1 Drying of ABS plastic
ABS plastic has high moisture absorption and sensitivity to moisture. Adequate drying and preheating before processing can not only eliminate the fireworks like bubbles and silver threads on the surface of the parts caused by water vapor, but also help plasticize the plastic and reduce surface color spots and cloud patterns on the parts. The moisture content of ABS raw materials should be controlled below 0.13%. The drying conditions before injection molding are: dry below 75-80 ℃ in winter for 2-3 hours, dry at 80-90 ℃ on rainy days in summer for 4-8 hours. If the parts need to achieve particularly good gloss or are complex, the drying time is longer, up to 8-16 hours. The presence of trace amounts of water vapor causing surface fog spots on the workpiece is often overlooked. Zhihao modified the hopper of the machine into a hot air hopper dryer to prevent the dried ABS plastic from absorbing moisture again in the hopper. However, this type of hopper needs to strengthen humidity monitoring to prevent overheating of the material in case of accidental production interruption.
2. Injection temperature
The relationship between temperature and melt viscosity of ABS plastic is different from that of other amorphous plastics. When the temperature increases during the melting process, the melting actually decreases very little, but once it reaches the plasticizing temperature (a suitable temperature range for processing, such as 220-250 ℃), if it continues to blindly heat up, it will inevitably lead to the thermal degradation of ABS, which has low heat resistance, and instead increase the melt viscosity, making injection molding more difficult and the mechanical properties of the parts will also decrease. So although the injection temperature of ABS is higher than that of plastics such as polystyrene, it cannot have a more relaxed heating range like the latter. Some injection molding machines with poor temperature control often have yellow or brown coke particles embedded on the ABS parts to some extent when producing a certain quantity, and it is difficult to remove them by adding new materials for air injection and other methods.
The reason for this is that ABS plastic contains butadiene components. When a certain plastic particle firmly adheres to some surfaces that are not easily washed away in the screw groove at high temperatures, it is subjected to long-term high temperature, causing degradation and carbonization. Since high temperature operation may cause problems for ABS, it is necessary to limit the furnace temperature of each section of the material cylinder. Of course, the applicable furnace temperature for different types and compositions of ABS is also different. Like a plunger type machine, the furnace temperature is maintained at 180-230 ℃; Screw machine, maintain furnace temperature at 160-220 ℃. It is worth noting that due to the high processing temperature of ABS, it is sensitive to changes in various process factors. So temperature control of the front end of the barrel and the nozzle is very important. Practice has proven that any minor changes in these two parts will be reflected on the component. The greater the temperature change, the more defects such as weld seams, poor gloss, flash edges, mold sticking, discoloration, etc. will occur.
3. Injection pressure
The viscosity of ABS melt is higher than that of polystyrene or modified polystyrene, so higher injection pressure is used during injection. Of course, not all ABS parts require high pressure, and lower injection pressure can be used for small, structurally simple, and thick parts. During the injection molding process, the pressure inside the mold cavity at the moment of gate closure often determines the surface quality of the workpiece and the degree of silver wire defects. Insufficient pressure leads to significant plastic shrinkage, increasing the chance of detachment from the surface of the mold cavity and causing fogging on the surface of the component. Excessive pressure and strong friction between the plastic and the surface of the mold cavity can easily cause mold sticking.
4. Injection speed
Using a medium injection speed for ABS material yields better results. When the injection speed is too fast, the plastic is prone to burning or decomposing into gas compounds, resulting in defects such as weld seams, poor gloss, and red plastic near the sprue on the workpiece. But when producing thin-walled and complex parts, it is still necessary to ensure a sufficiently high injection speed, otherwise it will be difficult to fill.
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 a 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, thin-walled parts, special heating of the mold should be considered. In order to shorten the production cycle and maintain the relative stability of the mold temperature, after the parts are taken out, cold water bath, hot water bath or other mechanical shaping methods can be used to compensate for the time of cold fixation in the mold cavity.
6. Material quantity control
When injecting ABS plastic into a general injection molding machine, the injection volume per injection is only 75% of the standard injection volume. In order to improve the quality and dimensional stability of the parts, as well as the uniformity of surface gloss and color tone, it is recommended to require an injection volume of 50% of the calibrated injection volume.