GB2265846A

GB2265846A - Method for coating a material web

Info

Publication number: GB2265846A
Application number: GB9307414A
Authority: GB
Inventors: Jukka Koskinen
Original assignee: Valmet Paper Machinery Inc
Current assignee: Valmet Paper Machinery Inc
Priority date: 1992-04-10
Filing date: 1993-04-08
Publication date: 1993-10-13
Anticipated expiration: 2013-04-08
Also published as: CA2093093A1; FI921588A0; SE506119C2; FI91298C; FR2691179A1; FI91298B; SE9301183L; DE4303577A1; SE9301183D0; GB9307414D0; GB2265846B; FR2691179B1; FI921588A

Abstract

A control method for an applicator and metering unit in which the coating mix is applied onto the web via a gap formed between the web and a blade tilted to an acute angle to the web, and the applied coating mix is smoothed and the excess mix is removed at the doctor blade, and on controlling the final coat weight by way of altering the amount of coating mix adhering to the web at the applicator unit, according to the method the linear loading pressure of the applicator unit blade (12) is at the start-up set to an approximately correct value for desired coat weight and held at a constant level during running, and the amount of coating mix applied onto the web is controlled during running by altering the amount of coating mix adhering to the web at the applicator unit (2), this arrangement achieves an accurate control of coat weight and the cross-directional profile of coat weight be readily controlled. <IMAGE>

Description

2265846 Method for coating a material web The present invention concerns a

method according to the preamble of claim I for coating a web with a coating mix and for controlling the coat weight. 5 This invention is related to the control of such an applicator and metering apparatus in which the coating mix is applied onto the web via a narrow gap formed between the web and a smoothing element and then the applied mix is levelled and the excess mix doctored away by means of a doctor blade. ' ' The GB patent application 8508431 discloses such a method and apparatus for coating a web. The method according to the publication uses a flexible blade for the preapplication of the mix, while the final doctoring is performed by means of 15 a scraping doctor blade. The coater apparatus is placed adjacent to the backing roll allowing the coating mix to meet the web over a great length of the web. Removal of entrained. air from the coatng mix is difficult in the disclosed embodiment, and the integrated construction of the doctor blade with the coater structure complicates service operations. Furthermore, coating mix behaviour in 20 the disclosed method is sensitive to disturbances in the application zone such as uneven distribution of the mix feed. Typical consequences therefrom include striping of web coat in the machine direction of the web. In a coater developed further from the abovedescribed construction, the coating 25 mix is fed onto the web in a laminar flow via a narrow gap formed between the web and a blade tilted to an acute angle with respect to the web. The mix is then levelled and the coat weight is controlled in a conventional manner by means of a doctor blade. Such an apparatus is described in the FI patent application 890249, filed earlier by the applicant. The laminar flow application of the coating mix 30 onto the web achieves a significant reduction of striping of the coated web and defects caused by entrained air bubbles on the coated web. Control of coat weight in the apparatus particularly with higher coat weights is, however. rather 2 complicated due to the fact that a relatively small change in blade loading causes a large change in coat weight. The construction design of the blade must be carried out very carefully to avoid hysteresis in blade adjustments.

It is an object of the present invention to achieve a novel method for coating a paper web.

The invention is based on controlling the coat weight by varying the feed of coating mix applied in the coating apparatus onto the web.

More specifically, the method in accordance with the invention is characterised by what is stated in the characterising part of Claim 1.

The invention offers significant benefits.

