ABS plastic occupies an important position in electronic industry, machinery industry, transportation, building materials, toy manufacturing and other industries because of its great mechanical strength and good comprehensive performance, especially for slightly large box structure and stress components, and the decorative parts that need electroplating cannot be separated from this plastic. Based on years of practical experience, the author makes the following analysis on ABS plastic injection molding process.
(1) Dry ABS plastic ABSplastic has high hygroscopicity and sensitivity to moisture. Adequate drying and preheating before processing can not only eliminate fireworks-like bubbles and silver filaments on the surface of parts caused by water vapor, but also contribute to plastic plasticization and reduce color spots and moire on the surface of parts.
The moisture content of ABS raw materials should be controlled below 0.13%. The drying conditions before injection molding are as follows: drying at 75 ~ 80℃ in dry winter for 2 ~ 3 hours, and drying at 80 ~ 90℃ in summer for 4 ~ 8 hours. If the product needs to achieve excellent luster or the product itself is complex, the drying time is longer, reaching 8 ~ 16 hours.
Mist spots on the surface of parts due to the existence of trace water vapor are often neglected. It is best to change the hopper of the machine into a hot air hopper dryer, so as to prevent the dried ABS from absorbing moisture again in the hopper, but the humidity monitoring of this kind of hopper should be strengthened, so as to prevent overheating when the production is interrupted accidentally.
(2) Injection temperature The relationship between temperature and melt viscosity of ABS plastics is different from other amorphous plastics. When the melting temperature rises, its melting actually decreases a little, but once it reaches the plasticizing temperature (suitable temperature range for processing, such as 220 ~ 250℃), if it continues to heat up blindly, it will lead to thermal degradation of ABS with low heat resistance, on the contrary, it will increase the melt viscosity, make injection molding more difficult, and reduce the mechanical properties of the parts.
Therefore, although the injection temperature of ABS is higher than that of polystyrene and other plastics, it cannot have a relatively loose range of temperature rise like the latter. In some injection molding machines with poor temperature control, when ABS parts are produced to a certain quantity, yellow or brown coking particles are often found embedded in the parts more or less, and it is difficult to remove and discharge them by injecting new materials into the air.
The reason is that ABS plastic contains butadiene. When some plastic particles are firmly adhered to some surfaces in the spiral groove at high temperature, they will be degraded and carbonized when exposed to high temperature for a long time. Since high temperature operation may bring problems to ABS, it is necessary to limit the furnace temperature of each section of the charging barrel.
Of course, different types and compositions of ABS have different suitable furnace temperatures. Such as plunger machine, the furnace temperature is maintained at 180 ~ 230℃; Screw machine, the furnace temperature is maintained at 160 ~ 220℃. It is worth mentioning that ABS is sensitive to the changes of various process factors because of its high processing temperature. Therefore, the temperature control of the front end of the barrel and the nozzle part is very important.
Practice has proved that any slight changes in these two parts will be reflected in the finished product. The greater the temperature change, the defects such as welded joints, poor gloss, flash, die sticking, discoloration, etc.
(3) Injection pressure The viscosity of ABS melt is higher than that of polystyrene or modified polystyrene, so a higher injection pressure is adopted during injection. Of course, not all ABS parts need to apply high pressure, but small parts with simple structure and large thickness can use lower injection pressure.
In the process of injection molding, the pressure in the cavity at the moment when the gate is closed often determines the surface quality of the part and the degree of silvery defects. If the pressure is too low, the plastic will shrink greatly, and the chances of getting out of contact with the cavity surface will be great, and the workpiece surface will be atomized. If the pressure is too high, the friction between the plastic and the cavity surface is strong, which is easy to cause mold sticking.
(4) Injection speed It is better to use medium injection speed for ABS material. When the injection speed is too high, the plastic is easy to burn or decompose and separate out the vapor, which leads to defects such as fusion joint, poor gloss and redness of plastic near the gate. However, when producing thin-walled and complex parts, it is necessary to ensure a high enough injection speed, otherwise it is difficult to fill them.
(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℃. When producing parts with large projection area, the fixed mold temperature is required to be 70 ~ 80℃, and the moving mold temperature is required to be 50 ~ 60℃. When injecting large, complex and thin-walled parts, special heating should be considered.
In order to shorten the production cycle and keep the mold temperature relatively stable, cold water bath, hot water bath or other mechanical setting methods can be used to compensate for the time of cold setting in the mold cavity.
(6) Material quantity control When ABS plastic is injected into general injection molding machines, the injection quantity per injection is only 75% of the standard injection quantity. In order to improve the quality, dimensional stability and uniform surface gloss and color tone, it is advisable to require the injection amount to be 50% of the calibrated injection amount.
