- April 01, 2001, William E. Hawkins, Contributing Editor
Melt extruded resin polymers on cloth and other webs sometimes result in transverse direction (TD) curl of the laminate. And this curl often causes poor roll formation when the base roll is slit into large-diameter, narrow-width slit rolls.
TD curl often is severe enough to cause the narrow-width slit rolls to dish dramatically as they are wound. This column in the February 2000 issue of PFFC suggested that TD curl would be present if the following relationship between laminated webs was not true:
(The thickness of web #1)2 × (stress/strain modulus of web #1) = (the thickness of web #2)2 × (stress/strain modulus of web #2).
This relationship must be true for melt extruded laminates also.
When a resin is extruded onto the base web, the melt shrinks as it cools to a solid. Heat is transferred from the melt through the base web to the cooling drum. Thus the base web experiences both machine direction (MD) and TD thermal expansion before the melt is cooled to a solid.
Thermal expansion of the base web is kept to a minimum by heat transfer efficiency of the cooling drum surface and the thermal conductivity of the base web. Often, the total resin contraction from cooling is great enough to cause inward directed transverse forces that are applied to the top surface of the base web. These forces tend to cause the base web to curl. See Figure 1.
TD curl might be reduced significantly in some melt extruded products if the base web could be elongated in the TD just as the melt touches down and held in the elongated TD configuration while the melt solidified. If such a process could be designed, there should be a more nearly balanced TD length of both webs when the TD tension is removed, since the base web would contract more than from thermal forces alone.
I propose the following concept that might be helpful to those converters that are suffering significant curl problems with their melt extrusion products:
TD tensioning of the base web might be achieved by utilizing opposing banks of close-coupled, narrow-width, tandem-mounted, angled spreading wheels with individual spring suspensions that operate on the outside edges of the base web (just outside the melt border).
Spreading would start before melt touchdown and continue around the cooling drum for however many degrees it takes for the melt to fully solidify. The spreading rolls would have to be covered with an elastomer that can operate at a high temperature. The wheels could be cooled if necessary. If more than 180 deg of wrap is required for the melt to solidify, then the spreading roll bank mountings could be split for engaging and disengaging.
Each spreading roll should apply about the same down pressure on the base web edge. The banks of spreading rolls would be mounted on horizontal sliding rails and be operated pneumatically so they would engage the base web edges automatically or manually. The spreading angle should be remotely adjustable so the operator could optimize the amount of base web TD elongation.
One negative with this concept is that more non-coated base edge is needed (about 1 in. each side) to provide a working surface for the TD spreading rolls. This requirement would result in more trim waste than is experienced with standard processes. However, if your process is in jeopardy from TD curl problems, this concept might be worth trying. The concepts are outlined in Figures 2 and 3.
William E. Hawkins has 30-plus years of process and equipment development in web handling, including experience on all types of converting equipment. He specializes i thin web applications. Contact him at 740/474-5840; email: firstname.lastname@example.org.