# Problem Hickeys

[Note: this post is a theoretical findings from sources to justify the common problem - Hickeys in printing]

Hickeys on plate

Hickeys on print

Hickeys - Solid particles from raw materials stick on plate and reproduce solid print with un-print border, they stick like sticker on plate surface.

Solid particles - source from paper fibres/lint - (Dust particles) and undissolved ink raw materials - mostly low quality inks contain slurry particles, or chemical reaction of materials skinning are some of the example. They travel from one ink duct to another or other unit ink train with help of paper easily, so some time we may suspect other colour Ink quality, for example black ink slurry/dust material travel to other colour ink train from bottom to top mostly with help of solid images (highlight area).

Commonly this kind of hickeys are unavoidable, frequent ink cleaning & minimum filling of ink duct may be solution to this problem.

Hickeys by Ink Skinning - In ink, chemical reaction of vehicle with Oxygen and form polymerization, as result they dry on ink duct surface as solid skin like particle.

Origin of ink skin-

In closed container (cans/barrels) - we see skin over paint cans, Oxidation occur in closed container were continuous process of polymerization occurs with help of inside air as result large area of skin is covered over the ink.
In open ink duct - this oxidation is same or may be different, in tiny area vehicles (monomers) combined to form polymerization - result tiny skin particles formed.

Ink Skinning :

Ink skin move from ink duct to ink train rollers and stick on the plate surface.

They spread throughout the surface of ink duct - we can carefully remove them with ink knife, like removing layer from boiled milk that kept aside with help of spoon.

As this skin is in plastic state, the physical structure can't be changed by rubbing and squeezing by our hand.

Where does it stick on plates?

They stick on 100% TAC, mostly shadow area of images.

Hickeys do not stick on plate when there is water beneath them on reaching plate surface, and get moved to blanket - paper - web path rollers etc.. this condition does not affect the printing quality.

If this case turn reverse we face Hickeys problem. Skin/dirt that stick on plate reproduce defective print.

Why my Web-offset cold set ink gets oxidation?

Cold-set ink for News paper printing (stock) should dry by penetration, but due to ink manufacturers compromise to mix vehicles that dry by oxidation in percentage level to ink for enhance colour depth / increase superior quality (gloss) / good density to competitive with heat-set ink counterpart. So basically we have to understand the market strategy behind this problem, maximum it won't affect but some cases 0.999 % we fall victim.

How special my Cold-set ink for High speed Newspaper production?

It is not like actual sheet-fed press, were gripper deliver the printed sheet directly after printing without any external contact to the printed surface of paper.

Were in High speed web offset press, printed image passes through number of rollers, turner bars etc with contact - so more caution should be taken while selecting additives for ink. The duration of drying and settling of pigment to the paper is more critical, consider these factors before altering / adding / mixing something apart from ink manufactures composition.

Precaution taken while facing ink skinning/ dirt problem?

When you use Castro Oil (solvent) to dilute duct, it breaks the polymer and spread throughout the image despite of not dissolving them, - result large amount of very tiny polymers (more hickeys) spread over the image and it increase the problem.
Petroleum gel - when adding to ink it reduce the tackiness, but on other hand increase or improve emulsifying performance of the ink. They may act as antioxidant but actually they are not meant for, some time there will be side effects too. [paracetamol tab won't cure viral fever !!]
When machine is in ideal for long time (over night condition) don't start with rotating the ink duct first, top surface of ink duct has skin formation that move first inwards and mix with good ink.
Stop using agitators in duct while ink get oxidation or more dust particles , they get mixed well with the ink and increase the problem.

When we use / mix excessive % of solvent that dry by oxidation in high speed web-offset machine (cold-set) what happens?

Printing with slur/ broken letters,
Uneven drying - may result in ink smell from the product,
Inconsistency in density throughout the image - due to vehicles of different type used,
Less rub resistance due to non drying of ink vehicles.

So what is this problem called?

My ink duct is not stable to hold the ink for certain time duration at particular atmospheric condition - called Duct stability -> result -> Ink skinning.

Duct stability - It is the testing method of ink, were to find the time taken for the formation of skin in open condition (like in ink duct).

We can find this problem by placing a spoon of ink in plastic container open for time interval in our press room, test them using small handy roller rub into thin film on flat material - you may find the skin in your roller or material (eg. paper) that you used to flatten ink to thin film. Number your specimens with time and you are able to find the duration of oxidation.

Is there any solution to overcome this problem?

Anti skinning agents - Additives can be added with extra care - example Non-Volatile anti skinning agent polyhydroxy phenoles such as Hydroquinone [C6H4(OH)2], pyrocatechol and pyrogallol. Guaiacol, a natural antioxidant which is relatively inexpensive can be used, but no satisfactory anti skinning agent for printing inks has heretofore been used.
Add additives carefully in % prescribed by Doctors (ink manufacturer) to the ink duct to solve this problem.
When we minimize the duct speed (curve setting) automatically screw opening will be more to supply the demand - so in this case hickeys escape or clear more from the duct. This condition is equal to frequent duct cleaning - by this way we can reduce hickeys problem. [method suggested by my Mentor]
Filling minimum required amount of ink for production in the ink duct
There are many suggestions like by covering the ink duct, Kerosene spray (thin flim) etc.. when machine is in ideal condition for over night. Some time this may not practically possible due to our press conditions.

Why I face this problem and not others, when same ink is used?

