Thermal expainsion calculation for 2pc casting mold

[MoldMagic]

Hello all,

This is my very first post here! glad to have found this place. I'm hoping you guys can help me with something, I'm trying to design a mold made from 253MA stainless steel for casting pure silver, but I want to make this a 2 piece mold with a top and calculate thermal expansion of the stainless as well as the pure silver so that when the silver melts and expands it will touch the top of the mold and cast with a flat top surface, once it cools it will have shrunk way down and have a nice flat top with which I could more easily stamp it. I just need the special calculations to know how much height do I need to achieve this magic moment during the casting inside a kiln, below will be all the information, please help me understand how to calculate this on my own as I'm just a lay person.

The mold cavity as follows: length 1.375" -1°/S x width .625" -1º/S x height .227" R.125" all 4 corners (this is my best guess adding for thermal expansion, but this is probably wrong, please double check the height) (-1°/S means the taper in the cavity) (R.125" stands for 4 rounded corners as 1/8th inch)

http://files.engineering.com/downlo...e8-4d1505fd7309&file=mold_cavity1_cleaned.jpg

http://files.engineering.com/downlo...ca-5ff2acb8d3e4&file=mold_cavity2_cleaned.jpg

Actual cubic volume of pure silver material is -->[.18"]<-- inch³ or 31.103 to 32 grams

Silver linear thermal expansion is: 10-6 in/(in ºF) -->[11]<-- (is that micron?)

Mold material 253MA Stainless Steel: Coeffiient of Thermal Expansion,(in/in°F x 10-6) -->[10.5 at 1600ºF] [10.8 at 1800°F]<-- (again, does that formula stand for micron?)

http://files.engineering.com/downlo...b94f-5160f6d13425&file=RA-253-MA_DS_US_EN.pdf

kiln temp will be -->[1761-1763°F]<--

 

[3DDave]

Most castings of this type have a riser to cause pressure to completely fill the mold. If you don't have a riser and overfill the mold it will leak. The variable coefficient of expansion over the temperature range means that it is just as easy to make the mold and try different amounts of silver.

 

[dgallup]

Re your coefficient of expansion question, those are given as in/(in-degree F) so it does not matter if your dimensions are in inches or meters or microns, the linear units cancel out. It is important to use the right temperature units as the Celsius or Kelvin degree is 1.8 times bigger than the Fahrenheit or Rankine degree.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[arunmrao]

As 3DDave has suggested you cannot produce a flat casting. It is bound to sink. You need risers to compensate for solidification shrinkage. Better add some allowance to take care of surface imperfections.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[MoldMagic]

Here is my theory in action, I just need to know what the algorithm is, someone please teach me, I'm confused by the responses. When the silver is molten it is expanded within the cavity and the mold itself expands, what I need to know is by how much is that expansion so I could make the cavity x high so when the silver is expanded it touches the top lid without overflowing, when it cools it will shrink a lot, but have a flat top surface.

===>TIP Casting machine<==

 

[John2025]

I guess it depends on your definition of flat. On the video you linked, you can see that the top surface is not flat because of the meniscus effect from surface tension along with some shrinkage depressions. it's the bottom surface that was in contact with the mold that's fairly flat.

 

[MintJulep]

The phase change (solidification) of molten silver to solid silver is not the same as thermal contraction.

Why not just cast you part "upside down" so that gravity is your friend rather than your enemy.

 

[MintJulep]

Also, look closely at your video at 1:14 and you will see that the top of the mold is not in contact with the ingot.

 

[bobjustbob]

You are making something more difficult than it needs to be. If you want a flat surface, it needs to be in the drag. When designing a mold we try to put important features in the drag to ensure we get the best fill and so that the feature is the first areas to solidify.

 

[MoldMagic]

@mintJulep, Yes I see that, what happens is during the molten phase the blob of silver is actually touching the top flat surface and as it cools it shrinks down so much, all that's needed is for the silver blob to touch the flat surface whilst molten, it will harden quickly as soon as the temp drops only a few degrees and what's left is a nice semi-flat surface. I don't need 90° square top I just need a nice somewhat flat surface for stamping, in fact I like the curved surfaces going to the flat top, it's a nice look.

 

[MoldMagic]

Please someone get back to me on how the thermal expansion of pure silver + thermal expansion of 253MA stainless steel would be calculated, I would greatly appreciate it.

 

[3DDave]

It cannot be calculated without data. There doesn't seem to be any data on either material over the range of expected temperatures. Expansion is only very linear over very small ranges and certainly not when one material changes phase.

If you make the mold, just change the amount of silver until it fills it. Since the mold will not be exactly the volume you calculated, so you will have to experiment to get the right amount anyway.