DE102014008183A1 - Cleaning nozzles of dried coating materials - Google Patents

Abstract

There is provided a method for cleaning a nozzle of a drop-on-demand coating head of solidified coating material, characterized in that the nozzle is deformed to peel solidified coating material adhered to the nozzle, and the peeled coating material together with liquid coating material out of the nozzle.

Description

The invention relates to the field of non-contact coating of surfaces or bodies with liquid coating materials, hereinafter also referred to as coating material or material, which solidifies on the surface or the body and thus results in a stable layer.

The inventive teaching is applicable to all classical inkjet printing and related fields, but particularly relates to the field of coating surfaces or bodies with material in layer thicknesses of up to 1 mm in drop-on-demand printing. In the area of coating technology, the focus here is on the coating of surfaces with materials such as emulsion paints or paints in drop-on-demand technology, as in US8,556,373 described. In particular, the invention also relates to the field of generic manufacturing or 3D printing, since here also binder-containing, fast-drying or crosslinking liquid materials are printed, with the help of which spatial bodies are built up additive.

The coating materials commonly used in coatings include pigments, fillers, binders and additives and are configured for rapid drying of the layers, which generates the problem of drying or solidification of material in the nozzles of the DOD coating head, i. H. solidified material adheres to the inner surfaces of the nozzles and eventually clogs them. Even small deposits can lead to an influence on the discharge amount and a deflection of the trajectory of droplets. Since the process of material accumulation is not completely avoidable, the nozzles of a DOD coating head must be rinsed in short time intervals, which requires a certain equipment cost and time. It would be desirable to have a simpler, integrated and time-saving possibility of removing deposits during operation.

Thus, the object of the invention is to provide a simple method for cleaning DOD nozzles of adherent solidified coating material, as far as possible without significant interruption of ongoing operation.

The object is solved by the preamble and the features of claim 1. Accordingly, a method is presented for cleaning a nozzle of a drop-on-demand coating head of solidified coating material, characterized in that the nozzle is deformed to solidified coating material, which at the Nozzle adheres, peel off, and that the detached coating material is conveyed out together with liquid coating material from the nozzle.

The invention discloses methods and apparatus for releasing solidified coating material adhering to the nozzle by deforming the nozzle, the nozzle being made of an elastic material, for example an elastomer.

The invention discloses methods and apparatus for deforming the contour of the inner nozzle surface, i. H. for locally changing the surface curvature or for stretching or compressing the inner nozzle surface, so that adhering material is detached.

The invention further discloses the use of a silicone as a low surface energy nozzle material. As a result, only a slight adhesion between the nozzle and solidified material is achieved, so that they easily detach from one another when the nozzle is deformed. At the same time, the low surface energy of the silicone ensures good droplet detachment of the exiting liquid coating material.

The invention discloses hydraulic and mechanical as well as ultrasonic application methods and devices for deforming the nozzle:
The hydraulic cleaning process exploits the fluid pressure of the liquid coating material itself, which builds up upon droplet discharge at the nozzle to deform the nozzle. At the same time, the thus released solidified coating material is transported away through the nozzle. Thus, the nozzle is cleaned with each drop delivery.

The mechanical cleaning method utilizes the action of mechanical elements, such as power transmission elements or plates, on a respective elastomer insert of a nozzle, thus causing deformation of the internal contour of the nozzle. In a particular embodiment, the mechanical element that causes the deformation of the elastomeric nozzle is at the same time a nozzle-closure element. This has in addition to the closing of the nozzle thus also the function of deforming the nozzle when opening and closing the nozzle. Thus, the common principle to close a fixed nozzle with an elastic sealing element is reversed here: According to the invention, an elastic, deformable nozzle is closed by a solid, non-deforming cover plate. Generally speaking, a power transmission element which serves to deform the nozzle simultaneously serves to close the nozzle. At the same time a deformation reaches the nozzle, which detaches adherent solidified material. According to the invention, a force transmission element can perform oscillating movements to loosen adhering material, as well as a liquid pressure can have a periodic time course, so as to also loosen adhering to the nozzle surfaces material.

