A New Method of Measuring Green and Fired Permeability of Investment Casting Shells
Abstract
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.
Introduction
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.
The above method may be acceptable for relating to hot permeability at pour, but can not be used to investigate autoclave dewax issues because of the ball inside the shell. Buntrock Industries Technology Lab undertook the task of developing a green (prior to dewax) permeability test. Objectives were to make the test easy to do, inexpensive, and repeatable. The test is described below.
Permeability Test Description
Purpose: To determine the green and fired permeability of an investment casting shell specimen.
Wax Specimen: 2” wide x ¼” thick x 5” long. Do not clean or etch wax. Depending on polymer used in the first dip, additional release agent may be needed. The idea is for the first dip NOT to stick to the wax as the shell will be removed from the wax and not autoclaved or flash fired to remove wax. Download and See Figure 1.
Dipping: Depends on purpose of test, but shell thickness is about ¼” – 1/3”. Scrape edges between backup dips to make shell grinding easier. Download and See Figure 2.
Shell Prep: After final dry, grind edges and carefully remove shell from wax. Cut into 4 samples roughly 2” x 2” x ¼”. A ¾” plastic 90 degree elbow fitting is then sealed to the face coat side of the shell. Sticky wax is used to seal the fitting to the shell. The entire face coat and also the edges of the sample are sealed to ensure airflow is through the shell. Download and See figure 3.
Apparatus: Download and See Figure 4.
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Air compressor with dryer. Any small air compressor will work.
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Air pressure regulator capable of reducing pressure to about 10 psi.
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U-tube Manometer 50” length.
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Flow meter to measure air flow (20 – 800 ml/min.)
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Flow control needle valve.
