The invention also relates to a method for applying a web treatment substance onto the applicator roll of a film-transfer coater.

A web of paper or paperboard can be coated or surface sized using a film-transfer coater, wherein the web being treated is passed through a nip between two opposed rolls. The coating furnish or surface size is applied with the help of a suitable applicator apparatus to the surface of one or both of the nip-forming rolls, wherefrom the substance is transferred to the surface of the web being treated in the nip between the rolls. The thickness and cross-machine profile of the size layer applied to the roll surface are controlled by a doctor blade or rotatable metering rod. The applicator apparatus incorporates an application chamber delineated in the rotating direction of the nip-forming roll on one side by a doctor member and on the other side by the chamber front wall. Laterally the application chamber is delineated by end wall seals.

The coating substance or surface size is fed into the application chamber via a duct adapted into the frame beam of the applicator apparatus. The volumetric feed rate and pressure of coating furnish into the application chamber are adjusted so high that a backflow of coating furnish or surface size is established in a reverse direction to the rotating direction of the applicator roll in the gap between the application chamber front wall and the applicator roll. The function of the backflow is to prevent the air boundary layer traveling on the applicator roll surface from entering the application chamber. If allowed to enter the application chamber, such external air may cause uncoated areas on the web.

As the gap between the applicator roll and the application chamber front wall generally is in the range of 3-6 mm, the volumetric flow rate of coating furnish must typically be set about 20 to 30 times greater than the amount of furnish adhering to the applicator roll in order to seal the gap completely. Even if the furnish flow rate is set smaller than that needed for sealing the gap with the web treatment substance, the excess flow rate still remains large as compared with that portion thereof forming a film on the applicator roll. In the case the gap is made large, it is not easy to increase the internal pressure of the coating application chamber due to the low flow resis- tance of the wide gap. Hence, an elevated pressure in the application chamber respectively increases the overflow or backflow through the gap. If the gap between the application chamber front wall and the applicator roll is made very small, e. g. , as narrow as 1.5 mm, even minor changes in the gap width due to manufacturing deviations, for instance, will cause cross-machine defects, known as profile errors, in the applied coating. Moreover, reducing the gap width may increase the application chamber pressure so much due to the elevated flow resistance of the narrowed gap width that the sealing of the application chamber edges may become problematic.

Furthermore, at very high coater run speeds even the backflow may be insufficient means to prevent the entry of air via the gap between the chamber front wall and the applicator roll into the application chamber where the entrained air can cause turbu- lence in the web treatment substance.

To increase the coater run speed, the application chamber front wall in a film-transfer coater can be implemented using a so-called dam blade that is pressed against the ap- plicator roll so that air cannot enter the applicator chamber via the gap between the dam blade and the applicator roll. While no actual backflow takes place past the dam blade facing the applicator roll, coating furnish is anyhow passed through flow openings made in the dam blade from the applicator chamber to in front of the dam blade. Hereby, a pond of lubricating furnish is formed in front of the dam blade thus preventing damage to the applicator roll or the dam blade due to the mutual friction between the dam blade and the applicator roll. If the dam blade of the applicator apparatus is situated higher than the actual doctoring member, coating furnish can

pass from in front of the dam blade over the dam blade holder back to the coating furnish circulation. Then, the pond of coating furnish in front of the dam blade may grow very large before furnish begins to flow over the dam blade holder. At high web speeds, coating furnish will readily splash from a large pond volume to the surroundings and the applicator roll thus causing inferior quality of coating/size application. Moreover, coating furnish may leak via the clamping groove of the dam blade holder to the exterior side of the applicator chamber. <BR> <BR> <P>If the internal pressure of the application chamber increases due to, e. g. , excessive feed of coating furnish, also the force pressing the dam blade against the applicator roll increases thus causing higher wear rate of the dam blade and possibly damaging the applicator roll.

It is an object of the present invention to provide for a film-transfer coater an entirely novel kind of applicator apparatus with a relief facility of the dam blade loading.

