Lead Author: Philip Pimlott
As Gravure Presses have embraced technology, becoming faster, more economical and less labor intensive, the Gravure Cylinder has endeavored to keep pace.
“Overpriced, long lead-time, outdated technology and cumbersome, resulting in the demise of Gravure and the rise of Flexography”.
Statements like this one are some of the most common criticisms leveled at the Gravure cylinder — but what are the facts and what are the options? When it comes to understanding the inherent strengths and benefits of gravure printing, it is important to examine three main characteristics of the gravure cylinder:
Let’s start with the Cylinder Base. This is an easy target for detractors of gravure to attack since it has not changed much since its inception. Cylinder bases are the machine parts made to fit the Press and support the engraving layer of copper at the appropriate circumference.
Bases can be made from various materials and are manufactured to specific circumference sizes.
Ordering new bases for a print job adds considerable cost. However, in fairness, these costs can be amortized across multiple press runs and subsequent jobs at the same, or similar repeats. The more a specific cylinder base is used, the less it actually costs over time. In other words, cylinder bases are an investment, so it’s important to know the different material types available as well as the advantages and disadvantages of each.
Steel Bases, for example are extremely durable and stable. A disadvantage is that they can only be used for a very limited range of circumferences and they are relatively heavy.
Steel bases are typically constructed by attaching (via shrink fit, or welding) shafts and heads to a steel tube. The constructed piece is then machined to the appropriate circumference and the correct shaft configuration for the intended press.
Shaftless cylinder bases are made in the same way, with the obvious exception of shafts. It is vital that the bases are constructed to precise tolerances for run-out, size, finish, and balance.
Run-out, or TIR is a measurement of the relative accuracy between all the diametrical planes of the base.
Size of the base is determined to accommodate the amount of copper that the Engraver requires for their process, typically between 0.012” and 0.020” in diameter.
Finish of the base body needs to be suitable for the copper plating process.
Dynamic Balancing ensures that the finished cylinder will run smoothly in the press at speed and also run true (and without vibration) in some of the production equipment used in the Engraving Process.
Steel bases can practically last forever, even when given the bare minimum of attention. It is for this reason, there are massive inventories of steel bases stored and shipped between Printers and Engravers.
Cylinder base manufacturers have honed their craft over the years and have developed supply chains and production systems that can deliver cylinder bases within a few days at a reasonable cost. The same can be said for the Engravers, where production efficiencies have led to a much lower price point, bringing focus to the proportion of the base cost to the overall price of a new Engraved Cylinder.
Several characteristics of steel need to be recognized and considered:
What’s the Alternative?
Many attempts have been and still are being made to replace the traditional cylinder base with a more viable alternative. Major Base manufacturers and third party suppliers have attempted to utilize alternative materials to produce replacement cylinder bases, or repurpose existing stock bases. Rigid foam materials, with conductive coatings, extruded elastomers (both conductive and non-conductive) and, more recently, injection molding of conductive material.
Market acceptance for these replacement technologies was challenged by the lack of confidence in the bond between the base material and the copper plating. The differential in Coefficient of Linear Thermal Expansion between copper and the replacement material caused delamination when the cylinder was exposed to extremely high temperatures during machining without sufficient coolant. Copper is an extremely good conductor and the non-metallic base materials were typically very poor conductors, as noted in the following examples.
|Material||Coefficient of Linear Thermal Expansion|
|Polyester – glass fiber-reinforced||25|
|ABS - glass fiber-reinforced||30.4|
|Polypropylene - glass fiber-reinforced||32|
|Polyurethane (PUR), rigid||57.6|
|Polyvinylidene fluoride (PVDF)||127.8|
At present, the best candidates seem to be glass reinforced ABS Polyester, and Polypropylene. Without the glass fiber reinforcement, the Thermal coefficient number increases by a factor of up to 5 (e.g. Polyester – 123.5 / Polyester – glass fiberreinforced – 25). This would suggest that glass fiber-reinforced materials would be suitable for any Steel base replacement technology being considered.
Understanding that price, weight, transportation, limited resizing potential, availability of raw material, and delivery times are all real issues with steel bases, what solutions have been offered, and how do they stack up to steel?
Aluminum has been considered a lighter-weight solution for the past 50 years or so. The challenge has always centered around creating a strong bond between the aluminum base and the copper required for engraving. There is a substantial difference between the Coefficient of Thermal Expansion of Aluminum and Copper. However, they are both good conductors and therefore react to temperature changes more rapidly than the non-metallic alternatives.
