Laser engraving is one of the most exciting new ideas to hit the embossing scene for the tissue industry.  Although tissue producers began experimenting with the use of laser engraved cylinders for embossing almost 20 years ago, advances in this technology are continuing to drive new developments in tissue embossing even today.  Some of these new ideas may be applicable to other grades of paper and board, and even to other materials. 

The earliest laser engraved embossing rollers were used as research and development tools to screen potential embossing patterns, primarily on the basis of appearance.  They were not very good models of real embossing rollers, especially in terms of runnability and product properties, because the laser could not reproduce the contours normally found in conventionally engraved embossing rollers.  This was called two-dimensional (2D) engraving, because it produced extremely repeatable and accurate element placement in the two horizontal dimensions, but much less control in the third dimension (depth). 

As laser engraving machines were designed with greater control of the vertical dimension, and engravable materials were developed that were more durable, laser engraving became a more prominent force in tissue embossing.  Today, laser engravers use a "3D" engraving process that is much more precise.  Laser engraved rolls are being used successfully in many tissue embossing applications: 

1. In the production of steel embossing rollers, where the laser engraved rolls are used as small tooling rolls or as full-sized master rolls. 
2. Evaluating the performance of new embossing patterns on experimental embossing lines.  In some cases, the laser engraved surface may be part of a removable sleeve. 
3. Accelerating the startup schedule of new embossing patterns in production.  These are full sized production embossing rollers with a laser engraved cover. 
4. Long-term use in production, in very rare and special cases. 

There are several advantages and disadvantages of laser engraving, when compared to conventional engraving of steel.  By conventional engraving of steel, we are including the use of computer controlled machining of steel, which is usually only applied to the smaller cylinders used as tooling. 

The technology of using laser engraving in the tissue embossing industry has been mostly an adaptation of the technology that was developed for the printing industry.  However, many advances have been made specifically for embossing.  Some of these were made by the manufacturers of the laser engraving machines and are commercially available with the purchase of those machines.  However, the individual companies that use these lasers to engrave cylinders for embossing have made a great number of technological advances as well.  This information is generally highly proprietary, which gives the older companies a significant advantage over newcomers in the field. 

Most lasers that have been used to engrave rollers for the tissue embossing industry are CO2 gas lasers that generate an invisible infrared beam.  The beam is focused upon a point just below the surface of the roller to be engraved, while the roller rotates in a lathe.  In some engraving machines, the beam traverses across the roller while the roller simply rotates.  In others, the beam does not move, and the roller travels axially while it rotates. 

The very early 2D engraving process produced a "square" profile, with vertical sidewalls and sharp corners (see Figure 1 below:  "Etch Profiles for Different Laser Engraving Technologies").  It was similar to the photo etching process, in that it used a mask to control what areas were etched, and the resulting surface had only two levels:  either fully etched, or not etched at all.  In the case of photo etching, the mask was a thin film of polymer and an acid was used to remove unprotected areas of a metallic substrate.  With laser engraving, the mask was a thin coating of reflective metal (usually zinc), and a steady laser beam was used to remove unprotected areas of a polymer substrate (usually polyurethane).  The artwork provided to the engraver to specify the embossing pattern was a two-dimensional image in just two colors:  solid black or solid white (no gray).  Black was used to represent the raised areas (no etching) and white was used to represent the areas that were to be etched away at full engraving depth.  The only engraving parameter was etching depth.  In those days, image processing was usually done photographically. 

The vertical walls and sharp corners of these early laser engraved cylinders made them unsuitable for engraving steel.  In an embossing nip, the sharp corners would initially cause excessive damage to the paper, and would then crumble due to the very high stresses at those points. 

