Silver wire or tiny bubbles on surface of injection molded items.

Silver wire or tiny bubbles on surface of injection molded items.

Resin Text/Mitel only milan

When plastic is perturbed by gas during filling process, silvery streaks or tiny bubbles often appear on surface of product, or bubbles form in thick wall of product. The source of these gases is mainly that raw materials contain moisture or volatiles or excessive lubricants, or it may be that temperature of material is too high and the plastic is heated for a long time to decompose and release decomposing gas.

Silver wire or tiny bubbles on surface of injection molded items.

TPV Battery Bubble Seal

1. Equipment

Nozzle opening is too small, material salivates or is drawn into nozzle, barrel or nozzle is obstructed or burred, and high-speed material flow generates frictional heat to decompose material.

Second, shape form

(1) Due to design defects such as: poor sprue placement, sprue too small, asymmetric sprue arrangement of multi sprue parts, small guides, unreasonable mold cooling system resulting in mold temperature fluctuations . If it is too large, flow of molten material in mold cavity is intermittent, which prevents passage of air. (2) The mold parting surface is missing necessary vents or vents are insufficient, clogged and in a bad position, and there are no technological gaps such as inserts and thimbles for ventilation, so that air is not ventilated into cavity. Plastic is included how it enters. (3) The surface roughness of mold is low and friction resistance is high, which causes localized hot spots and decomposition of passing plastic.

Third, technical aspects

(1) Material temperature too high causing decomposition. If drum temperature is too high or heat is not balanced, drum temperature should be gradually reduced. The temperature in feed section is too high, so some of plastic melts prematurely and fills groove of screw, and air cannot be released from feed port. (2) The injection pressure is small and pressure holding time is short, so that molten material does not stick close to surface of cavity. (3) The injection speed is too high, resulting in decomposition of molten plastic due to large shear and generation of decomposition gas; injection speed is too low, and cavity cannot be filled in time, resulting in insufficient product surface density and silver streaks. (4) Insufficient amount of material, too large feeding pad, too low material temperature or too low mold temperature will affect flow of molten material and molding pressure, resulting in bubble formation. (5) Use multi-stage injection to reduce silver streaks: medium-speed injection to fill trough→low speed to fill gate→fast injection→low pressure to fill mold at low speed, so that gas in mold can be removed in time to each section. (6) When auger is preformed, back pressure is too low and rotation speed is too high, so that auger retracts too fast and air is easily pushed to front of barrel along with material.

Fourth, raw materials

(1) CheeseIt is not mixed with various types of plastics, or granules are mixed with a large amount of powder, which easily entrains air during melting, and sometimes silver streaks appear. Raw materials are easily decomposed by heat when they are contaminated or contain hazardous waste. (2) The structure of recycled materials is loose, and amount of air retained in micropores is large, amount of recycled materials regeneration is too large, or ratio of recycled materials to new materials is too high (generally less than 20%)(3) The raw material contains volatile solvents or liquid auxiliary substances, such as auxiliary colorants white oil, lubricating silicone oil, dibutyl ether plasticizer, stabilizers, antistatic agents, etc. too much or mixed unevenly, fall into cavity in accumulated condition and form silvery stripes. (4) If plastic does not dry out or absorbs moisture from atmosphere, raw material should be completely dried and a drying funnel should be used. (5) Some grades of plastics alone cannot withstand higher temperatures or longer heating times. Catalytic cracking reactions may occur, especially in presence of trace amounts of moisture. For this type of plastic, consideration should be given to adding external lubricants such as stearic acid and its salts (up to 50 g per 10 kg of material) to minimize processing temperature.

5. Product designThe wall thickness is too thick, and cooling rate of surface and inside is different. The dimensions of gate, impeller and gate must be enlarged accordingly when making mold.

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