Most of the burr occurs on the parting surface, that is, between the moving die and the static die, the sliding part of the sliding block, the gap of the insert, and the hole of the ejector pin, etc., flowing into the molten material, forming the redundant flash burr on the workpiece.
Such flash and burr will play a lever role in forming, which will further increase the flash burr, resulting in the local depression of the mold, forming a vicious cycle of increasing flash burr. Therefore, if the flash burr is found at the beginning, the mold must be trimmed as soon as possible.
There are several reasons for burr
1. Insufficient clamping force
Compared with the projected area of the molded part, if the clamping force is small, there will be a gap between the dynamic and static mold due to the injection pressure, which will inevitably lead to flash burr. Especially when the side gate is placed on the hole near the center of the part, it is easy to appear burr because of the higher injection pressure required by the molding gate.
This defect can be eliminated by reducing the injection pressure or increasing the clamping force. If the plastic with good fluidity is used for low pressure molding according to the specific situation, sometimes it is very effective.
2. The local fit of the mould is not tight
First of all, the problem of loose clamping of dynamic and static dies is discussed. Although the mold itself is tight, when the injection molding machine with elbow bar type mold locking mechanism is used, the phenomenon such as uneven clamping of left and right sides occurs due to poor mold parallelism or poor adjustment of mold locking device, that is, only one side of the left and right sides are locked, and the other side is not tightly attached. At this time, the pull rods (two or two) must be adjusted Four roots are used to make it evenly spread.
Secondly, there are also due to the poor research and matching of the mold itself, resulting in loose adhesion. Especially when there are forming holes in the center of the part, burr will appear when the clamping force is not enough because of the supporting function of this part.
In addition, the sliding core, because the moving core is an action mechanism, often produces burr, so the matching of sliding core is very important. Especially for the left and right parting of the Haff die, the projection area of the side is also affected by the molding pressure, if the design can not fully bear the pressure, there will be burr. As for the insert gap and ejector pin hole, not only the burr vicious cycle increases, but also the ejection resistance increases.
3. Caused by mold deflection
If the original degree of the mold is insufficient, it will be flexed by the injection pressure. At this time, if there is a hole near the center, the burr will be generated around the hole; and when the center hole is used to open the side gate, the burr will also be generated around the hole and runner. The burr caused by this reason is caused by poor mold making, so it is difficult to repair. The burr can be reduced by adopting reinforcing die.
4. The fluidity of plastics is too good
In theory, it is not the reason of burr because of the good fluidity of plastic. However, when the plastic fluidity is too good, even the tiny gap plastic can also drill in, so it is easy to burr. To eliminate this kind of burr, the melt temperature or injection pressure can be reduced, and the mold temperature or injection speed can also be reduced,
5. Too much molten material is injected
This is not the direct cause of burr. In order to prevent shrinkage cavity, it is wrong to inject too much molten material. It is necessary to increase injection time or pressure holding time (refer to shrinkage cavity).
6. There are foreign matters on the mold surface
There are foreign matters on the die closing surface, which will inevitably produce burr. To solve this problem, it is necessary to clean the mold surface and make the die closing surface close to each other.
