Как устранить дефекты поверхности, такие как помутнение, на глянцевых пластиковых изделиях?

High-gloss and high-brightness plastic product have highly reflective surfaces, making them extremely sensitive to defects. Any minor imperfection can be easily noticed and cannot be concealed. Fine dust particles in the workshop environment may cause quality issues such as pits, sink marks or surface contamination. Therefore, cleanliness control is a critical factor affecting the yield rate.

Figure 1 defective plastic product

Figure 2 Improved plastic product

Phenomenon

From a visual perspective, the white haze defect shown in the picture on plastic product surface. This type of white water mark defect is particularly noticeable on black high-gloss plastic products. As shown in Figure 1, the product is made of PP material. Various white spots and irregular patches of different sizes are distributed around surface, resembling clouds or fog.

Theory

Based on above analysis, the theoretical explanation is as follows:

During material feeding stage, there are certain gaps between plastic particles. These gaps can carry air into the barrel during shearing process of screw. During screw shearing,  material undergoes physical changes, transitioning from a glassy state to a highly elastic state, and then from highly elastic state to a molten flow state.

This transformation process may only take a few seconds to several tens of seconds, depending on plastic injection machine conditions and products’ size. Completing such a complex transformation within such a short period of time can easily lead to injection molding defects.

When the gaps between plastic particles are compressed, the trapped air can dissolve into molten material. If the air cannot be effectively released from barrel during plasticization, the remaining gas will be trapped inside molten material and injected into mold cavity.

Whether the mold can effectively release gas depends on the quality of mold venting system. If the venting system is insufficient, defects such as haze, gas traps or burn marks may occur.

Obviously, white haze is also a form of gas trapping. Many injection molding technicians mistakenly believe that only burn marks are considered gas trapping defects, or they immediately enlarge the venting grooves whenever gas traps appear. This approach is not scientifically correct.

Burn marks occur when gas trapping becomes severe. Under high pressure and high-speed compression, the trapped gas cannot escape and causes local combustion. The combustion temperature can instantly reach 600–800°C. This represents most severe stage of gas trapping.

Решения

How can gas be released effectively to prevent white haze defects?

1) Mold

Gas venting cannot be completely solved by mold alone.

For PP materials, the processing temperature range is relatively wide and material has excellent flowability. If mold venting grooves are designed too large or placed in incorrect locations, they may not only fail to achieve proper gas release but may also cause flash defects. Therefore, correct venting design is the key to effective gas removal.

2) Process

Mastering proper injection molding adjustment techniques is highly effective in actual production.

1) Reduce Screw Rotation Speed

Reducing screw rotation speed can help achieve more uniform melt temperature and reduce the possibility of air being entrained.

If the screw rotation speed is too high, air is more likely to be trapped in material. At the same time, excessive shearing force from screw generates frictional heat, causing uneven melt temperature. This may lead to material degradation and gas generation.

2) Increase Mold Temperature

Increasing the mold temperature helps achieve high-pressure and slow-speed filling, allowing more time for gas release. If mold temperature is too low, plastic product surface may freeze prematurely. In this case, slow-speed filling cannot be achieved effectively, which may result in short shots or incomplete filling.

3) Reduce Injection Speed

The faster injection speed, the faster gas is compressed and the higher generated temperature. A slower injection speed compresses the gas more gradually, allowing the trapped air to escape more easily.

4) Clamping Force

The higher clamping force, the worse venting effect on cavity surface. A lower clamping force allows better gas release from cavity surface.

If you are encountering quality issues with plastic product or need to find a new factory for plastic product, Grandshine, as a high-end развитие плесени and injection формовка product factory, offers you a full range of services from product design, mold development to mass production and assembly.