EP1541765B1 - Coater - Google Patents

Abstract

In a process to apply a coating of liquid or paste to one side of a moving web, the descending web passes around a drum (7) onto a horizontal plane and is presented to a descending fluid or paste curtain. The gusset zone prior to and following the drum incorporates a gusset plug (8a, 8b). The gusset plugs (8a, 8b) minimise the movement of surface boundary layer air.

Description

The invention relates to a device for applying one-sided or two-sided application of liquid to pasty application medium, in particular aqueous pigment suspension, to a running surface, wherein the surface is the surface of a material web, in particular a paper or board web for direct application and the surface for indirect application a transfer element, for example, an application tape, which then transfers the application medium to the surface of the material web, with an applicator, wherein in the running direction of the running surface in front of the commissioned work Luftgrenzschicht- fighting device is arranged ng.

Although the air boundary layer entrained by the running surface may adversely affect the order result even in other types of applicators, the invention will be described below using the example of a curtain applicator. In such a device, the applicator located at a predetermined distance from the running surface from its machine-width application nozzle, the application medium, such as pigmented coating color or glue or starch, as a substantially gravity-induced moving curtain (curtain) from the running surface.

This technique is referred to in the art as curtain coating, wherein the order is carried out without excess of medium. This means that only as much medium is applied as ultimately to remain on the material web.

Modern machines for the production and / or finishing of a material web, in particular a paper or board web, have now for economic reasons widths of up to 10 m and more and are driven at high speeds of about 2000 m / min.

It is extremely difficult to be able to keep a curtain which is uniform over this width extremely stable, especially when coating the material web to ensure uniform application over the entire width of the web.

There is therefore a requirement to minimize the influence of the air boundary layer flowing with the high-speed surface.

From the DE-A1 10012257 a device for weakening the entrained by the running surface air boundary layer is known.
For this purpose, both a suction device, elastic scrapers, brushes, a blowing device, an electrode assembly and an ultrasonic source are used alternately.
In a further variant, this DE-A1 describes that the air boundary layer weakening device is designed as a sealing element, wherein this sealing element is pressed against the running surface and rolls on this substantially slip-free.
In addition, a strip extending in the transverse direction of the running surface is provided which precedes the weakening or Bekämfungseinrichtung and the removal of the uppermost layers of air over the running surface is used, which is to be relieved, in particular, the downstream attenuator in their work.

In contrast, it is an object of the invention to further increase the attenuation of the influence of the air boundary layer with a suitable device.

This object is achieved in that an air boundary layer weakening device is assigned at least one, extending in the transverse direction of the running surface, strip-shaped shielding.

This shielding element is associated with that area to which the running surface or the running material web runs to the commissioned work. At a Curtain coater is protected by the free-falling curtain directly and particularly effectively against the influence of harmful air boundary layers.

It is very useful in this case if the shielding both very close to the upstream assembly or the previous shielding or adjacent control device, as well as just before the commissioned work, in particular close to the delivered curtain.

The influence of the air boundary layer is thereby combated even more effectively. Such an additional bar causes only very low manufacturing and installation costs. It is particularly effective if it is arranged in front of and / or behind an existing weakening or Bekämfungseinrichtung.

As a weakening all known from the prior art devices such as suction boxes, rolling elements, such as rollers or suction rollers, scraper elements, electrode assemblies, ultrasonic devices alone or in combination come into question.

If an element rolling on the running surface or the material web, for example a roller, is selected, then it is expedient to chrome-plate this element or this roller. This creates a smooth surface suitable for unrolling. Alternatively, however, an outer surface may be provided which has a hardness of more than 90 Pussey <Jones and is therefore very soft. In particular, a better sealing effect is achieved with rough track surfaces.

It is conceivable that a plurality of shielding elements could be provided, which would make even an arrangement of the said weakening device superimposed on one another superfluous, which would mean a cost reduction of the device. This alternative is particularly suitable for less fast-moving and / or less wide machine.

In the context of the invention, it is particularly expedient for the at least one further shielding element to be arranged parallel to and at a small distance from the running surface, since this precludes the formation of air turbulence.
In this case, a distance of the shielding to the running surface of 0 to 5 mm, preferably 0.5 to 1 mm most effective. The distance can also be quite 0.