Very excessive Oxygen level in your place affect your ink, and those surrounded by population, traffic and pollution does not meet this condition. This in case you may have excessive rusting of machine parts than others.
Ink duct curve settings may differ from others - Minimize duct speed and increase the opening of screw clears hickeys.
Atmospheric condition is the master mind behind this problem.

What is Anti Skinning agent ? (Antioxidant)

Anti-skinning agents react with the free radicals formed during the oxidative polymerisation processes, as they are more readily oxidised than the drying oils or drying oil derivatives present in the coating. The anti-skinning agents hereby prevent the cross-linking from taking place, for which reason the drying of the coating is stopped.

The need for prevent skin formation might range from preventing in-can skinning during storage in closed container - by using volatile anti-skinning agents, [Methyl ethyl ketoxime, Cyclohexanone oximes ]
To prevent the ink from drying in the press duct - overnight duct stability only can be achieved by using non-volatile anti-skinning agents, more often referred to as antioxidants, [phenolics , Hydroquinone etc..]

Anti skinning agent in market - Antihaut spray (aerosol), Phenolated Compound Blendex are some of them available , but we have to consult with ink manufacturers before using these products for our setup ink.

What is Oxidation ?

Inks that use the process of oxidation to dry. Oxidising inks absorb oxygen from the air; the oxygen reacts with substances in the vehicle (usually drying oils such as linseed oil), which causes the vehicle to undergo polymerisation and results in the ink hardening into a solid film.

What is Polymerisation ?

A complex chemical reaction that takes place in drying printing inks in which several relatively simple, low-molecular-weight compounds (called monomers) combine to form a long, high-molecular-weight chain like molecule (called a polymer).

How does ink dry ?

Cold set offset ink dry by two methods;

by Absorption of vehicle into the stock (paper)
by Polymerisation of vehicles combine each other and form strong solid with pigment on surface of paper - in case of inks used for coated stock / screen printing / flexo and gravure printing.
In combination of absorption and polymerisation is also occurs in % according to stock they are printed.

In most cases, the first phase of ink drying is setting; immediately upon being applied to the stock, the liquid portion of the ink begins to evaporate into the air or to penetrate the stock, causing the ink to thicken. Setting is followed by actual drying via one or more possible mechanisms: absorption, oxidation, evaporation, or polymerization. The specific mechanism is determined by the relationship between the printing process itself, the ink vehicle system, and the substrate. Inks that are applied to an absorbent substrate such as newsprint dry only by absorption method. The liquid portion of the ink penetrates the substrate, leaving an ink film on the surface.

Depending upon the printing process, this ink film may undergo additional drying procedures. In oxidation, components in the ink’s oils chemically combine with oxygen in the atmosphere to form a semisolid or solid ink film. It often occurs in combination with absorption.

What is Vehicle in printing inks ?

“Ink varnishes are ‘vehicles’ for a reason; the vehicle is what conveys the pigment to the printed page. Also, the vehicle is the ink component most responsible for the final properties of the printed film. Ink vehicles are the heart of the ink. Vehicles and varnishes ultimately play a key role in how well the ink runs on press,

It is the binding agent added to pigment allowing it to dissolve within a solvent.

Distinguish <> Cold-set ink & Heat-set ink ?

There are two processes in offset web printing –heatset and coldset. To an average person, both heatset and coldest processes may produce the same product and look the same. The difference lies in the process. The essential difference with heatset is that it has the potential to produce superior quality print.

Heatset ink has solvents in it –when it reaches what is called ‘flashpoint’, it will evaporate. To achieve this, the paper passes through an oven dryer.
Cold-set web presses use far less energy and produce far fewer volatile organic compounds (VOCs) than their heatset counterparts produce

How Printing Inks are made?

From a powder to a liquid, learn how ink is made in these easy steps:

Step 1: The vehicle is weighed, transferred to a mixer, and heated by mixing. This process is necessary to make the product thinner than it normally would be at room temperature.

Step 2: Once the vehicle is thinned out, the pigment is added to the mixer. Every pigmented-ink starts as a finely ground powder and comes in a variety of colors. Most ink makers use the CMYK model, however, ink makers, especially in the screen printing business, are not subject to only these four colors.

Step 3: The smooth, blended product travels from the mixer to a transport cart.

Step 4: When the pigment is first added to the vehicle, some pigment particles stick together and form lumps. The ink is loaded up into a bead-mill machine, which is filled with tiny steel balls. When mixed, the balls break the pigment particles apart into tiny pieces and create smoother ink.

Step 5: After the ink is smoothed out, it goes through another machine called the three-roller mill. This machine is made up of 3 steel rollers that run in opposite directions. It smears the particles in the pigment even further apart and gives the ink the most color strength (the colorant’s ability to change the color of a colorless material) and a high gloss finish.

Step 6: Now that the ink is fully incorporated, it will go through a series of tests for quality control. The tests ensure that the ink is the same from batch to batch.

Step 7: Once approved by quality control, the ink is taken to another mixer. This is where extra ingredients such as waxes, drying agents, and other additives are mixed in.

Step 8: Next, the ink is run through the three-roller mill again. By repeating the process, the mill removes any final air pockets from the ink. This polishes the mixture and makes it even glossier.

Step 9: Finally, the ink is ready to be packaged into smaller containers. Once sealed up, the containers are given a label. From there, the ink is loaded up into trucks and ready for their next destination.

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