Note: Descriptions are shown in the official language in which they were submitted.
225742
A process for producing a casing providing a screen
against electromagnetic radiation
Background of the Invention
Field of the Invention
The invention relates to a process for producing a casing
providing a screen against electromagnetic radiation
according to the preamble of claim 1, and a casing
according to the preamble of claim 8.
Description of the Prior Art
Electronic components and also measuring, detection and
similar devices sensitive to interference by electromagne-
tic radiation require a screen against the electromagnetic
fields present at the operational site in order to ensure
a problem free operation.
They are therefore accommodated in screening cases which
comprise conductive material in the walls and act as a
Faraday cage.
Such casings are also used for equipment or components
which, themselves, emit electromagnetic rays that must be
excluded from the environment, in order for example, to
prevent the emission of secret information or the malfunc-
tion of external appliances.
Today such a screening against the emission or irradiation
of EMI must be more effective the more electronic appara-
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tuses are operated and the greater the proximity at which
these apparatuses must operate next to each other.
Finally, the continuous increase in performance and sen-
sitivity of such equipment also necessitates an improve-
s ment of the screening measures for which increasingly less
space is available, since the relevant appliances must,
moreover, be miniaturised. Thus, apart from the actual
operational properties of electronic appliances, the
"electromagnetic compatibility" is today a significant
factor determining quality.
If, as is mostly the case in practice, the casings are
multi-part constructions which must be capable of being
opened occasionally (e. g. in order to renew the energy
source or for maintenance purposes), it is necessary to
provide the parts of the casing to be separated from each
other during opening and to be re-connected again during
closing, with elastic conductive seals in order to achieve
an effective screening.
whilst, on the one hand, spring-like metal seals are known
for this purpose, they are, however, comparatively expen-
sive to construct and their operability may be greatly
affected by oxidation and soiling.
Furthermore, resilient sealing profiles made of elastomer
which is conductive or has been made conductive, which has
been mixed with carbon or metal particles in order to make
it conductive, are known from e.g. US 46 59 869 or DE-OS
28 27 676.
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Such sealing profiles are normally manufactured as
separate seals. They may be moulded or extruded as a con-
tinuous section and then placed into the casing to be
screened.
This is a labour-intensive operation and especially in
small casings it causes difficulties as seals with
correspondingly small dimensions are difficult to handle.
The provision of suitable guides (grooves) which facili-
tate the mounting on the casing requires an unreasonable
amount of space and is thus a hindrance to the further
miniaturisation of the appliances.
Intricately moulded seals likely to be needed for special
casings require specific positioning equipment which makes
the manufacture of the casing altogether more expensive.
The exact positioning is, moreover, time-consuming and
necessitates additional inspection.
The hot-moulding, in moulds, of such screening profiles
onto the relevant casing portions or parts and the setting
at a relatively high temperature and/or high pressure is
also known.
This prccess cannot be used with parts sensitive to
pressure and/or temperature such as printed circuit boards
or metallised plastics casings and, as a result of the low
tear resistance of the related materials, problems arise
during the removal from the mould resulting in a relati-
vely high number of rejects and, more particularly in
intricately shaped casings and seals, also frequently
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necessitating time-consuming and labour-intensive machining
on the pressed-out edges.
Summary of the Invention
The invention is for the purpose of providing a
process of the above kind for producing protective screens,
more particularly within the region of casing joints, which
may be adapted, in a simple manner, to the most varied
requirements, even in a miniaturised construction. It should
also be possible to use the process for casings to be produced
in larger quantities, in a simple manner and at low cost. The
casing produced according to this process should provide a
screening profile which meets the electromagnetic and
mechanical requirements and remains in good condition even
after a repeated opening of the casing.
The invention provides a process for producing a
casing providing a screen against electromagnetic radiation,
particularly for receiving electronic operational elements,
comprising a screening profile arranged in a given portion of
at least one casing part, said screening profile comprising
elastic and conductive material, characterised in that the
elastic, conductive material is applied in an originally pasty
or liquid state, by means of pressure from a needle or nozzle,
directly onto the portion of the casing part on which the
screening profile is to be arranged and is elastically
solidifying, adhering to the surface of the casing part.
The invention also provides a casing particularly
for operational elements, which shields its interior from
electromagnetic radiation, having a screening profile arranged
on a predetermined portion of at least one part of the casing,
said screening profile comprising elastic, conductive material,
characterised in that the screening profile made of the
elastic, conductive material is formed directly on said
predetermined portion of the casing part and is adheringly
connected thereto, and that the screening profile is arranged
in the edge region of a closable opening of the casing and,
in adaptation to the shape of the opening and the kind of
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closure, and has a configuration such that the casing may
easily be opened and closed.