In recent years the use of Combustion Powder Spray/Thermal Spray Coating type technology has offered a solution to this problem. In this process, the basic principle is that metal powders are fed through a flame and propelled by high velocity Oxygen or air onto a substrate. Refinements to this principal and the development of alloys have led to a Copper plating replacement process and the possibility of removing the need for Chromium plating.
Note: Some years ago, a method of spraying an initial layer of Zinc onto a fiberglass core was patented. The concept relied upon the relatively low melting point of Zinc and the ability to coat that layer with Copper. Cold Spray technology has also been explored as an option; here the high velocity of the spray creates the bond.
Electroformed Nickel sleeves are used in conjunction with air-mandrels as a lightweight substitute for the traditional steel base. In essence, the steel base element of the cylinder stays at the Engraver and the printer and the nickel sleeve is transported between them.
The nickel sleeve can be copper-plated and processed through the Engraver using the airmandrels and existing equipment. On press, it’s the same story; the nickel sleeves are mounted onto the air-mandrels and are loaded into the press as normal, or mounted onto cantilevered mandrels.
Particular care has to be taken in cleaning both the components prior to assembly, as the slightest inclusion can show up on the printed piece. Ink can penetrate between the sleeve and the mandrel, making them difficult to remove without damage.
Each sleeve can only be used for one size and typically, due to economics, only one time. Any application where a user has the same, or limited repeats to deal with (some label work, gift- wrap, decorative and product) may find this technology appropriate to their needs.
“Polymer and Elastomer” Bases
Another lightweight solution comes in the form of a rigid Polyurethane Foam body that is molded onto a standard tubular steel axle and is subsequently coated with conductive material to enable copper plating.
Utilizing the same technology as the Lightweight Cylinder, existing stock cylinders can be reused for different circumferences (larger).
Although there are obviously limited weight advantages, the other attributes and limitations associated with polymer-based cylinders apply.
Note: As well as molding build-up material onto the cylinder, there are options to wind extruded material spirally around the cylinder, or by injection molding; both conductive and non- conductive materials are applied by these methods.
Lightweight Build-Up System
Courtesy of Cylicron™
Traditional Resizing Methods
Steel sleeves can be added to steel based cylinders, effectively utilizing the old cylinder construction to attain a new (larger) size.
Note: Adding copper through plating, or reducing the steel body in a lathe achieves relatively small changes in circumference. Thicker layers of copper can also be applied through metal spraying processes.
ATTRIBUTES AS COMPARED TO STEEL BASES
|Aluminum||Nickel Sleeves||Polymer||Polymer Re-sizing||Traditional Re-sizing|
|Limited Resizing Potential|
|Availability of Raw Material|
+ Better — Worse = Equal
Looking at the table above, one would think that the adoption of Polymer cylinders would have been an ideal candidate for success.
However, for the reasons stated earlier, they were never commercially acceptable in North America.
To see why, one would also have to compare Polymer cylinders against Traditional steel base attributes:
Note: The caveat to some of these conclusions is the possibility of engraving directly into a polymer surface and coating the engraved surface via vacuum applied amorphous carbon. (Chrome surface replacement has been studied and successfully trialed to determine ink transfer and the ability to wipe non image areas cleanly.)
Beyond the materials that make up the cylinder, another key component of gravure performance is the way cylinders are prepared for engraving. Like bases, preparation methods vary in their ability to address specific printing concerns or desired outcomes as well as the necessities of maintenance, down time, and other operational aspects unique to each individual Printer.
Copper has long been the preferred (and original) substrate for engraving, readily available and readily recyclable. Intelligent use of copper can be very economical.
Copper will only adhere to a clean (oxide and grease free) surface, so chemicals are used to degrease and deoxidize the surface to be plated.
For a steel base, an intermediate layer has to be plated onto the steel surface prior to copper plating in the acidic solutions used. Cyanide plating fell out of favor some years ago because of health concerns. It has since been replaced by nickel or alkaline copper.
These deposits are usually only a few microns thick and are designed to protect the steel from erosion caused by the Sulfuric acid used in the final plating process, which would result in the plate not adhering properly. With the advent of Electromechanical Engraving, the character of the plated copper had to be adjusted. It is vital that the diamond stylus of the engraving machine cuts through the copper cleanly, leaving a smooth cut and no burr on the surface.
To accommodate this, additives were developed that gave the copper plate the close-grained characteristics required, sometimes referred to as “Hard Copper”.
