View Full Version : Pattern and Shrinkage for Cast Aluminum

Paul Alciatore
05-02-2005, 03:35 AM
Here's one for those of you who have casting experience.

I want to make an aluminum part in some quantity but need to make a mock up first. We are talking about something about the size of a fist or two; perhaps 1.5" to 2" in two dimensions and about 3 or 4 in the third. It will probably have hollows at both ends.

I don't plan to do the casting myself but rather will look for someone to do the casting. As for the mock-up, I figure a plastic part will do and that I can do myself.

So if I am going to make one or two plastic parts and then send it to a foundry for the production run, is it possible to do this with a single pattern? I know I need to allow for about 1 to 2 percent shrinkage for the plastic mock-up. Will that same amount be OK for aluminum? Will the same pattern even work for both? Or am I just completely off track?

And what would be a good material to make the master pattern with?

I would appreciate any comments.


Paul A.

[This message has been edited by Paul Alciatore (edited 05-02-2005).]

05-02-2005, 06:01 AM
Do a search for Dave Opincarne in the archives and email him,he's the expert.

05-02-2005, 07:54 AM
Paul, Working on an A36 casting, foundry said 1% my partner said shouldn't that be 2%? Second go around for the 9 piece run, 1.70%,that is volumetric, X,Y & Z.
Les H.

05-02-2005, 08:05 AM
I've heard 3/16" per foot shrinkage allowance for aluminum, which works out to about 1.5%. But I know diddly-squat about patternmaking and casting....

05-02-2005, 08:45 AM
Shrinkage for Alum suould be .014/.020 per inch.

05-02-2005, 11:00 AM
At the foundry I worked at, ours varied from 1.5% to 2.6%, as I recall, depending upon the shape of the final casting and how critical the dimensons were.

05-02-2005, 01:31 PM

Maybe some information there to help?

Paul Alciatore
05-02-2005, 04:50 PM
Interesting site. Thanks.

Paul A.

<font face="Verdana, Arial" size="2">Originally posted by QSIMDO:

Maybe some information there to help?</font>

05-02-2005, 06:19 PM
~5/16 per foot

05-02-2005, 08:29 PM
Am I going to have to recall my 1913 Buick water pumps? http://bbs.homeshopmachinist.net//frown.gif

Could the material (alloy) used make a difference? I used old aluminum engine block alloy. I seem to recall allowing 1%.

Dave Opincarne
05-02-2005, 08:42 PM
YIKES! 5/16 per Ft is way too much, that's closer to manganese steel which shrinks a lot. Generaly 1/8" is a good shrink for Al. A 1/10 shrink rule will work too. To determine the shrink factor divide the shrink scale by 12, so for Al it would be 12.125/12=1.0104, Multiply the desired outcome by this and you'll get the common rule diminsion needed for the pattern. Or go on e-bay and see if anyone has a shrink rule.

It's best to talk with the foundry since their are variables on their end. The shape of the part will play a factor as well. Any critical diminsions should be cast with machine or grind stock added and worked apropriatly.

I'm making some job and schedule changes at the moment but if anyone has any pattern questions feel free to contact me. If you have a general question post it here and let me know so that everyone can chip in and benifit.


05-02-2005, 10:55 PM
Paul, have you worked with this foundry before?
The one I work with wants the patterna on a match plate, including stubs for the sprue, riser, and also the gating. They educated me
about the rules (for iron). You didn't mention
whether the casting has uniform thickness, but from your description, i suspect it doesn't. You will want a riset to feed the thick part, or you could get defects.

Bottom line, talk to someone at the foundry, and ask them how to make the pattern.

BTW, I have had good results machining patterns from PVC. It glues well, machines well, and is durable. You will find foundries treat patterns a bit roughly, so durability is good.


05-02-2005, 11:02 PM
I generally use 1/8 to 3/16.

This number is verified by a bonified PhD.

lol...like that means anything http://bbs.homeshopmachinist.net//smile.gif


05-02-2005, 11:03 PM

Sorry off topic. Where do you get your PVC? I ordered a couple 1" Dia., 12" long bars from McMaster-Car to experiment with. They didn't seem to have a very large selection of shapes.

Thanks Mike

Paul Alciatore
05-03-2005, 09:20 AM
Thanks to all. I do not have a design yet and was looking for general info at this point. Want to have my head on straight when I do it. And I do plan to allow extra for machinning.

I do need to find a foundry so, no I haven't worked with one before. There is one locally but I doubt that they will be willing to do small batches. Any suggestions on one that will take on small lots and I can work with via telephone?

Paul A.

05-03-2005, 06:46 PM
Maybe you should join the group of foundry owners here. I built mine out of an old BBQ and it melts aluminum pretty easily. I would build a box and get some casting sand. I don't think I would recommend a cope and drag anymore, because I like lost foam casting where the model is sacrificed but you don't have to remove the model. I found that part of the process difficult and some shapes aren't easy to remove without the sand falling in. The foam is the easiest stuff to carve and cut. Once you've cast a piece or two you can adjust your next model for the shrinkage and get pretty good dimensional accuracy and a reasonable finish for the unmachined surfaces. There is a learning curve, but it is a very satisfying hobby. That amount of metal can be melted in a cast iron pot brought up to a dull red heat. Degass, skim and pour.


05-03-2005, 09:11 PM

Most of my pvc comes from a local plastics business. They sell cut pieces and sell the drop for $1 a pound.

I also get rounds from ENCO.