The invention is based on the concept of not
producing the screen profile separately but producing it
directly and without using a mould on the casing -- in one
step or preferably in several steps --, by means of a
hardening pasty or liquid compound with the required
properties which issues from one or more openings guided over
the geometrical extent to be sealed. Thereby, the handling
problems of premanufactured sealing profiles and, on the
other hand, the process-related disadvantages of compression
moulding are avoided, and there arises the possibility of
building-up profi-
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les having very different special cross-sectional shapes
and/or consisting of portions of different material pro-
perties (degree of elasticity, conductivity, thixotropy
etc.) according to demand. Here, the material consists of
a plastics compound which - at least in portions - con-
tains conductive inclusions, more particularly in the form
of metal or carbon particles.
If, for forming the profile, the guidance of the needle or
nozzle over the portion of the casing part, on which the
screening profile is intended to be mounted, is done by
machine, more particularly controlled by computer, a high
precision and great flexibility is ensured in shaping the
profile so that moreover intricately moulded casings or
openings of casings in small series may easily be provided
with the necessary screening seal in an economic manner.
Special profiles, for example comprising undercuts, reces-
ses etc., on the casing, are advantageously produced by
guiding the needle or nozzle several times or several
needles of nozzles at least over predetermined regions of
the portion on which the screening profile is intended to
be mounted, in order to produce a multi-layer screening
profile, thereby forming an exactly predetermined profile
section. The profile cross-section as requested is formed
herein in several operations in succession or nearly si-
multaneously. The shape of the profile above all is deter-
mined by a suitable choice of number, cross-sectional sha-
pe and dimension and scanning speed of the needles) or
nozzles) and the spatial relation thereof during the se-
veral coating steps, but furthermore by the choice of the
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material properties such as viscosity, thixotropy and cu-
ring speed.
A special design of the process consists in that the se-
veral strands of elastic as well as conductive material
are applied in a spatial configuration and a time-distance
each to the other such that a partial curing of an applied
strand has happened before a next strand touching the for-
mer one will be applied.
In this way cross-sectional profiles may preferably be
produced which have given elasticity properties and do not
acquire said elasticity because of their compressibility
but because of a bending deformation, as is the case in
bent lip sections or hollow sections.
It is, in particular, not necessary to provide each strand
of the material with conductive inclusions since linear
conductors already provide a great screening effect due to
the laws of the electromagnetic field. Of course, it is
advantageous to provide a strand of highly conductive mate-
rial so that it will touch the casing parts over a distan-
ce as long as possible. The sealing effect of this conduc-
tive strand is less important since one or more strands of
less or non-conductive but highly elastic material - which
primarily will fulfil the sealing function of the profile
- can be assigned to it.
The inventive measures also make it possible to produce
complicated seal constructions with dimensions which vary
along their extent, without special difficulties. Here,
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according to the relevant requirements, the cross-section
may vary along the edge to be sealed, within wide limits.
It is also possible to produce such constructions of
screening profiles which are interconnected in a way that
an individual production and mounting thereof, separate
from the casing, would not have been possible. Any joints
in the extent of the screening seal are thus obviated by
the measures according to the invention so that the
sealing effect is not interrupted.
Smaller casing zones or additions which are not made of
metal or metallised, which would produce a break in the
closed screen, may, in an operation carried out at the
same time as the other seal is produced, be coated grid-
like with the profile strands according to the invention,
so that homogenous screening conditions also exist in such
regions.
In this way elements of a Faraday cage may even be formed
from tracks of conductive plastics material and thus of
the sealing compound itself, if they are mounted in the
shape of a grid in a plane and conductively connected at
the intersections of the grid.
Due to the fact that different elastic materials are
applied when guiding the needle or nozzle several times
over the predetermined regions, at least one application
comprising conductive material, it is possible to produce
casings with seals whose conductive, corrosive and elastic
properties have been optimised to advantage.
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Casings allowing easier handling may be produced espe-
cially in such a way that the elastic conductive material
is applied, by computer control, directly onto the edge
region of a closable aperture of the casing so that the
screening seal assumes a configuration enabling an easy
opening and closing of the aperture.
In order to apply the screening profiles according to the
invention, computer-controlled handling appliances may be
used which allow a three-dimensional guidance of the
needle or nozzle, a fourth variable relating to the
metering of the still liquid or pasty material, as a func-
tion of the forward movement. By means of a fifth control
variable it is additionally possible to select a material,
i.e. various strands of material which may also be of a
different composition and may be applied alternatingly, or
simultaneously in "one single operation", so that the
material characteristics of the entire section, with
regard to its cross-section or its extent, may be varied
locally. These varying characteristics include the con-
ductivity, elasticity (bendability or compressibility)
and/or hardening or adhesive properties of the material.