The goal of the invention is attained by supporting and pressing the dam blade against the applicator roll by a support member adapted to rest against the rear side of the dam blade. The point at which the support member effects its support force is situated approximately midway between the mounting point of the dam blade on its holder and the tip of the dam blade resting against the applicator roll. The support member extends over the entire cross-machine length of the dam blade, it prevents web treatment substance contained in the application chamber from escaping to the exterior side of the application chamber via the contact line between the dam blade and the support member under normal loading conditions of the dam blade. In the case that the internal pressure of the application chamber increases, also the force pressing the dam blade against the applicator roll grows. If the internal pressure of the applicator chamber increases above a certain level, the dam blade bends and thus looses its contact to support member. Hereby, the excess pressure of the application chamber is relieved via the gap between the dam blade and support member. After the application chamber pressure has been relieved sufficiently, the dam blade again assumes its resting position against support member.

More specifically, the applicator apparatus according to the invention is character- ized by what is stated in the characterizing part of claim 1.

Furthermore, the coating method according to the invention is characterized by what is stated in the characterizing part of claim 6.

The invention provides significant benefits.

The internal pressure of the application chamber cannot grow excessively as the increasing pressure forces the dam blade off from contacting support member thus allowing relief of excessive pressure via the gap between the dam blade and support member. This arrangement prevents flexural failing of the dam blade under a transient pressure rise in the application chamber. The construction according to the invention is also uncomplicated and easy to implement in existing coaters.

In a preferred embodiment of the invention, on the dam blade between its support point and its mounting point in dam blade holder are made spillway openings that allow web treatment substance accumulating into the pond in front of the dam blade to be passed back to the web treatment substance circulation. As a result, the volume of the treatment substance pond formed in front of the dam blade is diminished permitting the coater to be run at a higher speed without the risk of splashing.

Because the web treatment substance is not any more passed over the dam blade holder back to the treatment substance circulation, the dam blade mounting groove in the dam blade holder stays clean, whereby detrimental chamber leaks of treatment substance via the mounting groove are eliminated.

In the following, the invention is examined in more detail with the help of the appended drawings in which FIG. 1 is a schematic side elevation view of a film-transfer coater ;

FIG. 2 is a schematic side elevation view of an applicator apparatus according to the invention; FIG. 3 is a front view of a dam blade suitable for use in an applicator apparatus according to the invention; and FIG. 4 is a side elevation view of another embodiment of an applicator apparatus according to the invention.

Referring to FIG. 1, a film-transfer coater 1 shown therein comprises two rotatable applicator rolls adapted into a nip contact with each other, a first applicator roll 2 and a second applicator roll 3 forming a nip through which a web 4 to be treated by surface size or coating furnish is adapted to pass. The travel direction of web 4 is denoted by an arrow. With the help of applicator apparatuses 5, a substance for treating the web 4, such as a coating mix or surface size, is applied to the surfaces of applicator rolls 2,3. The treatment substance layer is smoothed and metered to a desired thickness with the help of a doctor member such as a doctor blade or a rotating metering rod. The doctored layer of the treatment substance is transferred from the applicator rolls 2,3 to both sides of the web 4 being treated in the nip between the rolls 2,3. If only one side of web 4 is to be treated, the treatment substance is metered only on one of the applicator rolls 2,3.

Referring to FIG. 2, the construction of the applicator apparatus 5 of the second applicator roll 3 is therein described in more detail. The applicator apparatus 5 comprises an applicator chamber 6 delineated by an applicator roll 3. In the rotating direction of the applicator roll 3, the application chamber 6 is delineated by front and rear walls. The rotating direction of roll 3 is denoted by an arrow. The rear wall of the chamber is formed by a doctor member such as a doctor blade or a smooth- surface or grooved metering rod 7. The loading of metering rod 7 is effected by pneumatic inflatable tubes 8. The front wall of the chamber is upstream of the rear wall in the rotating direction of the applicator roll. The dam blade 9 serves as the chamber front wall. The ends of the application chamber 6 are delineated by end wall

