Slur is when a printed image is distorted due to running conditions on an offset printing press. The dot or type appears to be shifted. Slurring occurs in only two directions: lateral or circumferential. Each direction represents a different problem that must be remedied.
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| Slurring |
Slurring in the commercial printing market.
Most slurring is never noticed by the average viewer. It is usually detected by instruments used in the offset printing world or by special patches on the printed sheet mentioned below. However it is sometimes noticeable to the naked eye in commercial printing jobs. For example this beer label:
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| Slurring in the letters. |
Causes of Circumferential Slurring
I list each of the possible causes in order of probability. Slurring can be an easy fix or a deep problem.
1. Blanket torque. When the blanket has become a little loose, the circumference changes, and hence the speed of the printing unit. This means that the paper is pulled through at a slightly different speed and causes a slippage during the print run. Torque it to manufacturer specifications. See this article for more information on this.
2. Web tension. This is for web press printers. Tension problems can originate before or after the printing unit. If it is before the printing units, it may likely originate at the roll stand or infeed unit. Tension needs to be adjusted to the proper feed of the press. If the slurring is caused by tension after the printing unit, it needs to have just the right pull from the successive printing unit or the next driven nip point.
4. Register shift. When running color jobs, all the colors must print exactly on top of one another. The press will make adjustments, sometimes automatically, to make this happen. This is only a problem if slurring is continuously happening as the press tries to keep up with a wandering target.
5. Roller bearings. Inking and water rollers have bearings. When these bearings wear out, it will cause the form against the plate to "slide" and cause slur. The best identifier to this is the erratic patterns that it shows. The best way to check for this is to use a heat gun and check the temperature of the bearings while running.
6. Worn gears. This is a deep problem. I've seen it once. The press gears were so worn that slurring was happening in the circumferential direction. It is a last resort, but needs to be looked at after other possibilities have been checked.
Causes of Lateral Slurring
1. Paper movement. If the paper is moved during printing, a slur can take place. On sheet fed presses, the grippers need to be kept clean and in good working order. On offset web presses, the web moves and automatic adjustments are made to keep it in line. These movements will cause slur as the paper moves across the blanket. This is only a problem if the lateral slurring is constant.
2. Paper. Some papers are better made than others. Sometimes a poorly made paper (especially in web printing) can cause tension issues and thus slurring. One side of the paper may be baggy, while the other is not. Sometimes the paper will weave back and forth from side to side. Whatever the case, changing paper to a different batch will help reveal if this is the problem.
3. Register shift. Just like circumferential shifts, lateral shifts are made by registration moves in color printing. Sometimes adjustments can be made to an automatic register system as to how big the adjustments are made by the system or how fast it makes the moves.
4. Form oscillation. In order to deal with issues like ghosting, some plate forms will oscillate laterally. Settings that are not done right against the printing plate or other rollers will cause a slur to take place. Check the settings to eliminate this possibility.
What is done to detect slurring?
Offset printers will also print a color bar on the sheet with patches that will help them detect even a slight slur. Here is an example:
In this patch, the hourglass will detect circumferential slurring. The bowtie detects a lateral slur. Offset press operators will use these patches for early detection.
I've come up with remedies to help prevent slurring in another post. I hope this article gives you a clear definition of what slurring is.
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