The ultrasonic based cleaning method uses sonic vibrations to cause deformation of the inner contour of the nozzle and thus detachment of the coating material adhered in the nozzle. According to the invention, two embodiments are proposed here.

The invention discloses hydraulic steps for flushing detached, solidified coating material from a pressure nozzle 2 by means of liquid Beschichtungssoff.

By cleaning the nozzles of adhered solidified material by the method of the present invention, the reliability, life and working time efficiency of a DOD coating head can be substantially increased. The process according to the invention promises particular advantages in the printing of binder-containing, particle-containing and quick-drying or fast-curing coating materials.

The invention will be described in detail below, see 1 - 6 As a coating, coating or simply: material is here in the broadest sense of any liquid or any liquid mixture referred to, which can go into a solid material after a chemical, biological or physical solidification process and form a layer or structure on a substrate or a body can. Examples include: particle-containing, pseudoplastic, binder-containing, physically (including UV) or chemically crosslinking, two or more component systems, for example, based on epoxy or polyurethane or concrete monomers: paints, coatings, dispersions, Structural and structural compositions, cements, plasters, pastes, gels, adhesives, liquid foods and much more

A drop-on-demand (DOD) coating head in the context of the invention consists of devices for the electrically controlled generation of droplets, droplets or short-term jets of liquid contactlessly onto a substrate through nozzles, fluid outlets or orifices, herein simply referred to as "nozzles" or body are delivered. A DOD coating head is intended to include inkjet heads, jet valves or printheads in general. In the coating process, the material is discharged in liquid form from one or more nozzles. For small droplet volumes, contactless "microdosage" is also used.

Examples of generic coating heads are in 1a and 1b outlined. These usually have a plurality of nozzles, or in other words, electrically individually or in groups controllable channels. 1a illustrates in a sectional view a DOD coating head with its housing 1 , which works according to a valve principle. Liquid material is at the inlet P _FL at the inlet 4 is fed through a fast-acting fluid valve to the outlet, the nozzle 2 connected. The valve is formed in the example outlined from a rotationally symmetrical closure element 5 that interacts with a valve seat 6 as part of the housing keeps the intermediate fluid passage closed or releases. closure element 5 and valve seat 6 in the sketched shape form in the open state, a fluid passage with optimally small fluid resistance. closure element 5 and / or valve seat 6 may be made of an elastomer to achieve an improved sealing effect. To open the valve, the closure element 5 by means of an actuator from the valve seat 6 lifted, in this case by a pneumatic diaphragm actuator, which is controlled in dependence on the difference of a pneumatic control pressure p _pneu and the fluid pressure p _FL . Exemplary is dried material 3 in the nozzle 2 shown.

1b shows an embodiment of a generic DOD coating head 1 working on the inkjet principle. A piezo bending actuator 5 formed of a disc of piezoelectric material and a wall of the housing 1 , bulges when applying an electrical voltage to the piezoelectric disk in the fluid chamber and thus reduces their volume for a fraction of a millisecond and hurls a drop of the material from the opening 2 , As the actuator relaxes, material passes through the inlet 4 sucked, driven by the capillary pressure in the nozzle 2 ,

To solidified coating material 3 in a nozzle 2 , please refer 2a According to the invention, the nozzle is deformed and the removed coating material is conveyed out of the nozzle by means of liquid coating material. This usually happens, but not necessarily at the drop delivery. In 2a a DOD coating head is sketched, which works according to the jet valve principle. Because only in the area of the nozzle 2 elastic behavior should be present and not the entire housing 1 be elastic, are inventive nozzles 2 preferably designed as an elastomeric insert. Elastomer inserts can be manufactured either in the housing 1 used or, for example, in multi-component injection molding - Process or other integrated manufacturing process can be produced. In the present example contains the elastomer insert 7 both the nozzle 2 as well as the valve seat 6 , which is thus elastic and in interaction with a closure element 5 , which can be made of solid, non-elastomeric material, achieved a good sealing effect. Thus, according to the invention nozzles 2 in an elastomeric insert 7 which was in the case 1 is embedded.