seals (not shown). The application chamber 6 extends in the axial direction of applicator roll 3 over the cross-machine width of the coater. The dam blade 9 is mounted at its upper-edge mounting point 10 on a dam blade holder 12 that in turn is mounted on a dam blade support beam 11. On the dam blade support beam 11 is also mounted a support member 13 serving to back up the dam blade 9 and to press the same against to the applicator roll 3 so as to prevent the web treatment substance from flowing via the gap between the dam blade 9 and the applicator roll 3 to in front of the dam blade 9. The dam blade support point 14 is located between the dam blade tip 15 and the mounting point 10 of the dam blade on the dam blade holder. The dam blade support member 13 extends in the axial direction of the applicator roll 3 over the cross-machine width of the application chamber 6. The applicator apparatus 5 is mounted in a vertical position such that it operates substantially at the level of a horizontal plane passing through the axial center line of applicator roll 3.

The web treatment substance is fed via a feed duct 16 adapted to the applicator appa- ratus frame beam 11 to the application chamber 6, wherefrom the treatment substance is metered to the surface of the applicator roll 3. From the application chamber 6, the web treatment substance is also passed via flow openings 20, which are made to the dam blade 9 on the area thereof between its tip 15 and its support point 14, to in front of the dam blade 9 in order to form there a pond 17 of the web treatment substance.

To prevent the volume of the web treatment substance pond 17 formed in front of the dam blade 9 from growing excessively large, the dam blade 9 is provided with spill- way openings 21, via which the excess web treatment substance is passed into a spill- way duct 18, that is located behind the dam blade 9 and is delineated by dam blade holder 12, dam blade support member 13 and the portion of dam blade 9 between the dam blade support point 14 and the mounting point 10 of the dam blade 9 on the dam blade holder 12. Respectively, the spillway openings 21 are located within the area of the dam blade 9 situated between the dam blade support point 14 and the mounting point 10 of the dam blade in the dam blade holder 12. The spillway openings 21 are made across the width of the application chamber 6 at such a spacing and height that

the top level of the web treatment substance pond 17 remains at a desired level below the mounting point 10 of the dam blade on the dam blade holder 12.

The tip portion of the support member 13 resting against the dam blade 9 is made of rubber or other elastic seal material. Resultingly, the dam blade tip 13 can rest tightly against the dam blade 9 thus preventing any flow of the web treatment substance between the application chamber 6 and the spillway duct 18.

With no spillway openings made in dam blade 9, the web treatment substance pond 17 would extend to the top level of the holder 12 of the dam blade 9, whereupon the web treatment substance could flow over the dam blade holder 12 back into the treatment substance collection trough 19.

If the internal pressure of the application chamber 6 rises excessively high due to, <BR> <BR> e. g. , superfluous feed of web treatment substance, the dam blade 9 is pressed aggres- sively against the applicator roll 3, whereby the dam blade 9 bends until its contact to the seal tip of dam blade support member 13 is lost and the excessive pressure of application chamber 6 is relieved into the spillway duct 18. After the pressure of application chamber 6 has fallen sufficiently, the dam blade 9 again straightens itself into contact to the sealing tip of the support member. Thus, a flexural failure in the function of the dam blade 9 is avoided that could lead to a transient rise in the internal pressure of application chamber 6.

The invention may also have embodiments different from those described above.

The dam blade 9 need not necessarily have the flow openings 20 for passing the web treatment substance to in front of the dam blade 9, but instead, the web treatment substance pond 17 may alternatively be formed using other techniques such as spraying of the web treatment substance from separate nozzles, for instance. The embodiment according to the invention may also be applied to such film-transfer coaters in which the rotating direction of the applicator roll 3 is reverse to that of the embodiment shown in FIG. 2. Then, the doctor member 7 is located above the dam blade 9 (FIG. 4), whereby the spillway openings 21 may not necessarily be needed.