Patent Application
20060243201
1. Field of the Invention
The present invention relates to a doctor bed formed as a profiled strip having a continuous groove in the axial direction in order to hold a rotatable metering rod, which can be pressed against a moving surface and which meters and/or equalizes liquid to pasty medium applied to the moving surface. The moving surface, in the case of direct application of the medium, being one side of a paper, board or other fibrous web, and in the case of indirect application, being the surface of a transfer element which transfers the application medium to the fibrous web.
2. Description of the Related Art
A doctor bed of a metering, doctoring or equalizing unit is known from diverse documents, for example, reference can be made to DE-A1 3022 955. In this case, the metering rod is held in a doctor bed made of resilient material. However, other homogeneous materials of the doctor bed, such as polyethylene and polyurethane, are also known.
Normally, on its rear facing away from the metering rod, the doctor bed has a pressing element (pressure hose) to press it against the moving surface. By using the pressing element or the pressure hose, the thickness of the metered film on the moving surface can be adjusted and varied via the pressure in the pressure hose. In addition, the doctor bed contains flushing channels, into which a flushing or lubricating liquid, for example water, is introduced. As a result, the mounting of the doctor bed can be lubricated and any application medium which may have penetrated can be diluted and removed.
The smooth or profiled metering rod rotating in the doctor bed effects considerable wear of the doctor bed. The doctor beds, produced with accurate dimensions, therefore have to be replaced frequently, which causes considerable costs and effort.
The wear is all the greater and the frictional relationships become less and less beneficial the more the doctor bed is pressed against the metering rod in the region of its groove accommodating the metering rod, in order firstly to be able to guide the metering rod better over its entire length (machine width) and secondly to be able to prevent the aforesaid penetration of application medium. In this case, an enormously high drive power of the rotating metering rod is then needed, which again causes high costs.
What is needed in the art is a doctor bed that is improved in terms of its frictional relationships and an improved material.
The present invention provides a doctor bed wherein materials and material combinations are provided which exhibit a substantially lower coefficient of friction than the materials used hitherto.
According to an embodiment of the present invention a material is proposed which includes a homogeneous mixture of 70 to 99.8% polyethylene and 0.2 to 30% by weight of a wax of animal, vegetable, mineral or synthetic origin that is solid or liquid at room temperature; according to another embodiment of the present invention, a material is proposed which includes a homogeneous mixture of 70 to 99.8% polyethylene and 0.2 to 30% by weight of a grease of animal origin that is solid or liquid at room temperature; and according to another embodiment of the present invention, a material is proposed which includes a homogeneous mixture of 70 to 99.8% polyethylene and 0.2 to 30% by weight of a natural or synthetic oil.
The following waxes, greases or oils (to some extent, their product names are indicated) are suitable for the material combination according to the present invention: lubricant concentrates including hydrocarbon waxes; carboxylic acids and their amides but also carboxylates; to some extent also alcohols, such as HECOSLIP; paraffin waxes, such as AdSperse; Fischer Tropsch waxes; microwaxes, such as petrolatum, EnHance, Sasobit; raw paraffins, such as Rubitherm; vaselines, such as Hellfrisch Vaseline; white oil; silicone oil; wax oils; bitumen additives, such as Scenti; synthetic waxes, such as Hydrowax, Sasolwax; stearins, such as Varacid, VaraNat; natural waxes; and gel waxes, such as VaraGel.
Provision is made for the production of wear-resistant ultra high molecular weight polyethylene (UHMW-PE) according to the present invention to be carried out via sinter pressing. The production operation for doctor beds, for example, assumes stress-free production of the plastic block which, for example, can be achieved by way of appropriate cooling curves. During the compression (to 220 bar) and at the press temperature of 250° C., oils, liquid waxes and liquid greases tend to accumulate locally even at room temperature as compared with solid waxes and greases. Stepwise pressing with specifically different temperature levels prevents the precipitation of liquid. In the melt, homogeneous incorporation of the lubricating additives in finely dispersed form takes place.
In this case, no immediate compression at full temperature is carried out; instead with the press plunger placed without pressure and brief full temperature loading, a molten drawn skin is achieved, in which the interior is still in pure powder form wetted with liquid. Only after that is the pressure increased at lower temperature and the material pressed. A lower temperature is chosen in order to arrange for the liquid not to flow downward too quickly but at the same time to be able to press it finely into the powder (increase in temperature=reduction in viscosity).
