ABS, commonly known as acrylonitrile – butadiene – styrene plastic, is made of acrylonitrile, butadiene, styrene three monomers copolymerized. Due to the different proportions of the three monomers, ABS with different melting temperatures and flow properties can be prepared. If blended with other plastics or additives, it can be expanded into ABS with different uses and properties. Such as impact grade, heat resistance grade, flame retardant grade, transparent grade, enhanced grade, electroplating grade and so on.
The fluidity of ABS is between PS and PC, and its fluidity is related to the injection temperature and pressure, of which the injection pressure has a slightly greater influence, so the molding often uses a higher injection pressure to reduce the melt viscosity and improve the filling performance.
1. Plastic treatment
The water absorption rate of ABS is about 0.2%-0.8%, and for general grade ABS, it is baked at 80-85 ° C for 2-4 hours in the oven or 1-2 hours with a drying hopper at 80 ℃ before ABS injection molding processing. For heat-resistant ABS-containing PC components, the drying temperature is appropriately adjusted to 100 ℃, and the specific drying time can be determined by air extrusion.
The proportion of recycled material can not exceed 30%, and electroplating grade ABS can not use recycled material.
2. Injection molding machine selection
A standard injection molding machine (screw length-diameter ratio 20:1, compression ratio greater than 2, injection pressure greater than 150 MPa) can be used. If the color masterbatch is used or the appearance of the product requires high material, a small-diameter screw can be selected. The clamping force is determined according to 4700-6200t/m2, depending on the plastic grade and product requirements.
3. Mold and gate design
Mold temperature can be set to 60-65℃. Runner diameter 6-8mm. The gate width is about 3mm, the thickness is the same as the product, and the gate length is less than 1mm. The vent is 4-6mm wide and 0.025-0.05mm thick.
4. Melting temperature
It can be accurately determined by air injection method. For different grades, melting temperatures are also different, it is recommended to set as follows:
- Impact grade: 220℃-260℃, 250℃ is preferable
- Plating grade: 250℃-275℃, 270℃ is preferable
- Heat resistance grade: 240℃-280℃, with 265℃-270℃ is better
- Flame retardant grade: 200℃-240℃, 220℃-230℃ is better
- Transparency level: 230℃-260℃, 245℃ is preferable
- Glass fiber reinforced grade: 230℃-270℃
For products with high surface requirements, higher melting temperature and mold temperature are used.
5. Injection speed
Fire grade should use slow speed, heat grade should use fast. During ABS injection molding, if the surface requirements of the product are high, the rate of fire control of high-speed and multi-stage injection molding should be used.
6. Back pressure
Under normal circumstances, the lower the back pressure, the better, the commonly used back pressure is 0.5MPa, and the dyeing material needs to use a higher back pressure to make the color mixture uniform.
7. Retention time
At a temperature of 265 ° C, the residence time of ABS in the melt tube should not exceed 5-6 minutes. The flame retardant time is shorter, if you need to stop, you should first lower the set temperature to 100℃, and then clean the melt tube with universal ABS. The cleaned mixture should be placed in cold water to prevent further decomposition.
If you need to change ABS material from other plastics, you must first clean the melt tube with PS, PMMA, or PE. Some ABS products have no problem when just released, and there will be discoloration after a period, which may be caused by overheating or the plastic staying in the melting cylinder for too long.
8. Post-processing of products
Generally, ABS products do not need post-treatment, only electroplated products need to be baked (70-80℃, 2-4 hours) to passivate surface traces, and products that need to be plated cannot use release agents, and the products should be packaged immediately after removal.
9. Molding should pay special attention to matters
There are several grades of ABS(especially flame retardant grades), and the adhesion of the melt to the screw surface after plasticization is very large, and it will decompose after a long time. When the above situation occurs, it is necessary to pull out the homogenization section and compression of the screw to wipe the test, and regularly clean the screw with PS.
