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Unique Method to Reduce Gate Porosity in Investment Castings

It is common to expect shrinkage under investment casting gates or gate porosity due to solidification of gates, before the solidification of the casting part. There can be numerous reasons for this- poorly sized gates, poor gate placement, high pouring temperature among others. Foundries typically try to increase yield by reducing the sprue and gate sizes. However, the reduced cross-section leads to faster solidification of the gate. The part being fed by the gate gets starved of molten metal very close to its final solidification. This shows up as shrinkage porosity just under the gates. After machining, very small imperfections are seen all over the top surface of the casting, resulting from the shortage of molten metal.

It is known that if the silicon content of the casting is increased (within acceptable range), it aids in more uniform solidification. It is observed that Mild Steel scrap, commonly procured, has low silicon content. When ferro silicon addition was done to increase the silicon content, there was improvement in the solidified layer. However, there remained some gate porosity if gate size was not increased substantially.

The challenge is to keep the molten metal in the gate and runner, in the molten state, for as long as possible. Some literature suggests that insulating the gates can improve the outcome. However, the method is unclear. At Rajeev and Company’s Investment Casting foundry, a novel method was attempted. After pouring molten metal into the shells, the shells were covered with the surrounding sand in the sand tray. It was ensured that the shells were exposed to air while pouring, to allow the trapped air in the shells to exit through the natural pores. Thereafter, the shells were covered in sand and left undisturbed. The extra insulation that the sand offered, allowed the solidification of the metal to progress at a slower rate. This achieved the objective, namely, keeping the gate and runner in the molten state for a longer period. The castings thus manufactured had absolutely no gate porosity.