Accurate control of coat weight particularly for higher coat weights becomes easier according to the present invention than is possible in conventional doctor blade coating. In a particularly preferred embodiment, the smoothing element is a flexible blade tilted at an acute angle with respect to the web e.g. less than 20. The final coat weight is affected by a significantly higher factor by changes in blade loading than by changes in the amount of initially applied coating mix. A change in the amount of initially applied coating mix affects the final coat weight basically through altering the momentum inflicted on the doctor blade that tends to bend the blade away from the web surface. Thus, changes in the amount of initially applied coating mix are indirectly reflected in the loading force of the doctor blade required to remove excess coating mix from the web surface. In the present method the operating accuracy requirements on the constant-angle assembly of the doctor blade employed in the coater are relaxed because the loading of the assembly need not be altered during running. Consequently, the method is also free from blade tilt angle hysteresis which would diminish the accuracy of blade adjustment. Positional accuracy requirements of the support beam for the doctor blade proper are essentially relaxed, since the beam need not be rotated during running. Then, compensation for the beam's machining errors with the help of fineadjustment means becomes less crucial. As 3 the doctor blade is not anymore employed for the purpose of controlling the amount of applied coating mix, the blade geometry can be designed for a simplified arrangement of blade profile adjustment without the need for catering to the hysteresis phenomenon. Simultaneously, a more stable blade profile adjustment is attained. The life of the doctor blade is increased by virtue of the constant operating conditions at the blade tip. Limitations set to blade loading by the maximum tilt angle of the support beam are also relaxed.

The invention is next examined in detail with the help of exemplifying embodiments illustrated in the attached drawing.

The coater shown in Fig. I comprises a backing roll 15, whose rotational direction is marked by arrow H and whose surface is conformally covered by the paper web 1 to be coated. Extending over the cross-directional width of the web 1, under the backing roll 15, is placed a blade metering unit 2, whose frame is formed by a support beam 11 with an approximately triangular cross section. The support beam I I is pivotally mounted at a pivot point 10 to the support structures of the coater. The coating mix is fed along a feed channel 3, which is mounted onto the support beam 11 to the entrance side of the web I so as to extend cross directionally over the web 1, into a cavity-like space 16, wherefrorn the mix is propelled under pressure into a narrow feed channel 4. As evident from Fig. 1, when sectioned in the plane of the flow vector of the coating mix, the feed channel 4 has a slightly S-shaped section. The feed channel 4 exits to the stem part of a predoctoring blade 5. The feed channel 4 is extremely narrow in comparison to conventional feed channels; the exit opening of the feed channel 4 in particular has typically a height of only 3 - 5 mm. The predoctoring blade 5 is fixed at its stem between blade holders 6 and 18. The blade 5 leans flexibly on the web 1 at an acute angle a. The angle ct is typically less than 20". The blade holders 6 and 18 are implemented so as to avoid forming any significant step between the exit opening of the feed channel 4 and the stem of the blade 5. In particular the blade holder 18 on the side of the feed channel 4 has a wedge shaped cross section tapering toward the tip of the blade 5. The linear loading of 4 the predoctoring blade 5 can be regulated by a load control device 7. The load control device 7 is divided in the cross direction of the web 1 into independent control sections, whereby the loading of the blade 5 can be varied in the cross direction of the web 1 thus allowing for a control means of the applied coating mix quantity in order to obtain a desired coat weight profile in the cross direction of the web. Due to the high feed velocity exceeding 1 m/s, the excess mix will also spread toward the entry direction of the web 1. This excess mix is collected to a first mix collecting trough 9. Furthermore, the excess coating mix flowing over the doctor blade proper is collected to a second mix collecting trough 8.

The final doctoring of the mix on the web is performed using a doctoring unit 14, which comprises a doctor blade 12 proper attached to a second frame beam 17.

The loading of the doctor blade 12 can be regulated by a load control device 13.

As is evident from Fig. 1, distance D from the tip of the predoctoring blade 5 of the application unit 2 to the tip of the doctor blade 12 proper is rather large in the implementation according to the invention, typically being in the order 0. 5- 0.8 m. At typical web speeds used in practice, this distance corresponds to a duration of travel of approx. 30 -40 gs.

In the method according to the invention, the coater described above is operated as follows.

At start-up, the linear load at the doctoring unit 14 is set approximately to a value known to give a desired coat weight. The doctoring unit employed is advanta geously a doctoring unit of so-called constant-angle type. The linear load at the doctoring unit is basically held at the initial set value through the entire run. The end coat weight is adjusted by altering the applied quantity of the coating mix.