These older engraving machines were soon replaced with lasers that rapidly varied (modulated) the power of the beam to control etch depth, and did not require masking the substrate.  For the first time, it became possible to process the image 100% electronically (from computer to computer), without resorting to any physical image.  This was an enhanced version of 2D engraving, with the physical ability to create any embossing contour.  However, contour engraving requires 3D image processing, which was sold by the laser machine manufacturers as a very expensive software option sometimes known as the "embossing package".  Also, the software, computational power, and skills needed to create 3D artwork was not very available. 

Enhanced 2D engraving used the same black-and-white 2D artwork as the old technology did, but the laser controller on these new machines could be instructed to create a simple linear transition between the areas that were etched and those that were not etched.  This transition appeared in the engraved surface as a sloped section or ramp surrounding each raised area, shown in Figure 1. 

The ability to control the sidewall angle made these laser engraved cylinders more suitable for engraving steel.  Sloped walls also added strength to the areas that were compressed in an embossing nip, as in "rubber-to-steel" (R/S) embossing applications, where the "steel" roller was replaced with an engraved polymer.  This extended the useful life of the laser engraved roller, enough to make it practical in research applications, and also for some short production runs. 

The key to making laser-engraving work in the tissue embossing industry is in the material that is used on the surface of the cylinder to be engraved.  There are 3 main issues: 

1. This material must engrave well in the laser, through a special type of vaporization which is called "ablation".  This is where the material is ejected from the surface at high velocity, and the ejected material carries away most of the heat energy with it. 
2. The material must be durable enough to withstand the pressures that the cylinder will be subjected to, either when used to engrave steel, or when used to emboss paper. 
3. The thickness of the material must be greater than the depth of the engraving, by an amount that depends upon the embossing pattern and upon the properties of the material.  The engraving depth for R/S embossing can sometimes exceed 0.100". 

Steel certainly has the required durability.  However, it is currently very difficult to laser engrave steel cleanly at the depths required for tissue embossing.  This will change as lasers are developed with much higher power and much faster control. 

Ceramics laser engrave well at shallow etch depths.  However, when building a layer of ceramic up to the thickness required for embossing tissue products, it tends to crack and delaminate.  It is also difficult to laser engrave ceramic cleanly at the greater depths required for tissue embossing.  Eventually, someone will solve these problems as well. 

Laser engraving has opened up new possibilities in the design of embossing patterns.  Many ideas that were very expensive to implement before laser engraving arrived are now more practical.  Two of these ideas are "anti-nesting" and "anti-ridging".  Implementing either of these concepts normally involves increasing the distance in which a pattern repeats itself in the machine direction (MD), which increases the diameter of the smallest cylinder that can be engraved with that pattern.  This increase in size of the first tooling roll has a much greater affect upon the cost of conventional engraving than it has upon the cost of laser engraving. 

The thinking behind "anti-nesting" is that a roll of product can be made larger by preventing adjacent layers of product wrapped within the roll from "nesting" into each other.  Nesting is when the embossments of one layer fit into the embossments of the layer below it, much like bowls can be stacked on a shelf, thereby taking up less space.  Occupying less space is not desirable in a roll of consumer product, where value is often judged by the consumer based upon the size of the roll.  The only exception to this is at the outside surface of the product, where nesting would make the embossments less susceptible to damage, and therefore more visible to the purchaser. 

When pattern nesting occurs, it is usually seen in a roll of product as bands of denser layers alternating with less dense areas (see Figure 2 above:  "Bands of Nesting and Anti-Nesting").  Each of these denser bands has a circumference that is equal to an integer multiple of the shortest distance in which the embossing pattern repeats itself in the machine direction.  A shorter MD pattern repeat therefore increases the number of these nesting bands that appear between the core and the outside of the product.  When the MD pattern repeat is greater than the final circumference of the roll of product, nesting does not occur.  However, this requires that any tooling cylinder that is large enough to hold this pattern must be greater in diameter than the roll of product. 

