Moulding defects: causes and countermeasures for silver streaks

　Silver streaks are whitish thread-like streaks resembling silver that appear along the flow direction on the surface of the molded product and are often a problem with external components. They are most noticeable when the molded product is black, but they also occur on ivory, which is often used in housings for office automation equipment, and are a problem for those in charge of molding. Exterior parts are often made of ABS, AS resin and impact-resistant polystyrene, and when plastics from these materials are molded without drying while still absorbing moisture, the moisture vaporises in the molten material and the vaporised bubbles are crushed along the flow, becoming minute particles that rub against the mold surface.

　The silver streaks are therefore the true form of water vapour or gases generated by the molten plastic.

Countermeasures include keeping the cylinder temperature low to dry the material well and minimise gas generation from the molten material, suppressing decomposition gases from the heated material as much as possible and molding at a slower injection speed to avoid air entrapment in the mold.

　However, if the shape of the molded product is difficult to mold, the design of the molded product may also be the cause, as the material melting temperature has to be increased and the injection speed has to be increased.

Figure 1 shows the relationship between uneven wall thickness and silver streaks. If there are thin-walled areas, the temperature of the material is increased in order to fill these areas with material.

If the speed of injection is increased, the tendency for air and gas entrainment increases, causing silver streaks.

As shown in Fig. 1, when the degree of wall thickness increases, it becomes difficult to set molding conditions that prevent burrs, sink marks and silver streaks.

Fig. 2 shows air entrapment as the molten plastic traverses the ribs.

Silver streaks are less likely to occur when ribs are placed in the flow direction of the molten plastic, so they should be placed in the flow direction as far as possible. However, as shown in Fig. 3, ribs are sometimes placed as reinforcement for weakness in the direction perpendicular to the flow, which is sometimes unavoidable, so ribs should be as low as possible to prevent air It is better to prevent air entrapment.

An effective means of resolving silver streaks in terms of molding skills is ‘program control of injection’. As shown in Fig. 2, this is also known as ‘pattern control’, where the injection speed is reduced where it should be reduced and the required speed is maintained in other areas. The good use of this device can be expected to be quite effective.

Fig. 3 shows a case of a thin-walled container, where the mold temperature is low and the wall thickness of the molded product is thin, so the injection speed is accelerated to the limit, resulting in extremely high material flow velocity through the pin-point gate, which causes thermal decomposition of the material, resulting in silver streaks.

Fig. 4 shows the degassing situation from the screw of an injection molding machine. This degassing function, which is one of the functions of the screw, will not function at all if the temperature setting near the hopper is too high.

Material fed from the hopper to the screw should remain in the solid phase (solid) near the hopper (feeding section of the screw), and if the material is in a molten state here, the gas cannot move towards the hopper and is entrained in the molten plastic, causing silver streaks. Another phenomenon where air is entrained by the screw is when metering is close to the capacity limit of the injection molding machine and the screw repeatedly moves backwards and forwards at full stroke, entraining air from the hopper mouth.

　Also, care must be taken when rotating the screw at high speeds, as this can cause the material to overheat due to excessive screw rotation for high-speed molding.

The material used can also be a cause. In the author's experience, he was plagued by silver streaks and took all possible measures, but when he could not solve the problem, he changed the material manufacturer and used the same grade of material with less gas emission, which solved the problem at once.

If too much gas is generated from the material used, it will be difficult to prevent silver streaks, but if the cause is the material itself, the following can be considered.

Plasticisers

Stabilisers

Colourant dispersants

Ultraviolet absorbers

Antistatic agents

Thermal decomposition of the above-mentioned additives. If silver streaks persist despite all preventive measures, it is important to consider whether the decomposition of the above-mentioned additives is the cause.

The key to preventing thermal decomposition is to mold at as low a temperature as possible, but as mentioned earlier, in some cases the temperature has to be set higher at the molding site due to the complex shape of the molded product.

In addition, the injection molding machine used may be too large and the material may stay in the screw cylinder for too long, causing it to decompose. The causes can lie in many different places and often cannot be resolved without a wide range of considerations.

Gases generated from the molten material not only affect the appearance of the molded product, but in extreme cases of this phenomenon, the gases can obstruct the flow of material into the cavity, resulting in underfilling and welding. Selection of a material that generates as little gas as possible is another important measure.