A New Method of Measuring Green and Fired Permeability of Investment Casting Shells
A new method of measuring permeability has been developed using a flat plate instead of the traditional Ping-Pong ball specimen. Details of the test are given. It is possible to measure both green and fired permeability on the same sample. Data are presented for fiber enhanced and traditional non-fiber slurries. Fiber containing slurries are more permeable in both green and fired states. Like the ping pong ball test, there is a large amount of sample to sample variation in all slurries tested.
Foundries have long known the importance of having a shell that has sufficient permeability to allow air to penetrate the shell during the pouring of metal rather than remain trapped in the metal or at the metal/mold interface. More recently the issue of shell permeability in the green state is being considered as an important factor in the ability of a shell to dewax without cracking. The traditional method of producing a shell sample for measuring fired permeability is to insert and seal a hollow rod into a ping pong ball and then invest the ball and a few inches of the tube. After final dry, the invested ball and rod are placed into a furnace and the ball is burned out. Permeability can then be measured after cooling.