Very useful is a design such that the shielding element is arranged at an angle α to the running surface, which is about 0 to 5 °, preferably 2 ° and diverges toward the weakening device or a previous shielding to the running surface.

A further embodiment of the invention may also consist in that the shielding element or at least one further shielding element is arranged within the space which exists between the applicator unit and a material web tapering from above from the control or weakening device. In this case, an angle of inclination β is advantageous for creating an effective shielding. This can be about 45 °.
As a result, a barrier is created which counteracts the penetration of the air boundary layer.

Moreover, it is advantageous if the weakening or fighting device or an adjacent shielding element facing the end of the strip-shaped shielding element is adapted to the outer contour of the attenuation or control device or the previous shielding.
This ensures that the air boundary layer can not rebuild in the remaining gaps.

In the attenuation or control device, it is expedient if this as a sealingly pressed against the running surface or the material web and is formed on this slip-free rolling off sealing element. This element acts as a fixed point for the material web in direct order and can also serve as a deflecting element for an incoming from above material web or the running surface. In this case, the running surface would be an endless circulating belt, which then transfers the medium applied to its surface to the web of preferably paper or cardboard.
This type of order is known as an indirect order.
The rolling sealing element can be equipped very advantageously with a suction zone for the flowing air boundary layer. In addition, it can also serve as grounding in electrostatic charging of the running surface material web.

A further very advantageous embodiment of the invention may consist in that the shielding element can be extended to a screen which at least partially surrounds the applicator unit and / or the attenuation or control device. This creates optimal conditions in the surrounding area of the commissioned work, which on the one hand improve the climatic conditions with regard to a uniform medium application and, on the other hand, also offer protection of the environment from ejected coating media particles. In addition, the noise protection is improved.

In addition, it is possible to arrange a suction device within the enclosing shield, which additionally or alone acting, sucks the disturbing air.

Alternately, the supply of a medium (eg gas) is conceivable with this device, with the desired climatic conditions within the climate chamber formed are adjustable.

Incidentally, the device according to the invention can be structurally simplified by at least one shielding element as an integral part of the existing Bekämfungseinrichtung or the previous module is formed.

A further advantageous solution is that the running surface or the running material web in the direction of the commissioned work one at an angle γ of 5-30 °, but preferably 5-15 °, rising or at an angle δ of 5-30 °, but preferably 5-15 °, sloping course decreases.

In particular, a counterforce to the air boundary layer is formed at increasing speeds at high speeds, which can act additionally or possibly even alone.

A sloping course would then have to be selected if an extension of the application medium is preferred.

The object of the invention is also achieved in that a strip-shaped shielding element extending in the transverse direction of the running surface is arranged downstream of the applicator unit. Quite surprisingly, it has been found that an arrangement after the commissioned works creates a calming zone in which no unwanted air currents occur and thus the application medium can be applied undisturbed to the surface to be coated. This is especially useful in high-speed machines, because behind the commissioned work again air boundary layers can build up.

The device according to the invention can preferably be used for a so-called curtain applicator (curtain coater), which delivers the application medium to the running surface as a freely falling curtain or veil that moves essentially by gravity.

Such a curtain is particularly difficult to keep stable and evenly at wide (about 10 m and more) and fast running (about 2000 m / min) machines, because he is, inter alia, very much exposed to the influence of the air boundary layer.

The influence of the air boundary layer can be successfully minimized when the distance between the shield member and the discharged curtain is only 3 to 60 mm, preferably 20 to 40 mm.

Further advantageous embodiments will become apparent from further subclaims.

Moreover, the individual embodiments are not only separate from each other, but also used in combination with each other.

Overall, the device according to the invention is characterized by low production costs. Another advantage is the retrofitting of existing coating systems with the device according to the invention, because a smooth adaptation to existing machine sizes and the distances between individual components contained therein is possible.

The invention will be explained in more detail with reference to embodiments.

• Figuren 1 bis 13: grobschematische Seitenansichten verschiedener Ausführungsformen der erfindungsgemäßen Vorrichtung

Show it:

• Figures 1 to 13: a rough schematic side views of various embodiments of the device according to the invention

In the figures, the same reference numerals also denote the same components, for reasons of clarity, essentially only the reference numerals are given, which are changed compared to the other figures.