In the elastomer nozzle 2 is solidified coating material 3 adhered to the wall surfaces of the nozzle 2 , please refer 2a , According to the invention, the nozzle is deformed by a force or a pressure. In 2 , b1 is the case shown that the closure element 5 a force on the valve seat 6 , which is in the elastomeric insert 7 is exercising. The resulting deformation of the nozzle is a function of the pressing force of the closure element 7 on the valve seat 6 as well as the length and diameter of the elastomer insert 7 and the nozzle 2 , According to the invention, said parameters are configured so that there is still sufficient deformation of the nozzle 2 at the outlet is present. In addition to the static deformation plays in the illustrated embodiment, the dynamic deformation of the valve seat 6 a role due to the transmission of the momentum of the closure element 5 on the valve seat 6 takes place. This occurs when the closure element hits 5 on the valve seat 6 a vibration that manifests itself as a surface acoustic wave in the nozzle 2 into it spreads, see also the representation in 2 , b1. This by deforming the nozzle 2 detached solidified coating material is then conveyed to the nozzle along with the liquid coating agent the next time the fluid valve is opened, see Drops 18 in 2 , c. Since the cleaning sequence according to the invention described here is run through each individual fluid delivery, the inventive method is best suited to achieve a permanently high process reliability in the drop-on-demand application of fast-setting coating materials.

In 2 , b2 is the case shown that exerts an external force on the elastomer nozzle 2 , or the elastomer insert 7 is exercised. For this purpose, the nozzle protrudes 2 by a certain length, for example, 0.5 to 5 times the nozzle diameter, from the housing 1 out, so that it can be deformed with a plate, a scraper or by hand. In general, few movements or deformations are sufficient to adherent solidified coating material from the inner walls of the nozzles 2 completely detach. As in 2 , b2, a plate, which serves to close the nozzles, so with the nozzles 2 be brought into contact, that in the closed state, the nozzle is greatly deformed. The plate can be pushed by a sliding mechanism laterally over the nozzle, or perpendicular to the opening of the nozzle 2 be pressed. In the example given, the cover 11 used to stop the nozzles during a short or long term work interruption 2 to close. Especially with prolonged storage may be due to incomplete gas tightness of the nozzle and plate material or incomplete tightness between nozzle 2 and plate 11 Coating material on the inner walls of the nozzle 2 dry. It thus takes the contours of the plate 11 deformed nozzle 2 at. After opening the nozzle 2 by removing the plate 11 For example, a return deformation of the nozzle in the tension-free state takes place, wherein adhering coating material from the nozzle 2 solves. According to the invention, it does not matter whether the deformation takes place from a deformed state of the elastomer nozzle into the relaxed state or vice versa. Thus, a cover device 11 which the nozzles 2 Seals to the environment, configured to hold the nozzle 2 deformed in the closed state.

In the 2 , b1 and 2 , b2 outlined possibilities of deformation of the nozzle 2 or the polymer insert 7 overlap in the drawn configuration, thus, two deformation effects simultaneously act to adhere solidified coating material from the inner walls of the nozzles 2 replace.

According to the invention, this is done by deforming the nozzle 2 detached solidified coating material in the nozzle of a fluid delivery pulse, for example, in a subsequent opening of the fluid valve, or in a fluid shock in the case of the inkjet head out. Solidified material is transported under pressure from the nozzle. Sometimes the nozzles have to be actuated several times (fired) until they are free of solidified material.

It may be useful to introduce a cleaning cycle which is carried out at regular intervals or after storage: it is characterized by the positioning of a waste container in front of the openings of the nozzles 2 , Stripping solidified material from the inner surfaces of elastic nozzles 2 by single or multiple deforming thereof, and by one or more actuation (firing) of the nozzles for shooting out the detached solidified material into the waste container. All this can be repeated several times. Instead of the waste container according to the invention, a semi-open or closed channel system with optional (circulation) flushing can be used, which is part of a Nozzle cover mechanism is what the cover plate 11 contains.

