Patent Publication Number: US-6911085-B2

Title: Doctor blade dosing system

Description:
This is the U.S. national phase of International Application No. PCT/DE03/00738 filed Mar. 7, 2003, the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Disclosure 
   The disclosure relates to a doctor blade dosing system for a device used for coating material webs, particularly paper or paperboard, that has a doctor blade rod which serves as a dosing element while being held inside a slot of doctor blade bed that is mounted in a support that can be fastened inside the frame of the system. 
   2. Related Technology 
   It is a known procedure to coat continuous paper or paperboard webs by means of devices that have an application system for applying an excess of liquid coating material and a downstream dosing system fitted with a doctor blade rod as the dosing element that scrapes off the excess coating material until the desired application weight is achieved. The application and dosing procedures take place either directly onto the web or indirectly, first of all onto a roller that deflects the web and subsequently transfers the dosed film of coating material onto the web. 
   A doctor blade dosing system of the generic type is described in DE-A 3022955. The doctor blade rod is held in a doctor blade bed made of rubber-elastic material that is mounted in a support that is connected to the machine frame. At its rear side facing away from the doctor blade rod, the doctor blade bed is supported by a pressure tube so that the thickness of the dosed film on the roller or on the web can be varied to a certain extent by means of the pressure in the pressure tube. In the doctor blade bed, there are two open rinsing channels that run parallel to the doctor blade and that are open towards the doctor blade rod, into which a rinsing liquid, for example, water can be fed in order to lubricate the bearings of the doctor blade rod and to remove coating material that has penetrated there. Such doctor blade dosing systems in which the doctor blade bed is supported by a pressure tube are called roll doctor systems. They are used for direct dosing onto the web in an area where they are supported by a mating roll. So-called revolving doctor systems are likewise known in which the web is conveyed around the doctor blade in a free-running area. With these systems, the doctor blade bed is secured in a rigid holder without a pressure hose. 
   The doctor blade rods are normally made of steel and have a diameter between 6 mm and 40 mm. Since the pigment coatings normally used as the coating material for coating paper or paperboard are highly abrasive, it is a known procedure to coat the surface of the doctor blade rods with wear-resistant materials in order to increase the wear-resistance. Depending on the application purpose, doctor blade rods having a smooth lateral curved surface or doctor blade rods with circumferential grooves are used. During the operation of doctor blades with a smooth lateral curved surface, a narrow gap exists relative to the web to be coated so that the desired amount of coating material passes through this gap. Doctor blades with circumferential grooves are in contact with the web or roll during the dosing. The application weight that remains is determined primarily by the groove cross section through which the coating material can pass. 
   In order to hold the doctor blade rod securely in the slot, the known doctor blade beds have to be manufactured with precise dimensions. Since the doctor blade beds have a relatively large cross section, they are manufactured in a casting process in order to achieve the necessary dimensional accuracy and/or the required dimensional accuracy is attained by means of metal-cutting. The production is material-intensive and labor-intensive. 
   The doctor blade rod, which rotates during operation, causes considerable wear and tear to the doctor blade bed. Consequently, the material-intensive and labor-intensive doctor blade beds have to be replaced at regular intervals. Such replacement entails considerable costs. In order to reduce the replacement costs due to wear and tear without impairing the quality of the coating, the roll doctor system according to German Patent Application No. 10045515 has a support with a slot into which the doctor blade bed can be placed with the inserted doctor blade rod. The doctor blade bed is configured as the smallest possible wearing part. 
   SUMMARY OF THE DISCLOSURE 
   Therefore, the disclosure is based on the objective of improving a doctor blade dosing system according to German Patent Application No. 10045515 in such a way that the doctor blade bed with the inserted doctor blade rod can be replaced in a simple manner. 
   This objective is achieved according to the disclosure in that
         the doctor blade rod has a diameter of less than 25 mm,   the doctor blade bed is inserted in a removable manner into a slot on the front of the support, whereby the ratio of the cross-sectional area (measured in mm 2 ) of the doctor blade bed to the diameter (measured in mm) of the doctor blade rod is less than 60 mm, preferably less than 30 mm,   the support is made of a plastic material and is shaped like a clamp having at least one rearward extending clamp limb and one joint, whereby the holding slot that accommodates the doctor blade bed forms the clamp jaw, and   a clamping tube that presses the slot walls of the holding slot toward one another is arranged on a clamp limb.       

   According to the disclosure, the doctor blade bed is configured as the smallest possible wearing part, as a result of which it requires less material than the prior-art doctor blade beds. The fact that less material is required also allows the use of higher-quality-that is to say, more expensive- materials that translate into a longer service life for coating purposes. The support is not a wearing part. Therefore, it can be produced in a more complex manner, for example, by metal-cutting methods. It can be configured in such a way that it can be used universally and can accommodate different doctor blade beds for doctor blade rods having different diameters. Furthermore, the division of the support system for the doctor blade rod into two parts, namely, the doctor blade bed and the support, allows the use of various combinations of materials in order to improve the operational behavior of the dosing system. Thus, for instance, the doctor blade bed can be made to be especially elastic for an improved sealing behavior with respect to the doctor blade rod and particularly wear-resistant, while the support is made of a harder material so that its greater stiffness prevents vibrations in the dosing system. 
