July 9, 2020

Skills for adjusting streaks and welding marks on thick PVC injection moulded parts (1)

As mentioned in the previous column, to eliminate the streaks and weld marks of thick PVC parts, first of all, it must be ensured that when the melt is injected into the cavity from the gate, folding wave (snake type) or spiral injection cannot occur. The cavity must be filled smoothly and no shunts are allowed. Therefore, the tuning technology becomes quite important, and other measures are to make the tuning easier to achieve the above purpose.

First, a particularly slow injection must be used. This is a primary condition. Sometimes it is necessary to adjust the screw to be almost unable to move before it is possible to eliminate streaks.

However, if the injection is continued at a slow rate, it is difficult for the injection molded part to fill the cavity, and there will be serious matte color problems in the second half of the injection molded part. Therefore, when the slow injection is carried out until only a small part of the melt enters the cavity, the water ripple is confirmed to be eliminated (you can observe it with the “mold transparency method”). Medium-speed or medium-speed injection.

If the secondary speed is too fast, problems such as overflow or whitening may occur. At this time, the third stage of slow speed and low pressure can be used for final filling and pressure holding when the cavity is about to be filled. The speed and pressure parameters of the third stage can be determined according to the degree of defects of the injection molded parts, and can also be used The “positioning injection method” solves the problems of overflow and whitening. Of course, if the injection molded part is not too big, the speed of the second stage does not need to be too fast, it can be slightly faster than the slow speed of the first stage.

A small part of the melt injected slowly into the cavity around the gate at the beginning of the filling process, so when the second-stage faster injection is performed, this small part of the melt can play a blocking role. The melt will not be sprayed too quickly, so that the melt can be filled smoothly without shunting, and the problem of weld marks is also solved together.

In the actual tuning process, the key to the problem is the amount of this small amount of melt. A small amount will not act as a barrier. Welding marks and streaks will also be generated. Too much will make secondary injection difficult, resulting in problems such as uneven filling or matt color of injection molded parts. Therefore, in principle, in the absence of weld marks and water ripples, secondary and faster injection should be performed as early as possible, and sufficient injection pressure should be ensured.

This small amount of melt is actually related to the speed and pressure of the second injection. The higher the speed and pressure during the second injection, the more this amount is needed, otherwise the weld mark will be generated; otherwise, it may be less. The actual operation also requires careful observation according to the on-site situation to determine the starting point of the secondary injection and the speed and pressure of the secondary injection.

When adjusting the machine, you can first determine an approximate amount (about 10 ~ 15mm screw stroke), and then set the speed and pressure of the second injection. After the injection, observe whether the product has weld marks or lack of material, dumb color, etc., and then adjust the amount of the small part of the melt more or less. Using the “mold transparency method”, it is easy to determine the demand for this small portion of the melt.

When using the above tuning techniques to finally solve the problem, it proves that the problem is already quite serious, and we must use more of the aforementioned measures to cooperate to solve it, it is possible to achieve the desired level, it can not solve the problem, then only use another tuning technique described below.

The thick and thick injection molded parts, whether they are ABS, PP, or PVC, are prone to streaks and weld marks. The reason is because they are thick, so the above method is also applicable to the thickness of other hard plastic materials. The problem of streaks and weld marks on large products.

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