ABS plastic granules have high hygroscopicity and hygroscopicity. Fully drying and preheating before processing can not only eliminate pyrotechnic blisters and silver threads produced by water vapor on the surface of parts, but also help plasticize plastics and reduce stains and moiré 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: drying below 75-80 ℃ in winter, drying for 2-3 hours, and 80-90 ℃ in summer, drying for 4-8 hours. If the part wants to achieve excellent gloss or the part itself is complex, the drying time is longer, up to 8-16 hours. Due to the presence of microscopic water vapor, stains on the surface of parts are often ignored. Convert the hopper of the machine into a hot air hopper dryer to prevent the dry ABS from absorbing moisture in the hopper again. However, humidity monitoring of such hoppers should be strengthened to prevent overheating of the material during occasional interruptions in production.
The relationship between the temperature and melt viscosity of ABS plastic granules is different from that of other amorphous plastics. However, once the plasticizing temperature, if the temperature continues to rise blindly, it will lead to thermal degradation of ABS with low heat resistance. On the contrary, the melt viscosity will increase, the injection molding difficulty will increase, and the mechanical properties of the parts will also decrease. Although the latter has a wider temperature range than ABS, it cannot have a higher temperature range than ABS. Some injection molding machines with poor temperature control often find yellow or brown coke particles embedded in the parts when producing a certain number of ABS parts, which are difficult to remove by adding new materials in air injection.
The reason is that ABS plastic contains butadiene. When a certain plastic particle is firmly attached to the surface of the thread groove that is not easy to be washed out at high temperature, it will degrade and carbonize under the action of high temperature for a long time. Since high temperature operation can cause problems with ABS, it is necessary to limit the furnace temperature of the various parts of the drum. Of course, the applicable furnace temperature of ABS of different varieties and components is also different. Such as plunger machine furnace temperature maintained at 180 ~ 230 ℃, screw machine furnace temperature maintained at 160 ~ 220 ℃. It is worth mentioning that due to the high temperature of ABS plastic machining, it is very sensitive to changes in various process factors. Therefore, the temperature control of the front end of the cylinder and the nozzle is very important. It turns out that any small changes in these two sections will be reflected in these sections. The greater the temperature change, the defects such as weld seam, poor gloss, flash, mold sticking, discoloration, etc. will be brought.
The viscosity of ABS melt is higher than that of polystyrene or modified polystyrene, so a higher injection pressure is used for injection. Of course, not all ABS plastic pellet products should use high pressure, and small, simple, thick parts can use low injection pressure. During casting, the pressure in the cavity when the gate is closed often determines the surface quality and the degree of silver wire defects. If the pressure is too small, the plastic shrinkage is large, 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 very large, which is easy to cause sticking.
The results show that the ABS material with medium injection speed has better effect. When the injection speed is too fast, the plastic is easily scorched or gassed, resulting in poor welds, poor gloss, and red plastic near the gate. However, in the production of thin-walled and complex parts, it is still necessary to ensure a sufficiently high injection speed, otherwise it is difficult to fill.