It is not possible to prevent the formation of shrink holes completely. However, it is possible to shift the shrink holes to the areas outside the cast piece by means of casting- and foundry-related measures, the correct feedstock and various aids.
An overview of the key measures for preventing shrink holes
Ensuring directional solidification from the part of the cast piece that is furthest away from the ingate to the place where solidification takes place last, so that the formation of shrink holes is shifted to areas outside the cast piece or to the gating and feeding system.
- Consistently using feeding technology in casting materials with small solidification intervals, especially in eutectic alloys.
- Calculating the risers (e.g. Heuver's circle method) and applying the methods of simulating mold filling and solidification
- Using insulating and exothermic feeder sleeves
- Increasing local heat dissipation by creating chill molds or attaching cooling fins or by inserting cooling coils (interior cooling)
- Preventing strong differences in wall thickness and abrupt transitions from thick to thin cross-sections. Radii in particular should be sufficiently large. The direction of solidification should already be taken into account when designing the cast pieces
- Reducing the total volume deficit by reducing the casting temperature
- Changing the cooling speed by using mold materials with a good thermal diffusion coefficient
- If possible, changing the composition of the alloy in the direction of a smaller total volume deficit. In the case of cast iron, for example, chromo-saturation level 1 should be aimed at.
- Using molds that are compacted in a sufficiently firm way; in the case of cast iron with nodular graphite, feeding can possibly be foregone completely (pressure regulation method)
- Optimum inoculation
- Considering the superheating temperature and duration of the melt since this changes the nucleation state and therefore the morphology of solidification.
- Correctly positioning and dimensioning the gates; they should be placed where the hottest material is to flow through, i.e. where the cross-section of the mold must be heated.
- Selecting large gates so that the melt that flows back into the cast body when the volume contracts can flow back during the expansion. Placing the gates as far up as possible and thereby ensuring short paths to the gate.
- Aiming at high hydrostatic pressure for additional feeding
- In the case of aluminum casting, the effectiveness of grain refinement and refinement must be checked; using a different alloy, e.g. Al-Cu alloys, can also provide a remedy.
- Reducing the ingot mold temperatures; the same applies to the mold temperature in pressure die-casting.
Source: "Guss- und Gefügefehler" (casting and structural defects) by Stephan Hasse