Web Handling & Converting

Q: I plan to wind large rolls of delicate material on a two-drum winder. I am concerned that pressures might emboss the tender coating.

A: There are two pressures to be concerned with. The first is the external nip pressure between the wound roll and the drum. (Note a similar problem is the internal nip caused by core supported winding or unwinding). Here you might see drum grooving marks as one example or embossing of the front to back side. The second is the pressure between the layers due to winding physics. Here you would see the backside of the sheet embossing the coated side of the sheet that would fade away toward the top of the roll.

In either case, for 'large' rolls on a two-drum winder, both pressures will be on the order of 50 psi plus or minus a factor of two. However, you should have a safety factor that allows for gage variation and for time/temperature (in the case of interlayer pressures). Thus, your coating must withstand something like at least 200 psi without damage. If not, you need to either redesign the coating or find a different type of winding machine that has less nip load (case 1) or is capable of ultra loose winding (case 2). The rider roll should be at minimum (around 2 pli) well before the finish of the roll. Roll size could be reduced. Interleafs could be used but they are a huge nuisance.

So how do you tell which load case caused the marking and thus generate the correct remedy list. Simple, observe the marking as a function of radial position in a roll. If marking is due to external nip pressure, the marking will be very approximately uniform in severity through the roll (for winders which control nip uniformly through the wind). The solution for this case would be to reduce the nip load. If marking is slightly higher near the core, mostly uniform through most of the radii and finally disappearing toward the top of the roll; it is caused by interlayer pressure caused by winding. The remedies for this case would be to reduce any/all of the TNT’s. If the marking is found only near the core, it is caused by the weight of the roll supported by the winding/unwinding shaft. The solutions for this case are geometrical: Reduce finished roll diameter, increase core diameter.