Injection molding process of ABS

ABS, commonly known as Acrylonitrile Butadiene Styrene, is a copolymer of three monomers: acrylonitrile-butadiene-styrene. Different monomers proportions has different properties and melting temperatures. If the ABS with fluid properties is blended with other plastics or additives, it can be expanded to different uses and properties, such as shock resistance stage, heat resistant grade, flame retardant grade, transparent grade, enhanced grade, electroplating grade, etc.

The fluidity of ABS is between PS and PC. Its fluidity is related to both injection temperature and pressure, among which the injection pressure has a slightly greater impact. Therefore, a higher injection pressure is often used during molding to reduce melt viscosity and improve mold filling performance.

1. Plastic processing
The water absorption rate of ABS is about 0.2%-0.8%. For general-grade ABS, it needs to be dried in an oven at 80-85℃ for 2-4 hours or in a drying hopper at 80℃ for 1-2 hours before processing. For heat-resistant ABS with PC components, the drying temperature is appropriately increased to 100℃, and the specific drying time can be determined by air extrusion method. The using proportion of recycled materials cannot exceed 30%, and electroplating grade ABS cannot use recycled materials.

2. Selection of injection molding machine
Screw ratio: 20 : 1
Screw compression ratio: greater than 2
Screw injection pressure: greater than 1500 bar
Diameter: a screw with a smaller diameter if masterbatch is used or the product appearance requirements are high.
The clamping force: according to 4700-6200t/m2. Specific needs to be determined according to the plastic grade and product requirements.

3. Mold and gate design
The mold temperature: 60-65℃.
The runner diameter: 6-8mm.
The gate width: about 3mm.
The gate thickness: the same as the product.
The gate length: less than 1mm.
The gas vent width: 4-6mm.
The gas vent thickness: 0.025-0.05mm.

4. Melting temperature
It can be accurately determined by air injection method. Different grades have different melt temperatures. The recommended settings are as follows:
Shock resistance stage: 220℃-260℃, preferably 250℃
Electroplating grade: 250℃-275℃, preferably 270℃
Heat resistant grade: 240℃-280℃, preferably 265℃-270℃
Flame retardant grade: 200℃-240℃, preferably 220℃-230℃
Transparent grade: 230℃-260℃, preferably 245℃
Glass fiber reinforced grade: 230℃-270℃
For products with high surface requirements, it should use higher melting temperature and mold temperature.

5. Injection speed
For plastic with fireproof grade, it needs to use slow speed. For heat-resistant grade, it needs to use fast speed. If the requirements of the product surface are high, it should use high-speed and multi-stage injection speed to control.

6. Back pressure
In general, the lower the back pressure, the better. The commonly used back pressure is 5 bar. Dyeing materials require a higher back pressure to make the color mixing uniform.

7. Residence time
Under a temperature of 265℃, the maximum residence time of ABS in the melt barrel cannot exceed 5-6 minutes. The flame retardant time is shorter. If you need to stop the machine, you should first lower the set temperature to 100℃, and then use general-grade ABS to clean the melt barrel. The cleaned mixture should be placed in cold water to prevent further decomposition. If you need to change from other plastics to ABS materials, you must first clean the melt barrel with PS, PMMA or PE. Some ABS products have no problems when they are just demolded, and they will change color after a period of time. This may be caused by overheating or the plastic staying in the melt barrel for too long.

8. Post-processing of products
Generally, ABS products do not require post-processing. Only electroplating-grade products need to passivate surface marks by baking 70-80℃for 2-4 hours. The electroplated products cannot use release agents and should be packaged immediately after being taken out.

9. Notes to attention during molding
There are several grades of ABS, especially flame retardant grades whose melt has a strong adhesion to the screw surface after plasticization, and will decompose over time. When the above situation occurs, it is necessary to wipe the screw homogenization section and compression pull-out, and clean the screw regularly with PS, etc.