Note: Descriptions are shown in the official language in which they were submitted.
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P1153
A method for masking a component to be coated with a thermal spray
coating
The present invention relates to a method for masking a part of the surface of
a
component, which is to be coated by thermal spraying.
Thermal spraying is a coating process, in which a material, for example in
powder
form, is continuously melted. The resulting droplets are thrown onto the
surface to
be coated, whereby flattened droplets accumulate on the surface. The layers
built
up in this way lead to a coating, which may be, for example, harder, more
brittle
but also more porous than the uncoated component.
It is remarkable that any fusible material can be sprayed and that almost any
component material and almost any component geometry can be coated. The
degree of automation that can be achieved with the method is very high as well
as
the reproducibility and the quality of the layers that can be achieved.
In many cases, however, not the entire surface of the component should be
coated. Those parts of the surface which are not to be coated must therefore
be
covered, i.e. they must be masked. Unfortunately, the degree of automation
achieved in masking is so far very low. In many cases, the components are
still
masked manually.
On the one hand, overlay masks are used, as described, for example, in JP
3158451 or US 6645299. On the other hand, adhesive masks made of adhesive
tapes are used, which are affixed directly to the parts of the surface which
are not
to be coated.
W02010/031370 Al shows a short overview of the common masking methods in
connection with thermal spraying. There is also referred to the possibility of
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applying a masking of a paint or of a binder-containing mixture to the area to
be
protected, as described for example in US 4464430. There, however, masking is
done by means of dip coating by immersing the parts of the surface not to be
coated by thermal spraying in an immersion bath. Of course, this method is
limited
to very few geometries.
In W02010 / 031370 Al itself a masking is described, which is made of an
elastic
material being slightly undersized. Among the materials described there are
also
elastomers, for example. When applying the masking to the component, the
masking is very close to the component due to its undersize. This type of
masking
works especially well when the component has to be masked all around.
All methods described above have at least two disadvantages: on the one hand,
they cannot be automated, or if at all only with a great technical effort. On
the
other hand, in many cases they do not allow the desired precision in the
application. Thus, in many applications, those lines that define transitions
from
parts of the surface to be coated to parts not to be coated must be kept very
precisely and, above all, reproducibly. Such lines are referred to below as
critical
mask boundaries. It is clear that there can also be transitions defining
lines, which
require less precision. These are referred to below as uncritical mask
boundaries.
Thus, the present invention is based on the object of specifying a largely
automated masking method, which allows the critical mask boundaries to be
positioned with the required high accuracy.
According to the invention, the mask is realized at least along the critical
mask
boundaries by means of a paste which is dispensed by a nozzle. The term
"paste"
used in the present description means a liquid material having such a
viscosity
height that it can be applied to the surface of the component in the form of a
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sealing bead with any contour profile, without dissolving on the surface.
According
to the invention, the paste is curable.
Viscosity has a great influence on the precision with which a sealing bead
with its
geometric characteristics (generation of shading) can be produced. For this
reason, according to a preferred embodiment of the present invention, the
paste
and / or the component to be masked is processed and applied under a
predetermined tempered environment (cooled).
For example, curing can be made by evaporation of solvent contained in the
paste. However, according to a preferred embodiment, curing is at least
partially
realized by crosslinking, particularly preferably by photo-induced
crosslinking.
According to a particularly preferred embodiment, a UV curing paste is used.
This
is an advantage, among other things, because the paste just needs to be cured
shortly after it has been applied to the component. In the course of thermal
spraying, particularly if it is a plasma process, the component is exposed to
intense UV radiation due to the plasma process, resulting in further curing
and,
ideally, complete curing. Due to the fact that the paste only has to be cured
shortly, less expensive UV sources can be used and / or the UV irradiation
time
can be shortened.
