July 13, 2020

Skills for adjusting streaks and welding marks on thick PVC injection moulded parts (2)

Sometimes, the cavity space at the gate position is too large, which makes it difficult to eliminate weld marks and streaks. This requires an extremely low first-level injection speed for injection to reduce or eliminate the weld marks and streaks.

But if the injection speed is too slow, the heat dissipation time of the melt will be too long. At this time, even if the water ripples are resolved, the plastic parts leave a deep arc-shaped texture at the gate position (commonly known as ‘cold partition’), Causing the appearance of the product to be equally ugly. The cold ribs are caused by the fact that there is a large temperature difference between the supercooled melt injected first and the hot melt injected later in the second stage, and they cannot be completely fused with each other.

Therefore, when the streak is so severe that it can be solved with a very slow injection speed, it is difficult to solve the problem satisfactorily with the previous tuning technique (1). At this time, the following tuning technique can be considered.

When the slow injection enters the cavity with a small amount of melt, immediately switch to rapid injection, and give sufficient injection pressure. Because there is a little bit of melt in front to block, it is difficult to form a fold or spiral injection method around the rapidly injected melt to avoid the occurrence of water ripples. Due to the impact of rapid injection impact, the earliest injected melt will flush the condensed melt that has formed cold ribs into the cavity, so that the cold ribs at the gate no longer exist.

The key to using the tuning technique is to find the slow-fast switching point, that is to determine the amount of melt that enters the cavity slowly. At this point, the amount of melt is very important. If the amount is too large, it will not be washed away, and the cold partition will continue to be generated. If the amount is too small, the resistance will not be enough. Using the ‘mold transparency method’ is very helpful to pinpoint this.

Of course, one of the biggest drawbacks of this measure is the use of rapid injection, which has the consequence of causing more weld marks on injection molded parts. Because the amount of melt at this point is much less than the small amount of melt mentioned in the tuning technique (1), it is not enough to block the generation of weld marks. However, due to the use of rapid injection in the second stage, the weld marks are better because the melt temperature is still sufficient, so most of the weld marks become smaller. If the surface requirements of the product are not very high, these small weld marks are acceptable.

The principle of using rapid injection to make the welding line thinner, whether it is soft plastic or hard plastic, can come up with another solution to the welding line: when the welding line is really difficult to solve, simply take a first-level rapid injection. Without paying attention to the problem of water ripples, but it can make the weld line very small. If the gate is designed in a position where the wall thickness is relatively small or blocked, the streak should be very slight or non-existent.

After rapid injection, if there is a phenomenon of trapped gas, the weld mark will become thicker or even burnt or whitish. Therefore, the late stage of rapid injection should be turned to slow speed, and the ‘positioned injection method’ can also be used to prevent the occurrence of defects.

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Injection Molding, Plastic Mold Case, Plastic Molding Design, Plastic Parts
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