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Cast Iron Shrinkage Defect

The cast iron shrinkage has very large affect to the quality of iron castings. It will reduce the tensile strength and elongation, so the iron castings will be broken or penetrated at the shrinkage area.

According to my working experience in Dandong Foundry in China, there are three types of shrinkage.

1. sink on the surface

On the surface of rough castings, you could see the sunk pit. These pits are caused by the shrinkage beneath the surfaces. Just like the following photo. This kind of shrinkage will affect the dimensions of this area, and affect its physical properties. Usually, this kind of shrinkage is unacceptable.

2. large shrinkage holes inside of cast iron

You can not see this shrinkage on the surfaces, but after machining, it will be very obvious, you could see many small and large air holes in the shrinkage area. Of course, this kind of shrinkage is acceptable and very harmful to the application.

3. small shrinkage inside of cast iron

Sometimes, after machining, you could see very fine air holes, or sometimes, you can not see them by eyes, but can inspect them by magnifying glass. Or, sometimes, you could fine them by water or oil pressure testing. These shrinkage caused the low density to the cast iron, so they can not bear high pressure.

Obviously, the shrinkage is very harmful to the iron castings. The main reason of shrinkage is the unreasonable casting process, especially the gating sizes. After find the shrinkage, the iron foundries should adjust their gating system.

However, since most of shrinkage defects are beneath the surfaces of cast iron, how can we inspect them?

1. Normally, you could allow your supplier to do the rough machining, so they will find the shrinkage after their machining. This method is the most obvious and economic method since most of iron foundries in China have the rough machining capability in-house.

2. As for the pump bodies or valve bodies, you could do the water pressure test or ask your supplier to random test them. Although this method can not guarantee all castings are qualified, at least, it can affect some problems.

3. You could cut off the sensitive positions to check the shrinkage. According to our experiences, the shrinkage usually happen at the pouring gate area and some corner areas. So, after you find the shrinkage positions, you could inspect these positions in your batch inspection. You could just cut off these positions by sawing machine or require the supplier to cut off these positions with potential shrinkage according to your instruction.

 

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