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| What are the various types of Printing? |
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Basically conventional printing processes can be divided into 3 categories. |
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Letter press / Relief / Typographic. |
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Off set / lithography / planography. |
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Gravure / photogravure / Intaglio. |
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| What is Letter Press printing ? |
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In this process ink is applied to a raised printing plate surface by a system of rollers made of rubber. From the printing plate the ink is then transferred to the paper under pressure. During printing, the ink film splits, part of the film transferring to the paper and part remaining on the plate. The ideal situation from stand point of good printing is to transfer as much ink as possible to the paper because in this way the plate is cleaned for the next printing. |
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| What is Gravure Printing? |
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This is exactly the reverse of letter press printing. The ink is held in the engraved surface or recessed surface. In gravure process the plate is inked by rollers, the excess ink removed from the unetched surface of the plate. Paper is then pressed against the plate where it picks up the ink held in the recessed areas of the plate. |
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| What is Off-Set Printing? |
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In this process the printing area and the non-printing area are in the same plane. The printing plate first comes in contact with water (fountain water). The printing area does not catch water but the non-printing area catches water. Then the plate comes in contact with inking roll. The printing area receives ink and non-printing area does not receive ink, due to the presence of water. Then the image is transferred to a rubber blanket and in turn transferred to paper under pressure.
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| What are the requirements of Multi-color printing? |
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For a high quality multi-color printing the following are the necessary properties in the order of their importance: |
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Consistent performance on the press |
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Uniformity of formation |
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Smoothness |
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Ink receptivity |
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High surface strength |
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Uniformity in thickness |
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High opacity |
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Good Brightness |
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Good Color |
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Resilience |
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Balanced Moisture content |
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High Degree of performance |
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| What are the Important requirements of off-Set Papers? |
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Off-set papers be clean, strong and stiff. |
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Sheet smoothness is less important in off-set lithography than in letter press or gravure printing. |
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As with all printing papers optical properties, moisture content and bulk are important in Off-Set papers. |
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Resistance to blistering, fiber puffing and cracking at fold is important in web Off-set papers. |
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Problems such as scrap in the skid, dog ears, short sheets of paper and poor or unmarked splices and uneven roll in web paper are clearly runability problems. |
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Since the Off-set blanket contacts every square centimeter of paper, freedom from dirt and debris is very important in Off-set papers. |
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Off-set lithography demands greater surface strength of paper than other methods of printing. |
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Degree of sizing and moisture content of the sheet effects its behavior during Off-set printing. |
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The Off-Set blanket stretches the sheet, particularly in the areas carrying heavy ink converge. Off-set papers thus require greater stiffness than other papers. |
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Blisters result from a number of causes: rapid and excessive heating of sheet, excessive moisture content of sheet, low surface porosity, and low internal bonding. |
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Tearing resistance and Tensile strength are very important in web Off-set.Excess acidity in paper contributes to retarded drying of inks for sheet fed papers.
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| Which are the various printing defects associated with papers? |
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Hickies : Can occur with any of the main printing processes. They are random spots on the printed surface, each consisting of a dark spot surrounded by a white halo, and they are caused by solid particles of ink skin, paper debris, damper roller lint or debris from the ink rollers which attach themselves to the printing surface. |
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Lint or Debris Piling : Mainly occurs with letterpress and litho, and usually occurs when running uncoated grades. Loose fibers on the paper surface are picked up by the blanket, which in turn breaks up the print reproduction on the following sheets. |
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Surface Picking : Can occur on letterpress and litho printing process. The fiber or the coating of the paper is lifted from the surface. This can be due to too high an ink tack or insufficient binder in the paper stock. |
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Chalking : Occurs mainly with letterpress and litho printing processes and with coated papers. The dried print can be rubbed and so causes the print to smudge. This problem occurs because the ink vehicle drains into the paper, leaving the pigment with insufficient binder, or because the paper is particularly abrasive. Most matt-coated papers in particular display abrasive qualities. |
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Mottle : Can occur on any printing process. The print, especially in solid areas, appears to be blotchy. This can be due to uneven absorption characteristics of the paper stock, or poorly formulated ink. |
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Set – Off : Can occur on most printing processes. Traces of the ink on the printed side of a sheet are transferred to the back of following sheet in the deliver pile. |
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Ink-Rub : Is associated particularly with illustrated work printed on matt art paper. The ink on the sheet stays wet too long, causing set-off / smudging either in the delivery pile or in the later stages of the finishing / binding process. Some papers are worse than others in this respect. |
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Show-through : The printed image can be seen through from the opposite side of the sheet, due to the opacity of the paper being too low for the type of job being printed. |
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Strike-through : The effect is similar to show–through except that it is caused by ink penetrating through the sheet. |
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Ghosting : Can occur on most printing processes. The printed image on one side of the paper appears as a ghost on the printed image of the other side. This is due to chemical ingredients in the first print affecting the wetability characteristics of the reverse side of the paper. |
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Blade Scratches : Only occur on blade coated papers and are caused by dirt or coating mix getting caught under the doctor blade on the coating machine, and causing a fine channel to be formed. |
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Distortion : This can be in the form of wavy or tight edges in a stack or reel of paper |
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Wavy edges – Caused by the stack having a lower moisture level than the surrounding atmosphere. Therefore the paper takes in moisture from the atmosphere and in so doing causes the edges of the sheets to go wavy. |
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Tight edges – Caused by the stack having a higher moisture level than the surrounding atmosphere. Therefore the paper gives out some of its moisture to the atmosphere and in so doing causes the edges of the sheets to become tight and the center of the sheets to become baggy. |
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Creasing : An uneven moisture content in the sheet causes creases at the edges and back of the printed area. If the edges are wavy, creases tend to occur on leave edge corners, if tight, creases tend to occur in the center of the sheet.The remedy is to leave packed paper to adjust to room temperature before opening. |
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Tail End Hook : Is caused by the paper adhering to the blanket too tightly when pulled off by the delivery grippers. |
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Blistering : Top layer getting separated from the body of paper due to high pull of the blanket. |
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