Knight’s knowledge

Printing

Posted on 7 February 2012 | No comments

Jack Knight is director international technical service for INX International Ink Co. In his latest article, he talks about the role of the pressman

What is a pressman? Twenty years ago I came across an article written by J.C. Kernan about offset pressmen. I have adapted it so that it fits the guys who work in the metal packaging industry and wanted to pay tribute to pressmen around the world. I have honoured them in the words below and added a bit of humour.
Somewhere between the can salesman and the shipping department you will find an extraordinary character known as the pressman. Pressmen are all different, but each has the same creed, which is to do the best job they can and to protest loudly and strongly about anything that gets in their way while they are perfecting the craft of applying ink to metal.

A pressman can be found almost everywhere:
•     In the water
•     Between the ink rollers
•     Beside the gripper bars
•     Over the feeder
•     Under the delivery
•     On a coffee break
•     Arguing in the plate room.

Management needs them, the salesmen kid them, their helpers cuss them and plate makers hate them!
Pressmen are a strange breed of contradictions. They like single colour jobs, line work, greasy compounds they can mix into their inks, the occasional beer, and inventing new names for the foreman.
But they can’t very well tolerate the foreman, the pre-press department, off colour inks, replacing blankets, short runs, tough registration jobs, and above all the guy that talked them into being a pressman!

If you look in a pressman’s pocket you will usually find the following:
•   A few open end wrenches
•   An ink knife
•   Allen wrenches – a complete set except for the one he
needs
•   A depth gauge
•   An HFI sheet – he hates these but wants to make sure the
sorting department has steady work.

A pressman is a conscientious tradesman with grease on his face, a genius with oil on his shirt and an artist with ink under his fingernails. Yes a pressman is a complex creature. He says he hates his job, but he would never be happy without the smell of ink. He says he cannot stand the site of a press, but he knows in his heart that he would not give it up for all the tea in China.
And when he goes home at night, after all the headaches and frustrations of a difficult day, he can make himself feel like new by merely taking a can off the shelf at home and setting it on the kitchen table and saying to his kids, ‘Your old man printed that. I’m an artist. But most of all, I’m a damned good pressman.’ Please make sure you let them know how much you appreciate them and what they do for our process in providing decorated cans for the industry.

The role of a pressman
Here is what a pressman needs to know about his presses inking roller system:
A worn out roller, a roller with damaged bearings, rollers set with too much pressure or a roller that has not been properly cleaned can cause the following problems:
•   Colour inconsistency
•   Poor solids
•   Streaks
•   Dot gain.

Also if the rollers look shiny and have a glassy feel to them, they are glazed. When rollers are glazed, the texture in the ink rollers surface does not transfer ink effectively, which means you will need to run more ink and water on the press to achieve the required ink densities. Yes ink rolls have a texture to them when they are brand new. You want to maintain that texture for proper control.
Fountain solutions, wash-up solvent and ink are bound together and trapped in the ink rollers texture on the surface of the roll causing a glazed surface. Cleaning with solvent will not remove the trapped materials in the ink rollers surface texture.

Böttcher Rollers recommends the following steps to properly clean ink rolls:
Step one: Use a water miscible wash (solvent that mixes with water) to remove both the solvent-soluble and water-soluble particles that are near the top of the rolls texture. This leaves the insoluble particles.

Step two: Use a non-grit roller paste to remove the insoluble particles. Be sure to use a paste that is designed and tested for the type of rollers you have.

Step three: With the layer of insoluble particles removed, most of the remaining soluble particles will be exposed and can be washed out with a second water-miscible rinse. You can use warm water during the final rinse which will help dissolve any fountain solution and synthetic gum. Never use pure gum Arabic, the pH of the gum will increase with age and if you use it on a printing plate it will absorb moisture in the atmosphere which will oxidize the plate’s surface.

