Problems and Solutions of Injection Molding PC/ABS Products

1. Filamentary phenomenon

Filamentary phenomenon is the most common problem of PC/ ABS material, which refers to the phenomenon of the silvery-white silk-like stripe in the surface of the product along the flow direction.

 

The main reason is the interference of gas, in which the gas is mainly divided into three components:

Air: the air involved in the melting and ejection phase;

Moisture: the moisture contained in the material itself;

Pyrolysis gas: gas produced by high temperature hydrolysis / thermal decomposition.

 

Solution: firstly, checking whether the material is sufficiently dry; after the material is confirmed to be dry, the filamentary defect can be improved by adjusting the injection molding process. At the same time, this phenomenon is also related to the exhaust of the die.

 

2. Flow mark

The flow mark is appeared because of the poor fluidity of the material, and the appearance of the flow mark is different from that of the filamentary phenomenon.

 

Solution: The flow mark can be avoided by increasing the temperature of the material so as to improve the fluidity, and the fluidity of the material in the mold and the injection speed can be appropriately improved.

 

3. Contraction cavities and Indentation

The contraction cavities is caused by the lack of mold filling in the mold cavity.

 

Solution: Increase mold temperature and material temperature appropriately to improve material fluidity; Increase the injection time, injection pressure, and  injection speed to improve the mold filling, or increase the size of the gate and heating the gate runner to reduce and eliminate the contraction cavities of the products.

 

The indentation is due to the improper temperature of the material and the improper design of the product. When the temperature of the material is too low, it will not only cause contraction cavities, but also will have the problem of indentation. While the material temperature is too high, mold temperature is too high, will cause melting material in cooling excessive shrinkage, resulting in indentation.

 

Solution: Increase the injection speed.

 

3. Warping deformation

The warping deformation of the injection part is due to the unreasonable design, the improper position of the gate and the unreasonable processing conditions of the injection molding, so that the internal stress is generated inside, the shrinkage is not uniform or excessive.

If the mold temperature is too high or the mold temperature is not uniform, it will cause the mold release difficulty or the cooling non-uniform, which will also lead to warpage deformation.

Solution:

Processing:  prolong the injection molding cycle, reduce the injection molding temperature, adjusted the injection pressure and the injection speed adjusted, reduce the ejection speed, increase the ejection area, and balance the ejector force;

Product design: increase the thickness of the wall, add reinforcement bars and fillet reinforcement.

 

4. Marking

Markings are usually caused by melt rupture when the melt expands into the mold cavity during high-speed injection. 

 

Solution:

Moulding process: Increase material temperature and the nozzle temperature, slow down injection speed, etc.

Mold: improve the temperature of the die, arrange the overflow groove, increase the size of the gate, and modify the shape of the gate.

 

5. Spot

Solution: Add dispersant or oil; adjust melting material temperature and time.

 

6. Peeling

The peeling problem of plastic products is closely related to the rupture of fluids caused by high shear force. 

At low shear stress or velocity, small disturbances due to various factors are suppressed by the melt, while at high shear stress or velocity, disturbances in the fluid are difficult to suppress and develop to an unstable flow;

When a critical shear force is reached, the fracture of the fluid will be caused.

 

Solution:

Material: First of all, it is necessary to exclude the peeling caused by the mixture. The two components of PC and ABS are partially compatible, so it is necessary to add proper compatibilizer to improve the compatibility of the two components in the process of modification.

Mould: The principle of mold design should follow the direction of minimizing shear. Generally speaking, products with dense striped surfaces are more prone to skin peeling(caused by the friction shear between melt and inner wall of cavity during high-speed filling). At the same time, the small design of the gate size will lead to excessive shear while the melt passing through the gate, and then lead to product surface peeling.

Injection molding process: The main purpose is to avoid excessive shear. the high-speed high-pressure mode can be used to improve the product filling difficulty, and therefore, in the actual injection molding process, the flow resistance in the actual filling process can be reduced, so as to avoid excessive shearing caused by high-speed high-pressure.