There are a lot of things that can go wrong.
Here are some problems I have had recently.
Sand Too Wet
When the metal was poured, it splattered back up the sprue and on to the sand surrounding the sprue
hole as shown in the first photo. The second photo shows the casting, a bottle opener. You can see that
excess steam from the wet sand prevented the metal from completely filling the mold before it set up.
The metal was not hot enough when it was poured and "froze up" before it was able to fill the mold cavity.
As liquid metal cools, it shrinks. If there is not enough liquid metal to supply some part of the casting, a depression or cavity forms. Do you see the shallow depression near the center of the square section shown below? You can see more about this in the project section under Trailer Hitch Covers.
"When splattering occurs during pouring, solid globules of the metal are formed that become entrapped in the casting. Poring procedures and gating system designs that avoid splattering can prevent these defects." Source: http://me.emu.edu.tr/me364/2.pdf
My sand was too wet and I did notice a lot of splattering when I poured. This is the first time I ever had this happen.
I think this is the correct term for this.
Basically the sand in the pattern protruding from the cope broke off into the
drag. There are several possible causes for this. Most likely I rocked the cope
as I was lifting it which broke offthe deeper parts of the pattern.
The sand was a bit wet. You can see it stuck not only at each end but also around the edges of the pattern and flask. In this case I'm sure there was plenty of parting
dust between the cope and the drag. I finally used a "gagger" to add strength to this section.
A gagger is basically a piece of rusty wire, nail etc. or wood that is embedded in the mold to help reinforce a protruding section such as this. In this case I fixed the problem with a large rusty eye-bolt embedded in the sand that extended from the deep section in the drag up into the cope. As you can see in the last photo, the section lifted cleanly.
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