When surface quality of plastic parts cannot meet design requirements due to weld marks, technicians usually start with solution from aspects of melt temperature, injection speed, pressure, flow, and mold temperature. These are all realized by equipment, adjusted in small increments with reference to standard of molding conditions, approaching ideal value.

 

Obviously, items that can be adjusted by equipment such as melt temperature, flow speed, pressure, flow, and mold temperature 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 lies in the fact that when above methods can no longer be solved, it is necessary to modify mold to achieve desired effect, which is also focus of discussion.
There may be following situations where mold needs to be modified:
(1) Air bubbles are trapped in weld line, and vent holes need to be added on corresponding parting surface.
(2) Depth of weld line is always out of tolerance, and thickness of plastic part, that is, mold cavity, needs to be adjusted.
(3) Position of weld mark is biased toward middle of plastic part, and position of gate needs to be adjusted.
Improvement processes for above three situations are described below:
Formation of air bubbles at weld line is due to fact that when the two melts converge, enclosed gas is not removed in time, stays inside plastic part, forming pits on the surface of weld line, which can be colloquially called "gas trapped" .
Reason may be that parting surface of corresponding position is too tightly researched, so that gas cannot be discharged; it may also be caused by excessive uneven height of cavity (wall thickness of plastic part is different) after mold is closed.
In view of the former situation, it is often improved by adding or enlarging exhaust groove. Taking the most widely used PP material in China as an example, according to parameter that overflow gap of PP material is 0.03 mm, in order to avoid flash formation from overflow, exhaust groove gap is 0.01~0.02mm.
In order to facilitate cleaning of cavity during mold processing, position of exhaust groove is usually selected to be opened on parting surface of fixed mold, and try to open at the end of cavity as far as possible. In the case of excessively uneven cavity height, the only way to adjust cavity size is through "re-welding" and "grinding". This is the most difficult, and it is also the most feared encounter by mold technicians during debugging. Adjustment method is same as adjustment method in which weld line height is always out of tolerance to be mentioned below.
When it is impossible to improve or eliminate poor quality of plastic parts due to weld marks by adjusting equipment process parameters and method of opening exhaust groove, it is likely that size of cavity has to be adjusted. Of course, thickness of corresponding part of plastic part needs to be accurately measured. After measurement, work within range allowed by design.
Wall thickness of bumper plastic parts is not equal 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 situations: one is to increase cavity size, and the other is to reduce cavity size.
For the first case, it is easier to implement, and you can directly polish corresponding parts of mold cavity according to inspection data. The second situation is more complicated. In order to achieve purpose of reducing size of cavity, it is first necessary to build-up welding on the surface of mold cavity and then polish it.
Following specifically introduces method of reducing size of mold cavity:
In terms of difficulty of work, it is much easier to choose "repair welding" and "grinding" on movable mold than on fixed mold. Because quality of cavity surface of injection mold directly affects appearance of plastic part, and there is a large amount of heat generated during welding process, when there is no sufficient process guarantee, heat will often change structure of cavity surface, resulting in cavity surface Hardness is different, which affects appearance of plastic part. In practice, it is necessary to avoid modification of cavity surface of fixed model as much as possible.
General steps to modify cavity surface of moving mold are as follows:
(1) Through method of sticking glue on cavity surface, thickness of cavity needs to be increased or decreased roughly.
(2) Implement "repair welding" and "grinding" operations.
(3) Try mold again and adjust size of cavity surface according to molding effect.
The second step is difficulty and key. Following is detailed process of cavity surface repair:
A. Select welding material that matches base metal, determine welding range, reserve and protect polishing benchmark.
B. Alternate surfacing welding in zones. Be careful not to weld from beginning to the end, so as to avoid internal stress causing cracks on mold cavity surface.
C. Start polishing against reserved benchmarks, and pay attention to protecting surrounding parts.
D. Measure height of repair welding surface. After reaching requirements, fill reference vacancy to complete modification of cavity surface.