance is large, causing local hot spots and decomposing the passing plastic.
Technology
- The material temperature is too high, causing decomposition. 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, causing part of the plastic to melt prematurely and fill the screw groove, and the air cannot be discharged from the feeding port. .
- The injection pressure is small and the pressure holding time is short, so that the molten material does not adhere closely to the surface of the cavity.
- The injection speed is too fast, causing the molten plastic to decompose under large shear, producing decomposition gas; the injection speed is too slow, and the mold cavity cannot be filled in time, resulting in insufficient surface density of the product and resulting in silver streaks.
- Insufficient material quantity, too large feeding cushion, too low material temperature or too low mold temperature will affect the flow of melt and molding pressure, resulting in bubbles.
- Use multi-stage injection to reduce silver streaks: medium-speed injection to fill the runner → slow speed to fill the gate → fast injection → low-pressure and slow speed to fill the mold so that the gas in the mold can be removed in time in each section.
- When the screw is pre-molded, the back pressure is too low and the rotation speed is too high, causing the screw to retract too quickly, and the air is easily pushed to the front of the barrel along with the material.
Raw Materials
- If the raw materials are mixed with dissimilar plastics or the pellets are mixed with a large amount of powder, air is easily entrained during melting, and silver streaks may sometimes appear. When the raw materials are contaminated or contain harmful scraps, the raw materials are prone to thermal decomposition.
- The structure of the recycled material particles is loose, and the amount of air retained in the micropores is large; the recycled material is regenerated too many times or the ratio to new material is too high (generally should be less than 20%)
- The raw materials contain volatile solvents or liquid additives in the raw materials such as dyeing auxiliary white oil, lubricant silicone oil, plasticizer dibuty ester, stabilizers, antistatic agents, etc. are used in excessive amounts or are unevenly mixed, resulting in The accumulated state enters the mold cavity and forms silver streaks.
- The plastic is not dried or absorbs moisture from the atmosphere. The raw materials should be fully dried and a drying hopper should be used.
- Some brands of plastics cannot withstand higher temperatures or longer heating times. Especially when containing trace amounts of moisture, catalytic cracking reactions may occur. For this type of plastics, it is necessary to consider adding external lubricants such as stearic acid. and its salts (can be added to 50g per 10kg of material) to minimize its processing temperature.
Product Design
The wall thickness is too thick, and the cooling rates on the inside and outside are different. When manufacturing the mold, the size of the main channel, runner and gate should be appropriately increased.
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