Q-1: What is quality?
A: No definition of quality exists, even though we all assume to know what quality is. Somebody has stated that quality is: what somebody is prepared to pay for – thus a totally individual topic.
Q-2: What is consistent quality?
A: In theory consistent quality implies, that the quality between two or more identical products is not allowed to fluctuate. In practice this is impossible to obtain, so consistent quality implies that the quality between two or more assumed identical products is not allowed to fluctuate more than within some specific tolerances. In printing these tolerances are defined individually by each printer, so check the quality definitions from each printer when discussing the quality topic.
Q-3: What determines consistent quality in printing?
A: The size and structure of the ink dots as well as the quantity of ink in each ink dot. Any deviation of the specifics of an ink dot causes a fluctuation in the printing quality. Maintaining a consistent printing quality requires a consistent ink dot, which again depends on maintaining the density, dot gain, grey balance and contrast within the specified quality windows.
Q-4: What has an impact on the consistent ink dot?
A: Temperature, humidity and emulation between ink and water. The qualities of inks and substrates have also a significant impact on the printing quality as such. Inferior inks and substrates will fluctuate in quality and make it impossible to maintain a consistent printing quality even if temperature, humidity and emulation are under full control.
Q-5: What are the ideal pressroom conditions?
A: 22ºC (72F) and 55% RH (relative humidity). The air in the pressroom is heated or cooled by the press, and even the most sophisticated press temperature control system cannot maintain the correct press temperature at very high or low pressroom temperatures. Good temperature control systems on the presses are not enough. The pressroom as such must also be equipped with an appropriate heating and cooling system, and open windows and doors must be avoided.
Q-6: Why is the relative humidity so important?
A: Most substrates are based on wood, so the fibers in paper and board react to the relative humidity. Trees in the forests prefer around 55% RH, so paper and board tend to do the same.
Q-7: What is relative humidity?
A: Relative Humidity (RH) relates to the quantity of water in air. The tricky part is that the air’s absorption of water depends on the temperature so warmer air can absorb more water. This is why it sometimes feels very humid in hot areas even though it does not rain and why frozen air feels so dry. The consequence is that when hot humid air gets colder it starts to rain. When cold air gets warmer its RH drops, and this is why humidity systems that adds evaporated water to the air is so important for the consistent printing quality. It also explains why one should avoid to put a pallet of sheets directly from a cold truck or a cold storage room into the feeder of a warm press in a warm pressroom, because the sheet will immediately start to absorb water, change its structure on its way through the press and destroy the printing quality – if it at all is possible to get the press’ feeder to feed the dry sheets.
Q-8: What is lithographic- and waterless printing?
A: Lithographic printing is based on the fact that oil and water will not mix (which is a truth with limitations). So in lithographic printing the press first adds water to the plate, and where there is water there will not be inks. This happens on the surface of a flat offset plate and depends on its characteristics. In waterless printing the plate is configured in such a way that a thin layer substitutes the water. Normally a film of silicone is added to the plate, and when it is processed the silicone is removed where one wishes to have ink. In reality waterless printing is not a flat printing process, but is more similar to the gravure process, but as the silicone layer is so thin that the height difference is invisible it is considered as a flat process.
Suppliers of conventional plates are Agfa, Fuji and Kodak and a lot of smaller manufacturers. Suppliers of waterless plates are Toray and Presstek.
Q-9: Are inks temperature sensitive?
A: Yes. Conventional printing inks are oil-based. Oil changes its viscosity in relation to temperature, so inks do the same. When inks get warmer their viscosity decreases and they get more liquid, and more ink is fed to the press with the same ink key opening. This changes the quantity of inks on the sheet – and the printing quality. A thumb rule is that the viscosity changes roughly 10% when the temperature changes 1.0ºC (1.8ºF). Waterless inks fluctuate less in viscosity, because they do not absorb water from the non-existing dampening system.
