ABS plastic, known for its greater mechanical strength and overall performance, holds a significant position in industries such as electronics, machinery, transportation, building materials, and toy manufacturing. This is particularly true for slightly larger structures, force components, and decorative parts requiring plating. Based on years of practical experience, we analyze the ABS plastic injection molding process as follows.
(1) Drying of ABS Plastic
ABS plastic exhibits greater hygroscopicity and sensitivity to moisture. Adequate drying and preheating before processing not only eliminate surface defects like fireworks bubble belts and silver wires caused by water vapor but also facilitate plasticization, reducing color spots and cloudy textures on the surface of ABS plastic products. It is crucial to control the moisture content of ABS raw materials to 0.13% or less.
Pre-injection molding drying conditions include maintaining temperatures below 75-80°C during dry winter seasons for 2-3 hours, and 80-90°C during summer rainy days for 4-8 hours. For ABS plastic products aiming for exceptional luster or those with complex structures, longer drying times, up to 8-16 hours, are recommended. The oversight of surface haze caused by trace water vapor is a common issue. Converting the machine’s hopper into a hot air hopper dryer is advisable to prevent reabsorption of moisture by the dried ABS in the hopper. However, monitoring humidity is crucial for such hoppers, especially during occasional production interruptions, to prevent material overheating.
(2) Injection Temperature
The relationship between temperature and melt viscosity for ABS plastic differs from that of other amorphous plastics. While a temperature increase during the melting process results in a minimal decrease in melt viscosity, continuing to raise the temperature beyond the plasticizing temperature (optimal processing temperature range, such as 220-250°C) leads to heat degradation, increasing melt viscosity, making injection molding more challenging, and decreasing the mechanical properties of ABS plastic products. Therefore, while the injection temperature of ABS is higher than that of plastics like polystyrene, it does not have the same broad temperature range. Some poorly temperature-controlled injection molding machines may lead to the discovery of yellow or brown coking particles embedded in ABS plastic products after a certain production cycle, making their removal difficult.
This is attributed to ABS plastic containing butadiene components. When a plastic particle firmly adheres to the screw groove surface at higher temperatures, which is not easily flushed, prolonged exposure to high temperatures causes degradation and carbonization. Limiting the furnace temperature in each cylinder section is necessary. Different types and compositions of ABS require different furnace temperatures. For plunger-type machines, maintain a furnace temperature between 180-230°C, while for screw machines, it is 160-220°C.
It is worth noting that due to the high processing temperature of ABS, it is sensitive to changes in various process factors. Therefore, temperature control at the barrel front and nozzle is crucial. Practical experience has shown that even minor changes in these two parts can be reflected in ABS plastic products. Greater temperature changes result in defects such as fusion joints, poor gloss, flashing, sticking molds, discoloration, and more.
(3) Injection Pressure
The viscosity of melted ABS parts is higher than that of polystyrene or modified polystyrene, necessitating higher injection pressure. However, not all ABS plastic products require high pressure. Lower injection pressure can be used for small, simple, and thick ABS plastic products. The pressure during injection often determines the surface quality and silver wire defect severity of ABS plastic products. Insufficient pressure leads to increased plastic shrinkage, reducing contact with the cavity surface, resulting in surface fogging. Excessive pressure increases friction between the plastic and cavity surface, leading to sticking molds.
(4) Injection Speed
Medium injection speed is recommended for ABS material. Excessive injection speed can lead to plastic burning, decomposition, and the release of gasified material, causing defects such as fusion joints, poor luster, and reddening of the plastic near the gate in ABS plastic products. However, when producing thin-walled and complex ABS plastic products, ensuring a sufficiently high injection speed is essential for proper filling.
(5) Mold Temperature
ABS molding temperature is relatively high, and mold temperature needs to be adjusted accordingly. Generally, mold temperature is regulated between 75-85°C. For ABS plastic products with a large projection area, fixed mold temperatures should be maintained between 70-80°C, while moving mold temperatures should be between 50-60°C. When injecting larger, complex, thin-walled ABS plastic products, it may be necessary to consider specialized mold heating. To shorten production cycles and maintain relatively stable mold temperatures, compensatory measures, such as cold water baths, hot water baths, or other mechanical shaping methods, can be employed after removing ABS plastic products from the mold.
(6) Material Control
During injection molding of ABS plastic, each injection should reach only 75% of the standard injection volume. To enhance the quality and dimensional stability of ABS plastic products, as well as achieve uniform surface gloss and color, it is recommended that the injection volume be 50% of the standard injection volume.
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