EP1208992A2 - Ink-jet printer comprising a piezo printhead for ejecting lactate ink on plain print medium - Google Patents

Abstract

The ink-jet printer has a piezo-printing head (19) and the printing surface has a pre-heating surface (21) for pre-heating the printing medium, The printing surface has follow-up heating surface (22) for drying the applied lactate-ink. The printing surface is thermally insulated from pre-heating surface (21) and from the follow-up heating surface (22), these two surfaces (21, 22) being thermally decoupled.. The pre-heating surface (21) is adjusted to a temperature of from 24 deg C to 50 deg C independent of the type of printing material.

Description

The invention relates to an inkjet printer with a piezo print head for ejection of lactate ink on an uncoated printing medium according to the preamble of the claim 1 Art.

The following is the state of the art in the field of inkjet printers, printheads and inks therefor, and ink jet printing processes for their suitability for large-format printed products outdoors. Under the term "printer" In the present case, both so-called desk printers and plotters fall.

Inkjet printers, also known as inkjet printers, currently print primarily on so-called coated material. The properties of the coating material, including top coating called, determine the printability of the carrier material and the durability of the printed image. The material coating ensures that the ink is absorbed as quickly as possible a sharp image is created and this image against environmental influences and light if possible is stable. The printed image must also be dry and smudge-proof as quickly as possible. There are many different coating materials on the market for this. The carrier material itself usually only determines the mechanical properties of the medium. usual Carrier materials for inkjet printers and plotters are papers, foils and banner materials.

Common to all print media is that the carrier material is not printed directly can, since the ink usually does not form an adequate connection with the carrier material. The Coating the carrier material with coating material is a cost-intensive process. The refinement of the carrier material often costs a multiple of the carrier material. Coated print media are therefore less suitable for large-format print products outside.

Inkjet printers use different printheads. For inkjet printers with thermal print heads the ink is evaporated or strongly expanded by heating in a capillary and sprayed onto the print medium. This procedure is for large UV stable Color pigments are not suitable. Another disadvantage is that the thermal stress on the ink the colors changed. Some components of the ink evaporate before hitting the ink Print media. The heads typically have a short lifespan because of thermal effects Aging heavily. Finally, the printing speed by the cooling and Heating process physically limited. Thermal print heads are therefore for large format Outdoor print products not suitable.

Thermal jet printers with piezo print heads provide that the ink by mechanical Vibrations of the piezo crystal are expelled from the capillary and onto the pressure medium is injected. These print heads are therefore suitable for large UV-stable color pigments. Colors are not affected by the printing process. Almost all components of the ink hit the evaporation medium without evaporation. The ink is at ambient temperature processed and applied, and there is no unusual heating of the ink instead of. The piezo print heads have a long service life. However, the print speed is limited by the oscillation process of the piezo crystal. However, it can be as high Print speeds are achieved that in the slipstream of the first drop still some more drops of the ink can be refilled. These still connect before hitting the material into a large drop (variable drop size). Piezo print heads are therefore suitable for large-format printed products outdoors.

Water-based inks are the most commonly used with inkjet printers Inks, as they can be easily processed, especially with thermal printheads. However, these inks can only be used on coated print media. by virtue of Due to their low durability, they are only suitable for archiving to a limited extent. Your water solubility makes them unusable for applications in damp environments, especially outdoors.

Another type of ink for inkjet printers, especially those with piezo print heads, is coming pigmented inks. Oil-water mixtures are often used for pigmented inks used. However, they are largely based on water. These can be used Inks for coated materials only. They show good UV resistance and are suitable for medium-term archiving. As far as these inks are water soluble, come Depending on the coating, they may be considered for short-term use outdoors.

Solvent based inks have been used in the printing industry for many years, for example used for screen printing. For some time now, these inks have also been used on special inkjet or air brush printers used. These inks are on coated media usable. They have good UV resistance and can be archived in the long term. Moreover they are not water soluble. So they are basically for outdoor use best for. However, they require special protective measures during processing, because they are mostly extremely harmful to the environment and human health. Special suction measures and the use of active filters are officially prescribed. Disposing of the residual ink is also problematic. Because these inks have a low flash point, they can only be transported as dangerous goods. Critical, since it is subject to official requirements also their storage. Fast drying of the inks in the tube system and in the print heads of the inkjet printer makes the use of these inks additionally problematic. Therefore, the printer's ink system must be rinsed with solvents again and again, which is very expensive. Switching on can cost up to DM 100. For the These are used for the production of large-format printed products outdoors Inks less suitable.

It is also known that lactate-based lactate inks are actually family of adhesives can be sprayed with piezo printheads. Under lactate inks are presently based on lactate-based inks and also such inks understood that have components with properties similar to those of lactate. Such The advantage of inks is that they are sprayed directly onto uncoated print media can. They have good UV resistance and are for outdoor use suitable because they are not water-soluble. Ask because of their chemical composition they are not harmful to the environment. There is only a health risk due to direct contact with skin or eyes. They are easy to transport and storable. Drying these inks in the tube system and in the printheads is relative not critical, because it is controllable.

