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What is the importance of grain direction in printing ?

Papers and paperboards are anisotropic i.e they show different values for a same property in their orientation for e.g they expand more across the grain than along the grain direction. So the feeding in the printing machine is such a way that the grain direction is perpendicular to the feeding direction. This facilitates good feeding. Also the print length can be adjusted by changing the packing under blanket or plate cylinder. By adjsuting this the misregister caused due to the expansion or contraction of paper can be corrected.

Do I have to match the ink whenever I change the board ?

Yes, as a good manufacturing practice of printing operations, it is always advisable to match the ink to the substrate.The coating ingredients used, the type of coating processes, other parameters used change from machine to machine and mill to mill. These factors affect the properties and performance of the board. Hence, it is advisable to match the ink with the help of the ink supplier whenever you change the board supplier.

How do you minimise yellowing in UV varnish ?

UV varnishes are generally two types

  • Normal
  • Cationic

Normal varnishes tend to yellow more than cationic varnishes. In addition, a primer coat of Water based varnish before UV varnish also reduces yellowing. Usage of OBA free boards also tend to reduce yellowing.

How do you eliminate mottling in Printing ?

Mottling could be of many types like Back trap mottle, wet ink water interference mottle or gloss mottle. The tips here are techniques in hand to modify the ink settings when we need to solve the problem with the existing raw materials. However, detailed analysis is required to evaluate this problem.

  • If you have a spare unit, print double of the same print.
  • Increase the speed of the press.
  • Increase the printing pressure of the unit where the mottling is appearing.
  • Adjust the tack of the ink so that there is no back trap.
  • Reduce the water pick up of the ink which is used.
  • Reduce the feed of fountain solution in the unit where mottling takes place.
  • Reduce the pressure of the subsequent units to unit with mottling.
     
  When you want to print a high resolution printing what steps you need to take to  minimise dot gain in the printing pressure ?
  • Use the smoothest paper possible.
  • Use a ink, tacky enough, so that it does not smudge.
  • Give the minimum possible pressure between both Plate cylinder and Blanket cylinder, and Blanket Cylinder and Impressio der.
  • The substrate should not be too absorbent.
  • Matt surfaces minimise optical dot gain.
What are the factors which influences drying in printing ? 

In sheet fed offset surface pH of the substrate plays a very important role in ink setting and drying. A surface pH below 4.5 retards drying. A neutral or alkaline surface enhances drying of printing inks. Absorption of the surface also plays a role in ink setting. This needs to be balanced. A surface with very high absorption will absorb both solvents and resins and may cause flaking. A very low surface absorption will retard setting and may lead to set off problem. Press room humidity and temperature need to be optimised. A dry warm press room climate will enhance drying but may lead to other dimensional stability problems. Other factors which influence drying are pile height in the delivery stage of the printing machine. Temperature of the reel in web offset and gravure printing. Inks play an important role and need to be matched with the board surface. Dryers, wax type of solvents and resins play an important role in drying. A quick set ink will set fast and dry slowly but an oil based ink will set slow but dry fastly.

What is the cause of ply separation on the press & how can a printer control the same ?

Ply bond refers to the bonding strength between plies or within a ply. Several types of stresses can cause ply bond failure:
  • Tensile forces acting at right angles to the plane of the board (Z direction tensile force)
  • Outside shearing forces applied parallel to the plane of the board (ink tack is too high,printing forces are too strong, extrusion temperature is too high or the nip is under high pressure)
  • Shearing forces originating at the interior of the board (material is strained by being bent around small radius curves)
At printing stage
  • Reduce ink tack or replace ink
  • Reduce the press speed to reduce the shearing forces
  • Reduce squeeze pressure
  • Use quick release blankets
  • A printing machine where the board subjects to severe curvature can also give ply separation problem.

What is stiffness? How can two boards of the same grammage & caliper have different stiffness values? How does stiffness affect carton conversion ?

Stiffness, also called "flexural resistance", is a resistance bending under applied load. The stiffness of the board is directly related to the compression strength of the carton. It plays an important role in the performance of cartons in Form Fill and Sealing Machines, storage & handling, and in carrying the load of the material being packed.

The modulus of elasticity (MOE) & moment of inertia (MOI) are the two factors which control stiffness. The MOE & MOI differ with stock furnish. Boards are of different density - a board with same grammage can have different caliper. Stiffness is directly proportional to the cube of caliper. Therefore, two boards with similar caliper or grammage have different stiffness values.

What is print mottle and how do we control it ?