UV links are not oil-based. They are more like liquid plastic with a higher viscosity than oil-based inks, so they are also temperature sensitive. UV inks in the conventional printing process with dampening water are very difficult to control, because the lack of the oil in the inks makes the emulation of water into the inks highly uncontrollable and temperature sensitive. Waterless UV inks have no emulation of water and are, consequently, easier to control.
Q-10: What happens when the ink temperature increases?
A: The viscosity decreases, the ink dot will be more liquid, and the surface tension drops. So apart from getting more ink to the sheet the ink dot changes its structure to be more flat, cover a bigger area and the dot gain increases. This decreases the printing quality.
Q-11: What are the characteristics of a good ink roller temperature control system?
A: A good ink roller temperature control system must have a strong cooling capacity, a heating capacity, a fast flow of water through the chilled inking rollers and the capability to change temperature fast, plus the temperature should ideally be measured where the ink dot is generated – on the offset plate. But due the high inertia in the plate cylinder it is more efficient to measure on the first ink form roller in contact with the plate.
Q-12: Why is heating required?
A: the optimal temperature for most inks is around 27ºC (80ºF). This is above the recommend pressroom temperature. To maintain an optimal ink temperature the system must be able to heat the ink fountain, when the press is not printing.
Q-13: Why is a fast flow important?
A: Every cm³ or inch³ of water can only absorb a specific quantity of energy per time. Absorption of more energy can only be obtained by increasing the temperature difference between the chilling water and the material to be chilled or increasing the quantity of water per time. There is a limit to how cold the chilling water can be without creating condensation on rollers and tubes so the best system has the maximal flow possible through the chill rollers and a minimal temperature difference at a given cooling capacity.
Q 14: How are the rollers in the ink fountain connected?
A: A normal chilled ink fountain has 3 chilled rollers. There is a limit to how much water can be pumped through a roller per time so to obtain the fastest flow the chilled ink fountain rollers should be connected parallel. This triples the quantity of water through the rollers and minimizes the temperature difference between in- and out-flowing water. The parallel roller connection is marginally more expensive in tubes and fitting, but it requires a very strong and more expensive supply pump. When the rollers are connected in series the flow is reduced to 33% while the temperature between in- and out-flowing water increases with a factor 3, which has a negative impact on the ink viscosity and the consistent printing quality. But it is a significantly cheaper configuration.
Q 15: Should each printing unit have its individual chilling circuit?
A: The heat generation in an offset press depends on the friction between various mechanical parts and the friction between inks, rollers, plates, blankets and substrates. These frictions depend on the quantity of inks in a specific area, which again depend on the printed images. The heat generation from each individual printing unit fluctuates dramatically depending on the specific job, so a temperature control system should of course be able to handle these differences, and this can only be obtained with an individual chilling circuit per printing unit, also called a multi-zone system, which measures the temperatures in each zone as close to the plate surface as possible.
Nearly all ink roller temperature control systems in today’s market are single-zone systems, where the chilling to all units are supplied centrally with a central temperature control system measuring the temperature of the supply- or return water in the central tank. This concept is fine if one wants to prevent the press from overheating, but it has no impact whatsoever on maintaining a uniform ink dot on the plate surface, because the system does not know anything about what happens to the ink dot on the plate.
Q 16: What is an appropriate chilling capacity?
A: The required chilling capacity depends on how much energy is added to the press during printing. The law of physics state that all energy put into a system must come out again as heat or movement. The press moves the sheets through its system, but apart from this all other energy is changed to heat.
A good figure is that a 40” press printing with 15,000 iph requires around 6 kWatt of chilling per printing unit. If the speed increases to 20,000 the figure increases to 8 kWatt. Roughly 25% of the heat is removed by the evaporating dampening water, so for an 8 color press printing with dampening water the ink chiller should have a capacity of 36 kWatt, while it in waterless printing requires 48 kWatt.
Q-17: Why is the ability to change cooling water temperature quickly so important?