In FIG. 1 to 5 , an applicator device according to the invention is designated by 1. It includes a curtain applicator unit 2, the dispensing nozzle 3, the desired application medium M as a curtain 4 to a running in the direction L. moving surface O gives off.
In the illustrated embodiment, the surface O relates to a web of paper or cardboard 5, which is coated in a direct manner and on at least one web page, the application medium M, for example, aqueous coating color, hangs up as a coating layer 6.

To weaken or combat the registered with the high-speed material web 5 air boundary layer L _G is in the running direction L in front of the commissioned work 2 a fighting or weakening device 7 is arranged. This could for example be designed as a suction box. In the example shown, however, a roller rolling on the running surface O or the running material web 5 is selected. The roller or control device 7 (hereinafter referred to as "roller" or "device" 7) squeezes off the incoming air boundary layers L _G, so to speak, and at the same time forms a fixed point for the material web, which has a positive effect on the running behavior of the material web running in the running direction L. Paper or cardboard. The roller 7 can also be equipped with a suction zone S for additional removal of the harmful air, as Figure 2 shows.

It can be seen from FIGS. 1 and 2 , and FIGS. 2 to 5 and 7 to 13 explained below, that the material web 5 tapers from above the device 7, that is to say the roller 7. The roller 7 acts due to their Luftabquetschwirkung as a sealing element and at the same time as a deflection, so that the web 5 passes in substantially horizontal direction to the curtain applicator unit 2.

As FIG. 3 shows, the roller 7 can also be used as a grounding E when, for the electrostatic charging of the running surface O or the material web 5, an electrode arrangement 14 is provided on its opposite side of the web, which also influences the air boundary layer L _G minimizes.

In addition, FIG. 1 shows that at least one strip-shaped shielding element 8a of the device 7 is connected upstream in order to improve the control effect against the air boundary layer L _G. However, it is significant that at least one further strip-shaped shielding element 8b is provided in the running direction L immediately in front of the curtain 4, that is to say as the last element in front of the curtain 4. This shielding element 8b is arranged on the side of the web to be coated and, like the shielding element 8a, also extends parallel and transversely - ie orthogonally to the running direction L of the running surface Obw. The material web 5. Its length is adapted substantially to the distance between the roller 7 and the curtain 4.
An enlarged view thereof is shown in FIG . 4 . It can be seen that only a very small distance a remains between the shielding element 8b and the curtain 4. This is about 3 to 60 mm, preferably 20 to 40 mm.
In addition, the shielding element 8b is inclined at an angle α with respect to the surface O or the material web 5 running approximately horizontally in this region, where α is up to 5 °, preferably up to 2 °. The remaining to the running surface O or the web 5 gap b is extremely low, so that although low air turbulence of the entrained air boundary layers still exist, but lose their dangerous effect on the curtain 4. The arrangement is such that the gap b diverges in the direction of the device 7 and increases toward the curtain 4. The shielding element 8b can be suspended suspended.

It can also be seen from FIG. 4 that the strip-shaped shielding element 8b is fastened to a support body 9. The support body 9 is mainly used to ensure the rigidity of the shielding 8b, which is necessary especially for wide machines. In Figure 4 is also shown with dashed lines that the support body 9 may extend to the height of the applicator head of the commissioned work 2 in order to simultaneously protect the curtain over its entire height against air currents can. He thus exercises a dual function, namely stiffening and shielding.

FIG. 5 shows in a further variant that this support body 9 is arranged at a minimum distance z from the curtain applicator unit 2. In this case, it forms with the shielding element 8b a shielding unit 10, which can thus be moved independently of the vertical or longitudinal adjustment of the applicator unit 2 and can be installed and removed. It can also clearly be seen from this figure that a shape of the front wall surface adapted to the roller 7 forms a guide surface 10a for the air masses squeezed by the roller 7 and to be discharged from the device 1.
FIG. 5 also shows a shielding element 8c in dashed form. This shielding element 8c may optionally be present at the side of the material web 5 or the running surface O running away from the curtain, in order to protect the curtain 4 from undesired air flows. The installation of this vertically arranged element 8c is especially in high-speed paper and / or coating machines, which are operated today for reasons of economy at about 2000 m / min, provided. The shielding element 8c as well as the shielding element 8b or supporting body 9 or shielding assembly 10 is arranged at a minimum distance from the curtain 4, but leaves a viewing gap w to the running surface O or the material web 5 for the operator to observe the curtain application free.

