Introduction of ABS plastic injection molding process

ABS plastic plays an important role in plastic product manufacturing, electronics industry, machinery industry, and building materials due to its high mechanical strength and good comprehensive performance. Plastic decorative parts that require electroplating in plastic product manufacturing generally use ABS plastic, ABS plastic parts are widely used in life.

ABS is commonly known as acrylonitrile-butadiene-styrene terpolymer, which is copolymerized by three monomers of acrylonitrile, butadiene and styrene. Due to the different proportions of the three monomers, they can have different properties and melting temperatures. If ABS is blended with other plastics or additives, it can be expanded to ABS with different uses and properties, such as impact grade, heat resistance grade, flame retardant Grade, transparent grade, enhanced grade, electroplating grade, etc., the electroplating grade is generally used in the manufacture of plastic products. The fluidity of ABS is between PS and PC. Its fluidity is related to injection temperature and pressure. The influence of injection pressure is slightly larger. Therefore, the higher injection pressure is often used during molding to reduce melt viscosity and improve mold filling. performance. ABS injection molding performance: the general melting point of ABS is about 170 ℃, and the decomposition temperature is 260 ℃; the adjustable range of injection temperature is relatively large. During injection molding, the general operating temperature is 180-240°C; because of the presence of rubber components, it absorbs a small amount of water. During production, it needs to be dried, and can be dried at 80-90°C for 1-2 hours; at the same time, due to the presence of rubber components , poor thermal stability, it is easy to decompose, when injection molding, the raw material should not stay in the barrel for too long; the melt viscosity is larger than PS, but the gate and runner are general, and the product can also be filled; the product is prone to static electricity, and the surface Easy to vacuum dust. The shrinkage rate is 5‰; the overflow value is 0.05mm.

(1) Drying of ABS plastic
ABS plastic has high hygroscopicity and sensitivity to moisture, and sufficient drying and preheating before processing can not only
Eliminate the firework-like bubble tape and silver wire on the surface of the product caused by water vapor, and also help the plasticization of the plastic and reduce the surface color of the product
Spots and moires. The moisture content of ABS raw materials should be controlled below 0.13%. The drying conditions before injection molding are: dry season or winter
Dry at 75～80℃ for 2～3h, and dry at 80～90℃ for 4～8h on rainy days in summer, if the parts need to achieve a particularly good gloss
Or the part itself is complex, and the drying time is longer, reaching 8 to 16 hours. Due to the presence of trace water vapor, the fog spot on the surface of the workpiece is often
An issue that is often overlooked. It is best to convert the hopper of the machine into a hot air hopper dryer to avoid the dried ABS in the material.
The hopper absorbs moisture again, but this type of hopper should strengthen the humidity monitoring to prevent the material from overheating when the production is accidentally interrupted.
The proportion of recycled materials cannot exceed 30%, and the use of recycled materials cannot be used in electroplating-grade ABS.

(2) Injection temperature
The relationship between the temperature and melt viscosity of ABS plastic is different from that of other amorphous plastics. When the temperature increases during the melting process, the melting actually decreases very little. Once the plasticizing temperature (the temperature range suitable for processing, such as 220-250 °C) is reached, if the temperature continues to increase blindly, it will inevitably lead to low heat resistance. On the contrary, the thermal degradation of ABS increases the melt viscosity, making injection molding more difficult, and the mechanical properties of the parts also decrease. Therefore, although the injection temperature of ABS must be higher than that of plastics such as polystyrene, it cannot have a relatively loose heating range like the latter. For some injection molding machines with poor temperature control, when the production of ABS parts reaches a certain number, it is often found that yellow or brown coking particles are embedded in the parts, and it is difficult to use new materials to inject into the air, etc. way to remove it. The reason is that because ABS plastic contains butadiene components, when a plastic particle is firmly adhered to some surfaces in the screw groove that are not easy to be washed at a higher temperature, and is subjected to a long-term high temperature, it will cause degradation and carbonized. Since high temperature operation may bring problems to ABS, it is necessary to limit the furnace temperature of each section of the barrel. Of course, different types and compositions of ABS have different applicable furnace temperatures. Such as plunger machine, the furnace temperature is maintained at 180 ~ 230 ° C; screw machine, the furnace temperature is maintained at 160 ~ 220 ° C. It is particularly worth mentioning that, due to the high processing temperature of ABS, it is sensitive to changes in various process factors. Therefore, the temperature control of the front end of the barrel and the nozzle part is very important. Practice has proved that any minor changes in these two parts will be reflected in the parts. The greater the temperature change, the defects such as weld seam, poor gloss, flash, mold sticking, discoloration, etc. will be brought.

(3) Injection pressure
The viscosity of ABS molten parts is higher than that of polystyrene or modified polystyrene, so a higher injection pressure is used during the injection. Of course, not all ABS parts require high pressure, and lower injection pressures can be used for small, simple and thick parts. During the injection process, the pressure in the cavity at the moment when the gate is closed often determines the surface quality of the product and the degree of silver filamentous defects. If the pressure is too small, the plastic shrinks greatly, and there is a great chance of getting out of contact with the surface of the cavity, and the surface of the product is atomized. If the pressure is too large, the friction effect on the surface of the plastic cavity is strong, which is easy to cause sticking.

(4) Injection speed
ABS material with a medium injection speed effect is better. When the injection speed is too fast, the plastic is easily scorched or decomposed into vaporized articles, resulting in defects such as weld seams, poor gloss and redness of the plastic near the gate. However, when producing thin-walled and complex parts, it is still necessary to ensure a sufficiently high injection speed, otherwise it will be difficult to fill.

(5) Mold temperature
The molding temperature of ABS is relatively high, and the mold temperature is also relatively high. Generally, the mold temperature is adjusted to 75-85 °C. When producing parts with a large projected area, the fixed mold temperature is required to be 70-80 °C, and the movable mold temperature is required to be 50-60 °C. When thin-walled parts are used, special heating of the mold should be considered. In order to shorten the production cycle and maintain the relative stability of the mold temperature, after the parts are taken out, a cold water bath, a hot water bath or other mechanical setting methods can be used to compensate for the original cold fixing time in the cavity.

(6) Material quantity control
When the general injection molding machine injects ABS plastic, the injection volume per injection is only 75% of the standard injection volume. In order to improve the quality and dimensional stability of the parts, the surface gloss and the uniformity of the color tone, the injection volume is required to be 50% of the nominal injection volume.

(7) Selection of injection molding machine
Optional standard injection molding machine (screw length-diameter ratio 20:1, compression ratio greater than 2, injection pressure greater than 150MPa). If the color masterbatch is used or the appearance requirements of the product are high, a screw with a smaller diameter can be used. Clamping force is determined according to 4700～6200t/m, which depends on the plastic grade and product requirements.