What are the causes of excessive burrs on injection molded products?

Excessive burrs usually occur on the parting surface, i.e. between the moving mold and the static mold, the sliding part of the slider, the gap of the insert, the hole of the ejector rod, etc., forming excess melt on the injection molded product. Such a burr, which will be pried during the molding process, will further increase the burr, thus causing a partial depression of the mold, and the burr forms an expanding vicious circle. Therefore, if a flash point burr is found for the first time, the mold must be trimmed as soon as possible. Excessive burr is the main cause.
1. Insufficient clamping force
If the clamping force is small compared to the projected area of the molded part, a gap will appear between the moving mold and the static mold under the injection pressure, so that a flying burr is bound to appear. Especially when the side gate is placed on the hole near the center of the workpiece, the burr is very likely to occur because the molded gate requires a large injection pressure. Reducing the injection pressure or increasing the clamping force can eliminate this defect. It is also sometimes very effective if the plasticity is changed to low pressure molding according to the specific situation.
2. Poor mold fit
The static clamping problem of the hydrostatic mold and the hydrostatic die is discussed first. Although the mold itself is clamped tightly, in the case of injection molding machines with toggle clamping mechanism, it is often caused by poor parallelism of the mold or poor adjustment of the clamping device. For example, the left and right sides of the pattern are not. The phenomenon of balance, i.e., only the left and right sides are locked, while the other side is not closely connected. In this case, the tie rods (two or four pull tabs) must be adjusted to balance the secondary spreading, followed by the matching of the mold itself. Poor contact can cause slack. Especially when there is a mold hole in the center of the workpiece, due to the support of this part, when the clamping force is insufficient, burrs are easily produced.
In addition, it is a sliding core. Since the drag core is an action mechanism, burrs are often generated, so the fit of the sliding core is important. Especially for the left-right type haf pattern, the projected area on the side is also affected by the molding pressure. If the design cannot fully withstand this pressure, burrs will often occur. Regarding the insert gap and ejector hole, not only does a vicious cycle of burr increase, but also the ejector resistance increases.
3. Caused by mold deflection
If the mold is insufficient, offset will occur due to injection pressure. If there is a hole near the center, burrs will be generated at the periphery of the hole. When the center hole is opened, the hole and runner will also be produced. Burr. Burrs from this cause are difficult to repair because they are caused by improperly made molds. By using reinforced molds, the burr can be reduced.
4. The flowability of plastic is too good
This is simply because the plastic flows too well, and in theory, it is not the cause of burrs. However, when the plastic flows too well, even small gaps of plastic can be drilled out, so it is easy to produce burrs. To eliminate this burr, lower the melt temperature or injection pressure, and reduce the mold temperature or injection speed.
5. Injecting too much melt
This is not a direct cause of failure. In order to prevent shrinkage, it is wrong to inject too much melt and should be formed by increasing the injection time or holding time (refer to shrinkage).
6. Foreign matter on the mold surface
Foreign matter on the mold closing surface will inevitably produce burrs. To solve this problem, it is obvious that the surface of the mesh mold should be cleaned to make a tight fit on the mold surface.

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