The copper-plated cylinder needs to be blemish free and dimensionally correct. The more accurate and smooth the copper plated surface is, the less machining required. Levelers and brighteners are added to the plating solution to help the process; however, much is dependent on the surface of the cylinder, prior to plating.
Most plated cylinders are machined on some combination of a lathe and a grinding process, although there are many cylindrical milling machines (polishmasters) still in service.
The criteria for a machined copper plated cylinder are that: the copper metal is appropriate for the desired engraving process, the circumference is exact, and the cylinder is round and runs true to the shaft bearing surface.
The surface finish needs to be fine enough that it will not retain pigment in the surface texture, but has sufficient roughness to help lubricate the doctor blade in the press; a typical standard surface roughness measurement would be 0.04Ra. Over the years, copper plating has evolved into a precise science, using automated equipment supported by sophisticated control systems; a long way from the Dickensian death traps of old.
Simply put, a Ballard Shell is a layer of Copper plated onto a polished copper surface that has nickel oxide present, enabling the layer to be removed when the engraving needs to be changed. The trick is in having enough adhesion to retain a good bond during processing and printing, but not so much as to make removal difficult.
Ballard shells are routinely used in publication, where standard repeats and frequent copy change make the technique ideal. Any base can be used for Ballard Shell production, in fact the technique can negate some of the risks associated with Polyurethane bases and Nickel sleeves.
Primarily used for Direct Laser Engraving, Zinc was chosen for its tendency to readily melt (melting point of Zinc is 420°C [as opposed to 1,100°C for Copper] and it has a boiling point of 900°C), zinc vaporizes under the extreme heat of the laser light.
Zinc has also been found to be excellent for Electromechanically engraving, as it cuts very cleanly due to its crystalline structure. These engravings must be Chromium plated to protect the relatively soft zinc surface but cannot be refurbished by de-chrome / re-chrome, as with copper surfaced cylinders.
Laser Engraved Epoxy Resin and urethane/acetyl polymer coatings were developed and trialed in the early 1980s. These trials were abandoned due to unpredictable surface wear and performance on press. The criteria for the trials were demanding. The expectation was to replace traditional engravings for the high-volume magazine press runs of the day.
Since that time, different attempts have been made to resurrect this type of product, taking advantage of the progress made in material science. The future of Laser Engraved Polymer/Resin is still uncertain, and also brings the questions about using traditional bases and processing prior to coatings along with it.
Ceramic Cylinders are fairly new to Gravure, but are finding their niche in the industry. The thermal spray coating on these cylinders is actually Chromium Oxide, which produces a hardness of 1150 Vickers and a porosity of <3%.
The laser engraved cells resemble those found on anilox rolls, from which the technology was developed. Advantages include 10 – 20 times the life of a traditional copper and chrome cylinder and good ink transfer from very fine screen rulings. Currently used for coatings, adhesives and primers, some “product” gravure is produced from these cylinders.
Developed and marketed initially for use with Nickel Sleeves, this promised to offer a surface that could be engraved using standard equipment and print reasonable run lengths without the need for Chromium plating.
Development ceased in North America and Europe; there were options to apply the engravable Nickel to alternative carriers other than the original Nickel Sleeve. Press trials have shown that this technology certainly had the potential to succeed, either as a stand-alone product, or in conjunction with another innovation.
There are three main Engraving methodologies in general use today:
First introduced in the 1960s, electronic engraving became popular for packaging in the 1970s. The technology brought consistency and predictability to the gravure process for the first time.
Prior to electronic engraving there was indirect exposing of a screen and a positive to a gelatin coated paper (carbon tissue), or direct exposure of screened images onto a photosensitive coating on the copper cylinder.
The etching process for carbon tissue depended on a series of dilutions of ferric chloride that would etch progressive tone values to differing depths. For direct exposure, spray etching with a constant dilution of ferric chloride was utilized. Although direct exposure increased predictability, there remained risk of uneven etching and therefore subsequent revision of the cylinder.
Electronic engraving relies on a constant engraving head frequency and variable surface speeds and traverse speeds to deliver the desired line screens and angles.
The frequency (speed) of Engraving Heads increased over the years and, currently, engraving speeds of 24 kHz are achievable. Along with this development came more refined techniques that improved the fidelity of the engravings.
It is true to say that the results achieved by the latest electromechanical engraving machines bear no resemblance to the early models.
Advances in electromechanical engraving technology have addressed the issue of “sawtooth” edges on text, which has been used as a criticism of the gravure industry over the past forty years. Different stylus angles, which are measured at the cutting tip deliver differing cell depths for any given cell width, so a fine screen that is selected for detail can be made to deliver the cell volume required for color density by selecting a more acute engraving stylus angle.