3 shows several embodiments of nozzles 2 or elastomeric inserts 7 , In 3a is an elastomeric insert 7 shown, which includes only a part of the total nozzle length, in particular the Düsenaugang includes. He closes the output side plan with the housing 1 from. In order to deform it, a body must be located on the outlet side by means of a highlighting on the elastomer insert in the area of the outlet of the nozzle 2 be pressed. The valve seat 6 is located in the housing 1 ,

In 3b is an elastomeric insert 7 represented, which stepped in the housing 1 is in, and which the valve seat 6 contains.

In 3c is an elastomeric insert with one out of the housing 1 protruding, extended part 8th shown. The extended part can be deformed from the outside with little effort. Depending on the application, the typical nozzle diameters are between 0.05 mm and 0.5 mm, the nozzle lengths between 0.2 mm and 10 mm, the wall thickness of the polymer insert between 0.3 mm and 5 mm. The sketched nozzle protrudes by about 0.2 to 3 mm. Furthermore, nozzles can have a slight conicity internally, for example with an opening angle of 0.5 to 5 °. If the taper opening towards the outlet, this promotes the detachment and especially the removal of the dissolved coating material. Alternatively, the taper can also be inward opening. Should the nozzle be removed from the housing 1 only slightly protruding, the free, not with the case 1 connected length but still be large to a strong deformation of the elastomer insert 7 to reach, then can a recess 9 in the case 1 be provided, see 3d ,

According to the invention, the deformation of the elastomer insert 7 and the nozzle 2 in the case of the inkjet head after 1b also by the actuation pressure pulse, which causes the droplet delivery done. The fluid pressure can briefly assume values of a few bar when a drop is fired. In 4a an elastomer insert of such a DOD coating head is shown in the pressureless state. In 4b is increased the predominantly axial deformation of the nozzle 2 by the increased input-side pressure p at the nozzle 2 shown when firing. As in an axial deformation of the nozzle in the case of a rotationally symmetrical Elastomereinsatzes 7 only a slight deformation of the contour of the hole itself occurs, a non-rotationally symmetrical shape of the elastomer insert is proposed. In 4c is qualitatively a non-rotationally symmetric deformation of the nozzle 2 a non-rotationally symmetrical elastomer insert 7 after one-sided pressure p shown. Sometimes a single fluid shock is sufficient to solve solidified material and expel from the nozzle. Since the top of the nozzle, the pressure along the nozzle hole 2 and thus the deformation of the nozzle 2 spreads, also from the upper end of the solidified material is detached and wholly or partially transported out during one or more droplet ejections.

4d shows a further embodiment of an elastic nozzle according to the invention 2 , Along the fluid path within the nozzle is in addition a cross-sectional reduction, or bottleneck 16 , contain. This can be done at the nozzle inlet, at a position between the nozzle inlet and the nozzle outlet as in 4d represented, or at the nozzle outlet. The cross section of the bottleneck in the unloaded state may be round, oval, flat, star-shaped, or rotationally symmetric or mirror-symmetrical in any other way. When a pressure surge occurs at a droplet discharge at the nozzle inlet creates a back pressure at the bottleneck 16 , which allows efficient stretching of the nozzle 2 in the form of a cross-sectional enlargement, which also in the remaining areas of the nozzle 2 Can cause stretching. In this case, the shape of the cross-sectional area can change substantially, for example, a flat, oval shape can be transformed into a rounded shape. The continuation of the principle of cross-sectional constriction leads to a configuration as in 4e in which the cross-section is zero in the unloaded case, that is to say the nozzle is closed and the cross-section only widens so much at an input-side minimum pressure p_min that an open cross-section through which it can pass occurs. In the unloaded state, such a cross section is produced, for example, by a cut, that is, a material separation without material removal. In the case loaded under pressure from the nozzle inlet, the cross-section opens, resulting in an approximately oval shape. In a further variant, there are two or more sections, wherein, for example, the centers of the sections coincide and a star-shaped cross-sectional shape results in the case under pressure.