   The shaping of the support like a clamp with a pressure tube that presses against a clamp limb and thus presses the slot walls of the holding slot toward one another makes it possible to relax the support slot by lowering the pressure in the clamping tube in order to change the doctor blade bed. During the operation of the dosing system, the clamping tube is under pressure so as to securely hold the doctor blade bed with the inserted doctor blade rod. 
   The configuration of the dosing system according to the disclosure has the additional advantage that the tension with which the doctor blade rod is held in the doctor blade bed can be regulated via the pressure in the clamping tube. The friction of the doctor blade rod is a function of the tension during the rotation of the doctor blade rod in the doctor blade bed and said friction can thus be set at the lowest possible values. Thus, in the case of very wide (&gt;10 meters) coating installations, doctor blade rods with small diameters (10 mm to 12 mm) can be used whose ends are driven without the torsional forces becoming excessive. Moreover, detrimental vibrations of the doctor blade rod during operation can be avoided by changing the damping effect of the system via the pressure in the clamping tube. With the clamping tube, the system advantageously has an additional control element with which the operational behavior can be positively influenced with the tube pressure serving as the regulated quantity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings serve to explain the disclosed apparatus on the basis of embodiments depicted in a simplified form. The following is shown: 
     FIG.  1  - across section of an embodiment of a doctor blade dosing system according to the disclosure, 
       FIG. 2 - a  cross section of another embodiment, and 
     FIG.  3  - the replacement of the doctor blade bed in the embodiment, according to FIG.  2 . 
   

   DETAILED DESCRIPTION 
   The dosing system shown in the drawings is part of a device for coating a paper or paperboard web  1  with coating paint. It is preferably a so-called roll doctor system in which the web  1  is supported in the area of the dosing system by a mating roll  2 . By the same token, the doctor blade dosing system can be used for indirect dosing, whereby a pre-dosed film of coating material is first applied onto a roll and subsequently transferred from this roll onto the web. Another application possibility is with so-called revolving doctor systems in which the web is transported over the doctor blade rod in a free section without being supported by a roll. 
   In the direction of rotation of the roll  2  upstream from the dosing system, there is a suitable application system (nozzle coater, kiss-roll coater, etc.) from which the coating paint is applied in an excess onto the web  1 . 
   As the dosing element, the doctor blade dosing system includes a doctor blade rod  3  mounted so that a rotary drive can make it turn in a doctor blade bed  4  that is partially open towards the web  1 . The lateral curved surface of the doctor blade rod  3  is either smooth or else provided with circumferential grooves. Its diameter preferably ranges from 6 mm to 25 mm. Preferred diameters are 9.75 mm, 10.00 mm, 12.00 mm, 12.7 mm, 13.65 mm, 15.00 rum, 15.87 mm and 16.00 mm. The axial length of the doctor blade rod  3  corresponds to the working width of the coating device, which can reach 10 meters. Preferably, the doctor blade rod is made of stainless steel and its lateral curved surface is coated so as to be wear-resistant, for example, chrome-plated or coated with a ceramic material. 
   The doctor blade bed  4  is preferably made of a material that is elastic, at least to a certain extent. According to an embodiment, it is made of an elastomer (rubber or plastic), preferably polyurethane. The hardness of the material of which the doctor blade bed  4  is made preferably ranges from 50 Shore A to 65 Shore D, preferably it lies between 85 Shore A and 95 Shore A. Thus,the material is relatively elastic so that the bed  4  can lie in contact with the doctor blade rod  3  so as to create a seal, in order to prevent the penetration of coating paint into the bed  4 . The doctor blade bed  4  has a slot  18  into which the doctor blade rod  3  is inserted, whereby the doctor blade bed  4  encloses the doctor blade bed  4  at an angle range of at least 180° so that the doctor blade rod  3  is securely held in place. 
   As an alternative, it is also possible to make the doctor blade bed  4  out of polyethylene. The working to the required dimensional accuracy is then carried out with a cutting method, especially by milling. 
   The ratio of the cross-sectional area (measured in mm 2 ) of the doctor blade bed  4  to the diameter (measured in mm) of the doctor blade rod  3  is less than 60 mm, preferably less than 30 mm. Preferably, the ratio of the cross-sectional area of the doctor blade bed  4  to the diameter of the doctor blade rod  3  is 10 mm to 25 mm. 
   It has proven to be especially advantageous to have a doctor blade rod with a diameter ≦16 mm that is inserted into a doctor blade bed  4  with a cross-sectional area of 100 mm 2  to 400 mm 2 . The doctor blade bed  4  has a uniform cross section over the working width of the coating device, that is to say over the length of the doctor blade rod  3 . At least one channel  5  that is open towards the doctor blade rod  3  is made in the doctor blade bed  4 , and during the operation, water is fed into the channel to serve as a lubricant and cleanser. 