Setting the pressure
Now we have clean ink rolls that will be receptive to ink and water. So the next step is ink roller pressure. Setting rollers with too much pressure is a common mistake and an expensive one. Pressmen sometimes think that the pressure is what moves the ink and water. It’s not. It’s the shearing forces generated by the different speeds of rollers with different diameters.
More pressure does not help, and it can actually affect the quality of the print and lead to extra downtime and operating costs. Too much pressure will also affect dot gain on halftones and print quality on small reverse type being printed in solid ink areas of a design. Too much ink roll pressure will also cause the rolls to wear out prematurely. Minimal settings roll to roll or roll to the printing plate are best. Press manufacturers will specify the amount of pressure that they consider ideal. I recommend that you contact them for these recommendations or look them up in your operator manual. Pressure is a critical setting, big increases should be avoided, small changes are best. An increase in a roller stripe can result in a big increase of pressure to the rolls.
Finally monitor the ink roll durometer. If you notice a 10 point increase in ink durometer you have glazed ink rolls. If you are using alcohol free fountain solution then you should be using a lower durometer on your ink and water form rolls. You should use a higher durometer if you run alcohol. When alcohol is removed from the fountain solution, the viscosity of the solution becomes thinner. You need softer durometer rolls with lighter pressure stripes to help move the water down through the ink roller train and through the dampening system. Ultimately you should run less water using an alcohol free fountain solution versus using alcohol.
The bottom line to achieving a quality impression is to control your process. When a quality defect occurs it is necessary to stop printing and check the basics I have listed. Check and change, if necessary, one basic at a time to determine if it corrects the problem. If too many basics are checked and changed at the same time you will never know what corrected the problem. Remember, “A metal decorated container has a quality image that sells product on the store shelves.”

If you have a problem or process that you would like to discuss please feel free to contact Jack anytime via email at jack.knight@inxintl.com.
Alternatively you can contact Alec at alec@bellpublishing.com
More than 200 years and the can is still king!
Let’s work together to keep it that way.

Curing UV inks

Posted on 12 December 2011 | No comments

Jack Knight is director international technical service for INX International Ink Co. In his latest article, he discusses the UV process and answers questions from CanTech International readers

After a great turn out at Asia CanTech in Ho Chi Minh City, Vietnam, I spent the next three weeks travelling South East Asia. Not only did I see more customers than ever before during this period, I was also able to get to the questions that the metal decorators and can makers are asking answered in this article. Everyone I talked to said that their business had grown 20 to 30 per cent over the past two years and they predict that this will continue for the next 10 years.

The overwhelming questions were on the UV process. So let’s get to the science (no magic) as to how this power source turns a liquid to a solid. For help explaining the science I turned to Mark Gordon, energy curable chemist with INX. Thanks Mark.
James Zani from Cometa Can in Jakarta Indonesia asked the question ‘What spectrum is best for cure of INX UV Inks?’ The answer is, well, there isn’t one best answer and understanding what the spectrums are, which bulbs produce them and for which types of UV inks is important.

The UV spectrum is part of the electromagnetic radiation band between visible light (400NM) and X-rays (100NM). That spectrum is generally divided into three parts:

UVA     315 – 400NM (often called “long”)
UVB     280 – 315NM
UVC     100 – 280NM (often called “short”)

The significance of these three bands is mostly in relation to the UV source, the lamp. Most people use medium pressure mercury vapour (or H) bulbs, which has a spectral output that covers all three bands. The primary peaks of a mercury bulb are at 265NM (C band), 315NM (B band) and 365 NM (A Band). Still, the majority of its output is in B and C. Many photoinitiators will make use of these peaks and that is why mercury bulbs generally work quite well.

The type and amount of pigment in an ink has a huge impact on the cure properties because pigments also absorb UV wavelength and effectively compete with the photoinitiators. Dark colours and opaque whites are particularly affected by this. To overcome this, the formulator has to choose photoinitiators that make use of the longer UV wavelengths, UVA and UVB.

Another way to address this is to boost the amount of UVA and UVB available from the lamps. UV lamps can be modified with the addition of metal salts (halide lamps). A mercury bulb that has iron halide added (also called a D bulb) has much more UVA and UVB energy, but very little UVC. These are utilised to cure dark colours or thicker films, particularly if the ink has been modified to exploit these lamps.

For whites, the TiO2 absorbs all the UVC and UVB bands, and a good part of the UVA. An additive lamp where Gallium halide is added to the mercury bulb (called a V bulb) is best suited for this situation. This type of lamp only puts out energy in the longest part of the A spectrum and a little in the B. Only a couple of photoinitiators are efficient with this type of bulb. This is almost exclusively used for curing white inks only.

There isn’t really a best spectrum. The entire UV spectrum as a whole is used and it is up to the printer and ink supplier to figure out how best to utilise it. There are many more things to consider when using UV inks and the information given here is only a start.

In terms of what bulb to use, almost everyone uses standard medium-pressure mercury lamps. Consequently, most inks and coatings are formulated with that in mind. But with more people looking at additive lamps for the reasons stated above, it’s now possible for ink suppliers to provide products to address some of the cure issues that plague many printers. Matching ink chemistry to the type of lamp is a critical step that printers should not forget.