The following should continue on the deformation of the nozzle 2 in the elastomer insert 7 be received: In 5 are embodiments of elastomeric inserts 7 with elements 10 recorded for deformation thereof. In 5a is an elastomeric insert 7 represented, which is deformed by means of a nozzle radially arranged, movable deformation element in the form of a bolt. In this way, very strong deformations can be produced and a strong effect is achieved in the detachment of solidified coating material from the walls of the nozzle 2 , The elastomer insert after 5b has on the outer surface of the nozzle one very flat cone with central nozzle outlet, which is deformed when a cover element 11 , z. As a plate, laterally on the surface of the housing 1 moved and brought to the opening of the nozzle to cover. 5c illustrates the occurring deformation of the nozzle 2 and the elastomer insert 7 in the case of a further out of the case 1 outstanding nozzle shape in case of deformation by a lateral to the housing 1 sliding slide 11 , Additionally shown is a coating of the slider on the surface in contact with the nozzle. It is possible to use hydrophobic materials for coating, such as, for example, fluoropolymers, in particular PTFE, PFA or FEP, in order to prevent adhesion of the coating material. Also advantageous are elastomers, especially silokones because of their also low surface energy. In 5d is qualitatively the deformation of the same structure as in 5c but in the case of a closure movement of the cover 11 in the direction of the nozzle axis, as is the case for example by a rotational movement about the outlined pivot point.

Essential to detachment of the adhered solidified material is that deformation of the nozzles overcomes the adhesive forces between the material and the nozzle wall surface, which can be easily achieved, for example, by the use of an elastomeric low surface energy nozzle material such as the like. Silicone. Furthermore, it is favorable if the elastomer of the nozzles has a higher elasticity or lower hardness than the adhering solidified material. According to the invention, therefore, a deformation of the nozzle in the region of the contact surface to the adhering solidified coating material, so on the inner contour of the nozzle to make so strong that the forces acting from the deformation shear forces on the inner nozzle wall surface exceed the adhesion forces between the material and nozzle wall surface.

Furthermore, the material of the elastomeric insert 7 much more elastic than that of the housing 1 , However, it has a high Shore hardness of preferably 70-90, particularly preferably 80-90, so that sufficient for the drop ejection mechanical stability of the nozzle 2 is guaranteed. Furthermore, the material is preferably only elastically deformed by the deformation, ie deformation should not take place into the plastic, irreversible region as far as possible since this would cause fatigue of the material. Examples of other suitable elastomers are FFKM, NBR, EPDM, FKM / FPM, urethanes and much more

In a further variant according to the invention, a deformation of the nozzle 2 achieved with the help of ultrasound, which of the housing 1 attached ultrasonic transducers is sent out. Based on 6 Two possible configurations are described: 6a shows a first embodiment of the invention. The transducer used is a piezoelectric crystal 13a , which is langebracht side of the housing. The piezocrystal has electrodes on opposite surfaces, which are acted upon by an electrical alternating voltage. With one of the electrodes is the piezoelectric crystal 13a with the housing 1 bonded. At high frequencies in the MHz range predominantly longitudinal waves 14a in the case 1 in relation to the nozzle 1 radiated radial direction. In the example shown, the piezocrystal is operated in the (3,3) mode. According to the invention here is a deformation of the nozzle 2 involved in the detachment of adherent solidified coating material from the nozzle surfaces. The deformation is a function of the integral of the sound velocity at the location of the nozzle.