Another idea made more practical by laser engraving is "anti-ridging".  Ridging is a phenomenon seen on the surface of some roll products, where the roll appears to be corrugated, as with a tin can, with ridges running circumferentially around the product roll (see Figure 3 below:  "Ridging and Anti-Ridging in a Roll Product").  This is an example of uneven bulk generation within the product, where the higher bulk building elements of the embossing pattern tend to occur at the same axial positions within the product, and these elements do not nest.  As the product is being wound upon the core, the diameter steadily increases, and so does the length of paper that completely wraps around the roll.  Therefore, the relative placement of elements in the MD, or circumferential position around the roll is constantly shifting.  However, the CD or axial positions of all of these elements remains the same.  The accumulated effect of the higher bulk building elements within the pattern is to spread this excess bulk around the roll in the form of ridges.  Similarly, areas of low bulk cause valleys between the ridges. 

The solution is to spread these high and low bulk-building elements more evenly across the embossing roller.  Each pattern element should be placed on the embossing roller so that it shares the same axial or CD position with very few other elements.  This greatly lengthens the MD pattern repeat.  In the extreme case, where each pattern element has its own unique CD position, not shared by any other element, the MD pattern repeat is equal to the full circumference of the embossing roller, and the pattern is called a "once-around pattern".  In most cases, the most practical way to engrave such a pattern is with a laser. 

The more evenly the pattern elements are positioned across the CD of the embossing roller, the better the results.  Ideally, the spacing of these elements in the CD would be perfectly uniform, regardless of the MD positions.  For instance, if there were a total of 1000 pattern elements engraved onto an embossing roller with a face of 100", then there would be exactly one of those elements centered at 0.1" from one edge of the face, another element somewhere around the roller centered at 0.2" from that edge, another at 0.3", and so on all the way across the roller.  A non-uniform appearance can still be achieved by semi-random placement of the elements in the MD. 

Most embossing patterns are composed of repeating elements (or groups of elements) which repeat at even spacing along straight lines in the product.  In roll products, such as bath tissue and paper towel, these straight lines are often at an angle with respect to the MD and CD.  On the engraved embossing roller, the repeating elements follow helical paths around the roller, continuously from one edge to the other, with no breaks in spacing or direction.  When the direction and spacing of these helical paths are selected appropriately, whether by direct calculation or by trial and error, then the optimum uniformity of CD and/or MD positions of the elements will be found.  Avoiding alignment of elements in the MD through uniform CD spacing will reduce ridging to imperceptible levels.  Avoiding alignment of elements in the CD through uniform MD spacing will minimize vibration in the embossing nip. 

The routine use of laser engraved cylinders to emboss tissue first appeared in the experimental or R&D facilities of tissue producers, in support of the embossing pattern development process.  Although the best results were obtained with steel rolls engraved by the conventional process, the high cost and long delivery times severely hampered experimentation with new pattern ideas. 

In the early stages of the pattern development process, small embossed samples are made for the purpose of screening patterns on the basis of general appearance.  Very small quantities of samples embossed with each pattern are required, and it is practical to use photo-etched magnesium plates in a flat press for this purpose.  In the next stages of development, full size prototype rolls of product are often needed for testing of physical properties and for customer preference trials.  These product rolls must be made in a continuous rotary embossing nip.  In the case of R/S embossing, where one roll is engraved and the other roll is smooth rubber, laser engraving was often the most cost effective option. 

As the price of the engraving was reduced, a larger proportion of the total cost of a pilot embossing roller became the steel base, or the core, which was covered with the engravable material.  Although these cores were reusable almost indefinitely, they could only be reused by destroying the engraved cover, which many people were reluctant to do.  The number of available cores soon became a limiting factor in the number of embossing patterns which could be kept in inventory.  Also, the time that it took to manufacture a new core often introduced significant delays in the delivery of a new embossing roller. 