Double arrows in FIGS. 4 and 5 show the adjustability of the shielding element 8b in the horizontal and vertical directions or for tilting in order to set the angle of attack β already described. Also, the adjustment of the commissioned work 2 is shown with double arrows. The applicator 2 or the applicator head with application nozzle can be adjusted and / or pivoted in the horizontal, vertical or oblique direction, which is necessary in particular for setting up the start or stop position.

The distance b between the shielding element 8b and the running surface O or the material web 5 is very minimal and less than 10 mm. Preferably, the Distance b only up to 2 mm, most preferably only 0.5 to 1 mm. This small distance is sufficient to keep the unwanted air turbulence low, so that air boundary layers L _{G kept} away from the curtain 4, but at least their harmful influence is minimized. It is also possible to touch the running surface 0 or the material web 5.

So that the air boundary layer L _G can not build up again, as already mentioned, the end of the shielding element 8b extends into the gusset of the roller 7. For this purpose, the end of this shielding element (as well as the end of the shielding element 8a in FIG. 1) or its one end face is adapted to the outer contour of the roller 7. In the absence of the roller 7, the outer contour is analogously adapted to a subsequent shielding element 8d provided between shielding elements 8a and 8b, as FIG. 6 shows. This variant is suitable above all for slower and less wide machines with consequently less wide material webs 5 to be produced.

FIG. 7 shows a further embodiment of the device 1 according to the invention. Here, a shielding element 8e is arranged in the space R between the applicator 2 and the roller 7. In this case, an angle of inclination β is advantageous for creating an effective shielding. This angle β is in the example about 45 °.
As a result, a barrier is created against the penetration of the air boundary layer L _G into the space R toward the application area, especially when the material web 5 tapers from above to the applicator unit 2.

FIG. 8 shows a further embodiment variant in which, in addition to the inclined shielding element 8e located in the space R, the shielding element 8b shown in FIGS. 1, 4, 5 is also present and as a result of which the control effect is markedly increased.

Another very advantageous embodiment variant is shown in FIG .

Here, the shielding element 8b and with further shielding elements 8a, 8c, 8f and 8g to a device or roller 7 box-like, at least partially enclosing shield 11 expandable. This creates a compact space R from which harmful air can be removed and also improves noise protection. Within the enclosing shield 11, a suction device 13 is arranged in the example shown. This suction device can suck alone or together with the suction zone S in the roller 7, the disturbing air.

In the variant illustrated in FIG. 10 , the shielding element 8a has been omitted, and for this the supporting body 9 (see FIGS. 4 and 5) for supporting the shielding element 8b is present. Here, the individual shielding elements 8f, 8g and 8b and, as well as the support body 9 also form a shield, which is denoted by 15. With this variant, a shield in the region of the roller 7 is also to be created, wherein again a suction device 13 may be arranged in the space R.

By way of example, Figure 11 is intended to indicate that the shielding element 8a is even expandable with horizontally and vertically or arcuately arranged shielding parts 8h such that the entire device 1 is hood-like in an almost closed climatic chamber 16. The extension is possible on both sides of the running material web 5. In this variant too, the installation of the suction device 13 shown in FIGS. 9 and 10 is advantageous. Instead of sucking, with a device 13a and the supply of a medium in the chamber 15 or in space R in Figure 9 and 10 possible, whereby specially desired climatic conditions within the climate chamber formed are adjustable. Thus, for example, steam could be fed in, which ensures that the application medium M or the application layer 6 on the material web 5 does not harden so quickly and thus becomes more uniform.

In addition, in FIG. 11 , after the curtain 4, there is still a slight distance this arranged shielding 8c located, which is also removed from Figure 5 .

FIG. 11 also indicates that the climatic chamber 16 or the shielding elements 8h extend as far as the machine base 17 and the material web 5 is guided downward over a deflecting roller 18. Not shown is the further treatment option of the material web 5, such as drying, smoothing, other coatings, etc.

It should be mentioned that all shielding 8a to 8h and the support body 9 can be made of stainless steel or plastic. The latter material is useful in conjunction with electrostatics, as shown in Figure 3, or in a coronary discharge. In the case of a wide machine, further stiffening ribs, not shown, are also present on the shielding elements 8a-h.

The width B of all shielding elements 8a and 8h correspond approximately to the width of the surface O or the material web 5, but at least the desired coating width.

