Emulsification


Here is an old saying that oil and water don't mix, however in lithography if 
the two don't properly mix, you will not be able to print. In order to make 
the system work, you need a proper marriage among all the variables. All
 lithographic inks need to emulsify fountain solution in order to work
 correctly. If a lithographic ink did not emulsify fountain solution, the ink
 would not even transfer to the plate. Too much emulsification will result in
 scumming, too little will cause water marking. In the lithographic process,
 the plate is composed of two different types of areas. The non-image is a 
water loving area (hydrophilic) while the image area is oil loving (oleophilic)
. While this means that the water or oil will preferentially like one area 
greater than another, it does not prevent the water phase from going to the
 image area. The fountain solution for instance will wet out uniformly on the
 non-image area and bead up on the image area. When this occurs, if the ink
 was not able to emulsify the fountain solution, the ink would not transfer to
 this spot and thus cause snowflaking in the solids or missed dots in the 
halftones. Thus, the need for emulsification. 

While all lithographic inks need to emulsify fountain solution, there is a 
need to control this level on press. All inks will have a Awindow@ for 
optimum press performance.
However, this does not mean that the ink will not print with varying levels of
fountain solution emulsified. Thus, comes the term Ink and Water balance. 
If this balance is not properly maintained, various press problems will result.
There is no "universally correct" ratio. The correct ratio is dependent on the 
products in use, and on the various interacting systems on the press. As a
 rule it is usually safe to assume that near-optimal transfer will occur with
 minimal dampening and ink levels.

Signs of improper ink water balance:

1. Ink build-up on water form.
2. A very wet, shiny plate.
3. Ink spitting onto guards, floors, etc.
4. Solids look mottled, not uniform in density.
5. Solids appear weak, lack density.
6. Dots lack density.
7. Loss of detail in shadow portion of pictures.

Dampening systems can have an effect on ink water balance also. In an ink 
train dampening system, the process of emulsification provides the means 
where the fountain solution is transported from the dampening system in 
both finely emulsified and large droplets in the ink roller train, to the surface
 of the plate. At the surface of the plate, the large droplets break down, 
partially due to the pressure differences set up, and partially due to the 
physics and physical chemistry of the plate surface. The free water released,
 forms the aqueous film on the non-image area and thus aids the repulsion 
of the oily ink. This creates additional concerns in setting the proper ink 
water balance ratio.

Factors that influence water settings on the press :

1. Size of image area on plate

The amount of coverage on a particular page will be critical in setting the ink 
and water balance. The amount of water necessary will vary depending on 
the coverage. Many press operators will look to a dial setting for the 
fountain solution as a way to gauge if they are at the correct ratio during a
 run. The amount will vary depending on the need. Thus, the levels of ink 
and water will vary.

2. Image area

 Solid or screened. Similar as above, a solid will require different settings vs.
 a screened area.

3. Overall printing area across a cylinder.

This will be more prominent on process color units when dummy plates are 
being utilized, especially when brush or sock type dampening systems are
 being used. These types of dampening systems are more difficult to control
 since there are no page-to-page settings.

4. Temperature

This is a major variable in a pressroom that is not controlled. Varying
 temperature will have an effect on the level of fountain solution being used
. A lot of heat is generated on press. The only way for the press to cool 
down is by evaporating the fountain solution. Some presses (typically 
heatset) have water cooled rollers to aid in this process. Uncontrolled 
humidity in the pressroom will also cause varying evaporation rates. 
Temperature variation will also create different demands on the ink for 
emulsification, because of varying emulsion stability.


The above lists some of the factors which occur on press with a given set of 
products being utilized. Ink and water balance will shift also, depending on
 the consumable products used in the pressroom. We are not trying to list
 all of the variables here, but just to alert you that they exist. One of these, 
for example, would be the fountain solution type and concentration. Some 
fountain solutions promote clean running at low levels of emulsification, 
while with others the exact reverse occurs. The same is true of the ink, with
 different formulations requiring different levels of dampening to run cleanly
.

