For web offset the paper roll is scrolling at very high speeds and the paper path is passed from the roll through the printer device to the folder. Along the runway steel rolls are installed which stabilize the web by stretching it out to give it a little tension. In order for the paper path to pass through its high speed path and printing properly it must be dense but not broken. In most modern web offsets photocell sensors are installed along the track to ensure continuity of the web throughout the printing process. These sensors provide continuous feedback signal to the machines controls. When the signal is broken the machine stops to prevent paper being pulled into the rollers and rotating cylinders that cause damage to the machine.
During printing there are times when the web is broken. This interferes with the printing process because the machine will stop. Machine stoppages lead to high waste and low efficiency and productivity. At each start the first prints must be scrapped before a quality printout is obtained. The quality will eventually be affected by speed fluctuations due to stops and start-ups. In order to improve efficiency reduce waste improve quality and improve productivity stop should be minimized as much as possible. Web crashes are one of the main reasons for stopping web slides. The following are the 5 biggest reasons for web breaches in web offset printing.
Some paper rolls have manufacturing errors like uninterrupted roundness bad winding bad joints foreign objects glue inside or on the sides of the roller including loose roller core. In most cases such errors lead to web crashes while driving. The roller can also be damaged due to poor handling storage or transportation. Poor storage space can also affect the condition of the roll either that it becomes too dry or too damp and affects its performance on the press. Poor quality paper will also break when exposed to tension. Hjulfel and paper quality are therefore major reasons for offset printing.
Reel is wrong
Another important cause of offset pressure breakage is an error on the coil. The wheel stand is designed to apply just the right tension on the roller required by the pressman. With the help of pneumatic and electronic controls the tension rollers apply tension on the roller and the mesh to extend it from the roller is all the way into the printer unit and in the folder. An error in the electronic or pneumatic controls or during calibration of the tensioning rollers will result in either too much voltage or low voltage. This will lead to a web break and the machine will stop. Due to an error the reel can not switch from one coil to another which automatically leads to a web violation. Therefore the fitness of the wheelchair can be a source of offense pressure web breaches.
Poor voltage regulation
During the press operation the press man adjusts the voltage parameters according to pressure and paper conditions. Depending on his assessment he can remotely increase or decrease the paper tension at the main control console. On this console the press can manipulate the paper tension at the roll holder after the print device or in the folder on a central screen with one touch of a button. If he increases or decreases the voltage the track will break and the machine will stop. Poor voltage control is another important reason for offset printing.
Bad ink and water balance
Litographic offset process is based on the principle that water and oil are not mixed. In simple terms the ink which is oil-based and damping solution is water-based applied to the printing plate surface. The image areas on the plate are oleophilic and will attract the oily ink and expel the attenuation solution while the non-formed surfaces of the plate are hydrophilic and will attract the damping solution and wipe off the oily ink. The balance between the attenuation solution and the ink applied is crucial for a clean print quality. If too much water is applied excess water will be absorbed by the paper. Waterproof paper will easily break under tension. Excessive use of water under printing is therefore the main cause of web breach in web offset printing.
Bad joint pattern
Paper rolls are available in different sizes. Weights and diameters vary depending on paper usage. Different manufacturers produce paper rolls depending on customer and market requirements. Depending on how many copies to be produced many rolls may be required. When a reel is consumed the reel must be automatically replaced to a new reel to avoid pressure drop. In order for this change to occur smoothly and at high speed a splice pattern must be made. The track from the output coil will be pasted into the new coil which then continues to unwind and feed.