The surface defects of the moldings include the effect of a gramophone record. In this case, concentric or parallel grooves are formed on the surface of the molded part. They are mainly the result of a too fast cooling rate of the melt stream front resulting from too low mass temperature, too low injection speed or too low temperature of the mold walls. In order to eliminate this disadvantage, it is necessary to prevent excessive cooling of the stream front and to modify the appropriate injection parameters and to avoid small cross-sections of the flowing stream .
On the surface of the molded part, as a result of too rapid cooling of the melt stream front caused by too low injection speed, concentric or parallel
grooves. They resemble grooves on a gramophone record and hence the name of this defect in moldings.
How is the effect of a gramophone record created?
It is the result of:
– mass temperature too low,
– too low injection speed,
– too low temperature of the tool wall.
During injection of the plastic mass into the cold tool, due to the high cooling rate behind the front of the plastic stream in the mold, the solidified boundary layer is quickly formed. It also contributes to the cooling and solidification of the jet front areas close to the tool walls. When filling the mold with rapid cooling (especially at low injection rates), high-viscosity or solidified areas of the jet front obstruct the direct flow of the plastic melt towards the wall. As a result, the hot melt does not penetrate into the mold wall and the central area of the jet front extends.
Under the pressure pressure, the jet front flows back to the wall of the tool. The strongly cooled edge areas of the material stream front do not come into contact with the walls of the tool over the entire surface. Instead of mapping the surface of the mold on the surface of the moldings, there are grooves that appeared in the flowing face of the alloy. The effect of a gramophone record is also favored by the periodic stopping of the melt flowing through undercuts, etc.
In order to eliminate this surface defect of moldings, the presented step-by-step diagram should be analyzed.
The article was prepared on the basis of materials from the Dopak Research and Development Center and the KraussMaffei company.
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