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Temperature profile on a chill roll
March 12, 2008
I recently spoke about the heat transfer on the roll surface of a cast line. The figure below shows the temperature of the cast film on both the top and bottom surface of the film and how they vary as the film is moving through the line and transferring from one roll to the next roll. In this case the initial melt temperature is 550 oF and the chill roll surface is 60 oF while the downstream rolls are approximately 150 oF. Heat is being lost from both the roll and air surfaces but the heat loss to the roll is greater due the much higher heat transfer coefficients relative to the air surface. As the film is transferred to the second roll (surfaces swap position from air to roll 2 and roll 1 to air) we see the air side drop quickly to the second roll surface temperature while the chill roll side reheats when it is exposed to the air.
The plot is temp vs. time so this curve can help in choosing roll diameters as line speed is increased to maintain the same quenching conditions at the stripping point as I discussed earlier. If the temperature profile at 15 sec represents the stripping limit then faster speeds with the same roll will decrease the contact time on the roll and stripping will fail. Therefore to go faster with the same roll heat transfer coefficient (same roll shell thickness and water flow rate) I need to increase the roll diameter to maintain 15 second dwell time at the higher speed. There are of course other ways to improve line speed with a given roll so long as we can lower surface temperature or increase water flow rate.
These calculations can be done in several ways and are relatively straightforward.
Posted by Eldridge M. Mount on March 12, 2008 | Comments (3)
In response to: Temperature profile on a chill roll
Jimmy commented:
Hi Eldridge, Which is more efficient, chill roll directly in contact with ink coating or the subtrate in between? I understand that chill roll is used after dryer oven exit to remove heat. It brings the film to ambient temperature; will a direct contact help in removing more solvent retention in the ink coating? Our new machine is capable of threading the film in both conditions. Any additional info...
In response to: Temperature profile on a chill roll
Eldridge Mount commented:
Jimmy, first the chill roll, because it lowers the temperature of the film, will stop the evaporation of solvent from the film so it will not improve solvent removal which requires the heat of the oven. Which side to cool from depends on the rate of cooling you need. Typically the film would be cooled from the unprinted side to prevent ink pickoff by the roll surface. In this instance the ink will cool last. But in general the heat transfer coefficients are high in a roll and for thin films the average film temperature comes to the roll surface temperature very quickly. If the ink is properly dried after the oven it may not matter which surface is contacting the chill roll, but I would think that contacting the ink layer directly would cause more potential problems than cooling from the unprinted surface.