MD and CD Folding of Webs



Web folding can be categorized as follows:

  • Longitudinal folding in machine direction,
  • Transverse folding in cross direction
  • Diagonal folding at an angle

Often a combination of the above folds or folding followed bywinding makes the finished product. For example:

  • Quarterfolded napkins are first folded in MD and then in CD
  • Books are folded in MD, then CD and then again in MD
  • Certain medical sponges are first folded in MD and then diagonal
  • Paper tissues are sometimes folded in MD and then rolled

Although there are a number of methods to fold a web, we will onlyconcentrate on two methods typically used in tissue paper convertingoperations:

  • MD folds on a continuous web by means of folding plows
  • CD folds on severred webs by means of folding drums


MD FOLDING



MD folds invariably use the technique referred to as 'plow folding'.In this method the web is drawn over a stationary plow withedges inclined at some angle to the flow direction. As theweb travels over the plow it folds over along a line which isparallel to the flow direction.

Two designs of the plows are prevalent:

  • Fold pan or former made of plane sheet metal
  • Folding rods or guide bars

The design of the fold plow is purely geometrical: the web pathalong any streamline parllel to the flow direction is constant.One can then generate a set of equations using trig functions.This calculation is sufficient to design a typical Vee former.
For designs which require the web go over rods, the equationsmust take into account the radius of the rod. The resultingtranscendental equations have to be solved numerically.
The plow method requires a definite tension in the web to track itover the fold plows. In addition to this tension, the web issubjected to compressive stress normal to its plane as it goesaround the edges of the plow. These forces are detrimental tobulk and stretch of the web.


CD FOLDING...



...of a continuous web (e.g. Hudson-Sharp)

The web is zig-zag folded by means of a pair of coacting foldingdrums. The drums are mirror image of each other,and each drumincludes grippers and strippers. The folded pack is sliced inthe middle to create two lanes of folded product.

...of severred sheets (e.g. Bretting, PCMC)

The web is first severred into discrete pieces which are thentransferred to a folding section comprising of one or morefolding cylinders or drums. Sheets are folded as they transferfrom one cylinder to another. Transfer takes place at the foldline by means of pushers and grippers or vacuum.

Unlike MD folding, this process does not require any sheet tensionfor folding. Hence, properties like bulk and stretch are notaffected unless the folded item is passed through creasing rolls.


SERVICES FROM WEBCON



FOLDING PLOWS



Webcon will

  • redesign the existing plow for better performance, or
  • recommend alternatives best suited for the purpose.

A trial design is often the first step for selecting the option.

Webcon has developed mathematical formulation for the following:

  • Vee Plows
  • Double Vee Plows
  • In-line J-fold plows
  • In-line C-fold plows
  • Tri-fold plows
  • Back-fold plows
  • Interfolding plows

Autocad drawings are prepared for the complete design:

  • Layout drawing including guide rollers at entry and exit
  • Developed shape of the sheet metal for cutting
  • N-C Milling tape for the developed shape if requested
  • Detail drawings of the plow and bracketry
  • Guide bars and bracketry



CUTTING AND FOLDING CYLINDERS



The design of these cylinders is specific to a machine. Webcon hasdesigned or improved a number of

  • Cutting cylinders (shear as well as crush)
  • Folding cylinders (mechanical as well as vacuum)

Examples are:

  • PCMC cutting cylinders
  • Hudson-Sharp horse-shoe cams
  • Bretting shear cutters
  • Blade clamps for cutting cylinders
  • Plunger type MD and CD folding mechanisms
  • Custom cutting cylinders and folding rolls for proprietary machinery.
Copyright 2000 Webcon, Inc.