In the above-described Figures 1 to 11 in each case from the top of the curtain applicator 2 tapered web 5 is shown, which is coated directly with the discharged from the curtain 4 Medium M (for example pigment-containing pigment dispersion).
However, the material web 5 could also be coated in an indirect manner with the said application medium M, as FIG. 12 shows.
For this purpose, there is a running surface O in the form of a belt 22 running over guide rollers 20 and 21. This belt 22 receives the application medium M from the curtain 4 and then releases it to the material web 5. Also in this variant, an air boundary layer control or attenuation device in the form of a roller 7 is provided, which rolls on the belt 22. Also as in the previous examples, the shielding member 8b is provided. Of course it would also be possible to provide the shields 11, 15 or climatic chamber 16, but are not shown in FIG. 12 for reasons of clarity.
The variant shown in Figure 12 is possible for one-sided application, but particularly useful if the web 5 is to be treated simultaneously on both sides. For this purpose, two adequate devices 1 are provided in mirror image. The two guide rollers 22 together form a Nip N (press nip), which passes through the material web 5.

Finally, FIG. 13 shows that the running surface O or the material web 5 can, in all the variants shown, after passing through the device 7 in the direction of the applicator 2, take a gradient rising or falling at an angle to an imaginary horizontal H.
When the gradient increases, an angle φ of greater than 90 ° is achieved between the curtain 4 and the running surface O or material web 5, whereas with the curve decreasing, the angle φ 'is less than 90 °.
Preferably, the angle γ 5 -30 °, preferably 5-15 ° with increasing surface O or the web between it and the horizontal. As a result, a counterforce to the registered air boundary layer L _{G is} formed.
In the case of a sloping surface O or material web 5, the angle δ is also 5-30 °, preferably 5-15 °, as a result of which stretching and thus homogenization of the curtain 4 is achieved.

Incidentally, the material web 5 does not have to run vertically correctly when directly applied, but instead can also run at a different angle to the roller 6, as also shown symbolically in FIG. 13. In the example chosen, the angle λ, starting from an imaginary dashed vertical V, is approximately 30 °.

REFERENCE NUMBERS

1

applicator

2

Curtain applicator

3

dispensing nozzle

4

curtain

5

web

6

application layer

7

Control or weakening device rolling off sealing element, roller

8a-8h

shielding

9

supporting body

10

Shielding assembly

10a

guide

11

shielding

13

suction

13a

Device for feed

14

electrode assembly

15

shielding

16

climatic chamber

17

machine base

18

deflecting

20.21

guide roll

22

tape

a

Distance between shielding element and curtain

α

Tilt shielding element to the running surface

B

width

b

Distance between shielding element and running surface

β

angle of attack

e

grounding

H

horizontal

L

direction

L _G

Air boundary layer

M

application medium

N

Nip, press gap

O

running surface

R

room

S

suction zone

V

vertical

w

viewing gap

z

distance

δ, γ

Angle between running surface and horizontal

φ, φ '

Angle between curtain and running surface or web

λ

Angle between web to vertical

Claims (24)