Lactate inks meet the long-standing need, as simply as possible and environmentally friendly without health risks inks on uncoated material to print with inkjet printers, not least because of the lack of coating material previously unavoidable immense manufacturing costs can be saved.

Unfortunately, it has been found in practice that the printing result when used of lactate inks for conventional ink jet printers and ink jet printing processes of course phenomena is qualitatively unacceptable.

WO 00/24 583 A1 describes an inkjet printer with a print head for ejection known from conventional printing ink on a printing medium in the area of a printing surface. The printing area is a preheating area for preheating the print medium from upstream and a post-heating surface for drying applied to the print medium Ink added from below. The preheating area and the post-heating area are in this known printer formed by the jacket of one and the same heatable roller, which is why preheating and post-heating are coupled in this known inkjet printer are. Due to this coupling, it cannot be guaranteed that the preheating inside the temperature range required for a satisfactory printing result and the Post-drying takes place independently of this in the most suitable temperature range.

A similarly constructed inkjet printer with preheating and post-heating, which is also is provided by the jacket of one and the same heatable roller is from DE 196 11 700 A1 known.

The invention is therefore based on the object of providing an inkjet printer which with lactate ink on coating-free print media high quality printed products guaranteed.

This object is achieved by an inkjet printer with the features of claim 1.

The preheating process removes any moisture from the pressure medium evaporated. By applying heat, the pores of the pressure medium, which is preferably as PVC film is present, opened. This allows the lactic acid based ink to penetrate deep into the material penetration. Due to the adhesive behavior of the lactates, the ink literally becomes in the pores screwed.

Correctly setting the preheat temperature directly affects whether the ink runs on the material or the colors bleed into each other. As ideal temperature values temperatures of preferably 28 ° C to 35 ° C have been found. Depending on the material thickness you can however, with thick banner materials up to 45 ° C.

The temperature control must fluctuate as little as possible around the ideal value respectively. If the temperature is too high, the medium will curl and the printhead can block and be mechanically damaged. If the temperature is too low, it runs Ink on the medium, the colors mix and the edges bleed.

In order to ensure that the inks dry as quickly as possible, the print medium is immediate reheat after printing. The faster the afterheating When the ink dries, the print becomes sharper.

If the post-heating takes place too early, the print head is again subjected to thermal stress and dries yourself one. Here, too, the print platform must be thermally decoupled.

A possible deformation of the medium through the afterheating is not critical, since that Material unhooks after a few centimeters and the waves no longer damage of the print head.

Temperatures of are preferred as ideal temperature values for the after-heating 40 ° C to 45 ° C proven. Depending on the thickness of the material, thick banner materials can be used up to 55 ° C may be required. Temperatures above this value should be avoided if possible to avoid injury to the user when touching the heating plate.

Due to the after-heating, the print image is already 10 to 15 cm behind the print head dry and smudge-proof. The risk that the printed product will be blurred if the medium touching the ground due to the printing length is not given. A winder is optional.

Normal room temperature conditions must be ensured in the area of the piezo print head his. The area of the printing area must be free from any heating of the printing medium be decoupled. If the printhead were also heated, the ink would already be in the Cure the printhead or in the hose system. The same would apply to the technical characteristics of the piezo crystals change. Then a trouble-free printing would not be guaranteed.

For this reason, the print platform must be thermally decoupled from the preheating and post-heating. Additional cooling of the print head with a fan may prove necessary to avoid undue heating of the printhead.

Due to the slightly different viscosity of the lactate inks and the higher risk of drying out compared to conventional inkjet inks, a more frequent cleaning cycle of the heads is required. Usually the print heads move every 1 m ² into a so-called capping station to be cleaned. Cleaning cycles every 20 to 30 cm have proven useful in practice.

The heating of the pressure medium in the preheating and in the reheating area must be possible done evenly across its entire width.

Heating with radiant heat from above is impractical, as this means only that Material surface of the print medium is heated. This prevents moisture from escaping and the pores of the print medium open insufficiently. The heating must pass through the print medium from below. This makes his whole Material heated evenly. This is especially true for self-adhesive films made from paper backing, Glue and PVC exist, important.

Most printers have a half-moon shape in the area of the rear and front material guides Metal plate to ensure perfect material transport. This Metal plate can be heated directly.

Attempts with infrared heating lamps or heating elements from water boilers have failed because the heating effect is too large and cannot be controlled. Overheating was practically always the consequence. The power consumption was also much too high at around 1 kW.

Resistance heating wires with thermostable insulation, for example Teflon or silicone sheath. The silicone coated resistance heating wires are Can withstand temperatures up to 180 ° C. Thus, at the maximum required 55 ° C warming sufficient security reserves available. The heating wires are used for additional shielding preferably wrapped with a metal braid and with aluminum tape from glued into the metal plate below. The wire mesh is connected to the earth conductor. in the In the event of a wire break or a burnout of the silicone insulation, the house fuse will or the residual current circuit breaker tripped. This poses a risk to people locked out.

