Coating units & coater circulators
1Q: More and more tower coaters work with the Chamber Doctor Blade (CDB) technology. Is this really a better way to apply coating compared to conventional roller coaters?
The CDB technology applies a more consistent layer to the sheet, which reduces the coating consumption, increases the gloss and reduces the drying time. This improves the coating quality, makes the production easier and reduces the production costs.
2Q: Can you explain the difference between the CDB coater and the conventional roller coater in more details?
The CDB coater has an anilox roller with a ceramic layer with a screen of a lot of small, uniform laser engraved cells in the surface. The coating travels in these cells, which are emptied by surface tension every time the cells touch the blanket (or coating plate) and replenished with fresh coating from the chamber.
A roller coater has an open pan and 2, 3 or 4 chrome and rubber rollers. There is a nip (pressure) between the rollers, so the coating travels from the pan to the blanket on the surface of the rollers by a mix of surface tension and pressure.
3Q: The ability to adjust the nip between the rollers makes it easy to apply various layers of coatings and change the coating result. Does the consistent, repeatable layer of the CDB technology imply that we must have several anilox rollers and change between them every time, we want another coating result?
The core advantage of a CDB coater is the consistent layer of coating, which remains the same, not depending on printing speed, coating viscosity or adjustment of the coater. Via an evaluation of the wished coating jobs it possible to establish, which anilox roller is the correct for the jobs, and for most printers one or two rollers are enough. Only in very special cases are more than two rollers necessary.
It is not necessary to adjust a CDB coater to maintain a consistent coating quality, but because the roller coater’s roller settings change over time, it is, for this type of coater, necessary with ongoing adjustments to maintain the consistent nip and coating quality. This is time consuming and generates waste.
4Q: How come that that the coating layer in the CDB coater does not fluctuate with the printing speed. Our experience from dampening systems and roller coaters is that everything involving rollers require speed compensations?
In a roller coater the nip between the rubber and chrome rollers determines the coating quantity. The dynamic pressure from the coating just before the nip and the elasticity of rubber allow relatively more coating through the nip with increasing speeds. Rubber’s elasticity depends on temperature, so the quantity of coating is function of both the printing speed the press’ temperature. To maintain a consistent quantity with a roller coater requires a lot of ongoing corrections.
The quantity of coating in one anilox roller cell is always the same, so when the press speed increases, the anilox roller speed increases proportionally, and the quantity of coating on the substrate remains constant.
5Q: Can we spot coat with both roller coaters and CDB coaters?
This depends on what you consider as “spot coating”. Adding coating to a part of the substrate based on cutting in the blanket can be made with both coater types, but the result from the roller coater is inferior.
Coating a real partial image (i.e. flexo printing) requires a coating plate instead of a blanket and can only be made with a CDB coater, because this is the only concept transporting the coating without pressure. The coating plate has an elevated image (like old book printing) and the roller coater presses the coating away from the elevated image surface towards its edge. This gives an inconsistent image with a distorted edge. A thicker coating layer amplifies this problem.
6Q: Can we use both roller coaters and CDB coaters for pigmented coatings?
Pearlescent (Iriodin) or metallic pigments require a constant circulation and stirring of the coating. If not, the pigments will settle on the bottom of the tank or pan. The roller coater has an open pan, where the pigments can – and will - settle, so it cannot be used for pigmented coatings. This goes for both full face and spot coating.
Most pigmented coatings are used for spot coating with a coating plate (i.e. flexo printing). The pressure in the roller coater squeezes the coating away from the elevated image on the plate and towards the edge. Pigments in the coating amplify this problem, so the roller coater cannot be used for pigmented spot coating.
7Q: What is the lifetime of a blade?
The blade lifetime depends on the type of job, coating material and the anilox roller. A good estimate is an average lifetime of between 7 and 14 days. We have seen blades lasting much longer, but also shorter.
The quality of the anilox roller has an impact on the lifetime of the blades. A very rough anilox roller surface can wear a blade out in a minimum of time. Engraving the screen generates a rough surface with a lot of edges. To supply a smooth and consistent surface it is important with a final roller polishing. This is a costly part of the production process, but if this step is “forgotten”, it is the blades in the CDB system, which perform the “polishing”. This will of course generate a dramatic blade wear, until the roller is polished.
Q8: What is the lifetime of a seal?
