Adding chill blocks/heatsinks for Aluminium casting

[not-a-number]

I have some thick sections in my casting that are shrinking a lot so I would like to add chill blocks to solidify that section quicker.


Does anyone have any resource on best practice or could advise on a 'rule of thumb' for sizing?

Im guessing the goal of the chill block is to solidify the large section at the same time as the thinner section. If I over sized the chill block could that cause problems?

[labr@]

Chill blocks might help but they might make things worse. It's very dependant on the actual shapes involved. Posting some images of the pattern may help others with providing useful advice.

Have you tried to design a feeding system that will provide liquid metal to the heavy sections until they are solid? If there is room in the box then this may be a better approach to start with.

[not-a-number]

Here are some images. Most of the thicker sections were actually acceptable. The one that really shrunk was the area that was twice as thickas the rest. So I could possibly get away with just fixing this area though fixing all areas would be preferred.
The way I have it now I cant think of a great way to get a feeder to those areas in the middle. Maybe I could run a blind riser/feeder one in the center core.

I had put risers right at the top. I could change that to 4 risers at the thick points.


https://i.imgur.com/rHvxskJ.jpg

https://i.imgur.com/c28qg2o.jpg

https://i.imgur.com/nqAotVZ.jpg

https://i.imgur.com/JQUFAiu.jpg

https://i.imgur.com/UxPjgBG.jpg

https://i.imgur.com/2wggXU0.jpg

[labr@]

Well it's easier to understand why you are thinking of using chill blocks now - there doesn't seem to be any easy way of applying risers to some of those areas.

At the risk of causing more confusion here are a few random thoughts:

- The centre boss has a rough surface but the rest of the casting looks quite smooth. Could there be dross or other material getting in and settling there? I know this is unlikely because the mould and core surfaces look good, there is a strainer and also that is the type of area where you would expect the mould surface to break down due to more heat. However if something is getting in then it could be a contributing factor and it would pay to be sure that the problem is just shrinkage.

- If the centre boss is to be drilled out then perhaps casting the hole will help as there will be a smaller mass of metal to feed. Be aware that sometimes this can backfire if the hole has a small diameter because a narrow core will stay almost as hot as the metal surrounding it and can result in shrinkage at the edge of the hole.

- On a similar track, do the bosses need to be solid? If they are only for support and do not need to be drilled out to accept fasteners then perhaps they could be converted to circular ribs.

- The design does look suitable for using chill blocks. Just be aware that the casting can be prone to small laps on the surface that is against the chill block. The surface there will also be harder but with Al this should not be a problem.

You should also be aware that my experience with Al is very limited and I've been out of the industry for over 20 years so if someone else suggests something that seems logical and practical then go with it!

Good luck and do let us know how you get on.

[not-a-number]

Thanks for the great info and insight.




- The centre boss has a rough surface but the rest of the casting looks quite smooth. Could there be dross or other material getting in and settling there? I know this is unlikely because the mould and core surfaces look good, there is a strainer and also that is the type of area where you would expect the mould surface to break down due to more heat. However if something is getting in then it could be a contributing factor and it would pay to be sure that the problem is just shrinkage.


I dont think its caused by something getting in there. It really looks like its dry and rough because of its shrinking. No thin areas have it though. Could be wrong though.




- If the centre boss is to be drilled out then perhaps casting the hole will help as there will be a smaller mass of metal to feed. Be aware that sometimes this can backfire if the hole has a small diameter because a narrow core will stay almost as hot as the metal surrounding it and can result in shrinkage at the edge of the hole.


Yes this is a good point. I was thinking of doing this as all thick areas are machined out. Though they would still be quite a bit thicker. The change I would have to do to the patterns is not ideal so was looking at other options first. I could probably make a separate 'hole' core and drill out my main core to install it (save me modifying patterns)



- On a similar track, do the bosses need to be solid? If they are only for support and do not need to be drilled out to accept fasteners then perhaps they could be converted to circular ribs.


This could work. There are a few fasteners around the bosses so that sand detail would be quite delicate.



- The design does look suitable for using chill blocks. Just be aware that the casting can be prone to small laps on the surface that is against the chill block. The surface there will also be harder but with Al this should not be a problem.


This is probably fine. Most bosses have machining allowance on the face except the inner top ones that are just cosmetic.



The other tricky thing is I dont have a lot of depth for a chill block. (Unless I make the mould box bigger). So the blocks would need to be big diameter to get enough mass.


Something like this:


https://i.imgur.com/NBmjQjTh.jpg

https://i.imgur.com/broDDtdh.jpg

[not-a-number]

Does the chill block need to be the same volume as the part that it is chilling?

Or is the goal only to decrease the thick area temperature by X amount? So if I have a 50gram boss I may only need a 20gram chill?

Im guessing you dont want to chill that area too fast? Or does it not matter?

[labr@]

I don't know the formula to calculate the ideal chill block size. However it looks like the cooling action needs to be fairly strong to be effective because most of the contraction in the boss needs to happen while the rest of the casting is still liquid so that it can feed more metal into the shrinking area.

The way you show the chill blocks in the previous post looks like a good option.

[not-a-number]

I think I'll try adding risers to the top thick areas and chill blocks to the middle.
For the chill blocks I'll size them to be the same mass as the bigger boss. (so one will be 1:1 mass and the smaller will be 2:1. See what works better)

https://i.imgur.com/GerZHdC.jpg

https://i.imgur.com/e4F8AsD.jpg

[not-a-number]

The chill blocks worked well. The risers worked well on 2 bosses but not perfectly on the bigger bosses. So I could add some chills there too.
The slight defects are on a face with machine allowance so the part is usable.
I plan to do 2 more pours.

https://i.imgur.com/LoQaljT.jpg

https://i.imgur.com/1Jil6dx.jpg

https://i.imgur.com/Q1Moi8q.jpg

https://i.imgur.com/pJWKgLW.jpg

https://i.imgur.com/k4Kr3bp.jpg

https://i.imgur.com/CXqpkAL.jpg