ABS plastic has large mechanical strength and good overall performance, in the electronics industry, machinery industry, transportation, construction materials, toy manufacturing, and other industries occupy an important position, especially the slightly large box structure, and force components, the need for plating decorative parts are inseparable from this plastic.
Based on years of practical experience, the author analyzes the ABS plastic injection molding process as follows.
(1) ABS plastic drying ABS plastic hygroscopicity and sensitivity to moisture is greater, in the processing of adequate drying and preheating, not only to eliminate water vapor caused by the surface of the parts smoky bubble, silver but also help plasticization, reduce the surface of the parts color spots and clouds.
ABS raw materials to control the moisture below 013%. Drying conditions before injection molding is: dry winter season at 75 ~ 80 ℃ below, dry 2 ~ 3h, summer rain days at 80 ~ 90 ℃, dry 4 ~ 8h, such as the parts to achieve a particularly good gloss or the parts themselves complex, drying time is longer, up to 8 ~ 16h. The presence of trace water vapor caused by the surface of the parts fog spot has often ignored a problem. It is best to convert the hopper of the machine into a hot air hopper dryer so that the dried ABS will not absorb moisture again in the hopper, but such hoppers should be monitored for humidity to prevent overheating of the material when production is interrupted occasionally.
(2) Injection temperature The relationship between temperature and melt viscosity of ABS plastic is different from that of other amorphous plastics. However, once the plasticizing temperature is reached (220-250℃), if the temperature is raised blindly, the thermal degradation of ABS, which is not very heat resistant, will increase the melt viscosity and make the injection more difficult, and the mechanical properties of the parts will be reduced.
Therefore, although the injection temperature of ABS is higher than that of polystyrene and other plastics, it is not possible to have a more relaxed temperature range than the latter. In some injection molding machines
with poor temperature control, when producing ABS parts to a certain number, more or less yellow or brown coke particles are found embedded in the parts, and it is difficult to use new material to empty injection and other methods to remove the discharge.
The reason for this is that ABS plastic contains butadiene, which causes degradation and carbonization when a plastic particle adheres firmly to some surface in the screw tank that is not easily flushed at higher temperatures and is subjected to long periods of high temperature. Since the high-temperature operation may cause problems for ABS, it is necessary to limit the furnace temperature of each section of the barrel. Of course, the applicable furnace temperature varies for different types and compositions of ABS.
For example, for the plunger-type machine, the furnace temperature is maintained at 180-230°C; for the screw machine, the furnace temperature is maintained at 160-220°C. In particular, it is worth mentioning that ABS is sensitive to changes in various process factors due to its high processing temperature. Therefore, the temperature control of the front end of the barrel and the nozzle part is very important.
It is proven that any small change in these two parts will be reflected in the part. The greater the temperature variation, the greater the defects such as fusion seam, poor gloss, flying edge, sticky mold, and color change.
(3) Injection pressure The viscosity of the molten ABS part 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 should be injected with high pressure, but small, simple, and thick parts can be injected with low pressure.
During the injection process, the pressure in the cavity at the moment of gate closure often determines the surface quality of the part and the degree of silver wire defects.
If the pressure is too small, the plastic will shrink a lot, the surface of the cavity will be out of contact with the surface of the part, and the surface of the part will be fogged. Pressure is too large, the plastic and the cavity surface friction is strong, easy to cause sticky mold.
(4) injection speed ABS material using medium injection speed is better. When the injection speed is too fast, the plastic is easy to scorch or decompose and precipitate gasification, thus appearing in the parts of the fusion seam, poor gloss and red plastic near the gate, and other defects.
However, in the production of thin-walled and complex parts, it is still necessary to ensure a high enough injection speed, otherwise, it is difficult to fill.
(5) Mold temperature ABS molding temperature is relatively high, the mold temperature is also relatively high. When producing parts with a large projection area, the mold temperature should be 70～80℃ for fixed mold and 50～60℃ for moving mold. In the injection of larger, complex configuration, thin-walled parts, should consider special heating of the mold.
To shorten the production cycle and maintain the relative stability of the mold temperature, after the parts are removed, a cold water bath, hot water bath, or other mechanical shaping methods can be used to compensate for the original cold fixed in the cavity time.
(6) material volume control general injection molding machine injection ABS plastic, the injection volume of each injection only up to 75% of the standard injection volume.
To improve the quality and dimensional stability of the parts, the surface gloss, and color uniformity, the injection volume is required to be 50% of the standard injection volume is appropriate.