Introduction of ABS injection molding

Introduction of ABS injection molding

Because of its high mechanical strength and good comprehensive performance, ABS plastics occupy an important position in the electronic industry, machinery industry, transportation, building materials, toy manufacturing and other industries. Especially for the slightly large box structure and stress components, the electroplating decoration parts are inseparable from this plastic. Based on many years of practical experience, the ABS plastic injection molding process is analyzed as follows.

(1) The drying of ABS plastics has high hygroscopicity and sensitivity to moisture. Sufficient drying and preheating before processing can not only eliminate the fireworks like blisters and silver wires on the surface of parts caused by water vapor, but also help to plasticize the plastics and reduce the color spots and moire on the surface of the parts.
The moisture content of ABS raw material should be controlled below 013%. The drying conditions before injection molding are as follows: in dry winter, under 75-80 ℃, for 2-3 hours; in rainy days in summer, at 80-90 ℃, for 4-8 hours. If the parts want to achieve excellent luster or the parts themselves are complex, the drying time is longer, up to 8-16 hours.
Because of the existence of micro water vapor, the fog spot on the surface of parts is often ignored. It is better to refit the hopper of the machine into a hot air hopper dryer, so as to prevent the dry ABS from absorbing moisture again in the hopper. However, the humidity monitoring should be strengthened for this kind of hopper to prevent the material from overheating when the production is interrupted occasionally.
(2) The relationship between injection temperature and melt viscosity of ABS plastics is different from other amorphous plastics. However, once the plasticizing temperature is reached (suitable temperature range for processing, such as 220-250 ℃), if the temperature continues to rise blindly, it will lead to the thermal degradation of ABS with low heat resistance. On the contrary, the melting viscosity will increase, the injection molding will be more difficult, and the mechanical properties of the parts will also decline.
Although the latter has a wider temperature range than that of ABS, it can't have a higher temperature range than that of ABS. Some injection molding machines with poor temperature control often find yellow or brown coking particles embedded in the parts when they produce a certain number of ABS parts, and it is difficult to remove them by adding new materials into air injection.
The reason is that ABS plastic contains butadiene. When a certain plastic particle adheres firmly to some surface which is not easy to be washed out in the screw groove at high temperature, it will degrade and carbonize under the action of long time high temperature. Since the high temperature operation may cause problems to ABS, it is necessary to limit the furnace temperature of each section of the drum.
Of course, the applicable furnace temperature of ABS with different types and compositions is also different. The temperature of screw type furnace is maintained at 180 ℃ for example. It is worth mentioning that because of the high processing temperature of ABS, it is sensitive to the changes of various process factors. Therefore, the temperature control of the front end of the barrel and the nozzle is very important.
It has been proved that any minor changes in these two parts will be reflected in the parts. The greater the temperature change, it will bring weld seam, poor gloss, flash, die sticking, color change and other defects.
(3) Injection pressure the viscosity of ABS melt is higher than that of polystyrene or modified polystyrene, so higher injection pressure is used in injection. Of course, not all ABS products should be applied with high pressure, and low injection pressure can be used for small, simple and thick parts.
In the process of casting, the pressure in the cavity at the moment of gate closure often determines the surface quality and the degree of silver wire defects. If the pressure is too small, the plastic shrinkage is large, and the chance of contact with the cavity surface is large, and the surface of the part is atomized. If the pressure is too high, the friction between the plastic and the surface of the cavity is strong, which is easy to cause die sticking.
(4) The results show that the injection rate of ABS material with medium injection speed is better. When the injection speed is too fast, the plastic is easy to scorch or separate out gasses, resulting in weld seam, poor gloss and red plastic near the gate. However, in the production of thin-walled and complex parts, it is still necessary to ensure that the injection speed is high enough, otherwise it is difficult to fill.
(5) Mold temperature ABS molding temperature is relatively high, mold temperature is also relatively high. Generally, the mold temperature is adjusted to 75-85 ℃. When the workpiece with large projection area is produced, the fixed mold temperature is required to be 70-80 ℃, and the dynamic mold temperature is required to be 50-60 ℃. In the injection of large, complex and thin-walled parts, special heating of the mould should be considered.
In order to shorten the production cycle and maintain the relative stability of mold temperature, cold water bath, hot water bath or other mechanical shaping methods can be used to compensate the time of cold fixation in the mold cavity after the parts are taken out.
(6) When the injection molding machine injects ABS plastic, the injection volume is only 75% of the standard injection volume. In order to improve the quality and dimensional stability of the product, the surface luster and color uniformity, the injection volume should be 50% of the calibrated injection volume.