When the surface quality of a plastic part cannot meet the design requirements due to weld marks, the technician usually starts with the melt temperature, injection speed, pressure, flow rate, mold temperature, etc. These are all achieved through equipment. Condition criteria are adjusted slightly to approximate the ideal value.
Obviously, the melt temperature, flow rate, pressure, flow rate, mold temperature, etc., which can be adjusted by the equipment, are relatively easy to implement. On the one hand, it is easy to adjust. On the other hand, it can be repeated many times. The difficulty is that when the above means When it cannot be solved, we must achieve the desired effect by modifying the mold. This is also the focus of discussion.
The following situations may be required to modify the mold:
There are air bubbles trapped at the weld marks, and an additional vent hole is required on the corresponding parting surface.
The depth of the weld mark is always poor, and the thickness of the plastic mold, that is, the mold cavity needs to be adjusted;
The location of the weld marks is biased towards the middle of the plastic part and the position of the gate needs to be adjusted.
The following describes the improvement process of the above three scenarios:
1. The formation of bubbles at weld seams is due to the fact that when the two melts converge, the enclosed gas is not removed in time and remains inside the plastic part. The pits are formed on the surface of the weld, which can be informally referred to as 'trapped air'. .
The reason for this may be that the parting surface of the corresponding location has been over-tightened so that the gas cannot be discharged, or it may be caused by the excessive unevenness of the height of the cavity (the wall thickness of the plastic part is greatly different) after the mold is closed.
2. For the former case, it is often improved by increasing or increasing the exhaust groove. Taking the most widely used PP material in the country as an example, the overflow gap of the PP material is 0.03 mm. This parameter avoids flash formation. Edge, exhaust slot clearance is 0.01 ~ 0.02mm ideal.
In order to facilitate the cleaning of the cavity in the mold processing and molding process, the position of the exhaust groove will most often be selected on the parting surface of the fixed mold, and it shall be set as far as possible in the final filling of the cavity. For the excessively uneven size of the cavity height In this case, the size of the cavity can only be adjusted by means of 'weld welding' and 'sanding'. This is the most difficult and is most afraid of encountered by the technicians in the debugging process. Its adjustment method and the following will be discussed. The method of adjusting the weld height is always the same.
3. When adjusting the equipment process parameters and opening and unloading slots can not improve or eliminate the quality of the plastic parts due to welding marks, it may be necessary to adjust the size of the cavity. Of course, it is necessary to accurately measure the thickness of the corresponding plastic parts. After the measurement, work within the allowable range of the design.
The wall thickness of bumper parts is not an equivalent value in different parts, but a gradual amount. The reason is that taking into account the specific shape of the plastic parts and the fluidity of the molten plastic, the gradual wall thickness is conducive to molding. Usually 2.60~3.50mm.
Mold cavity size is modified in two cases: One is to increase the cavity size, the other is to reduce the size of the cavity.
For the first case, it is easier to implement. According to the test data, the corresponding parts of the mold cavity can be directly ground. The second case is more complicated. In order to reduce the size of the cavity, the mold cavity needs to be firstly The surface is surfacing and then polished.
Here's how to reduce the size of the mold cavity:
From the degree of difficulty of work, first consider that choosing to perform 'repair welding' and 'polishing' on a moving mold will be much easier than on a fixed mold.
Since the cavity surface quality of the injection mold directly affects the appearance of the plastic parts, a large amount of heat is generated during the welding process. Without sufficient process measures, these heat changes the tissue components of the cavity surface, resulting in a cavity surface. Different hardness, and then affect the appearance of plastic parts, in practice to avoid the modification of the cavity surface of the fixed model.
The general steps for modification of the cavity surface of the dynamic model are as follows:
By applying the mold on the surface of the cavity, the thickness of the cavity needs to be increased or decreased.
Implement 'repair welding' and 'polishing' operations;
Try the mold again, adjust the dimensions of the cavity surface according to the forming effect.
The second step is difficult and critical. The following is a detailed process of cavity surface repair:
Select the welding material that matches the parent metal and determine the welding range. Reserve and protect the grinding standard.
Sub-area alternately surfacing, pay attention not to weld from the end to the end, so as to avoid internal stress caused by mold cavity surface cracks.
Keep abreast of the reserved baseline, and pay attention to the protection of surrounding areas.
Measure the height of the repair welding surface. After meeting the requirements, weld the reference vacancy to complete the modification of the cavity surface.