Do I need to account for the shrinkage in a mold design for casting aluminum? - Page 1
Forum section: electronics / general

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 26, 2023, 12:44:01 pm
content: Beginner quesiton, but does the shrinkage need to be accounted for in the mold? I would think that it couldn't possibly shrink since the mold is in the way, does it result in increase porosity because of that? Or do I need to account for the shrinking, if so, in what way? just increase the size of the mold by some factor evenly?

author: jmelson
author_rank: Super Contributor
timestamp: July 26, 2023, 03:06:23 pm
content: Quote from: ELS122 on July 26, 2023, 12:44:01 pmFirst, I'm not an expert. But, I have worked with some guys who are. It depends on the mold. If this is a sand-cast mold, then the mold is soft. The aluminum when poured into the cold mold with freeze a "skin" against the mold, then the molten aluminum in the center of the mold features will contract as it cools. You can watch this process as the level of aluminum in the risers will sink as it cools. The cooling aluminum will crush the sand mold somewhat differentially, depending on thick/thin areas and so distort the shape. I think common practice is to make the mold 10% larger for aluminum, and different for other metals. If you are using a rigid mold, then you have to put in draft angles so the part can be pulled out of the mold. Cast aluminum is always porous, that's what wrought aluminum is all about. The rolling process compresses all the porosity out of it.Jon

author: jpanhalt
author_rank: Super Contributor
timestamp: July 26, 2023, 03:22:11 pm
content: Since it is likely a low-volume casting, I suspect it will be a sand mold. You need draft angles for those too, or you can't remove the model without disturbing the sand.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 26, 2023, 03:35:51 pm
content: Different materials will shrink by different amounts, e.g. aluminium will shrink more than steel.This is typically taken into account when making the pattern. Pattern makers traditionally used to use a special ruler that had measurement units that took the scaling into account. Obviously CAD software has features that can scale appropriately these days.I believe it's actually all a bit more complicated than that if you dig in to the details, though. Things like different shrinkage rates according to features of the part, etc. Such things are beyond my understanding.

author: Bicurico
author_rank: Super Contributor
timestamp: July 26, 2023, 04:01:48 pm
content: Yes, shrinkage will definitely be an issue! The metal will contract for two reasons: due to the cooling and, mainly, due to phase changes during solidification.In order to determine which phases will be present during the process, you use equilibrium diagrams that match the composition of your alloy.For a given alloy, you will be able to consult literature for the specific shrinkage.However, it is not as simple as just scaling the CAD model accordingly! Because the shrinkage will not be uniform, as the temperature gradient within the mold is different, the last liquid metal to solidify won't be fed further liquid metal to compensate the shrinkage, forming a defect. To avoid this, you need to use feeders that solidify last and provide liquid metal to the part you want to obtain. For this to work, you need to determine how many gates, feeders, including their volume and position, you need. This is done through archaic methods, experience or through simulation software like ProCast, SolidCast, MagamaSoft, etc.Finally, modelling a part for foundry needs to take into account the draft angle and excess thickness for finishing, normally milling faces, rectifying cavities and drilling holes.If you have no idea what you are doing, consult a metallurgical engineer or a foundry.Aluminium can be cast in low pressure (sand or metallic molds) or high pressure (metallic molds - similar to plastic injection). Both processes are quite distinct and some parts are only suitable for one of both processes.Regards,Vitor