When the surface quality of the plastic parts cannot meet the design requirements due to the weld marks, the technicians usually start from the aspects of the melt temperature, the injection speed, the pressure, the flow rate, and the mold temperature. And these are all realized by the equipment, and the small adjustment of the molding condition standard is made, and the ideal value is approached.

Obviously, the items of melt temperature, flow speed, pressure, flow rate, and mold temperature that can be adjusted by the equipment are relatively easy to implement. On the one hand, it is convenient to adjust, and on the other hand, it can be repeated many times. The difficulty is that when the above means can no longer be solved, it is necessary to modify the mold to achieve the desired effect, which is also the focus of discussion.

There are several possible situations in which the mold needs to be modified:

1. There are air bubbles in the weld mark, and it is necessary to add a vent hole on the corresponding parting surface.

2. The depth of the weld line is always out of tolerance, and it is necessary to adjust the thickness of the plastic part, that is, the cavity of the mold.

3. The position of the weld line is biased toward the middle of the plastic part, and the position of the gate needs to be adjusted.

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The following describes the improvement process of the above three cases:

1. The formation of bubbles at the weld line is due to the fact that when the two melts merge, the enclosed gas is not removed in time, but remains inside the plastic part, forming a pit on the surface of the weld line, which can be called "sleepy". gas".

2. The reason may be that the parting surface of the corresponding position is too tightly welded, so that the gas cannot be discharged; or the cavity height dimension is excessively uneven after the mold clamping (the wall thickness difference of the plastic parts is large).

3. For the former case, it is often improved by increasing or increasing the venting groove. Taking the PP material which is the most widely used in China as an example, according to the parameter that the overflow gap of the PP material is 0.03 mm, in order to avoid flashing It is ideal to form a flash edge with a gap of 0.01 to 0.02 mm.

In order to facilitate the cleaning of the cavity during the mold processing and molding process, the position of the venting groove is mostly selected to be placed on the parting surface of the fixed mold, and is placed as far as possible in the final filling of the cavity.

For the case where the cavity height is excessively uneven, the cavity size can only be adjusted by the "welding" and "grinding" methods, which is the most difficult and the most difficult for the mold technician to encounter during commissioning. The adjustment method is the same as the adjustment method in which the height of the weld mark is always out of tolerance.

When the process parameters and the method of opening the exhaust groove are not improved or eliminated, the size of the cavity may have to be adjusted due to the poor quality of the plastic part due to the weld line. Of course, the thickness of the corresponding part of the plastic part needs to be accurately determined. After the measurement, work within the allowable range of the design.

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The wall thickness of the bumper plastic part is not an equivalent value in different parts, but a gradual amount. The reason is that considering the specific shape of the plastic part and the fluidity of the molten plastic, the gradual wall thickness is favorable for molding. The wall thickness is usually between 2.60 and 3.50 mm.

The modification of the mold cavity size is divided into two cases: one is to increase the size of the cavity, and the other is to reduce the size of the cavity.

For the first case, it is easier to implement, and the corresponding part of the mold cavity can be directly polished according to the test data. The second case is more complicated. In order to reduce the size of the cavity, it is first necessary to weld on the cavity surface of the mold and then sand it.

The following describes the method of reducing the size of the mold cavity:

From the difficulty of the work, it is much easier to first consider the choice of "welding" and "grinding" on the moving mold. Since the quality of the cavity of the injection mold directly affects the appearance of the plastic part, and a large amount of heat is generated during the welding process, when there is insufficient process guarantee, the heat tends to change the tissue composition of the cavity surface, resulting in the cavity surface. The hardness is different, which in turn affects the appearance of the plastic parts. In practice, the modification of the model cavity surface should be avoided as much as possible.

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The general steps for modifying the cavity surface of a dynamic model are as follows:

1. By testing the mold on the cavity surface, the thickness of the cavity needs to be increased or decreased.

2. Implement “repair welding” and “grinding” operations.

3. Try the mold again and adjust the size of the cavity surface according to the molding effect.

The second step is the difficulty and the key. The following is the detailed process of cavity surface repair:

1. Select the welding material that matches the base metal, and determine the welding range, and reserve and protect the grinding standard.

2. Partition alternately surfacing, be careful not to weld from the beginning to the end, so as to avoid internal stress causing cracking of the cavity surface of the mold.

3. Start grinding according to the reserved reference, and pay attention to the protection of relevant parts around.

4. Measure the height of the repaired surface. After the requirements are met, fill the reference vacancies to complete the modification of the cavity surface.