Another question I often hear from decorators who are making the transition from UV to conventional is why are the UV inks not as glossy before and after applying overprint varnish. The answer comes from the differences in the curing/drying process. Conventional inks dry by baking them for 10 minutes creating a hard shell over the pigments using the same process as the overprint varnish. UV inks are 100 per cent solids.

When they are cured in a fraction of a second, they bind the pigments in a continuous semi-solid film without the ability to create that same hard-shell that baking provides. This leaves the ink porous so that when the overprint varnish is applied it dives into the ink film leaving less varnish on the surface to reflect light giving the finished product with less gloss than the conventional process. The final results are very close for gloss but with some systems you can see a difference. ❑

If you have a problem or process that you would like to discuss please feel free to contact me anytime via email at jack.knight@inxintl.com
Alternatively you can contact Alec at alec@bellpublishing.com

Colour control

Posted on 20 April 2011 | No comments

In this article I will talk about the affect ink and water balance has on colour control. In lithographic offset printing we are mixing ink, which is an oil base, with water together as part of the process. This is where the skill of a press operator and years of experience comes into play. In theory ink/oil and water do not mix together but our press operators make this phenomenon happen every day.

If the press operator does not balance his ink and water level a number of quality issues will develop. Foremost is off colour, the ink can become water logged, emulsified, reducing the colour strength resulting in colour variation. The emulsified ink will not flow down the ink roller train evenly resulting in an uneven ink distribution across the rolls, resulting in colour variation.

So the first process control that must be put into place starts with water. Should tap water or treated water be used as the base of the fountain solution? Tap water changes with the seasons so in all cases treated water is your best and only bet. You can use a deioniser to remove the hardness from the water or a reverse osmosis system that filters the water until it is free of contaminates. RO water is corrosive so it is advisable to add some salts back into the RO water to achieve a conductivity reading of 50 micromhos.

Adding solution

Now that the water is controlled lets start adding fountain solution, FS, to the water. The first thing that must be done is to determine what effect, if any, the FS has on the ink. Some solutions can attack the ink causing the pigments to separate out; this is noticeable in the FS chiller tanks. If you see a lot of pigments floating around in the tank or if the filter you have in the tank has a lot of pigment in it then work with your FS or ink supplier to determine if the solution is compatible with the ink.

Once you are satisfied you have a compatible system run a conductivity test to determine the conductivity of your FS at different concentrations. The FS supplier will specify the amount of concentrate that should be used. It is important to note that FS is concentrated. A recommended mix could be a minimum of three ounces per five gallons of water with a maximum of six ounces per five gallons. Check with your supplier for the correct amount. It is important to start at the minimum level of concentrate to see how it performs while printing.

Now that we have our water and FS mixed up we need to add a wetting agent. Either alcohol or alcohol substitutes are both wetting agents that reduce the surface tension of the non-image area of a printing plate. Alcohol is a solvent, which should be used at a maximum concentration of five per cent, any more will cause the ink to breakdown especially if you run UV inks. If alcohol is used at a high enough level it will affect UV ink cure, it also has health and environmental concerns so it is not the first choice. Now run a conductivity test to determine the conductivity of your FS and alcohol substitute at different concentrations to determine the conductivity levels. Now you have an FS operating range to work with.

Fountain mix

• Maintain conductivity readings for UV inks at 1,800 – 2,800 max.
• Keep fountain and tank clean of scum build up.
• Use only deionised water for fountain mix.

Water balance

• Set water balance to the minimum to keep plate clean. UV inks are more receptive to take on water.
• Setting water balance more than necessary can cause UV inks to have a reduction of cure. The result being (soft ink film) with possibility of offsetting in stack.
• Dampers should be clean and set correctly.
• Maintaining an even ink film (using densitometer) across the sheet will make water balance easier to maintain at a lower level.

Other colour variation contributing factors

• Check the colour of the white base coating.
• Monitor the mixed ink to make sure the recipe was mixed correctly.
• Examine the printing press to make sure the ink rolls were cleaned properly, if they were not they will affect the ink and printed colour.
• Make sure the ink is not emulsified on press resulting in colour variation.
• Inspect to see if there is enough ink in the fountain, a low level will change the pressure on the ink fountain blade
• affecting ink flow to the roller train.
• Investigate to see what affect the varnish will have on the colour. Some varnishes have a tendency to yellow the
• printed image.

If you have questions please submit them to me at asktheinkman@yahoo.com or to Alec at alec@bellpublishing.com.