Analysis of the molding processing steps and key points of flame-retardant PC/ABS

PC/ABS alloy is a kind of engineering plastic with excellent performance. It improves the stress cracking and processability of PC. It also has excellent heat resistance, impact resistance, chemical resistance and dimensional stability.

Halogen-free flame retardant PC/ABS does not use halogen flame retardants and is more environmentally friendly. The flame retardant grade can reach 1.5mmV0, 2.0mm5VB and 3.0mm5VA, or even 0.75mmV0. It is widely used in computer monitor shells, notebook shells and other IT products, as well as the shells of office equipment such as copies and printers.

Halogen-free flame-retardant PC/ABS can use traditional processing equipment for injection molding, extrusion molding, secondary processing, etc., but injection molding is the most widely used. Because flame-retardant PC/ABS contains PC components and flame retardants, a small amount of moisture can easily cause PC to hydrolyze during high-temperature molding processing, and excessive heat (including shear heat) may cause partial degradation and small molecules to precipitate, resulting in products Appearance problems such as silver wire, yellowing, and color difference, as well as strength problems such as product brittleness and paint cracking.

In order to form better products, the following forming processing guidelines are summarized for reference.

1. Material drying

Flame-retardant PC/ABS plastic will absorb moisture in the air, and its moisture content is between 0.2 and 0.3%. In order to achieve the best effect of injection molded products and reduce the possibility of degradation, PC/ABS must be dried before molding to reduce the water content of the material to below 0.04%, preferably below 0.02%, to improve processing stability and machinery performance.

Generally, flame-retardant PC/ABS plastics are pre-dried for 4-6 hours at the recommended temperature to achieve the required moisture content. The specific drying time is related to the drying equipment used, but the longest cumulative drying time cannot exceed 48 hours, otherwise the material may degrade and change color.

There are three commonly used drying equipment: box dryer, hopper dryer and dehumidifying dryer. Halogen-free flame-retardant PC/ABS is recommended to be dried with a dehumidifying dryer.

2. Selection of injection molding equipment:

When determining the specifications of the injection molding machine, in addition to considering the clamping force, for halogen-free flame-retardant PC/ABS, a machine with a low maximum injection volume should be selected as far as possible. When the total injection weight (all cavities, including runners and sprues) is equal to 50~80% of the maximum injection capacity of the injection molding machine, the best results can generally be obtained. Generally, the longest heat residence time of flame-retardant PC/ABS should be less than 10 minutes, and the best heat residence time is less than 5 minutes.

3. Setting of injection process parameters:

1. The setting of the molding temperature is based on ensuring that the flame-retardant PC/ABS is fully plasticized, and the low temperature area should be used as much as possible to prevent material degradation. In principle, when using the recommended upper limit of the molding temperature, the residence time of the melt should be as short as possible to avoid degradation. The molding temperature of halogen-free flame-retardant PC/ABS is generally 230℃-270℃.

2. The screw speed and the back pressure are too high. The screw speed will cause excessive shear heat, so that the actual temperature of the molten material is much higher than the set temperature, which may lead to material degradation. Similarly, excessive back pressure will also cause excessive shear heat. The back pressure of flame-retardant PC/ABS is generally adjusted to 3-10kg/cm2. Generally, the number of screw rotations is set by completing the measurement as much as possible within the cooling time, and the recommended rotation speed is set to 30-70RPM.

3. When the injection speed is selected, the appearance of the product, the exhaust of the mold and the resistance of the plastic flow in the cavity are mainly considered. It is recommended to use the stepped speed setting according to the product and mold structure to ensure smooth filling and product appearance.

4. Whether the holding pressure and time are set reasonably or not, directly affects the size of the internal stress of the part. In general, the appropriate holding pressure is 60-80% of the injection pressure. The dwell time is set according to the time that the gate completes its solidification as it cools, and the screw advances again and no longer applies pressure to the molded product.

 5. Mold temperature Mold temperature control is very important to determine the filling degree, appearance and residual stress of the final part. It is recommended to use a mold temperature machine to control the mold temperature when molding flame-retardant PC/ABS. The recommended mold temperature is 50-80°C. As far as the surface of the part and the cycle period are concerned, when the mold temperature is in the middle of the recommended temperature range, better results can be expected.

6. Cooling time The setting of cooling time is based on that the product will not be deformed after being solidified and opened. It can be set a little longer, and then slowly reduced according to the product and the cooling effect. The minimum cooling time for the product without deformation is the most Good cooling time.

In short, the most important processing precautions for halogen-free flame-retardant PC/ABS are two points: 1. The material must be fully dried, otherwise it will easily cause material hydrolysis; 2. Strictly control the molding temperature of the material and the residence time at high temperatures. Otherwise it is easy to cause thermal degradation of the material. The rest of the process parameters are coordinated with each other. According to the differences in product and mold structure, comprehensive consideration and setting can be obtained to obtain products with stable quality, better appearance and optimal performance.