Laying out rectangular cores on foundry pattern

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Greg_Lewis
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Joined: Wed Jan 15, 2003 2:44 pm
Location: Fresno, CA

Re: Laying out rectangular cores on foundry pattern

Post by Greg_Lewis » Mon Jan 13, 2020 11:51 pm

FKreider wrote:
Mon Jan 13, 2020 1:21 pm
....

That being said; the volume/mass of a part has an effect on how the molten metal cools and in reality it might not shrink uniformly.
Been there. I strongly suggest a chat with the foundry that will be pouring the metal. They should have a good idea as to how this will work out.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

K. Brouwers
Posts: 63
Joined: Wed Mar 21, 2007 11:57 am
Location: Philippines

Re: Laying out rectangular cores on foundry pattern

Post by K. Brouwers » Tue Jan 14, 2020 8:21 am

Hello,
I think you will find a draft of only 1.5 degrees is not enough for sand casting. Your core boxes might be do able at that angle but you might need to use investment cores with sawdust or flour mixed in to assist in getting them out of the finished casting as the sawdust will burn upon pouring the metal and weaken the investment. Putting wires into your cores will also help them keep their shape during handling and casting. It is best when designing your patterns to try keep the wall thicknesses constant as mentioned above, sometimes this is not possible and risers might be necessary to act as reservoirs for the molten metal to fill the thicker parts of the castings. if its only in one area you could gate the part there and use a large pouring riser to hold the extra metal to feed the casting while it is cooling. Are you making a match plate for your castings or loose patterns? Could you put a few photos up of what you are doing. We all love photos. :)
Happy pattern making
Karel

Mike Walsh
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Location: St. Louis, MO

Re: Laying out rectangular cores on foundry pattern

Post by Mike Walsh » Tue Jan 14, 2020 10:28 am

Gating and Risering should be left to the foundry's discretion.

I have experienced core prints with a more severe draft angle. Say 10 degrees. This allows for more play when setting cores. The tighter the draft angle, the more accurate you must be in setting the cores. But if you go with a more generous angle for the core print, you have more flexibility in setting the cores.

work with the foundry on this. They can help.

Tharper
Posts: 37
Joined: Mon Jan 23, 2012 8:51 pm

Re: Laying out rectangular cores on foundry pattern

Post by Tharper » Tue Jan 14, 2020 5:35 pm

For shrinkage I scale it up as a whole pattern the appropriate percentage - easy to do if your 3D printing the cores etc.
I like 2.5 degrees for draft and of course add machining allowance as required.

As mentioned by others risers and shrink bobs should be left to the foundry. Early on I found core boxes a challenge
So I made male masters of the core pieces and then made simple plaster-of-paris castings for the core boxes.
With any imperfections fixed and sealed with shellac and rattle can paint they worked well and held-up
well.

jlakes85
Posts: 215
Joined: Thu Feb 23, 2012 6:07 am

Re: Laying out rectangular cores on foundry pattern

Post by jlakes85 » Wed Jan 15, 2020 6:13 am

Tharper,

I poured core boxes in either quik cast 8002 or plaster of paris (when i ran out of the repro..) i definitely prefer the repro..

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