Understanding Rotary Converting (die cutting)

The concept behind rotary die cutting is simple: pass one or more layers of material between rollers to ‘convert’(change it in some way) it. This can be as simple as unrolling a large roll to wind it into several small rolls or as complex as making a transdermal patch from a variety of materials and layers.

A rotary die is made from a solid cylinder of tool steel.  The blades are machined or EDM etched, removing material in the cutting area where there are no blades.  On each side of the body are uncut zones 'bearers' (from 9/16" to 1" wide depending on the size of the die) and smaller diameter 'journals'.  The bearers are precisely ground to maintain the proper distance between the blades and the smooth, hardened tool steel 'anvil' cylinder.  It is the combination of the bearers and the fact that the blades are machined from solid steel that gives rotary die cutting its inherent accuracy and long life.  The 'journals' are there so that the die can be held in place with bearings and so a gear can be attached.

Some of the processes which can be performed with rollers include:

Die Cutting: a general term covering any cutting of material in some repeated pattern.

*Thru cutting: the blades cut through all the layers.  Since the blades touch the anvil, thru cut blades have a 'flat' at the top with angled sides.  Depending on the material cut, the 'flat' or 'land' can be form .001 inches to .004 inches wide.  A number of other factors can be varied according to the material to be cut and the finished part requirements. 

• To avoid distortion of a part, one side of the blade can be made almost vertical while the angle on the other side is increased to maintain blade strength. 

• The depth of the cavity between the blades can be varied to accommodate ejector foam or thick materials. 

• For abrasive or hard to cut materials, coatings (chrome), material treatments (cryogenic), and higher grades of tool steel are available.  

• Some blades can be made as replaceable inserts.  This is especially useful for simple blades such as sheeters (straight cuts across the web) or for vacuum dies in which the cut piece is to be removed by being vacuumed through the center of the die.

• Air eject dies are a special case of the thru cut die in which holes through the center of closed shapes (circle, rectangle, or special closed pattern) through which air pressure is applied to help remove cut pieces.

*Kiss cutting: the blades cut only selective top layers.   These blades are made sharp since they will see only the relatively soft material to be cut.  To cut only selective layers of material, a precise distance must be maintained between the blades and the anvil roller.  This is the role of the 'bearers' on each side of the die.  The die maker determines the proper clearance between the kiss cut blades and the top of the bearers (and therefore the distance to the surface of the anvil) so that the top materials will be cut without damaging the bottom layers.  This height will vary according to the thickness and compressibility of the materials.

*Slitting: is a special case of thru cutting where a continuous, straight cut is made in the direction the material is    traveling (web direction, a web being any material that is wound on a roll)

*Scoring: is a partial cut (kiss cut) version of slitting or heavy mark made in the to aid in folding.

*Sheeting: is a periodic thru cut made perpendicular to the web.

*Butt cut: is a periodic kiss cut made perpendicular to the web.

*Perforation: intermittent cut in any direction or shape.

*Ultrasonic die cutting can cut and seal at the same time.  Ultrasonic technology uses vibration to generate local heat.  They are like out-sized speakers which vibrate tuned blocks of metal rather than a thin membrane.   This is supposed to be ultra (above) sonic (hearing) range, but anyone who has been around them can attest to the loud squealing sound when the face of the horn (vibrating block of metal) comes in contact with the material to be converted and the back up die.   The horn only vibrates a few thousandths of an inch in amplitude, but it can be surprisingly effective in generating local heat which can cut, bond, and seal edges of compatible thermoplastic materials.  Most Ultrasonic work is done with a flat supporting pattern against which the flat horn pushes the material to be converted.   Wherever the supporting pattern is raised, cutting and sealing will occur, leaving the rest of the material unheated and undamaged.  Most applications involve raising the ultrasonic unit, indexing the material, lowering the ultrasonic unit and turning it on for a fraction of a second before starting the next cycle.  CRC has developed the technology to cut and seal material in a continuous process with a specially designed rotary die.  This process is especially well suited for cutting plastic cloth, woven and nonwoven.  The possibilities are endless when you consider that you can cut some areas and seal others to form pouches or reinforcement.  It is a very clean process with little cutting debris or chances for contamination.

Lamination: bonding two or more layers of material.

PSA lamination (Pressure Sensitive Adhesive): uses adhesive applied to  one of the layers to bond to the other layers with pressure applied by  rollers.

Adhesive lamination : bonds 2 layers of material after a layer of bulk adhesive has been coated onto one of the layers. This can be PSA or heat activated adhesive or cold seal.

Heat lamination: uses time (dependent on the speed of the web), temperature, and pressure to bond materials, often in patterns.  The bonding agent can be a heat activated adhesive or the material itself if it is thermoplastic and compatible with the mating material.

Knitting: bonding fibrous materials (i.e. coffee filters) with mechanical pressure between knurled rollers.