A: The heat generation from the press changes with the printing conditions. When the press accelerates from idling to a fast printing speed it consumes a lot of energy and generates a lot of heat, so it needs a lot of cooling. When the press reaches its production speed and the acceleration stops, the energy consumption and heat generation, which is more or less proportional to the printing speed, is reduced, so less cooling is required.
With a constant, fast flow of cooling water through the chill rollers the only way to increase or decrease the cooling capacity is to increase or decrease the temperature of the cooling water. This implies that the best temperature control system has one temperature circuit with a minimal quantity of chilling water with the fastest possible flow per printing unit.
Q 18: Where should the ink temperature be measured?
A: Ideally the ink temperature should be measured with IR temperature sensors where the ink dot is generated, which is on the surface of the offset plate. Due to the high mass in the plate cylinder the temperature fluctuations on the plate surface are slow, so in practice this is not the ideal place to measure. Experience shows that the best place to measure the ink temperature is on the surface of the ink roller in contact with the offset plate. The measurement should be performed with a high-quality, fast reacting IR sensor. Any fluctuation in the ink temperature on the surface of plate roller will immediately adjust the required cooling (or heating) capacity in the ink roller temperature control system.
Q-19: Is dampening water temperature sensitive?
A: The viscosity of water changes with roughly 2% each time the temperature changes 1.0ºC (1.8ºF).
Q-20: What is dampening water sensitive to?
A: Dampening water is sensitive to water-hardness, conductivity, pH, which all impacts the critical surface tension and acidity. It is the surface tension, which determines if the dampening water will stick to the offset plate or not. The specific value of the parameters must be within specific windows, but a constant figure without fluctuations is much more important than the figure as such, because it is the fluctuations, which generate problems with the printing quality. Good values for the above parameters are: water-hardness: around 12 ºdH, conductivity: around µS 1,500, pH: between 4.8 and 5.2 and surface tension: below 45 dynes/cm.
Q-21: Why add IPA – Isopropyl Alcohol - to the dampening water?
A: The surface tension of tap water is around 72 dynes/cm while the surface tension of IPA is around 22 dynes/cm. The surface of the offset plate is around 45 dynes/cm. Adding IPA to the water brings the surface tension of the mixed liquid below 45 dynes/cm, so the dampening water can stick to the offset plate.
Q-22: Does IPA – Isopropyl Alcohol – have an impact on the viscosity?
A: Adding IPA to the dampening water increases the viscosity of the mixture, which implies that more dampening water is transported to the plate, if IPA is added. IPA is very special, because it is the only liquid which both reduces the surface tension and increases the viscosity.
This implies that an extremely accurate concentration of IPA in the dampening water is fundamental for maintaining a consistent dampening water quality.
Q-23: Can IPA be eliminated without causing problems?
A: Yes, a lot of modern fountain solutions can substitute IPA, especially by reducing the surface tension. But the additives will not increase the viscosity, so the surplus of dampening water based on adding IPA must be compensated by letting the dampening system add more water based on roller settings and roller speeds.
Q-24: Why is water hardness important?
A: The optimal water hardness should be just below 12ºdH (German hardness degrees. 1ºdH corresponds to 10 mg CaO/liter water). Both too hard and too soft water gives printing problems. Too hard water creates f.ex stripping on the rollers making it impossible to print, while too soft water makes it difficult and sometimes impossible to get the sheets dry.
Q-25: Can water hardness be measured?
A: Yes, the water hardness is directly linked to the conductivity, which is easy to measure. 12 ºdH corresponds to a conductivity of 280µS (mySiemens).
Q-26: Is tap water consistent?
A: No, the quality of tap water fluctuates between different areas, and it can also fluctuate within the same area and out of the same tap. Consequently, raw tap water is not suitable as basis for good dampening water.
Q-27: How is good basis water for dampening water created?
A: By running raw tap water through a Reverses Osmosis system (RO). This removes all salts from the tap water and generates basis water with a pure 0 in hardness. Then it is just to add an accurate quantity of salts to obtain the wished hardness.