According to another embodiment of the present invention, masking is performed
in two steps. Areas to be masked are accordingly masked using one of the
methods, as they are already known from the state of the art, wherein,
however,
no masking with the correspondingly known means is provided in the area of the
critical mask boundaries. So, this is a partial masking, wherein the areas
around
the critical mask boundaries are left blank. The areas around the critical
mask
boundaries are then masked according to this embodiment by means of the
method mentioned above, i.e. masking paste is applied to these areas by means
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of at least one nozzle, thus completing the masking. In doing so, it is
advantageous to consider that there is an overlap with the partial masking
when
the masking is completed, in order to ensure that no unmasked areas are formed
between partial masking and completion masking.
In this method, it is particularly advantageous that the masking of relatively
large
areas, which would take a lot of time with the paste process, can be applied
quickly by means of a relative to the position relatively inaccurate method
and the
critical edge areas of the critical mask boundaries can be applied very
precisely
using the paste process. The inaccurate method can be automated relatively
easily. The automation of the paste process is also possible. In the paste
process,
it is necessary that the place of the nozzle exit is moved relative to the
areas close
to the mask boundaries to be masked in the course of paste dispensing. In this
case, either nozzle or component can be mounted on a robot. Preferably, the
component would be mounted on the robot arm for smaller components to be
masked, because in this case, the connection of the nozzle to the paste
reservoir,
which is usually ensured by hoses, is not exposed to any movement. However,
for
larger components that are more difficult to move, it may be advantageous to
mount the nozzle on the robot arm. More generally, means for positioning and /
or
orientation of the component and / or means for positioning and / or
orientation of
the at least one nozzle may be provided.
Due to the robot-controlled movement in space, free-form paths can be
traversed.
As already described above, a critical mask boundary is described by a precise
sealing bead. The non-critical mask boundaries can be divided into at least
two
categories, namely applying of e.g. simple covers. However, in some cases
covers
cannot be attached to undercuts, or would possibly restrict the dispensing
process.
For this reason, according to a preferred embodiment, a low-viscosity pasty
material is used in a second dispenser. Due to the low viscosity, it coalesces
and
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can therefore be applied over a wide area. Initially, the precise sealing
beads are
placed.
As already explained, precise masking in the area of the critical mask
boundaries
5 is essential in many applications. However, the inventors have discovered
that
sometimes in these areas after removing the masking no layer applied by
thermal
spraying was left, although the masking was applied very precisely. From this
observation, the idea was born to try, to realize the masking along the
critical mask
boundaries not slowly increasing but with high steepness. This is based on the
assumption that if the thermally sprayed layer is realized as a continuous
layer
extending the critical mask boundary at the transition from the area to be
coated to
the area not to be coated, components of the sprayed layer are carried away
beyond the mask boundary when removing the coating.
To avoid such a continuous layer, the paste is therefore preferably adapted to
the
material of the component to be masked in such a way that a contact angle of
at
least 90 is formed. Then, the mask thickness does not continuously increase
at
the critical mask boundary but forms an at least vertical, if not even
overhanging
wall. As a result, the thermally sprayed layer is substantially interrupted at
the
critical mask boundary. When the masking is removed, no parts of the layer are
carried away beyond the critical mask boundary. If the contact angle exceeds
90 ,
there is an increased shading effect, which contributes to the separation of
the
spray layers left and right of the critical mask boundary. In order to further
enhance
this shading effect, in a particularly preferred embodiment of the present
method, a
second sealing bead is placed over the first sealing bead. This second sealing
bead is applied in such a way that a reinforced overhang is realized resulting
in a
reinforcement of the shading effect.
Here, the contact angle is defined similar to the contact angle at a boundary
between a liquid and a solid, as, when the paste is applied, a contact angle
is
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formed, wherein the contact angle does not change substantially during the
entire
process, regardless of whether the paste cures or not. The angle is defined in
the
following as the angle of contact, which is enclosed by the surface of the
component and a tangent at the surface of the sealing bead or a tangent at the
surfaces of the double sealing beads at the boundary between the component and
the sealing bead.
Fig. 1 illustrates the corresponding situation in which two overlying sealing
beads 3
are applied to a component 1 in such a way that the contact angle is a > 900
(visible at the left edge of the figure). A contact angle a <900 would not be
optimal,
because this can lead to problems of layer separation of the layer 5 (visible
at the
right edge of the figure).