The next advance was the use of laser engraved removable sleeves.  Again, tissue embossing development personnel borrowed a technology that had been developed for the printing industry.  A sleeve, usually nickel or fiberglass, was slid onto a special steel base, using compressed air to expand the sleeve.  The sleeve was then coated with the laser engravable material.  The coating was then prepared and laser engraved the usual way.  The finished sleeve was slid off the base (using air again), and only the sleeve was shipped to the customer.  Since a new steel base was not required, a great deal of money and time was saved. 

Using laser engraved cylinders as embossing rollers in production is only practical in very special conditions at this time.  The primary problem is poor durability of the laser engraved material, especially when attempting to use it in an environment where steel embossing rollers are customarily used.  In these cases, the useful life of the engraved cover can be anywhere between a few hours and several weeks. 

Laser engraved cylinders have been used to accelerate the startup date for a product rollout with a new embossing pattern.  Once a decision had been made to proceed with a new pattern, the tissue producer would order enough laser engraved cylinders to run a few embossing lines, and would order steel cylinders for the remaining lines.  The laser engraved rollers would arrive in 6 to 8 weeks, allowing early startup, and would be replaced by the steel embossing rollers when they arrived several weeks later.  In some cases, the laser engraved covers were removed and the steel bases engraved as conventional steel embossing rollers. 

Several of the laser engravers realized that supplying the embossing rollers used for trial work would provide many benefits beyond the profits generated from the small rolls usually involved in the testing process.  Some of these benefits are listed below: 

1. Get to know customers within the paper industry and their pertinent personal involved with embossing. 
2. Have the opportunity to learn from these contacts the different embossing applications and which specifications are critical to which embossing process. 
3. Also learn from these contacts what aspects of laser engraving needed to be improved, and the priority of each improvement. 
4. Learn the size and weight of the production rolls that would ultimately be required if and when they could make the necessary improvements in their process.  This would assist them in evaluating the need for and the specifications of new laser engraving equipment. 
5. Once the changes they had to make were identified, they could be prioritized.  Work could then begin to find or develop what would be necessary to make the required improvements. 

Basically, these improvements were as follows: 

1. Find or develop an extremely wear resistant roll covering with good laser engraving qualities. 
   • A covering that is as wear resistant as chrome plated steel is unlikely to be found.  This is especially true if the condition of the chrome plating is monitored for wear and the steel roll is de-plated and re-plated when it shows spots where it is starting to wear through.  Re-plating is relatively quick and inexpensive. 
   • While a compound equally wear resistant to chrome plated steel may be elusive, the material with acceptable wear resistance may be on the horizon.  Acceptance of somewhat shorter production roll life may be appropriate, especially when you consider equalizing factors such as trauma and pattern changes, which also limit emboss roll pattern life. 
2. Find a method to do more sophisticated contour engraving. 
• Most laser manufacturers have now developed special 3D "embossing" packages.  These packages can be provided as conversions to existing equipment or sold as an option with new equipment.  Most of the laser engravers that are serious about working in the emboss roll market now have this capability. 
3. Develop a means to produce a finer finish (smoother surface) on and in the pattern. 
• Tweaking of their procedures, plus Items "A" and "B" above has already made considerable improvement to the finish on and in the laser engraved emboss roll patterns achievable.  As of this time the surface finish of a quality laser-engraving job is not as smooth as a quality conventionally engraved job.  This may or may not be a critical area depending on the requirements of the end products being embossed. 
4. The laser engraving process has some associated problems with element radiuses at the bottom of female cells, tight tolerances of sidewall angles, etc. 
• Most of these limitations are only critical with matched steel (S/S) embossing applications but not rubber-to-steel (R/S) embossing applications.  The laser engraving software, and improved process and compound developments have all given the laser engravers improved control over this problem. 

A laser engraver experienced in pattern development and embossing processes should be the first to tell you that laser engraving cannot satisfy every specification.  Some pattern specifications require conventional engraving processes, whereas still other specifications may require a marriage of both processes.

One result of the laser engraved trial rolls is reduction of cost for both the paper industry and the conventional engravers.  In most cases the conventional engravers were selling tooling and trial rolls at a loss.  This was done to help procure the eventual order for the steel engraved embossing rollers to be used in production. 