   The doctor blade bed  4  is inserted in a removable manner into a slot  18  of a support  6  and said slot is open towards the web  1  as can be seen especially clearly in FIG.  3 . The cross-sectional area of the slot  18  is adapted to the outer contour of the doctor blade bed  4  so that the latter is securely held in place by a positive or non-positive connection. Preferably, the opening of the slot  18  of the support  6  is limited by at least one projection  19 , preferably by two projections  19 , and the doctor blade bed  4  has a corresponding number (two in the embodiment) of matching widened sections  20  that are moved into the support  6  behind the projection or projections  19  when the doctor blade bed  4  is inserted, so that the doctor blade bed  4 , after being inserted, is positively clamped in the support  6 . 
   The support  6  is made of a plastic material, for example, polyethylene or polyurethane. The hardness of the material ranges from 50 Shore A to 65 Shore D. It is shaped like a clamp, having at least one rearward extending clamp limb  13  and one joint  15 . The holding slot  18  that accommodates the doctor blade bed  4  forms the clamp jaw. On the back of the support  6 , arranged on a clamp limb  13 , there is a clamping tube  16  that presses the slot walls of the holding slot  18  toward one another and that can be filled with compressed air. 
   In the embodiment according to  FIG. 1 , the support  6  is configured with two rearward extending clamp limbs  13 ,  14 . The clamping tube  16  is arranged between the two clamp limbs  13 ,  14 . 
   In the embodiment according to  FIGS. 2 and 3 , the support  6  has one clamp limb  13 . The clamping tube  16  is mounted on the main part of the support  6  into which the slot  18  has been made. It presses the clamp limb  13  upwards in order to clamp the doctor blade bed  3  and consequently, it presses the upper slot wall of the holding slot  18  downwards. 
   Preferably, as a joint  15 , an area of the support  6  between the clamping tube  16  and the holding slot  18  is configured as a weak spot with a reduced thickness. In the embodiment according to  FIG. 1 , for this purpose, a slit  17  is made into the support  6  extending from the area of the clamping tube  16  in the direction of the holding slot  18 . In the embodiment according to  FIGS. 2 and 3 , the support  6  is configured so as to be correspondingly thin in the area between the holding slot  18  and the clamping tube  16 . The thickness of the support  6 , measured in the direction of the clamping tube  16  [−] (in  FIG. 1  the distance between the slit  17  and the doctor blade bed  4 ) [−] is 0.5 mm to 10 mm, preferably 1 mm to 3 mm, at the weak spot. This ensures an adequate elasticity of the clamp limbs  13 ,  14 . 
   The support  6  is detachably fastened to the foot  8  on a support beam  7  as part of the frame of the dosing system. In the embodiment according to  FIG. 1 , the support  6  is clamped tight with a rotating clamping element  9  that is arranged cross-wise over the working width of the coating device, extending in a slot that is open towards the foot  8  and that is in an angled bearing part  10  that is screwed onto the support beam  7 . In the embodiment according to  FIGS. 2 and 3 , the foot  8  widens towards a fastening part  21  that is screwed onto the support beam  7  by means of screws  22 . 
   On the back of the support  6 , there is a pressure tube  12  that is supported on the frame of the system, that presses the support  6  in the direction of the doctor blade bed  4  and that extends crosswise over the length of said support  6 . The pressure tube  12  is supported on the frame of the system and presses the back of the support  6  with the doctor blade bed  4  and the doctor blade rod  3  in the direction of the web  1  to be coated or of the roll, in order to build up the contact pressure of the doctor blade rod  4  against the web  1  or against the roll. In order to make this possible, an area of the foot  8  of the support  6  is made so as to be elastic, at least to a certain extent. In the embodiments, the elasticity is effectuated by a thinner cross section between the fastening place on the bearing part  10  and the main part with the holding slot  18 . 
   In the embodiment according to  FIG. 1 , the pressure tube  12  is fastened at the top on the vertical part of the bearing part  10 , while in the embodiment according to  FIGS. 2 and 3 , it is on the back of the support  6 . 
   During an operational standstill, the doctor blade bed  4  with the inserted doctor blade rod  3  is placed into the slot  18  of the support  6  while the clamping tube  16  is without pressure. As is shown in  FIG. 3 , the clamp limb  13  is pre-tensioned in such a way that the slot  18  autonomously opens somewhat, without pressure in the clamping tube  16 . After insertion of the doctor blade bed  4 , compressed air is fed into the clamping tube  16 . As a result, the slot walls of the support  6  are pressed against each other and the doctor blade bed  4  with the doctor blade rod  3  is affixed in the support  6 . The pressure is selected here in such a way that, on the one hand, the bed  4  and the rod  3  are securely held in place and, on the other hand, the friction of the doctor blade rod  3  is sufficiently low. If vibrations occur during operation, the pressure is varied for damping purposes.