In web offset printing, the reel of paper is unwound at very high speeds and the paper web is passed from the reel stand through the print unit to the folder. Along the web path, steel rollers are installed which stabilize the web by stretching it out to give it some tension. For the paper web to pass through its path at high speed and print correctly, it needs to be tight but should not break. In most modern web offset machines, photocell sensors are installed along the web path to ensure continuity of the web all through the duration of the printing process. These sensors give continuous feedback signal to the machine controls. When there is an interruption of the signal, the machine stops to avoid paper being pulled into the rollers and rotating cylinders causing damage to the machine.
During printing there are instances when the web breaks. This disrupts the printing process because the machine will come to a stop. Machine stoppages lead to high wastage and low efficiencies and productivity. With every start up, the initial prints have to be scrapped before a quality print is obtained. Quality will eventually be affected with speed fluctuations due to stoppages and start ups. To improve efficiency, minimize waste, improve quality and improve productivity, stoppages should be minimized as much as possible. Web breaks are one of the main causes of stoppages in web offset printing. The following are the 5 major causes of web breaks in web offset printing;
Some paper reels have manufacture’s defects such as out of roundness, poor winding, poor joints, foreign objects, glue inside or on the sides of the reel, loose reel core among others. In most cases, such defects will lead to web breaks during run. The reel may also have damages due to poor handling, storage or transport. Poor storage can also affect the condition of the reel either making it too dry or too moist affecting its performance on the press. Poor quality paper will also break when subjected to tension. Reel defects and paper quality are therefore major causes of web breaks in web offset printing.
Reel stand fault
Another major cause of web break in web offset printing is a fault condition on the reel stand. The reel stand is designed to apply just the correct tension on the reel as demanded by the pressman. Using pneumatic and electronic controls, the tension rollers apply tension on the reel and the web to make it stretch out from the reel stand all the way into the print unit and to the folder. A fault in the electronic or the pneumatic controls or in the calibration of the tension rollers will result in either too much tension or low tension. This will lead to a web break and the machine will stop. Due to a fault, the reel stand may not be able to change from one reel to another automatically leading to a web break. Therefore, the condition of the reel stand can be a cause of web breaks in web offset printing.
Poor tension controls
During press operation, the pressman adjusts web tension parameters according to press and paper conditions. Depending on his judgment, he may remotely increase or reduce paper tension at the main control console. On this console, the pressman is able to manipulate the paper tension at the reel stand, after the print unit or at the folder on a central screen at the touch of a button. If he increases or reduces the tension too much, the web will break and the machine will stop. Poor tension control is hence another major cause of web break in web offset printing.
Poor ink and water balance
Lithographic offset printing process relies on the principle that water and oil do not mix. In simple terms, the ink which is oil based and dampening solution which is water based, are applied onto the printing plate surface. Image areas of the plate are oleophilic and will attract the oily ink and repel the dampening solution while the non-image areas of the plate are hydrophilic and will attract the dampening solution and repel the oily ink. The balance between the dampening solution and ink applied is critical in getting a clean quality print output. If too much water is applied, the excess water will end up being absorbed by the paper. Water soaked paper will easily break under tension. Excessive use of water during printing is therefore major cause of web break in in web offset printing.
Poor splice pattern
Paper reels come in different sizes. Weights and diameters vary depending on paper mills. Different manufactures produce paper reels depending on client and market demands. Depending on the number of copies to be produced, many reels may be required. As one reel is consumed, the reel stand will have to change to a new reel automatically to avoid press stoppage. For this change to take place smoothly and at high speed, a splice pattern has to be made. The web from the expiring reel will be pasted to the new reel which will then continue to unwind and will be fed into the print unit for production. For this change over and pasting to be successful, the arrangement of the pattern has to be correct. The gum and the splice indicator have to be place at the right position. A pattern that is poorly made will end up in a web break because the pasting will not be accurate or strong to withstand the tension on the paper. Hence poor splice patterns are major causes of web breaks in web offset printing.