Dimensional stability

[pavaluca]

Hello everybody,

I have to design a high precision squeegee support foe a digital press. Squeegee dimensions: 1"x0.118"x22". Preciosion and dimensional stability of the support is of the essence. 5 microns are way too much. Could anybody advise, please, what would be the best material selection among:

- Wrought Al alloys
- Casting Al alloys
(anodization necessary)
- Rolled SS 316
- SS 316 casting

What would be the recommended heat treatement in each case to acieve stress relaxation and dimensional stability.
The mecanical loads are very low so any alteration of the original mechanical properties is acceptable.

Many thanks for any suggestion.

 

[GRoberts]

The aluminum will have a lower coefficient of thermal expansion than the austenitic stainless steel. However, if you need something with an extremely low coefficient of thermal expansion, try invar (trademark) material. It is a nickel based alloy designed for dimensional stability in tooling components.

 

[TVP]

If you already have a fixed design (length, width, and thickness) then the material with the lowest CTE will give you the least distortion. GRoberts is incorrect when he states that aluminum will have a lower CTE than austenitic stainless steel. The following are representative values of CTE for some materials:

carbon & low alloy steel ~ 11 to 12 x 10^-6 m/mK

austenitic stainless steel ~ 16 to 18.5 x 10^-6 m/mK

wrought aluminum alloys ~ 23 to 23.5 x 10^-6 m/mK

Invar or Nilo-36 ~ 1.3 to 1.5 x 10^-6 m/mK

There is a great deal of information on Invar available from Carpenter at their website under the Technical Information header:

http://www.cartech.com/

Nilo-36 is a similar Fe-Ni alloy to Invar available from Special Metals. More information is available on their website under the Controlled Expansion Alloys header:

http://www.specialmetals.com/products/index.html

 

[GRoberts]

Thanks for pointing out my error TVP, I am too used to thinking in welding terms, for which austenitic stainless typically has more distortion because it expands/contracts more between room temperature and its melting point than aluminum. But as you pointed out, it is not because of the coefficient of expansion, but because of the difference in melting points.

 

[pavaluca]

Thank you for yous answers. Invar would be great, but I am still concerned with its corrosion resistance. Our printing inks are pretty aggresive (contain KCl). SS 316 provides a good resistance, so I need a metal with equivalent corrosion resistance. From the recommended web pages apparently Invar has not the same corrosion resistance as SS 316, but I may be wrong. My problem is not as much the thermal expansion, since the squeegee support is free to expand, but rather the distorsion. Residual stress relaxation is of the essence, I think. If I will use Stainless Steel castings I will still need heat tretement for stress relaxation before the final precision grinding, I guess. I will check Invar corrosion resistance with carpenter anyway. Thank you again for your answers.

 

[TVP]

Perform a search of the Eng-Tips site for more information on stress relieving stainless steel. This topic has been addressed several times. Good luck.