Why are there dents in extruded profiles?

In the production of extruded profiles, dents are the most frequent adverse phenomena. The volume shrinkage of plastic injected into the mould occurs when it is cooled. The early cooling part, i.e. the surface hardens first, and bubbles will be generated inside. The so-called dents are that the slow cooling part produces conspicuous concave surfaces in the direction of bubble shrinkage.

Large shrinkage materials are also prone to dents. When changing the forming conditions to eliminate dents, the setting conditions should be set in the direction of small shrinkage. That is, the die temperature and the barrel temperature decrease, and the injection pressure increases, but it should be noted that the residual internal stress may be caused by this.

Because the indentation is better not to be conspicuous, if it does not affect the appearance, it is intentionally processed into corrosion patterns on the die, such as grain, granular and so on. Also, if the forming material is HIPS, reducing the mold temperature to reduce the smoothness is also effective. But once these methods have dents, it is difficult to repair polished products.

Solutions for extrusion profiles with dents

Instant: Increase injection pressure, prolong injection holding time, reduce barrel temperature and die temperature, and force cooling in the place where dents occur.

Short-term: Make up the runner where the dent occurs. When there is a narrow area along the edge of the material where the dent occurs, the edge should be thick.

Long-term: The thickness difference of design products should be completely avoided. Reinforcement tendons prone to dents should be as short as possible in narrow and long shapes.