Q-28: What about pH?
A: The pH controls the acidity of the dampening water. The correct pH is between 4.8 and 5.2. It is possible to measure the pH, but most modern fountain solutions are “buffered” so the chemistry, which maintains the pH, maintains its quality until the fountain solution is consumed. This eliminates the purpose of measuring pH.
Q-29: How to control the quality of the dampening water?
A: Apart from controlling the hardness of the basis water, the quality of the dampening water can be controlled via its conductivity. If the concentration of the fountain solution changes the conductivity changes, and if the dampening water gets dirty the conductivity also changes, so if any parameter in the dampening water changes, the conductivity changes. Good dampening water has a conductivity of around 1,500 µS, and a consistent figure is more important than the exact value of the figure.
One danger here is that if the concentration of fountain solution decreases the conductivity decreases, and if the dampening water gets dirty the conductivity increases, so if the two activities happen at the same time the conductivity could remain constant. Therefore it is important to monitor that the fountain solution container is not empty and that the fountain solution doser pump works properly, and that the filters are changed regularly to prevent them from being dirty.
It is also important to observe that IPA has no conductivity, so increasing the IPA concentration makes the conductivity drop. So an accurate IPA doser pump is fundamental for maintaining a proper conductivity and consistent dampening water.
Q-30: What are the characteristics of a good dampening water system on the press?
A: A good dampening water system must be able to supply a sufficient, consistent and accurate quantity of dampening water to the plate not depending on the printing speed and the consumption of dampening water. This requires chrome- and rubber rollers of the best quality, good maintenance and a very accurate roller setting. Some systems substitute the chrome rollers with ceramic rollers, as these rollers have a higher surface tension. Besides, the dampening water pan must allow a fast flow of dampening water.
Q-31: Are highly accurate pumps for IPA and fountain solution required?
A: If IPA is added to the dampening water its concentration has a fundamental impact on the water quality, so a highly accurate IPA doser pump is fundamental. IPA is consumed faster than water, so IPA must constantly be replenished to the circulation dampening water.
The fountain solution is consumed with the same speed as water, so fresh fountain solution must be added together with fresh water to the circulation system. Fountain solutions are added in very small quantities (2%-4%) and have a dramatic impact, so an accurate concentration, which implies an accurate pump, is required.
Q-32: What is the correct dampening water temperature?
A: It depends if IPA is added or not. If IPA is added a temperature of around 10ºC (50ºF) seems to be correct, while temperatures of up to 15ºC (59ºF) seems to be applicable in non-IPA printing concepts.
A low temperature reduces the IPA evaporation speed, and a low temperature increases the cooling capacity of the dampening water as the cooling energy generated by the evaporation increases.
Q-33: Where to measure the dampening water temperature?
A: The consistent and accurate dampening water temperature is critical on the surface of the offset plate where the ink dot is created. Due to the high inertia in the plate cylinder it is difficult to measure the dampening water temperature on the plate, so it is advisable to measure it on the plate roller surface.
Nearly all dampening water circulators measure the temperature in the dampening water tank, which is far away from the press. This is absolutely not the best place to measure the dampening water temperature, as a lot can happen before the water reaches the plate, so measuring the temperature in the tank does not tell anything whatsoever about the dampening water temperature on the plate.
Q 34: Should each printing unit have each own dampening water circulator?
A: The temperature of the dampening water on the rollers and in the dampening water pan is impacted by the heat generation in the printing unit. With a constant flow speed this implies that the temperature of the supplied dampening water should fluctuate with the heat generation in the printing unit. This can only be obtained with a dampening circulator per printing unit.
One dampening water circulator per printing unit would also make the filtration better, because only one type of ink would be pumped through one circulator.
Q 35: How do I configure the optimal inking and dampening water chilling system?
A: Study the specifications of the systems very carefully. Ask the correct questions to your press supplier.
You are very welcome to consult Cool Graphics Aps. We developed the most advanced systems several years ago.