Due to its high mechanical strength and excellent overall performance, ABS plastic holds an important position in the manufacturing of plastic products, electronics, machinery, and building materials industries. In the manufacturing of plastic products, plastic decorative parts that require electroplating generally use ABS plastic.
ABS, generally known as acrylonitrile-butadiene-styrene terpolymer, is made by copolymerizing three monomers: acrylonitrile, butadiene, and styrene. Depending on the proportion of these three monomers, different properties and melting temperatures can be achieved. When ABS is blended with other plastics or additives, its range of uses and properties can be further expanded, such as impact-resistant grade, heat-resistant grade, flame-retardant grade, transparent grade, reinforced grade, electroplating grade, etc. In the manufacturing of plastic products, the electroplating grade is generally used. The fluidity of ABS is between PS and PC, and its fluidity is related to the injection temperature and pressure, with the impact of injection pressure being slightly greater. Therefore, higher injection pressures are often used during molding to reduce melt viscosity and improve mold filling performance.
The general melting point of ABS injection molding is around 170℃, with a decomposition temperature of 260℃; the adjustable range of injection temperature is relatively large. The usual operating temperature during injection is 180-240℃.
Due to the presence of rubber components, ABS absorbs a small amount of moisture, and needs to be dried during production. It can be dried at 80-90℃ for 1-2 hours.
Additionally, because of the rubber components, ABS's thermal stability is poor and it decomposes easily. Therefore, the raw material should not stay in the barrel for too long during injection; although the melt viscosity of ABS injection molding is higher than PS, the gates and runners are usually adequate to fill the molds; the ABS injection molding products are easily subject to static electricity and dust collection on the surface. The shrinkage rate is 5‰; the flash value is 0.05mm.
Drying of ABS Plastic
ABS plastic has high hygroscopicity and sensitivity to moisture. Sufficient drying and preheating before processing can eliminate surface fireworks-like bubbles and silver streaks caused by moisture, help plasticize, and reduce surface color spots and patterns on parts. The moisture content of ABS injection molding raw materials should be controlled below 0.13%. The drying conditions before injection are: drying at 75-80℃ for 2-3 hours during dry or winter seasons, and at 80-90℃ for 4-8 hours during rainy summer days. For particularly high-quality gloss or complex parts, drying times of 8-16 hours are needed.
The presence of minute moisture causing surface fog spots on parts is often an overlooked issue. It's best to turn the hopper of the machine into a hot air hopper dryer to avoid re-moisturizing the dried ABS in the hopper, but such hoppers need enhanced humidity monitoring to prevent material overheating during production interruptions. The ratio of recycled materials should not exceed 30%, and recycled materials cannot be used for electroplating-grade ABS.
ABS Injection Molding Temperature
The temperature-viscosity relationship of ABS plastic differs from that of other amorphous plastics. When the temperature rises during melting, its viscosity actually decreases very little. Once the plasticizing temperature (appropriate processing temperature range, e.g., 220-250℃) is reached, any further temperature increase will cause the heat degradation of ABS, which has relatively low heat resistance, resulting in increased melt viscosity, making injection more difficult, and lowering the mechanical properties of the parts.
Thus, although the injection temperature of ABS is higher than that of polystyrene and other plastics, it does not have as wide a range for temperature increase. In some injection molding machines with poor temperature control, yellow or brown scorched particles often appear embedded in the parts after producing a certain number of ABS injection molding parts, and it's difficult to remove them by adding new material and doing air shots. The reason is that ABS plastic contains butadiene components, and when certain plastic particles adhere firmly to some surfaces of the screw groove at high temperatures and are subjected to long-term high temperature, degradation and carbonization occur.
Since high temperature operation may cause problems with ABS injection molding, it's necessary to limit the temperatures of the different zones in the barrel. Of course, the applicable barrel temperatures vary for different types and compositions of ABS. For plunger-type machines, maintain barrel temperatures at 180-230℃; for screw machines, maintain at 160-220℃. Notably, due to the higher processing ABS injection molding temperature, it is sensitive to variations in various process factors. Therefore, temperature control at the front of the barrel and in the nozzle section is crucial. Practice shows that any small change in these two sections reflects on the parts produced. The greater the temperature variation, the more issues like weld lines, poor gloss, flash, sticking molds, discoloration, etc. arise.
ABS Injection Molding: Injection Pressure
The viscosity of molten ABS injection molding is higher than that of polystyrene or modified polystyrene, so higher injection pressures are used during injection. However, not all ABS injection molding parts require high pressure. For small, simple, thick parts, lower injection pressure can be used. The pressure in the mold cavity at the moment when the gate closes often determines the surface quality and the degree of silver streak defects on the parts. If the pressure is too low, the plastic shrinks significantly and often separates from the mold cavity surface, causing surface fogging of the parts. If the pressure is too high, the friction between the plastic and the mold cavity surface is intense, which easily causes sticking to the mold.
ABS Injection Molding: Injection Speed
Moderate injection speed works best for ABS injection molding material. If the injection speed is too fast, the plastic can easily burn or decompose, releasing volatile substances that create weld lines, poor gloss, and redness near the gate on the parts. Nonetheless, to produce thin-walled and complex parts, a sufficiently high injection speed is necessary to ensure the mold is filled.
ABS Injection Molding: Mold Temperature
The molding temperature of ABS is relatively high, so the mold temperature is also relatively high. The general mold temperature should be adjusted to 75-85℃, while for parts with large projected areas, the fixed mold temperature should be 70-80℃, and the movable mold temperature should be 50-60℃. For the injection of large, complex, thin-walled parts, mold heating should be considered. To shorten the production cycle and maintain relative mold temperature stability, methods like cold water bath, hot water bath, or other mechanical setting methods can be used after part removal to compensate for the original cooling time in the cavity.
ABS Injection Molding: Material Quantity Control
When injecting ABS plastic, the standard injection amount for each shot should be 75% of the rated injection amount. To improve the quality and dimensional stability, surface gloss, and uniform color tone of the parts, the injection amount should ideally be 50% of the rated injection amount.
ABS Injection Molding: Selection of Injection Molding Machine
A standard injection molding machine can be selected (with a screw length-to-diameter ratio of 20:1, compression ratio greater than 2, and injection pressure greater than 150 MPa). If using color masterbatches or if high part appearance is required, a screw with a smaller diameter can be chosen. The clamping force should be determined at 4700–6200 t/m², depending on the plastic grade and part requirements.
