casting defects 1

[durablack2]

Can anyone identify the cause of the bumpy surface seen on these cast parts?

They are made of ductile iron and made via shell mold process. The shaft diameter is about 5/8".

Is this cold shot? Or is this referred to as something else? Why would one part come out with this bumpy surface and another not (assuming they were made at the exact same time on a bed of molds).

 

[Dhurjati Sen]

Dear durablack2,

Did you check the shell mold?

Did you check the ladle analysis?

Regards.

DHURJATI SEN

 

[Compositepro]

I had to look-up what the "shell-mold casting process" is. The problem solving logic is pretty simple though. Those bumps can only exist if the shell mold has pits in the surface. Pits can only exist if:

1. The pattern originally had bumps on the surface (not likely).

2. Bits of sand adhered to the pattern when it was removed, leaving pits in the mold surface. If these patterns are then not cleaned properly, you will then get problem #1.

3. The sand/resin mixture spalls from the surface due to out-gassing when the molten metal is poured into the mold. This could be due to poor mixing of the sand and resin. Resin rich spots will be less porous and not allow adequate release of the off-gassing.

4. Too much moisture in the sand. Is there a possibility of the molds getting contacted by water spray. Even workers dripping sweat onto the mold at some point (though this seems unlkely to create the observed pattern of bumps.

5. The sand/resin mixture did not flow and compact well when coating the pattern, thus leaving pits in the mold surface, or voids just under the mold surface that collapse when metal is poured.

6. Too much lubricant on the pattern, which then gets deposited on the mold surface.

 

[EdStainless]

These are cosmetic defects.
They do not impact the strength of the part, just the look and feel.
#3 and #5 are the most likely, along with pouring too hot.

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P.E. Metallurgy, Plymouth Tube

 

[durablack2]

Thanks for all the info I believe #3 and 5# are probably the most likely causes too. These products are outsourced to a vendor overseas and I have never been to the foundry so I'm not sure what the working environments are like.

I should also mention these were prototypes so I was expecting much better results. Should hear back from the vendor soon with an explanation as to why exactly this occurred. I will keep everyone posted.

 

[Tmoose]

"I was expecting much better results."

In your purchase specifications were any quality standards included ? Like ASTM A802 or MSS SP-55, even though they officially are for steel castings.