Stories of engineers / Large format printer maintenance. Image banding while printing

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Image banding while printing.

Stripes might appear in the following situations:

1)            Unstable operation of print heads

2)            Unevenness or inconsistent supply of material

3)            Incorrect operation of material reeling and unreeling system

4)            Image banding because of the wrong regime of printing

5)            Inconsistent aggregation of print heads.

Knowing physics and origin of image banding problem, it is possible to solve it by using following tips:

1)  Problem with unstable operation of print heads. It's one of the basic problems that cause image banding. Is notable for the fact that at first, right after printing, we get a good quality image but after some time banding becomes apparent. Frequently, it happens because of air which got into the print heads or wrongly adjusted ink-vacuum system. This problem was considered in detail in the following article.

2) Problem with uneven or inconsistent supply of material. It consists of a number of similar light or dark stripes on the image. Its width can vary. It is due to the fact that when printing, printer put out an image by passages, after each passage, material supplying shaft, stretches out material on a certain distance. Frequently, supply of material is not adjusted. In different printers, a certain preset index (подстроечный коэффициент) Feed Compensation is responsible for it. Its task is to increase or minimize the pace of supplying. Unfortunately, not everyone understands that while changing chargeable material, pace of supplying can also change. The mass of a stretched material changes, so engine needs more effort to stretch material on the same distance. As a result, shaft stretches material for a smaller distance and we get superposition of passages on one another. Consequently, we can notice dark stripes on the image. There also can be an opposite situation, when while printing on a very light material, supplying shaft can push printed material too far, and we get white stripes on the image.

Here I mentioned the simplest variant of the problem, but they vary. For example, let's take one of the chinese printing press. I won't mention any brands. Problem consisted in the fact that periodically while printing, black and white stripes appeared with a certain pattern. Problem was in the crookedness of supplying shaft. Or in rollers, located under the printer, that supported this shaft.

Important value consists in adjusting drivers for material supply, it happens sometimes, that drivers are adjusted to a lower capacity for engine, and that is why in acceleration or deceleration regimes of material supplying shaft, spontaneous miscarriages of feed pitch occur. And we can notice so called floating feed.

Also, you should pay attention to a backlash in the asterix of the supplying shaft, tension of belt, and you should keep an eye on the elements that require lubrication.

Clamping rollers are also very important, which when material is poorly pressed, don't allow to press the material to a supplying shaft tightly, and material slip.

3) Incorrect operation of material reeling and unreeling system. You should pay attention to reeling and unreeling material system, loop must form correctly. There mustn't be any excessive tension, no bounded together layers on the banner, vinyl.

4) Problem of image bending because of the wrong printing regime. Quite frequently printer sellers claim excellent speeds of printing. However, they don't mention the quality of printing. It is worth noting, how the speed is usually reached:

a. Printing in a small quantity of passages

b. Printing in two directions

c. Increasing speed of carriage movement and speed of material supply

If, for example, while printing, print head initially had some broken nozzles, then they will give out blank spots on the image. And the little passages the printers makes, and the more broken nozzles are there, the more noticing will the defect of printing be. Also when printing in a small quantity of passages, you should pay more attention to material supply.

While printing in two directions, very frequently there is a fact of not aggregating the printing regime bidir. That means, dots lay in incorrect places, and we see fuzzy, striped image. Also printing in two directions can give out a result of color changing from the direction, movement of carriage. It is linked with the fact, that majority of examples, doesn't have an inverse manner of disposition of print heads. For example, we have a 4 color printer CMYK, and its heads are located in that manner: K C M Y. It means that when printing in the right direction, black head will inject ink first, and then goes Cyan, Magenta, Yellow. We get a layer of four colors. When printing in the left direction, yellow head will start injecting ink, than Magenta, Cyan, and Black. We get the same drops of ink, but with another layer, where black is on top. This fact is visible on the ultraviolet printers, which operate with one line of heads.

5) Another problem is in the inconsistent aggregation of the print heads, to the coordinate Y. It is very important to physically put all the print heads in one line, so there won't be any defect of a head relative to the normal. So as if we move the head, we will always get a line of an unwanted color, which will give out an unwanted result.

Thus, knowing all the causes of the origin of stripes, you should try to:

1)            Keep an eye on the condition of the print heads, ink-vacuum system.

2)            Control material supplying adjustments, when you change the print material.

3)            Pay attention to the operation of reel and unreel system of material, look after the quality of usable material

4)            Choose an optimal regime of printer operation; keep an eye on head aggregation and on its coefficient adjustments.

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