Common angles are 115° to 140°. For some specialized engraving, stylus angles below 90° may be utilized, in these cases the very tip of the stylus may be ground to a “chisel” edge to provide strength and engrave a larger, but shallower highlight cell. To avoid moiré patterns in the printed product, pseudo-screen angles are achieved by adjusting the elongation and compression of the engraved cells along with the screen ruling.
Elongation and compression are achieved by altering the surface speed of the cylinder against the constant frequency of the engraving head.
The pitch (screen ruling, or lines per linear inch) of the engraving track determines the potential size, or width of the engraved cell. If you can imagine a screw cut thread, the relative coarseness of the thread is dependent on the traverse speed, the faster the traverse speed the courser the thread, which in turn allows the tool to cut deeper.
Now if you imagine that the tool can also track in and out, in relation to the centerline, you can visualize how the cell shape is formed.
This process improved on the original direct exposure technology by utilizing more substantial coatings and laser ablation. The laser head can be controlled to deliver very precise shapes that conform to the outline of the desired image, therefore creating extremely fine detail in small font text, for example. Because this technology ablates the coating very rapidly, line screens of 2,000 lines per centimeter may be achieved within reasonable process times.
The etching process can be ferric chloride, or electrolytic etching using saline solutions. For linework, this process is well established and accepted for the delivery of high-fidelity printing. Due to the etching process, concerns remain regarding the method’s ability to deliver consistent cell volumes and cell walls across areas of line work. As a result, technology for half-tone work under this method continues to evolve.
Originally developed for the Publication industry, the benefits of this method further increased consistency against the multi-ribbon (multi-head) Helio-Klischograph Engraving machines and the etching processes that were typically used to balance color from page to page.
Subsequently, Max Daetwyler Corporation developed a system that successfully engraved into zinc-plated cylinders using laser technology.
Speeds were increased to 70 kHz and developed to engrave into copper; there remains the possibility of laser engraving into many substrates. The possibility still exists that the industry may return to a non-metallic substrate, just as was originally intended in the creation of the process.
The laser engraved cell has several advantages over Electronic Engraving. Because of the “bucket” shaped cell, as opposed to the inverted pyramid delivered by a diamond stylus, Laser has the ability to deliver ink in a more uniform and economical fashion.
The industry has chosen half-autotypical screens which alter both the area and depth of cells to describe tonal values, whereas “conventional” laser engraving used cells of a relatively consistent area and differing depths (more like the carbon tissue of old).
This produced an almost photographic quality (continuous tone) to illustrations, but with the advent of faster solvents, was challenging to print in the highlight areas.
Because of the mechanical restrictions of the engraving and exposing machines, they are limited to a finite number of “screen angles” by altering the traverse, rotation and head frequency.
By combining screens and angles, illustrations can be printed without the risk of Moiré patterns, or color shift. In the case of direct laser engraving, only two angles are available. The exception is “conventional laser,” because each cell is relatively the same size and 100% coverage at differing ink thicknesses, there is no fear of screen clash and therefore, no need for different screen angles.
The choice of screen rulings, that is the number of rows of cells per linear inch, or centimeter, is determined by the type of work to be printed along with the ink, or coating and the substrate. The more rows, or lines per inch/cm, the higher the resolution of the printed image.
In electromechanical engraving, ensuring that the correct amount of ink is transferred to achieve the color densities requires different stylus geometry.
The more acute the angle of the diamond stylus tip, the deeper the engraved cell for a given area. For indirect laser, the depth of the cell is controlled by the etching time.
Direct laser determines cell depth by altering the power of the laser light. As the available power of lasers increase, newer machines are successfully engraving directly into copper, bringing a new level of resolution to conventional processing lines.
Not bad for a process that has supposedly not changed in fifty years!
The processes that are currently in daily use are capable of delivering a fine product for a reasonable cost. In the future, challenges and opportunities will include new applications for gravure and shorter press runs, requiring faster changeovers.
Further development using current methods and materials will continue, will create value and should be recognized. However, if the industry is to experience a game-change, new methodologies and materials will most likely be required. Encouraging innovation and embracing opportunities presented by economical, portable, high performance replacement image carriers could bring significant benefits to the industry as a whole.
One such example would be industry-led technology developments that can deliver more economical solutions for press-side engraving that perform well on press, without the requirement for chrome – an advancement that would represent a giant leap forward.