The appearance of ink on coated board is controlled by two factors - absorbtivity of the ink vehicle into the coated surface, smoothness of coated surface. Print mottle is the non-uniformity of the printed surface that affects the quality of print. It can be a small variation in either gloss or colour. It can also occur in solids, half tone areas, or both. Mottles are of four types:
  • Back trap mottle
  • Wet ink trap mottle
  • Gloss mottle
  • Water interference mottle

Back trap mottle occurs when the ink film losses its ability to transfer from the blankets to the printed paper as the sheet progresses through the multiple units of the press. This can be caused under one or more of the following conditions:

  1. Multiple colour printing units
  2. Non uniform ink absorption by paper
  3. Fast setting ink
  4. Fast setting stock

Wet ink trap mottle occurs with the increasing tack of ink . Ideally, the first down ink should have a tack that is higher than the ink going over it. In case the tack builds unevenly in an ink film as it splits further down the press, the equilibrium between the blanket-ink-board is upset. A non-uniform ink film occurs and back trap mottle results.

Water interference can directly impact the ability of the ink to transfer or interfere with the solids printed in the last press. If the surface is not absorptive enough to water, poor ink transfer occurs. To prevent this, the fountain solution should be run with minimum concentration.

Gloss mottle occurs when there is gloss variation in the paper stock and is reflected in the print.

How can 'hickeys' be controlled at the press room ?

Hickeys are specks in the print that look like doughnut or white spots. They occur when a contaminant sticks to the plate or blanket and interferes with print quality. Loose surface dust is a major cause of specks in printing. When loose dust sets on the paper surface, it prevents ink from reaching the sheet and sticks to the blanket. Loose dust can be either,

  • Air dust that accumulates on paper surface at the paper mill or at the printing press or
  • Paper dust which is composed of both fiber and coating materials generated during sheeting, cutting, slitting and trimming and during stacking & feeding in the press room.
Air dust can be avoided by good housekeeping. If dust is already on the paper, the first unit may be run blank, without any ink or fountain solution. The dust gets deposited on the blanket but needs frequent cleaning.


What is picking ?

Picking is the fracture of coating layer. It occurs when tack of the ink exceeds the cohesive forces of coating on to the base paper or the printing pressure is too high to pick the coating or the paper surface.

Piling occurs if the paper coating is too sensitive to fountain solution and the coating dissolves and gets deposited on the blanket.

If number of impressions / hr is high, coating material can build up rapidly and interfere with ink transfer.

Piling is the result of the ink in the first units of the press, building tack on the surface of the paper and eventually causing paper failure on the blankets further down, in the press. Piling shows up immediately and is more concentrated at the tail of the print image. It can be controlled by increasing wet pick strength of coating and frequent wash-ups at the press.

What is the significance of fountain solution in printing process ?

Fountain solution formulated for offset lithography consists of five basic ingredients: Water - to protect non-image area

Acid - to reduce pH for accepting ink in the image areas & water in the non-image areas
Gum Arabic - gum adheres to non-image area, keeps it from accepting inks, and protects it if it dries during press idle time Wetting agent - to decrease surface tension of water, allowing the plate to run with minimum water; keeps non-image area free from attacking ink.

Fountain solution separates image and non-image areas; Too low concentration of the solution may cause plugging to image area, tinting, summing and too high concentration may cause increase in dot gain, ink piling, poor ink drying and wet trap mottle.

PH and Conductivity are two important parameters parameter in fountain solutions.
     
  What is the difference between quality and measurement ?

The term quality is subjective and is more to do with feel than to measurement. Quality is fitness for purpose. If the product meets all the demands of its application it can be called a quality product. Measurement involves tools and we can arrive at quantifiable values and is objective

What is the understanding required between a print buyer and a print supplier ?

The print buyer should clearly spell out the quality expectation levels with the supplier. The measurement parameters in the final print job should be clearly spelt out. Once the measurement parameters are agreed the acceptable tolerances should be arrived at so that it meets all its application needs.

How can you categorise the quality levels in a print job?

  • Economy : Single / Double color jobs like bill books, computer stationeries , simple memos etc.
  • Regular : Single and multicolor jobs like direct mailers, periodicals , magazines, books
  • Premium : premium product catalogues, annual report covers, display cartons
  • Super Premium : Coffee table books , premium magazines, cartons of premium products

What are the print quality variables which affect the job and are measurable?

  • Colour density
  • Colour match,
  • Dot/loss
  • Screen percentage
  • Print Register
  • Half Tones
  • Seperations
  • Coatings
  • Finishings
  • Any Minor Flaws

What is an OK sheet?

OK sheet is the printed sheet which is approved by the print buyer for all aspects of printing such as the

  • Details of text
  • The colour and the density of the colour.

The size and dimensions of the printed sheet and any operation

This can be treated as a standard sheet for all the future production. For repeat jobs the print supplier should protect the OK sheet preferably in a light proof wrapper. Even then it is advisable to periodically review the OK sheet by both the parties. For repeat jobs it is also advisable to define the tolerance in terms of light and dark sheets.

What is Print Register ?

Print register defines the positioning of the components of the print with each other and to the sheet. Print register variations are caused by poor setting of front lay , side lays and the pressure setting of the gripper fingers with respect to the thickness of the printed sheet. Register can be measured with register marks and magnifiers with minimum magnification power of 10 X. The variation can be measured in millimeters or in thou.


What is print density ?

