Ink and coatings

Jack Knight from INX International returns with a back to basics guide and provides a general description of the lithographic process and equipment

I want to start by providing a definition of lithography. Lithography is defined as a “printing process involving the use of aluminium printing plates whose surface is partly water repellent/ink receptive image area and partly ink repellent/water receptive non-image area.” It is a process that uses the principle that ink and water do not mix. The printing plate is so constructed that ink is left on the image area of the printing plate and the water helps keep the ink from spreading on to the non-image area of the printing plate.

Lithographic equipment

Coater – a coating roller applies a uniform film of coating on to the tinplate sheets before printing. A coating line consists of a sheet feeder that delivers the sheets from a stack of metal to the coater, one sheet at a time. A coating roller, covered with urethane, transfers the coating on to the sheets. Special coatings are used on the inside of the sheet to either protect the metal from the contents filled into the aerosol or food can or to protect the contents from the metal. Depending on the design either a clear coat or a white coat is applied to the outside of the sheet as the printing surface, transparent inks over clear coat and opaque inks over white coat. The coatings are applied individually then conveyed through a thermal oven for 10 minutes which cures the coating at a prescribed temperature and after curing cools the sheets down. A conveyor carries the sheets away from the oven into an automatic stacker that piles the sheets on to a skid.

Printing press – a press line consists of a sheet feeder that delivers the sheets from a stack of metal to the printing press one sheet at a time. The sheets are then printed and followed by a coat of clear varnish that protects the inks from abrasion. The varnish also provides mobility through the can lines after the cans have been manufactured and reduces transit abrasion during shipment. After the sheets are varnished they are then conveyed through a thermal oven that cures the varnish at a prescribed temperature and after curing cools the sheets down. A conveyor carries the sheets away from the oven into an automatic stacker that piles the sheets on to a skid.

Lithographic printing press  
Several different types of printing presses are used around the world. Some common types of metal decorating printing presses used are Hoe, Fuji, Harris, Mailander, Crabtree and Planeta. Though these presses have design and operating features that are somewhat different than the others all six function essentially the same way in terms of general lithographic principles.

An offset lithograph press has six main operating units. The following identifies them and briefly explains their function:

a. Feed table – the function of the feed table is to receive sheets of tinplate fed in by the feeder and then deliver them to the printing press properly positioned and in time with the press cylinder rotation to receive the inked image of the design being printed.

b. Dampening system – the dampening system functions to provide a controlled and continuous flow of fountain solution, a mixture of water and acid, to the printing plate. The fountain solution is attracted to the non-image area of the printing plate and repelled from the image area of the plate.

c. Ink distribution system – through a series of rollers covered with rubber, the ink distribution system functions to supply a continuous, uniform and consistent flow of ink to the printing plate.

d. Printing press cylinders – there are three cylinders on each printing press stand. Each cylinder performs its own separate and distinct function. The plate cylinder acts as a carrier of the printing plate. The blanket cylinder serves as a carrier that receives the inked image from the printing plate and transfers it to the sheets of tinplate. The impression cylinder functions as a pressure base or back up during the printing operation supporting the sheet of tinplate. The impression cylinder also houses the gripper assembly, which acts to control the position or registration of the sheets just prior to the blanket cylinder transferring the image to the sheet.

e. Discharge assembly – this unit takes the decorated sheet away from the blanket and impression cylinders and conveys it to the varnish coater and oven.

f. Drive mechanism – consists of an electric drive motor, drive shafts that provide the power needed to operate the five main operating units of the printing press.

Lithographic press configurations

Some lithographic printing presses are arranged to apply one colour of ink on a sheet during a “pass”, this press is called a single colour press. Two colour printing presses apply two different colours during “one pass”. This press is called either a two-colour press or a tandem press. There are also three, four, six and eight-colour printing presses.

General explanation of the lithographic process

This explanation of the lithographic process is only meant to provide a general understanding of the offset printing process.

Printing ink of a specified colour is placed in the ink fountain located at the top of the press. The ink ductor roller transfers a controlled, metered amount of ink from the ink fountain roll and transfers it to the first of a series of ink distribution rollers. The distribution rollers transfer the ink from one roll to the next milling out the ink evenly across the width of the press rolls. The ink is distributed down through the ink distribution rollers to four ink form rolls. The four ink form rolls transfer the uniform ink film on to the printing plate.

Fountain solution is applied to the printing plate by the dampening system. The dampening system applies a uniform, continuous and consistent film of water on to the printing plate. The fountain solution is attracted to the non-image areas of the printing plate.

A rubber-surfaced blanket that is mounted on the blanket cylinder receives the inked image from the printing plate and transfers it to the sheets of tinplate. The sheets of tinplate are fed between the blanket and impression cylinders in a controlled position to assure registration.

Offset lithography means the image on a printing plate reads from left to right. After the image is transferred from the printing plate to the blanket the image on the blanket reads from right to left or backward. The image is then offset from the blanket to the sheet of tinplate reading left to right.

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.

I hope to see you all at Asia CanTech in Bangkok in October.

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