The lifetime of the seals depend on the type of job, type of coating, type of anilox roller and how the sealings in general are treated in relation to appropriate washing cycles. It also depends on the material of the seal. A good estimate is an average lifetime of 7 to 14 days for seals based on plastic, but we have seen sealings lasting much longer. Seals of foam last roughly one day.
Seals of plastic can normally be cleaned, while they remain in the chamber, with the automatic cleaning functions in the coater circulators (both for waterbased and UV use). You can also take the seals out of the chamber, clean it manually and re-use it. A proper cleaning can increase the lifetime significantly. Seals of foam er difficult to clean.
Q9: Can I use the same seals for waterbased and UV coatings?
“Combi-seals” with a special compound for a mixed use between waterbased and UV coatings exist.
But most seals are dedicated for waterbased or UV coatings with an extra long lifetime.
Q10: Some press manufacturers recommend a coater circulator as an option only. Why should we invest in a coater circulator?
Because modern coater circulators do much more than just pumping coating to the coating unit. They also have integrated washing programs, and some can be equipped with conditioners, which, among others, can control the temperature of the coating and/or stir pearlescent (Iriodin) and metallic coatings.
The coater circulator is an important, efficient and integrated part of the total CDB system. The coater circulator saves water and washing liquids, and increases the coating quality and the overall productivity of the press, and makes the operator’s life easier and more efficient. All in all, something that boosts the overall profitability.
Q11: Can I use the same coater circulator for waterbased and UV coatings?
A coater circulator for waterbased coatings cannot be used for UV coatings. The automatic washing programs will add water to the UV coatings and destroy it. A coater circulator for UV coatings does not have an integrated water washing system and is not suitable for waterbased coatings.
It is much faster and safer to use dedicated coater circulators for waterbased and UV coatings or use a Combi system, which can handle both types of coatings with a fast change over without problems.
Q12: You mentioned that the anilox roller must be maintained properly. What do you mean by this?
An anilox roller has screen, which is full of small cells. If these cells get dirty – i.e. something remain in them, which is typically dry waterbased coating – they will not supply the same quantity of coating to the blanket, and a consistent coating quality does not longer exist. Finer screens mean smaller cells, which are more difficult to clean.
An anilox roller must always be cleaned properly immediately after use, and if this is not done, contamination and detoriation of the roller can – and will - take place.
Manual cleaning is normally dirty and boring, so it is not always made as often as it should be. The way to overcome this is to use a coater circulator with an automatic cleaning system. In case of waterbased coating hot water and detergent is used, and in case of UV coating, a special UV solvent is used.
There are different cleaning liquids available on the market. Some are better and some are worse.
Q13: I have heard that even with normal coatings some CDB systems tend to foam more than roller coaters. Is this correct?
This is a tricky question, because several independent reasons can cause foaming. Coatings with high viscosities tend to foam, if they absorb air. But also coatings with lower viscosities tend to foam, if they constantly are mixed with a lot of air. This is the case, if the chamber is not full of coating. The turbulence inside the chamber will simply mix the coating with air and make it foam and make it impossible to coat. The lack of a full chamber is directly related to a bad coating circulation system – or simply the concept of the chamber. Here we can only refer to Tresu’s superior coater circulators and the special chamber outlet systems, which secure a fast flow and a full chamber.
Finally it is important to consider the coating itself. There are no free lunches, and cheap coatings have simply not the same running qualities as high quality coatings and might tend to foam in a CDB system, because they cannot get rid of the air they have absorbed. Here an agitator in the tank might help.
The roller coater does not bring air back into the coater pan, and therefore coatings suited for open pans can have another affinity towards air. Therefore it is highly recommendable that you consult your coater supplier to get the correct coating for a CDB system.
Q14: Going back to the foaming issue. If air gets into the coating, it must be much more difficult to get the air out of coatings with higher viscosities. Have to overcome this problem?
In general, the most important action against foaming is a full chamber and a fast flow through the chamber.
But in case of coatings with a high viscosity – high-gloss UV coatings and waterbased coatings with some kinds of pearlescent (Iriodin) or metallic pigments in the coating – it is nearly impossible to get the air-bubbles out of the coating, when they are first in. As the conventional anilox roller always takes air back into the chamber, we faced a difficult challenge here.
The solution is to use a chamber under pressure combined with a line screen anilox roller. Such chambers can normally be installed on all presses with a tower coater, which already has a CDB chamber doctor blade.