The present invention discloses the use of the material combinations according to the present invention, including polyethylene with proportions by weight of waxes or greases or oils, which, in addition to the doctor bed already described of any desired design and shape, can also be used for the production of doctor wiping strips as they are known (one such is described in terms of the arrangement in the subsequently published DE 10 2004 028 754), and also applicator nozzles and edge limiting elements (one such element is described in the subsequently published DE 10 2004 028 755) in an application device, both publications incorporated herein by reference.
In addition, the material combinations described can also be used for supporting elements for the applicator nozzles and metering or doctor units in addition to doctor beds.
The following advantages apply to the doctor beds:
It is not necessary for the complete doctor bed to include the material combination according to the present invention; instead, it can be expedient if this particular material is fabricated in the form of an insert part which is inserted section by section and runs in the axial direction, as indicated in the subsequently published DE 10 2004 024 079, incorporated herein by reference. This insert part then exhibits lower friction in relation to the rest (normal material, for example resilient material or PU or PE).
Hitherto, the greases, waxes and oils proposed here have been used only during the production of detergents or specific coating materials. The suitability for forming homogeneous mixtures for the aforementioned components has proven to be surprising, on the other hand, and it is to the credit of the inventors of the present invention to have recognized this.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
Referring now to the drawings, and more particularly to
By using a pressing element 7, doctor bed 2 including metering rod 3 is pressed against a moving surface 8 which in the case of direct application is one side of a fibrous web and in the case of indirect application is the surface of a transfer element which discharges application medium 9 onto the fibrous web, and in the process determines the thickness d of the medium applied. As a rule, a pressure hose is used as pressing element 7, via the adjustable internal pressure of which a pressing force F is set.
According to one embodiment of the present invention, doctor bed 2 is produced from a material combination including a homogeneous mixture of 70 to 99.8% polyethylene and 0.2 to 30% by weight of a wax of animal, vegetable, mineral or synthetic origin that is solid or liquid at room temperature, in particular consisting of long-chain fatty alcohols and fatty acids or carnauba wax and paraffin.
In a second embodiment, doctor bed 2 is produced from a material combination including a homogeneous mixture of 70 to 99.8% polyethylene and 0.2 to 30% by weight of a grease of animal origin that is solid or liquid at room temperature, such as stearins.
In a third embodiment, doctor bed 2 is produced from a material combination including a homogeneous mixture of 70 to 99.8% polyethylene and 0.2 to 30% by weight of a natural or synthetic oil, to which up to 40% by weight of thickeners can be added.
The material combinations selected ensure better frictional relationships than hitherto because of the lower coefficient of friction, so that either the previous drive power can be reduced or an additional pressing element 10 can be accommodated in the doctor bed 2, in a groove 11 in its upper part 2.1, by which elements metering rod 3 is firstly mounted better as a result of this additional pressing and secondly better tightness with respect to penetrating application medium 9 is achieved. This would normally be possible only with an increased drive power of metering rod 3 but this is presently compensated for by using the material proposed in the present invention.
The additional pressing element 10 can be formed as a pressure hose. If the pressure hose is pressurized with air, upper part 2.1 therefore presses against metering rod 3 and the aforementioned advantages are achieved.
Furthermore, a covering strip 12 for the aforesaid pressure hose, that is to say the pressing element or else sealing element 10, can be as shown in
An application device 13 is illustrated, in which an applicator nozzle 15 is placed on a supporting element 14, which is hollow-cylindrical here. Applicator nozzle 15 includes two nozzle lips 15.1 and 15.2 which, between them, form a metering gap 15.3 for applying application medium 9 to surface 8 moving in the running direction L. Both applicator nozzle 15 and supporting element 14 are produced from the material combination according to the present invention and, as a result, guarantee a long service life.
It should also be mentioned that what are known as bounding elements (not illustrated), which are also designated edge limiters or else format limiters, and, as the name already states, define a coating-free edge and a specific format or application width on the moving surface 8 during the production and/or finishing of the latter, can be produced from the material according to the present invention.
It was previously the case that, because of the contact of the bounding element with the rapidly moving surface 8 and the high friction, this element and also the roll cover of the moving surface 8 were partially damaged. By way of the material according to the present invention, the aforementioned disadvantages are reduced considerably and, here too, the service life of the aforesaid components is increased as a result.
In
While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 020 848.7 | May 2005 | DE | national |
