When surface quality of plastic parts fails to meet design requirements due to weld lines, technicians usually start with melt temperature, injection speed, pressure, flow, mold temperature and other aspects. And these are all realized by equipment, with small adjustments according to molding condition standards, approximating ideal value.

 

Obviously, items such as melt temperature, flow rate, pressure, flow rate, and mold temperature that can be adjusted by equipment are relatively easy to achieve. On the one hand, it is convenient to adjust, and on the other hand, it can be repeated many times. Difficulty is that when above methods cannot be solved, desired effect has to be achieved by modifying mold, which is also focus of discussion.
There may be the following situations when mold needs to be modified:
There are air bubbles in weld line, and it is necessary to add vent holes on corresponding parting surface.
Depth of weld line is always out of tolerance, and it is necessary to adjust thickness of plastic part, that is, cavity.
Position of weld line is biased towards middle of plastic part, and position of gate needs to be adjusted.
Improvement processes for above three situations are described below:
Formation of bubbles at weld line is due to fact that when the two melts converge, enclosed gas is not removed in time, but remains inside plastic part, forming a pit on the surface of weld line, which can be colloquially called "trapped gas" .
Reason may be that parting surface of corresponding position is too tight, so that gas cannot be discharged; it may also be that height of cavity is too uneven (wall thickness of plastic parts varies greatly) after mold is closed.
For former situation, it is often improved by adding or increasing exhaust groove. Taking the most widely used PP material in China as an example, according to parameter of PP material's overflow gap of 0.03 mm, in order to prevent overflow from forming flying side, ideal gap of exhaust groove is 0.01~0.02mm.
In order to facilitate cleaning of cavity during mold processing and molding process, position of exhaust groove will be selected to be opened on parting surface of fixed mold in most cases, and it should be opened at last filled part of cavity as much as possible.
For case where height of cavity is too uneven, cavity size can only be adjusted by "repair welding" and "grinding", which is the most difficult thing for mold technicians to encounter during debugging. Adjustment method is same as the adjustment method that weld line height is always out of tolerance mentioned below.
When it is impossible to improve or eliminate poor quality of plastic parts due to weld lines by adjusting process parameters of equipment and opening venting grooves, size of cavity may have to be adjusted. Of course, it is necessary to accurately measure thickness of corresponding part of plastic part, and then work within range allowed by design.
Wall thickness of bumper plastic parts is not an equal value in different parts, but a gradual amount. Reason is that considering specific shape of plastic part and fluidity of molten plastic, gradual wall thickness is conducive to molding. Wall thickness is usually 2.60~3.50mm.
Modification of mold cavity size is divided into two cases: one is to increase size of cavity, and the other is to reduce size of cavity.
For the first case, it is easier to implement, and it is enough to directly grind corresponding parts of mold cavity according to detection data. The second situation is more complicated. In order to achieve purpose of reducing size of cavity, it is first necessary to weld surface of mold cavity and then grind it.
Following is a detailed introduction to method of reducing size of mold cavity:
From degree of difficulty of work, first consider that it is much easier to choose "repair welding" and "grinding" on movable mold than on fixed mold. Since quality of cavity surface of fixed mold of injection mold directly affects appearance of plastic part, and a large amount of heat is generated during welding process. Without sufficient technological measures to ensure, these heat often change structure of cavity surface, resulting in different hardness of cavity surface, which in turn affects appearance of plastic part. In practice, modification of cavity surface of fixed mold should be avoided as much as possible.
General steps for modifying cavity surface of moving mold are as follows:
Through method of applying glue on cavity surface, thickness of cavity needs to be increased or decreased roughly.
Carry out "repair welding" and "grinding" operations.
Test mold again, and adjust size of cavity surface according to molding effect.
Second step is difficulty and key. Following is detailed process of repairing cavity surface:
Select welding material that matches base metal, determine welding range, reserve and protect grinding reference.
Alternately surfacing by partition, be careful not to weld from beginning to the end, so as to avoid cracking of mold cavity surface caused by internal stress.
Start grinding against reserved benchmark, and pay attention to protection of surrounding parts.
Measure height of repair welding surface. After meeting requirements, fill reference vacancy to complete modification of cavity surface.