Three basic methods are available for controlling the amount of coating mix applied onto the web in the applicator unit 2, namely, alteration of the feed rate of coating mix pumped via the channel 3, the doctor blade linear loading pressure or the width of the gap between the web or the width of the return flow slit between the web and the applicator unit. The best control result is obtained by altamg either the limar lwffillg pressure of the h] cr the widffi QE the reWm flow slit. The applicator blade 5 is attached to the support beam 11, which forms the frame of the apparatus, via a control device 7 of the blade linear load. In its operating position the applicator blade 5 tilted to an acute angle in relation to the web 1 floats on a hydrodynamic bed formed by the flowing coating mix. A portion of the coating mix material fed in a laminar flow onto the web 1 passes via the gap between the blade 5 and the web 1, thus adhering to the web. The excess mix is deflected backward against the machine direction H of the web 1.

The control of the total amount of coating mix adhering to the web is advanta geously managed by altering the width of the return flow slit. According to this scheme, increasing the slit width reduces the back pressure of the return flow, thereby reducing the proportion of coating mix adhering to the web. Reduction of the slit width correspondingly increases the back pressure and elevates the internal is pressure in the coating mix bed formed between the applicator blade 5 and the web 1. Due to the increased pressure, the blade 5 is forced slightly upward from the web and more mix can thus flow out along with the moving web 5.

0 The same control result is achieved by altering the linear loading pressure of the applicator blade 5. Moreover, in addition to the machine-direction control of the coat weight profile, the control device 7 of the applicator blade 5 can be employed for the cross-direction control of the coat weight profile. The profile control can be implemented in all types of blade holders equipped with a profile control facility, and as the control is directly applied to the amount of coating mix adhering to the web surface, a more precise and stable control of the coat profile is attained than by performing the control at the doctor blade 12.

Larger run-time changes in coat weight are coarsely made by altering the linear loading pressure at the coater unit 14. Thus, the nominal end weight of the coat 0 C applied onto a web is adjusted according to the invention at the doctoring unit, while the precision fine-control of the final coat weight to desired limits is performed by way of altering the amount of coating mix applied onto the web.

6 The blade tilt angle ce of the blade 5 can be set even smaller than 15'. Optimal settings of the blade tilt angle and amount of applied coating mix can in advantageous cases permit omission of the doctor blade 12 proper.

The use of a conventional short-dwell coater following the metering unit 2 is possible for grades requiring heavier coat weights.

7 Cl ai ms 1. A method for applying and smoothing a coating mix onto a web moving on a rotating back roll using an assembly comprising:

(i) an applicator unit for applying coating mix onto the web comprising a smoothing element for smoothing the applied coating mix and a narrow-slit feed element which exits without any essential threshold close to the smoothing element; and (ii) a'doctoring unit downstream of the applicator unit in the direction of motion of the web, including at least one doctoring blade adapted to be pressed against the web so as to smooth the coating mix onto the web, characterised in that the amount of coating mix applied onto the web is controlled during operation of the assembly by varying the amount of coating mix applied onto the web at the applicator unit, the linear loading pressure of the doctor blade being set, before application of the coating mix, to an approximately correct value for the desired coat weight, and subsequently held at an approximately constant value during operation of the assembly.

Claims

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2. A method as defined in Claim 1, in which the smoothing element is a

flexible blade tilted at an acute angle with respect to the web.
3. A method as defined in Claim 2, in which the acute angle is less than 20.
4. A method as defined in any of Claims 1 to 3, in which the amount of coating mix applied onto the web at the applicator unit is controlled by altering the linear loading pressure of the smoothing element.
5. A method as defined-in any of Claims 1 to 3, in which the amount of coating mix applied onto the web at the applicator unit is controlled by altering the mutual distance of the gap between the applicator unit and the web on the entry side of web into the gap.
6. A method as defined in any of Claims 1 to 5, in which the amount of coating mix applied onto the web is altered in the transverse direction to the length of the web for controlling the cross-directional profile of the coat weight.
7. A method as defined in Claim 6, in which the amount of coating mix applied onto the web is altered in the transverse direction to the length of the web by controlling the profile of the smoothing element.

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8. A method as defined in Claim 6, in which the amount of coating mix applied onto the web is altered in the transverse direction to the length of the web by controlling the profile of coating mix fed via the feed element.