ABS plastics occupy an important position in the electronics industry, machinery industry, transportation, building materials, toy manufacturing, etc. due to their large mechanical strength and good comprehensive performance, especially for slightly large cabinet structures and force components. The decorative parts that need electroplating are inseparable from this kind of plastic. Based on years of practical experience, the author analyzes the injection molding process of ABS plastic as follows.
(1) ABS plastic dry ABS plastic has high hygroscopicity and sensitivity to moisture. It can be fully dried and preheated before processing, which can not only eliminate the fireworks-like foam band and silver wire on the surface of the workpiece caused by water vapor. It also helps plasticize plastics and reduce surface stains and moiré. ABS raw materials should control moisture below 013%. Drying conditions before injection molding are: dry winter seasons below 75~80°C, drying 2~3h In summer, the rainy days are at 80~90°C, and the drying is 4~8h. If the parts are to have a particularly good gloss or the parts themselves are complex, the drying time is longer, up to 8~16h. The surface of the parts is caused by the presence of trace water vapor. Fog spots are a problem that 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, but this type of hopper should strengthen the humidity monitoring, in case of accidental production interruption. To prevent overheating of the material.
(2) Injection temperature The relationship between temperature and melt viscosity of ABS plastic 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 (suitable processing temperature range) If it is 220~250°C), if it continues to heat up blindly, it will lead to thermal degradation of ABS which is not too hot, but the melt viscosity will increase, the injection molding will be more difficult, and the mechanical properties of the parts will also decrease. Therefore, ABS Although the injection temperature 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 more or less when producing ABS parts to a certain amount. Or less or less yellow or brown coking particles are found on the parts, and it is difficult to remove them by adding new materials to the air.
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 temperature of different types and configurations of ABS is also different. For example, the plunger type, the furnace temperature is maintained. At 180~230°C; screw machine, the furnace temperature is maintained at 160~220°C. It is worth mentioning that because of the high processing temperature of ABS, it is sensitive to the change of various process factors. Therefore, the front end of the barrel and the nozzle part Temperature control 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 weld seam, poor gloss, flash, mold, discoloration and other defects. .
(3) Injection pressure ABS melt parts have higher viscosity than polystyrene or modified polystyrene, so higher injection pressure is used during injection. Of course, not all ABS parts are applied with high pressure, which is small and simple in construction. The thickness of the workpiece can be used with a lower injection 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-like defects. The pressure is too small, the plastic shrinks. Large, the opportunity to get out of contact with the surface of the cavity is large, the surface of the workpiece is atomized. The pressure is too large, the friction between the plastic and the cavity surface is strong, and it is easy to cause the sticking.
(4) Injection speed ABS material has a better effect on medium injection speed. When the injection speed is too fast, the plastic is easy to burn or separate out the gasification, so that the weld seam appears on the part, the gloss is poor and the plastic is red near the gate. And other defects. But 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) The mold temperature ABS has a relatively high molding temperature and the mold temperature is relatively high. The general adjustment mold temperature is 75~85°C. When producing a workpiece with a large projection area, the mold temperature requires 70~80°C. The mold temperature is required to be 50~60°C. When injecting larger, complex-shaped, 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, remove the parts. After that, cold water bath, hot water bath or other mechanical setting method can be used to compensate the time of the cold type in the cavity.
(6) Feed quantity control When the injection molding machine is injected 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 uniform color tone, the injection volume is required to be the calibration injection. 50% of the amount is appropriate.