01 Mar 2014 Why does laminate curling change at different atmospheric temperatures?
In a previous edition of Converting Quarterly, AIMCAL’s substrate guru Eldridge Mount raised the question of why the curl in a laminate varies at different atmospheric temperatures. The short answer is: “It’s complicated”. But fortunately we have computers and tablets that don’t mind complications and the Laminate Curl Calculator sorts out all the complexities for you. The examples below are in metric units but you can run the apps in US units if you prefer.Here we have one aspect of the complications in laminate curl. We have two webs, say PET and PP being laminated together at the same tension (as you are often advised). Unfortunately the resulting laminate is massively curled – around a radius of 25mm. What’s gone wrong? The 50µm PET with a modulus of 4GPa is being stretched by 0.05% whilst the 25µm PP with a modulus of 1GPa is being stretched by 0.4%! It’s not surprising that once the laminate gets out of the nip it will curl in the direction of the PP to relieve the strain. You can quickly find that the root cause is the use of the same tension. To reduce the curl you have to strongly reduce the tension in the PP web. The reason you need an app is that none of this can be sorted out by intuition. If, for example, you make the PET 25µm and the PP 50µm you don’t gain all that much. Although the strains are less mis-matched, you now have much less of the high-modulus PET so the resistance to curl is less. No wonder most laminators find it hard to balance curl!
But what has this got to do with temperature? If you click the THC button in the LCC app you find a similar-looking app:
We have the same PET and PP, but now we start with the assumption that the laminate was flat but that the temperature has risen, along with the humidity. PET has a Coefficient of Thermal Expansion of 15µm/m/°C and PP is much higher, perhaps 100. If the temperature had risen 10°C equally on both sides with no change in humidity, then the curl would be 105mm. However, PET and PP both expand with humidity, though PET expands more than PP. The net effect of the 30% increase in RH along with the 10° temperature rise happens to be 162mm curl. Again the actual value depends strongly on the relative thicknesses and moduli of the laminate films as well as temperature and humidity changes. As Eldrige so nicely explained, there’s a lot going on and it can be very puzzling. With the two apps you will still have the complications, but they will start to make more sense.
This blog first appeared in the Converting Quarterly Magazine from AIMCAL and I am grateful to the Editor, Mark Spaulding, for permission to re-use the text.