ABS Plastic Injection Molding Process Knowledge
1. Drying of ABS plastic
ABS plastic has high hygroscopicity and sensitivity to moisture. It can be fully dried and preheated before processing. It can not only eliminate the fireworks-like bubble and silver wire on the surface of the workpiece caused by water vapor, but also help plastic plastic. Reduce the surface color and moiré of the parts. ABS raw materials should control moisture below 0.13%.
The drying conditions before injection molding are: dry winter season is below 75-80 °C, drying is 2~3h, summer rainy days are at 80-90 °C, drying is 4-8h, if the workpiece is to achieve particularly excellent gloss or the part itself Complex, drying time is longer, up to 8 ~ 16h. Due to the presence of traces of water vapor, surface fogging on the surface of the part is often overlooked. It is best to convert the hopper of the machine into a hot air hopper dryer to prevent the dry ABS from re-absorbing moisture in the hopper. However, such hoppers should be enhanced in humidity monitoring to prevent overheating of the material during accidental production interruption.
2. Injection temperature
The relationship between temperature and melt viscosity of ABS plastics is different from other amorphous plastics. When the temperature of the melting process increases, the melting actually decreases little, but once the plasticizing temperature is reached (the temperature range suitable for processing, such as 220-250 ° C), if the blind temperature is continued, the heat resistance will not be too high. The thermal degradation of ABS, on the other hand, increases the melt viscosity, makes injection molding more difficult, and the mechanical properties of the parts also decrease.
Therefore, although the injection temperature of ABS is higher than that of plastics such as polystyrene, it cannot be as loose as the latter. Some injection molding machines with poor temperature control often produce yellow or brown coking particles on the parts when producing ABS parts to a certain amount, and it is difficult to use new materials to inject into the air. The way to clear it out.
The reason is that ABS plastic contains butadiene, when a plastic particle adheres firmly to some non-scored surfaces in the groove at a higher temperature, causing degradation and degradation caused by prolonged high temperature. Carbonization. Since the high temperature operation may cause problems for ABS, it is necessary to limit the temperature of each section of the barrel. Of course, the applicable furnace temperatures for different types and configurations of ABS are also different. For example, the plunger machine maintains the furnace temperature at 180-230 °C; the screw machine maintains the furnace temperature 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 seams, poor gloss, flash, sticking, and discoloration.
3. Injection pressure
The viscosity of the ABS melt is higher than that of polystyrene or modified polystyrene, so a higher injection pressure is used at the time of injection. Of course, not all ABS parts are subjected to high pressure, and low injection pressure can be applied to small, simple, and thick parts. During the injection process, the pressure in the cavity at the instant of gate closure often determines the surface quality of the part and the degree of silver-like defects. The pressure is too small, the plastic shrinks greatly, the chance of disengagement from the surface of the cavity is large, and the surface of the workpiece is atomized. Excessive pressure, the friction between the plastic and the cavity surface is strong, and it is easy to cause sticking.
4. Injection speed
ABS material has a better effect on medium injection speed. When the injection speed is too fast, the plastic is easily burnt or the gas is separated out, so that the weld seam, the gloss difference and the plastic redness near the gate appear defects on the workpiece. However, in the production of thin-walled and complex parts, it is necessary to ensure a high enough injection speed, otherwise it will be difficult to fill.
5. Mold temperature
The forming temperature of ABS is relatively high and the mold temperature is relatively high. Generally, the mold temperature is adjusted to 75 to 85 ° C. When producing parts with a large projection area, the mold temperature is required to be 70 to 80 ° C, and the dynamic mold temperature is required to be 50 to 60 ° C. When injecting larger, contoured, thin-walled parts, special consideration should be given to heating the mold. 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 method can be used to compensate for the time of the cold type in the cavity.
6. Quantity control
When the injection molding machine is usually filled with ABS plastic, its injection volume is only 75% of the standard injection volume. In order to improve the quality and dimensional stability of the parts, the surface gloss and uniformity of the color are required, and the injection amount is required to be 50% of the marked injection amount.