How to achieve correct ink water balance :

The following steps should be used as a guide on setting the proper ink and 
water balance on your press:

1. At the unit reduce water level 5 points at a time. Allow 3 to 5 minutes for 
the press to react.
2. Observe ink level and reduce as necessary to maintain proper density.
3. Continue this process until you reach the minimum water settings.
 Minimum settings are when the background looks dirty and the ink density 
is in specification or slightly below.
4. If you get toning in background, bring water level up slightly.
5.Once water settings are correct, proceed to adjust ink to proper density.
 Check with a densitometer, don't guess.

When the proper ink and water balance is achieved on press one will gain 
many advantages. The following are some of the benefits in achieving this
 proper balance:

Advantages of correct ink water balance :

1. When water and ink are at minimum levels, ink settings are more
 consistent (less fluctuation) throughout run.
2. Trapping of solids is better.
Example: Solid magenta - solid yellow = reds better
Solid cyan - solid yellow = greens better
3. Single Color - signatures are better.
Laydown smooth, not mottled.
4. Better 4-color reproduction.
5. REDUCED build-up on ink rails.
6. REDUCED misting.
7. REDUCED lint build-up on blankets.
8. REDUCED plate wear.
9. Optimal pages per pound of ink.
10. Lower cost, less waste, increased productivity.
11. Reduce rub off of ink.
12. Reduced pipe roller buildup
13. Reduced set-off.
14. Reduced Dot Gain

Comments

Post a Comment

Popular posts from this blog

Packing and Cylinder Pressure

PRESSMAN TIPS  Packing and cylinder pressure seem to be the number one culprit in printing related problems. It is absolutely necessary for the press operator to check and maintain proper printing pressure between plate, blanket, and impression cylinders. Press manufacturers should be able to supply this valuable infor mation. Most of the time the packing information is located on a plate on the press or in the operating manual. How much pressure is too much? Most operators feel that is .004" squeeze is good, then .008" must be better, and if that doesn't work then an extra .002" - .004" will certainly help. This will without a doubt effect quality and production. Most presses should operate with .004" - .006" squeeze between cylinders. Any more than this can cause problems like dot gain, emulsification, bearing shock marks (streaks), poor register and fit to name a few. It would take several paragraphs to explain how each one of these problems ...
Read more

Pre-Press-Color halftone & Screen angles

In reproducing colour pictures that were originally continuous tone, printing requires us to overlay the CMYK images as halftones. If the halftones are AM (amplitude modulated), they will be formed from regular patterns of fixed frequency. The need to rotate the angles of the halftone patterns comes from the fact that printing cannot precisely plac e down one halftone dot on top of a previously printed other-coloured dot. Commercial-quality colour printing typically has 60 dots per cm in both the horizontal and vertical direction. Slight inaccuracy in placing four identical patterns on top of each other will result in an unpleasant moiré or screen clash. By rotating the screen patterns at 30° from each other it is possible to reduce the frequency of the moiré to a level which is below the visual threshold – it becomes too small to be obtrusive. That reduced moiré is the pattern which is commonly called the printing rosette. As the halftone patterns are crossed lines at 90°, we only hav...
Read more

Ink Mileage

Ink mileage is usually defined as the number of copies produced with a kilogram of ink, or more commonly quoted in the newspaper industry as the kgs of ink required to print a thousand impressions on paper. . Estimating ink mileage has always been a difficult task. This is because there are several interest ing factors that control the amount of ink required to achieve the final ink density. The factors that most influence the ink requirements are as follows: 1) Strength of the ink 2) Substrate 3) Print Density Control 4) Ink and Water Balance 5) Print coverage Strength of the ink The key factor here is the level of pigment in the final ink formulation which determines how much ink is required to achieve density. The pigment is the ingredient in the ink that either absorbs or reflects light. When ink is printed on paper, what we are trying to do is to change the light reflection off of the paper so that the reader perceives color. The pigment type has certain characteristics to ...
Read more