Question

How does the Holding Pressure reduce the material shrinkage in Injection Molding process? Explain this in terms of the internal structure of material

Answer 1

Sink marks are small craters or depressions that develop in thicker areas of the injection molded prototype when shrinkage occurs in the inner portions of the finished product. The effect is somewhat similar to sinkholes in topography, but caused by shrinkage rather than erosion.

Sink marks are often caused when the cooling time or the cooling mechanism is insufficient for the plastic to fully cool and cure while in the mold. They can also be caused by inadequate pressure in the cavity, or by an excessive temperature at the gate. All else being equal, thick sections of the injection molded part take longer to cool than thin ones and so are more likely to be where sink marks are located.

Increase injection speeds and pressure to the optimal level, which will ensure the cavities are filled properly (while not allowing the molten plastic time to start cooling in the wrong spot). The temperature of the molten plastic or the mold itself can also be elevated to ensure the plastic does not cool down sufficiently to cause the defect.

In micro- and thin-wall injection moulding the process conditions affect the developed internal structures and thus the resulting part properties. This paper investigates exemplarily on polyamide 66 the interactions of different cooling conditions on the morphological and crystalline structures. The investigations reveal that a slow cooling rate of the melt results in a homogeneous morphology and a higher degree of crystallinity and also a favoured crystalline structure. Consequently, the dielectric behaviour and light transmitting part properties are affected.