Flame lamination and Corona treating changes the surface energy of the materials and will bond some materials.

Embossing: putting a repeated pattern of dents into a material with pressure and sometimes heat.

Stripping: peeling off or evacuating portions of the material which was cut in a previous process.

Folding: the material is possible in the web direction. It is also possible to make folds in the cross direction, but that is more complicated.

Coating: is the application of a layer of bulk material in a repeated pattern.

*Printing: is the laying down of ink in a precise pattern.

*Flood coating: covers the entire surface of the material with ink.

*Adhesive coating : applies a layer of adhesive to the material.

Hot melt extrusion: also known as 'slot coating'

Hot melt roll coating: Applying the glue by making contact with a roller covered with hot/molten adhesive.

Cold glue roll coating: Applying cold glue to a web by bringing it in contact with a roller coated with a liquid adhesive. Cold glues can be either cohesive or adhesive.

Pattern adhesive coating: using gravure rollers or special silk screen equipment to put glue in selected areas in a repeated pattern.

Slitting: Cutting the material in the web direction (along its length). Most materials to be converted come in rolls. These rolls are usually produced in a wide (60" typical) format but used in smaller (i.e. 6") wide rolls. A slitter is a dedicated machine which divides the larger "master" roll into widths needed for the final steps of the process. There are 3 main techniques for cutting materials:

Razor cut: for thin films.

Crush cut: for heavy or thick materials, foam, cloth. In this process, a dull round blade rolls against the material, crushing it against a hard smooth anvil roller.

Shear cut: for papers, and delicate materials. Shear cutting is a scissors like action where the blades are rotating disks pressed against each other with only a slight overlap. The material is fed between the tangent to the two disks where the shearing action occurs.

The beauty of rotary converting is that the processes can be stacked in series and parallel to make complex parts in a minimum number of passes.

Example #1 Labels. P = process

P1= print first color, P2= print second color, P3= kiss cut label shape, P4= strip matrix (unwanted material between labels), P5= Sheet the individual groups of labels.

Example #2 Band Aide™ like product.

P1= cut vent holes, P2= strip release liner to expose adhesive, P3= laminate absorbent materials at the center of each strip, P4= fold the edges of the new release liner, P5= laminate the new release liner, P6= cut the shape, P6= strip the matrix, P7= space the parts and laminate them between the packaging material, P8= pattern laminate package, P9= die cut the package.

This is a relatively complex operation but has tremendous potential. If each part is ¾" wide, and 2 parts are made side by side, there will be 32 parts per foot. With a production rate of 50 feet per minute and allowing 10 minutes per hour for changing rolls, 80,000 parts will be made per hour.

There are of course many subtleties for each process and many things to consider, but in the spirit of proving a general overview here, we will move on to an other major consideration for rotary converting: material handling. This includes holding the rolls, controlling the tension in the material, and guiding it.

Materials destined to be converted (referred to as a web) are usually wound on cores (tubes, typically cardboard or plastic). The inside of the cores usually match some industry standard. The most common diameter is 3", but some materials will be damaged by such a small diameter and are put on 6", 12", and even 16" and larger. The material must be purchased with a core size that the equipment can handle or the equipment must be modified to accept the roll.

The need to guide the material is obvious, but the task is much less so. It requires a well aligned machine with straight webs. Even so, if the roll of material is not perfectly aligned, it will have to be guided to position. Stiffer materials can be guided with some level of accuracy with mechanical guide rollers. More delicate materials sometimes require a web guide to position them accurately. A web guide is a set of steering rollers controlled by a servo motor which gets its feed-back with an edge sensor on the edge of the web.

Tension control is critical, difficult to understand and see, and more difficult to control. Excessive tension (pulling/ stretching force on the material) can damage material. Too low a tension will not allow the material to conform to the rollers and will wander from side to side: some tension is needed to be able to guide the material through the machine. Tension is also critical for achieving a part of the right size. Rotary dies are made oversize and tend to overfeed the material. Some tension is needed to slip the web back enough to achieve the correct finished dimensions in the web direction: tension is the control for fine tuning the size of the part in the web direction.

Packaging: As the parts come off the machine, they must be handled properly. Some parts can be dropped in a box randomly and others must be individually packaged, counted, and stacked. Packaging can be a significant part of the converting cost.

The simplest way to take parts off a press is to drop them in a box. The parts can be weight counted. This method is not acceptable if the parts will be damaged by being folded or by the weight.

Rewinding the parts on a roll is also quite simple as long as the web is strong, that is that the cuts in the material do not make it so weak that it is easily damaged.

More involved packaging might include stacking the parts, counting them, enclosing the counted parts into bags, special containers, and special labeling.  We can even insert the parts automatically into individual sealed pouches as is commonly done with medical devices.