Experiments have shown that resistance heating wires with an internal resistance of Around 20 ohms per meter length ensure the best results. With a printer width For example, a heating wire of around 950 cm long becomes several cm in lengthways attached to the metal plate from below. The power consumption is then around 220 Watt. The final length of the filament depends on the type of printer and its construction dependent. Four to six turns have proven to be sufficient in practice. The The metal plate is heated to a width of approx. 12 cm and a length of approx. 140 cm. Due to the slow printing speed (feed) of the Ink-Jet plotter, this becomes Print medium warmed over a period of approx. 20 - 45 seconds. For regulation preferably a temperature sensor with electronic control is used.

The thermal decoupling of the printing platform from the preheating and post-heating surfaces is achieved by a temperature-stable felt or Teflon tape. This will avoid that the heated metal plate is in direct contact with the printing platform. Because the case metal plate and the printing platform mostly made of different materials, for example steel and aluminum, the decoupling is easy to achieve.

Numerous conventional inkjet printers, especially plotters, can be used without any problems Retrofitting with the preheating surface according to the invention and the post-heating surface, then using lactate ink to produce large format print products in the Outdoor area based on uncoated print media using a piezo print head or - to be able to operate printheads.

The invention is explained in more detail below using the drawing as an example; the only Figure of the drawing shows schematically in cross section an inkjet plotter. This Plotter comprises a base body 10 supported on a base with a base 11 at the bottom. The upper end of the base body 10 is designed as a pressure stage 12. A support 13 protrudes laterally from the base body 10 and winds it onto a core 14 Supply roll 15 carries from a printing medium, for example a PVC film. The The direction of conveyance of the pressure medium 16 is indicated by an arrow A in the figure.

The printing stage 12 is connected upstream in relation to the conveying direction of the pressure medium 16 a crescent-shaped curved inlet metal plate 17. The printing stage 12 is connected downstream a similarly shaped discharge metal plate 18.

A piezo print head arranged above the print stage 12 is used for printing on the print medium 16 19, which has cooling in the form of a fan 20.

According to the invention are the inlet metal plate 17 and the discharge metal plate 18, via which the pressure medium 16 runs away, designed to be heatable and thermally from the printing platform 12 isolated. The inlet metal plate 17 thus places a preheating surface 21 with respect to the Printing stage 12 fixed, while the discharge metal plate 18 defines a post-heating surface 22.

To heat the metal plates 17 and 18 and thus the pressure medium passed over them 16 to enable, are heating devices in the form of resistance heating wires under these plates intended. These resistance heating wires are only in the area in the figure the preheating surface with the reference number 23.

These heating devices are supplied by a control device, not shown, that they have a precise temperature of the preheating surface 21 or the post-heating surface 22 guarantee, namely across the entire width of these areas and in an even distribution on this.

The heating of the inlet metal surface 17 ensures that the piezo print head 19 discharged lactate ink literally fits into the Pores of the print medium 16 are screwed so that this ink is effectively prevented runs on the print medium 16 or the colors bleed into one another.

The afterheating ensures that the pressure medium 16 immediately after the Printing process is dried quickly, which benefits the edge sharpness of the printed image.

Claims (10)

Inkjet printer with a piezo print head (19) for ejecting lactate ink onto an uncoated printing medium (16) in the area of a printing area,
characterized, that the printing area is preceded by a preheating area (21) for preheating the printing medium (16) from its side opposite the piezo print head (19), the preheating area (21) depending on the type of material and thickness and / or conveying speed of the printing medium (16) can be set to a temperature of 24 ° C to 50 ° C, that the printing area is followed by a post-heating area (22) for drying the lactate ink applied to the printing medium (16) from the side of the printing medium (16) opposite the piezo print head (19), wherein the printing area is thermally insulated from the preheating area (21) and from the post-heating area (21) such that it is thermally decoupled from the heating of the preheating area (21) and the post-heating area (22). Inkjet printer according to Claim 1, characterized in that the preheating surface (21) has a temperature of 28 ° C to 35 ° C depending on the type and thickness of material and / or the conveying speed of the printing medium (16). Inkjet printer according to one of claims 1 or 2, characterized in that the post-heating surface (22) has a temperature of 35 ° C to 60 ° C depending on the type and thickness of the printing medium (16). Inkjet printer according to Claim 3, characterized in that the post-heating surface (22) has a temperature of 40 ° C to 45 ° C depending on the type and thickness of the printing medium (16). Inkjet printer according to one of claims 1 to 4, characterized in that cooling (20) is provided for the piezo print head (19). Inkjet printer according to one of claims 1 to 5, characterized in that the preheating surface (21) or the post-heating surface (22) extends essentially over the entire width of the printing medium (16). Inkjet printer according to one of claims 1 to 6, characterized in that for the preheating or reheating of the printing medium (16) in the preheating surface (21) or in the post-heating surface (22) a flat heating device is provided below the printing medium (16). Inkjet printer according to claim 7, characterized in that the heating device is covered on its upper side by a metal plate (17, 18) over which the printing medium (16) is guided. Inkjet printer according to one of claims 7 or 8, characterized in that the heating device for uniform heating of the metal plate (17, 18) or the printing medium (16) comprises resistance heating wires (23). Inkjet printer according to one of claims 7 to 9, characterized in that the heating device is electronically controlled.

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