Flatness

 

 

The foil, which exits the mill, has to have a good flatness otherwise it will result in serious quality issues. The foil has to follow the target curve set for the aluminium foil rolling mill. The target curve and working roll roughness, mill force, unwind and rewind tensions, mill spray oil pressure and oil flow rate has to be chosen correctly for each reduction and has to be suitable for the following reduction. If this is not correctly chosen then this will result in drastic problems and quality issues that are not desirable. Creases (wrinkles), pinholes, loose pockets, web breaks etc. Bad flatness can also be seen on the carpet diagram.
Diagram examples that are shown here are from 6,3 micron width 1500mm AA8079.

 

Imagine you have a thickness gauge centered in your mill { a normal situation}.  But your flatness control is not optimal, then your thickness is not optimal ( you are only measuring in the middle). So how does this affect your quality and production?

The thickness is only measured in the center of the foil width, so if the flatness is not consistent across the width then the thickness will also be different. This could affect your tensile strength results( depending on the position where your samples were taken )
 If you cut 10mm strips of foil in the length direction you will see that the thinner thickness is longer than the thicker. Bearing this in mind, let’s look at the rewind tension.
If you have a rewind tension set to pull say 43N/mm2 this will be much higher because you are only pulling on a part of the material, so if only 50% of you flatness is good it would mean that your rewind tension is higher than shown, or double your set tension, and thus result in breaks.( rewind tension is a calculation, based on the entered information .Example 43N/mm2 would be  for width 1500mm / thickness 0.006mm ( 6 microns),  387N tension.) But if the flatness is say only 50% it could mean that the tension would be approx. 86N/mm2 ( 774N) the thinner parts of the web width will be enclosed between thicker material [ loose pockets  ] and thus could form wrinkles / creases.

  • Gray area is thicker ( tight )
  • Blue area is thinner ( loose)

 

If you have the correct mill settings / reductions / proper work roll roughness/correct mill oil and mill oil additives then your flatness will be good, while producing at a good speed the deviation in thickness will be minimal because when flatness is ok the mill settles and will not have to accelerate / decelerate due to flatness deviations, the warmth in the mill byte is equal, and thus the thickness becomes stabile with minimal deviation.{ see diagram flatness, red is thickness deviation }

If anyone would like to know more then I would be able to give lessons / information at your plant on “How to optimize your mill for better flatness and in some cases,more production“.