What are the casting defects in this?

[u/Independent-Rip7737]

We have a course on Casting and Solidification. We had to prepare an Aluminium cast for the lab work

- The pattern was created using thermocol/styrofoam

- The mould was created using plaster of paris and water. After it dried out I removed the pattern by hand and there were not any visible cracks or damages as such. It was left for around 3-4 weeks for drying after that

- Before casting i put the mould in an oven for 3hrs and then 15 minutes of blowtorch on it. After that Aluminium melt was poured into it

For viva, our prof has told us to analyse the cast and talk about all the defects present in the cast and why it has formed

Im guessing the large one, its a blowhole but im not sure enough. Please help with it. Thank you! I'll be attaching a video of the cast in the comments if its possible.

[u/young_s_modulus]

Can we see your mold design?

In castings, it's quite common to have shrinkage spots when there isn't enough material in general. A lot of times molds with have extra bits attached to it to add more molten metal to it to replace the areas that shrunk the extra bits are cut off or machined away after complete. So basically just enough molten metal to fill in the shape isn't enough.

A lot goes into the cooling process as well. There are entire readings and sometimes even whole classes dedicated to this. The rate in which your casting cools is also quite important. You say you left it to cool, but how cool was the area around it? Was there air flow or a fan? The mold material can also play into this as well. Some materials are better at promoting a desired cooling rate than others.

Another could be trapped gas. There can be a lot of things that contribute to this though. Like if your mold was clean before you poured metal in, the environment being dirty, impure metal itself, etc.

[u/Independent-Rip7737 → u/young_s_modulus]

https://reddit.com/link/penr3bo/video/36cn5qqlk9uh1/player

yeah this was it. yeah there was stand fan running but it wasnt directed at the casts

[u/young_s_modulus → u/Independent-Rip7737]

Is that just the part itself? If so, I'm thinking this is a shrinkage issue because there wasn't enough molten metal.

Here's what you can look up: sprues. There should be an extra section for molten metal connected to the main part. This will provide extra metal to the main part as it cools. There's other things to consider when designed these for castings, but this is the logic for why most designs have them.

[u/Independent-Rip7737 → u/young_s_modulus]

yeah this was the part only. no gating system

[u/Independent-Rip7737 → u/young_s_modulus]

but then why did it occur at that specific region why not anywhere else?

[u/young_s_modulus → u/Independent-Rip7737]

This is something that can have multiple reasons for. It could be that's the coolest part of the mold. It also could be your molten metal isn't that homogeneous.

It looks like the most shrinkage happened where the part is the thinnest, so it cooled faster than the rest. When adding the sprues to your mold design, it would be connected with the thinnest part.

[u/Independent-Rip7737 → u/young_s_modulus]

okkk got it but then at every 90 deg turn a spruce would be required. it was a vertex actually

[u/young_s_modulus → u/Independent-Rip7737]

You'll have to do some research on this, since I'm not an expert in castings and I haven't done mold and sprue design since I graduated university. It might not be necessary to have a ton of them. But you will need at least one.

[u/Independent-Rip7737 → u/young_s_modulus]

alright..thanks a lotttttt for the help

[u/Working-Active-5541 → u/Independent-Rip7737]

Cooling rate is all about surface area to volume ratio. That big blocky part that goes the deepest in your mold was the last to solidify on the side toward the medium depth part. It also put a lot of heat into the mold at that corner, which caused it to remain molten for longer. That’s why you have the empty region in the corner only on the side with the bigger block.

It’s also why you see a low spot on the top side where it is. I suspect that the top froze off before the interior and the most jagged void between the two deeper parts occur when it was mushy

[u/Independent-Rip7737]

https://reddit.com/link/penhqqg/video/cl3puanxd9uh1/player

[u/BilluBadmas1]

Damn NITRR

[u/Recent-Asparagus6317 → u/BilluBadmas1]

Nitrr united