Stories of engineers / Dust around printing image.
Dust around printing image.
There are several reasons that cause such problems:
1) Prints heads are placed too high above the printing surface
2) Ink doesn't correspond to normal quality
3) Statics on the material
4) Operation of print heads is not in the regime (clogged up nozzles, crooked nozzles, too high or too low tension of heads)
5) Too high speed of printing
Let's scrutinize first and the most common problem:
1) Prints heads are placed too high above the printing surface. Very often, when selling wide-screen printers, manufacturers install carriage with print heads especially a bit higher above the material, than it is allowed. Thus, it is done to minimize unauthorized scratching of the print head upon the rough material. Unfortunately, such cautions lead to worsening of printing. Because a drop of ink on the way out of the print head get through a certain distance in a way to the material. It is natural, as long is the distance, as more factors influencing it appear. An ink drop can scatter into multiple little drops, can deviate from the initial trajectory, a flow of air can blow it away, which are generated from the carriage movements. Most often it develops dust and indistinctness of an image.
Distance of carriage above the material of the majority of printers is approximately from 3 to 5 millimeters. It depends on the model of printer. There is a regular occurrence that the smaller a size of ink drop of printer head is, the smaller the distance between the head and material must be. For example, drop of ink of 4 pkl size. Recommended height above the material: not less than 1 millimeter. With a size 14 pkl of a drop - not less than 3 millimeter. Although, you have to be careful while changing height of print heads. If you put the carriage too low, there is a risk of touching the material by a print head, and if you put it too high, there is a probability of springing up a dust cloud.
Once I had to solve a client's problem. He complained about dust around the text. He said that is wasn't there before. On-site, I asked for examples of printed text with problems. It was printed on the thin, white, glossy plastic sheet. The dusting around small text was clearly visible. First thing that I asked about was what he used to print on before. Turned out he used a thick banner. He didn't change any settings, loaded a plastic sheet and got a result with dusting. First thing that I had done is diminished a distance between printing head and material. I also made a new profile for current material. Banner has another structure, which absorb more ink than a plastic sheet, so there was too much ink for a sheet. That's why it had a negative effect on quality.
2) Ink doesn't correspond to normal quality. While solving this problem, attention must be paid to the usable ink. Firstly, you have to check expiration date of ink. If it is coming to an end, it is advisable to stop using this ink. With the course of time, there are different chemical processes happening in the ink, which can lead to clot formation, unwilling suspensions and sediments, they can get into print heads, clog or damage inner nets, membranes, piezocrystals. Also, you have to pay attention to storage of ink and its transportation. For example, when ink is kept in too high temperatures, chemical processes in ink happen faster than usual and it leads to reduction of expiration date. If temperature is too cold, sedimentation of pigment can happen as well as restructing of pigment. As a result, viscosity of ink is changed and print head is unable to make a drop in a normal regimen. Such cases when ink is defective right from the manufacturers' factory are quite frequent.
There was an occasion, when customer bought a new Epson DX 4 print head. But after 4 days, he complained about the fact, that yellow print head printed fuzzily and halation can be seen. On-spot, firstly, I examined an old print head. Nozzle side, where yellow ink comes out, was covered in grey tinge. It looked like it worked with sand. When I removed the yellow head, I discovered the same result. It is clear that two heads on the same spot cannot break just so. After detailed examination of ink jars I found sediment on the bottom, which looked like tiny grains of sand. The ink was 3 years old. Client convinced me that it was an excellent ink and that he never had any problems with it. Arguments, that it was the ink weren't working. I advised to change the yellow ink. And install a trial head. After 2 weeks of usage, there weren't any deterioration of work with the new ink. Henceforward, before replacement of a new head, client promised not to save money on call for an engineer and consider competent people's opinion.
3) Statics on the material. This problem mostly occurs on ultraviolet printers, because they print on flat plastic surfaces. On such surfaces, fixed charges are gathered and they occasionally start to attract tiny drops of ink. As a result, you can notice little clouds, which have no clear form, around image and in casual places. You can struggle against this effect in several ways. One of them is using antistatic rule or special brush, spraying material with antistatic solutions. I, myself, use an old and proved method, before printing I rub all the problem places with a damp cloth, wetted in alcohol. By this, I get rid of statics and remove all the dust from material.
Once I had an order for printing on the light plastic, which is used for commercial purposes, in supermarkets such signs are hanged under the ceiling. There was a problem when printing an image. Colored clouds appeared chaotically, though there was no regularity, or clear regularity. Some came out ideally, other were with such clouds that puzzled everybody. First things that came to mind, was to rub material with water. We waited until it got dry, printed, there were no clouds. Made 10 test samples in the same way, there were no problems. Placed material not rubbed with water and got all kinds of clouds. So, we found the problem: statics on the material.
4) Operation of print heads is not in the regime (clogged up nozzles, crooked nozzles, too high or too low tension of heads) This problem frequently happens after ink replacement. Every ink is slightly different from the other by its structure. Print head is customized for printing with average ink, which has its own viscosity and printing temperature. That's why when replacing ink you have to pay attention to the work of print heads. Frequently, you have to heighten or reduce head's tension, change temperature, or change vacuum magnitude, which create meniscus on the print head. For example, if ink is thick, they need more energy to release a drop. To get more energy, you need to increase voltage, or increase temperature. On the contrary, if ink is more liquid, to release a drop we need less energy, so that temperature and voltage can be low. Condition of a print head has a high impact on quality of printing, if nozzles are clogged up or strike unevenly, the process of drop formation is malfunctioned and instead of smooth and accurate drop we get obscure splashes. Needless to say, that in such cases there will be no accuracy of an image.
5) Too high speed of printing. When using a printer at the highest possible speed, head starts to work in a rougher regimen, frequency of piezocrystal work increases, vortical stream between carriage and material increase, and trajectory of a drop starts to change. It all leads to deterioration of printing quality. That is why it is highly important to perform previous points, to have an acceptable quality on the maximum speed of printing. If even one of the pints isn't performed, then high-speed printing can be forgotten for good.
There was an occasion when client needed to print high quality promotional posters on the self-adhesive material. A printing press with Xaar 128 print heads was used. It was an old machine, but they still tried to use it. Her only operation was huge banners for advertisement hoarding. And customer, a friend of a client, cannot refuse, no other machinery and it was urgent. So we need to get the best out of it, correspondingly. First things that I had done: washed heads thoroughly with hot solvent, combined it all over again, created new profiles for this material. Lowered the carriage to material as low as I could, printed in more passages, in the regimen of one sided printing. It was printing for a rather long time. But it went really good. Everybody loved it.
In conclusion, I'd like to say that you cannot get rid of dust completely; you can only reduce it to minimum following the rules which are written above.
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