Some of this laser equipment is used for a process that results in an engraved steel embossing roller without the use of smaller tooling.  Basically this is a three- step process. 

1. Coat the steel roll with a special thin coating, which will protect the steel from acid and is capable of being removed by the laser. 
2. The laser is programmed to burn away the protective coating to remove it from areas they wish to etch with acid. 
3. The steel roll is then acid etched to begin putting the pattern into the steel.

This process is repeated until the pattern is fully etched into the steel roll.  Some of the engravers that use this process are still outsourcing laser engraving of tooling and lab line emboss rolls.

A tool or tooling roll is a small engraved cylinder which is used to engrave another cylinder through a process called "making a transfer", where a reverse copy of the engraved pattern is transferred to the new cylinder.  The new cylinder may be another tool, a steel master roll, or the final embossing roller (also known as the production roll).  With the exception of dinner napkin embossing patterns, tooling rolls are usually smaller than 6" diameter by 8" face. 

In most cases of conventional engraving an engraved hardened steel tool is used to make new steel tooling.  Extremely high pressure is used to press a hardened engraved steel tool into a new non-hardened steel tool blank.  As the raised elements of the hardened tool are forced into the non-hardened tool blank, the areas of contact become compressed and work-hardened.  The work-hardened contact points cannot be allowed to become too hard.  As the contact points become harder progress slows because pressing is less effective.  In addition, the contact points may become hard enough to damage the hardened tool.  To prevent this from happening, the burrs must periodically be filed off the surface of the new tool and protective wax rolled on the surface.  This protective wax is applied with a roller onto the new tool so as to protect the roll surface but not get into the cells being formed at the contact points.  The surface of the work-hardened tool is protected and contact areas are etched with acid to remove the hardened material from the contact areas.  By etching away the hardened material the pressing action can once again effectively compress the steel at the contact area.  These pressing, filing, waxing, and etching processes are repeated until the pattern is fully pressed into the non-hardened tool. 

When a laser engraved tool is used in place of a hardened steel tool to engrave steel, the processes used are quite different.  The laser tool is much softer than a new tool blank (regardless if the new tool is pre-hardened or not) and therefore no pressing can be used.  The fact that even the hardest coverings being used for laser engraving are compressible will cause element distortion if too much pressure is applied.  The covering material also has memory and barring damage (and given enough time) will return to a like-new condition after the pressure is removed.  If this happens, inspection of the new tool will show the cell openings in the new steel tool will be wider and shallower than elements on the laser engraved tool.

A master roll is a large engraved cylinder which is used to engrave the production roll (embossing roller).  The master roll is usually wider than the production roll.  See Figure 4 below "Tooling Rolls and Master Rolls". 

The process differences between using a laser engraved master roll or a steel engraved master roll are not as great as when engraving tooling because both use only etching to engrave the production roll.  The pressure used when engraving with a laser engraved master roll is still considerably less than what can be applied when using a steel master roll.  When using a laser engraved master it is more critical to avoid a crown from developing in the production roll throughout the entire engraving process.  Some of the techniques used to eliminate a crown near the end of the engraving process with a steel master cannot be used with a laser engraved master. 

Many engraving operators whom I’ve talked to tell me that once they got used to laser engraved masters, they actually prefer them over steel master rolls.  They tell me that they can usually complete a production roll in fewer hours with a laser engraved master roll than with a steel master roll. 

Laser engraved rolls have found a place in today’s paper industry with the potential of playing an even greater role in the future.  Making this potential real will depend upon continuing developments in several areas: 

• Composition of the surface to be engraved. 
• Capabilities of the laser engraving machines. 
• Laser engraving process technology. 
• Using laser engraved rolls to make steel embossing rolls. 
• Learning how to reduce the wear on laser engraved rolls when used for embossing.