1. Device for the one-sided or two-sided application of liquid to pasty application medium, in particular aqueous pigment suspension, with an applicator (2) to a moving surface (0), the surface (0) in the case of direct application being the surface of a material web (5), in particular a paper or board web, and in the case of indirect application being the surface of a transfer element, for example an applicator belt (22), which then transfers the application medium (M) to the surface of the material web (5), and an air boundary layer combatting or weakening device (7) being arranged before the applicator (2) in the running direction (L) of the moving surface (O),
   characterized in that
   at least one strip-like shielding element (8b), which extends in the transverse direction of the moving surface (0) and is placed immediately before the applicator (2), is arranged after the combatting or weakening device (7).
2. Device according to Claim 1, characterized in that at least one further strip-like shielding element (8a) is placed before the air boundary layer combatting or weakening device (7).
3. Device according to Claim 1 or 2, characterized in that the at least one shielding element (8a, 8b) is arranged parallel to and at a short distance from the surface (0) running towards the applicator (2), away from the device (7).
4. Device according to one or more of Claims 1 to 3, characterized in that the moving surface (0) is a moving material web (5), which preferably runs towards the device (7) in the vertical direction from above.
5. Device according to one or more of Claims 1 to 4, characterized in that at least one further shielding element (8e, 8g) is arranged in the space (R) existing between the applicator (2) and the device (7), which element forms an angle of inclination β of about 45° with respect to the material web (5) .
6. Device according to one or more of Claims 1 to 5, characterized in that the device (7) is constructed as a sealing element or as a roll (7) pressed in a sealing manner against the moving surface (0) and rolling on the latter without slip.
7. Device according to Claim 6, characterized in that the rolling sealing element (7) is equipped with a suction zone (5).
8. Device according to Claim 6 and/or 7, characterized in that the rolling sealing element (7) is used as an earthing means in the event of electrostatic charging of the material web (5).
9. Device according to one of Claims 6 to 8, characterized in that the rolling sealing element (7) is suitable to form an air cushion between itself and the material web (5).
10. Device according to one or more of Claims 1 to 9, characterized in that the end of the strip-like shielding element (8a, 8b) that faces the device (7) is matched to the external contour of the device (7).
11. Device according to Claims 1 to 10, characterized in that the shielding element (8a, 8b) is at a distance (b) of 0 to 5 mm, preferably 0.5 to 1 mm, from the moving surface (0).
12. Device according to Claims 1 to 11, characterized in that the shielding element (8b) is arranged at an angle α with respect to the moving surface (0) or material web (5) which is about 0 to 5°, preferably 2°, and diverges towards the moving surface (0) in the direction of the device (7).
13. Device according to one or more of Claims 1 to 12, characterized in that the shielding element (8b) is at only a minimal distance from the device (7) and/or, if appropriate, further shielding elements (8a, 8d, 8e, 8f, 8g) and from the medium (M) discharged by the applicator (2).
14. Device for the one-sided or two-sided application of liquid to pasty application medium, in particular aqueous pigment suspension, with an applicator (2) to a moving surface (0), the surface (0) in the case of direct application being the surface of a material web (5), in particular a paper or board web, and in the case of indirect application being the surface of a transfer element, for example an applicator belt (22), which then transfers the application medium (M) to the surface of the material web (5), and an air boundary layer combatting or weakening device (7) being arranged before the applicator (2) in the running direction (L) of the moving surface (O),
    characterized in that
    at least one strip-like shielding element (8c) which extends in the transverse direction of the moving surface (0) is arranged after the applicator (2).
15. Device according to one or more of Claims 1 to 14, characterized in that the shielding element (8a, 8b, 8c, 8h) can be widened towards a shielding means (11, 15) or an air-conditioning chamber (16) enclosing the applicator (2) and/or the device (7).
16. Device according to one or more of Claims 1 to 15, characterized in that the shielding element (8a, 8b) is an integral constituent part of the combatting device (7).
17. Device according to one or more of Claims 1 to 16, characterized in that the shielding element (8a to 8h) is fabricated from stainless steel or plastic.
18. Device according to one or more of Claims 1 to 17, characterized in that a suction device (13), which can also be constructed as a medium supply device (13a), is accommodated within the enclosing shielding means (11, 15).
19. Device according to one or more of Claims 1 to 18, characterized in that, after passing the device (7) in the direction of the applicator (2), the moving surface (0) assumes a course rising at an angle γ = 5-30°, preferably 5-15°, or falling at an angle δ = 5-30°, preferably 5-15°.
20. Device according to one or more of Claims 1 to 19, characterized in that the outer surface of the device (7) constructed as a rolling sealing element or roll is smooth and chromium plated.
21. Device according to one or more of Claims 1 to 19, characterized in that the outer surface of the device (7) constructed as a rolling sealing element or roll is soft and has a hardness of < 90 P & J and is therefore deformable.
22. Device according to one or more of the preceding claims, characterized in that the applicator (2) is a curtain applicator, which discharges the application medium onto the moving surface (0) or the moving material web (5) as a freely falling curtain (4) induced to move substantially by the force of gravity.
23. Device according to one or more of the preceding claims, characterized in that the distance (a) between the shielding element (8b, 8c) and the curtain (4) is 3 to 60 mm, preferably 20 to 40 mm.
24. Device according to Claim 22 and 23, characterized in that the shielding element (8c) is arranged at a short distance from and parallel to the curtain (4) at such a height that the shielding element (8c) leaves a visible gap (w) from the moving surface (0).

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