Print density is the darkness of the printed ink layer. It is measured with densitometers or colour patches. Density is measured in a logarithmic scales ( For e.g. Density of 1.00 means that 1/10 of the light striking an area is reflected. Density of 2 means 1/100 of the light striking an area is reflected)

Density variation can be to the entire sheet or specific to given areas in a sheet. Some sheets may be darker/lighter than the others. Density vary with colour , substrate used, inks, machine used. Typical density values on a coated substrate are yellow : ± 90 Cyan & Maganta : ± 1.4 and Black : ± 1.6

How do you define screen percentages ?

Screen Percentages is the percentage of print over white in a specified area. The printed area is covered by dots. Screen percentages. Variation can be caused by the errors in printing pressure between blanket and impression cylinder, wrong exposure or development time in platemaking and image setting.. Screen percentages can be measured with tools as densitometer ( dot area function) color patch, printed screen area.

What is dot gain or dot loss?

Dot gain/loss is the enlargement/reduction in the dot . This takes place because of various factors like the roughness of the substrate, improper printing pressure, low viscosity and tack of the ink

How to limit the ill effects of dot gain/loss ?

Dot gains is predictable and is non avoidable. It is upto the printer to make proper allowance to limit its damage. Dot gain/loss in excess results in dark/light image. Dot/ gain/loss should remain same through out the run to avoid any variations in the print

How do you measure dotgain /loss?

Dot gain/loss is measured with the help of GATF star target. which is included in the colour patches. Percentage of dot gain can be calculated with the help of densitometer by comparing the dot on the plate or the standard with corresponding dot on a press sheet .

How is colour matching done?

Colour matching is done by comparing the printed sheet with the swatch book or OK sheet . The colour matching should be done with the same substrate on which the final job is going to be printed. Colour matching applies to both cut colours and the process colours.

When and why do variations occur in colour matching?

The variations occur when

  • the ink used is different.
  • the recipe for the cut colours got changed.
  • the colour is matched under improper lighting
  • there is a variation in colour and ink absorption of the substrate used.
  • the dot gains/loss is more than what is acceptable.

What are the tools required to do colour matching?

  • Controlled lighting conditions ( colour matching Box or room with light whose colour temperature is 5000 ° K)
  • Colour swatch books or pantone guides
  • The standard proof or the OK sheet.
  • A spectrophotometer

How can we measure colour objectively ?

Colour can be measured objectively as L, a, b values with the help of spectrophotmeters. The difference in the color value between standard and the print is measured as ?L, ?a, ?b,.

E which is the variation in color is given by the formula v ?L² +?a² +?b²

What are halftones?

Halftones refers to the screened portion of a image which often get affected by the reproduction quality of the dot. This could be due to density variations in the print area which is caused by the printing machine used., errors in scanning, halftone photography, platemaking, substrate smoothness and ink absorbency of the paper used.

How do we measure halftones?

Measure the density range by subtracting the density of the darkest area to that of the lightest area . The edges of the dots are measured for its sharpness. Compare shadow areas with that of the proof or standard sheet.
Are the shadow areas clean or plugged.?

What are separations?

Seperations are similar to half tone but for a multicolour job

What are the other minor flaws which should be considered in evaluating a print job
Includes scumming, set off, hickies, smudges, wrinkles, doubling, slurring, ghosting etc.

What are the Coating influence to be considered in evaluating a print job?

Varnishing, UV varnishing, Film lamination, should be uniform without any peel, blistering, curling, or any discolouration

What are the Print finishing influences to be considered in evaluating a print?

Accuracy of dimensions, cuts, drills, folding, trims, scoring. This should be measured with a scale.
   
 
   
  What is the guide line for different quality levels of all the above parameters ?
A guide to quality levels of different Print parameters
  Economy Regular Premium Super Premium
1. Print Register ± 0.04 mm ± 0.025 mm ± 0.012 mm ± No variation allowed
2. Dot gain ±10% ± 5 ± 1 1
3. Density ± 7% ± 5% ± 3% ± 1%
4. Screen Percentages ±10% from the nominal percentage ±5% from the nominal percentage ± 2% from the
nominal percentage		 no variation from the nominal percentage
5. Colour Match < 2 < 1.5 < 1 < 0.75
6. Half tones Density range 0.9, No shadow detail, slightly fuzzy dot Density range 1.2, Some shadow detail, sharp dot Density range 1.6,
Full shadow detail, very sharp dot		 Density range 1.8, Almost Match the original
7. Seperations Not applicable Almost match the digital proof approved Density range 1.2 Almost match transperancies
Density range 1.6		 Almost match the original Density range 2.0
8. Minor Flaws On max10% of the sheets On max 5% of the sheets On max 2% of the sheets ON none of the sheets
9. Coatings Not critical Uniform without slight cast or flaws Uniform without
any cast or flaws		 Uniform without any cast or flaws
10. Finishing ±1.6mm ± 0.8 mm ± 0.4mm ± 0.4mm
   
 
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