Since longitudinal waves are transmitted in solids with a high radiation impedance, the energy of the wave predominantly manifests itself in a high sound pressure instead of a high sound velocity. Cheaper is the use of surface waves, such as Rayleigh waves, as they allow much higher deformations of the material due to their much lower (magnitude) radiation impedance. In 6b this case is sketched. On the nozzle outlet side of the housing 1 a flat piezocrystal is glued to one of its two active electrodes and is driven electrically with an alternating signal in the MHz range. The crystal is flat in profile and is operated in the (3,1) direction, ie it stimulates perpendicularly to the electrode surface (3-direction), while the strain caused by the transverse contraction of the crystal is utilized parallel to the electrodes (1-direction) for the generation of ultrasound. Thus, shear vibrations are transmitted to the housing via the adhesive layer. Depending on their frequency, these propagate primarily along the surface until they encounter an edge which represents an acoustic impedance jump and causes a reflection of the wave according to the laws of sound propagation. In the example shown are two grooves 15 represented, on which the surface wave is respectively reflected back, between which the wave thus wanders back and forth. A housing corner is acoustically equivalent to a groove in this case, so both are commonly referred to as reflection sites. It makes sense if the distance of the reflection points is a multiple of half a wavelength of the surface wave, since higher sound beats and expansions of material can be achieved by periodic, in particular harmonic signal response of the excitation by superposition. In this case, a reflection cell does not necessarily have to be considered a straight corner or Edge be executed. Also, round or parabolic shapes of grooves are conceivable which cause a reflection of incident waves toward one or more focal points, which are in particular one or more nozzle openings. Thus, according to the invention is the nozzle 2 within the sound field of the surface wave, in the case of a standing sound field at the point of a maximum of the sound velocity, ie at a sound pressure node. This is preferably located at a distance of a multiple 1... N half the wavelength of a reflection point. According to the invention, the deformation of the housing material in the region of the nozzle 2 used to remove adherent solidified coating material. In this case, the deformation effect of the wave into the material decreases with increasing distance from the surface and is about 1-2 wavelengths in Rayleigh waves. However, since the fastest drying occurs near the surface, this effect is not undesirable.

By reducing the frequency and reducing the housing thickness along with the nozzle length, the acoustic conditions change, so that the Rayleigh wave is the well-known Lamb wave, which corresponds in principle to a bending wave and for the inventive method for deformation of the nozzle 2 use.

Another effect of the ultrasonic excitation of a DOD nozzle in the region of its mouth, i. e. in the region of the nozzle outlet, according to the aforementioned method, in particular by surface waves, in particular by Rayleigh waves, based on the cavitation of the liquid coating material. The ultrasonic excitation by surface waves leads in the area of the nozzle outlets to relevant fast components along the nozzle axis and perpendicular to the end face of the fluid outlet. These generate shear stresses between the liquid coating material and the nozzle wall, which effectively supports the detachment of drops. One can use this method to aid in droplet separation from the nozzle surfaces, especially in the case of DOD printing of viscous or tacky coating materials.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• US 8556373 [0002]

Claims (11)

A method for cleaning a nozzle of a drop-on-demand coating head of solidified coating material, characterized in that - the nozzle is deformed to peel solidified coating material adhered to the nozzle, and - that the detached coating material together with liquid coating material from the Nozzle is carried out. The method of claim 1, wherein the nozzle is contained in an elastomeric insert which is deformed by the coating agent pressure. The method of claim 1, wherein the nozzle is contained in an elastomeric insert that is deformed by a mechanical force. The method of claim 3, wherein the mechanical force is applied by means of a power transmission element 10 predominantly takes place in the radial direction of the nozzle. The method of claim 3, wherein the mechanical force is applied by a nozzle sealing cover 11 he follows. The method of claim 3, wherein the mechanical force is applied to a protruding part 8th the nozzle is done manually or by means of a power transmission element. The method of claim 1, wherein the nozzle is deformed by means of ultrasound, in particular by surface waves such as Rayleigh or Lamb waves. Drop-on-demand droplet generator for carrying out the method according to one of the preceding claims, characterized by a deformable free jet nozzle 2 of elastic material, means for deforming the jet nozzle and means for releasing solidified coating material by means of liquid coating material from the nozzle 2 hinauszuspülen. Drop-on-demand droplet generator according to claim 8, characterized in that nozzles 2 in an elastomeric insert 7 are included, which in the housing 1 is embedded. Drop-on-demand droplet generator according to claim 8, characterized in that elastomer insert 7 a valve seat 6 a metering valve contains. Drop-on-demand droplet generator according to claim 8, characterized by a Abdeckvorichtung 11 which the nozzles 2 is sealed to the environment and configured to deform the nozzle when closed.

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