A firm closure through adhesion can also be achieved by
means of the screening sealing elements, if adjacent
material strands have corresponding characteristics, e.g.
if they are the two components of a two-component adhe-
sive.
In other advantageous embodiments of the invention,
instead of parts of the casing, parts of printed circuit
boards projecting beyond the outer surface of the
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appliance may also take over screening functions, and for
adaption to adjacent screening members may be provided
with the features according to the invention.
Other advantageous further developments of the invention
are characterised in the sub claims and are hereinafter
more fully explained in the description of preferred embo-
diments of the invention, with reference to the Figures.
Brief Description of the Drawings
Figure la and lb basic diagrams of two embodiments of the
process according to the invention;
Figure lc a detail of Figure la;
Figures 2a to 2k schematic, partial cross-sectional
diagrams of screening profiles which are part of embodi-
ments of the casing according to the invention and may be
produced using embodiments of the process according to the
invention, as well as
Figure 3 a schematic drawing of the length of an edge with
screening profile of a casing according to the invention,
in one embodiment.
Description of the Preferred Embodiments
Figure la shows an aluminium screening casing 1 for an
electronic circuit component 2, which has a cavity 3 for
inserting the circuit component, said cavity being closed
with a lid 4 after the insertion of the component.
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Figure la also sk~ows how a screening profile 8 is applied
to the edges of the cavity 3 by an application needle 6
airtightly connected to a piston-cylinder device 5, said
application needle being guided, together with the piston-
s cylinder device 5, by a computer-controlled robot arm 7
which exerts a pressure p onto the piston 5a of the device
5, at a small and precisely maintained spacing from the
casing 1, and at the speed v, along the surrounding edge
3a. The automatic arm may be guided in the three spatial
directions x, y and z.
Cylinder 5b of device 5 is filled with a quickly air- and
room-temperature-drying silicone mixture 8a at ambient
temperature with included metal particles, which, as a
result of the pressure exerted onto the piston 5a, is
pressed ("dispensed") through the channel 6a of the needle
6 onto the surface of the casing, to which it adheres and
where it hardens under the influence of air, to form the
elastic screening profile 8.
The cross-sectional dimensions and shape of the screening
profile 8 are primarily determined by the physico-
chemical-properties of the conductive plastics material
used, more particularly the hardening speed, viscosity,
surface tension with regard to the casing material, and
the thixotropy thereof, as well as by the cross section of
the channels, the pressure exerted onto the piston, the
speed of the needle movement and by the environmental
influences such as temperature or air humidity at the
manufacturing site and may be predetermined by a suitable
selection of said parameters.
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In the casing 1, shown in Figure 1, comprising a flap
cover 4 mounted on one side to a hinge, it may be of
advantage if the application needle 6 is guided along one
edge portion of the opening 3 at a higher speed than in
the other portions. A profile with a smaller cross sec-
tion than that in the other edge portions would be formed
here and the lid could be closed more easily. It is possi-
ble to adjust the characteristics of the plastics mate-
rial, particularly by adding fillers (carbon or the like),
metal binders, surfactants and hardening catalysts or
cross-linking agents. The kind and grain size of the ad-
mixture ensuring conductivity, such as carbon, silver,
silver- or gold-coated copper particles or the like, not
only influence the electrical but also the mechanical and
processing properties of the conductive elastic material.
Fig. lc shows an enlarged representation of the unit 5 as
a detail of Fig. la. In addition to the elements refe-
renced by identical numbers there are shown a plunger 5a
in a cylinder 5b containing a supply of the elastic and
conductive material 8a later forming the screening profile
8 in a still pasty and unsolidified state.
In the arrangement of Fig. la the robot arm 7 bearing the
device 5 can be guided along the edge portion of the ope-
ning 3 several times to apply several strands of the sili-
con gum side by side and/or one on top of the other or in
other spatial relation (as exemplified below). In this way
profile cross-sections having especially advantageous sea-
ling and/or screening properties can be formed.
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Alternatively hereto - as shown in Fig. lb - the use of a
multi-nozzle coating device is possible which can be used
to apply several material strands approximately at the sa-
me time.
The casing parts shown in Fig. lb correspond to those of
Fig. la and will, therefore, not be described once again.
The coating device 150 comprises four coating modules 151
to 154 each comprising a nozzle 161 to 164 for ejecting a
material strand and, furthermore, a (not specially desi-
gnated) piston-cylinder device similar to the coating de-
vice 5 of Fig. la. The modular structure of the device
enables the combination of any number of modules, on de-
mand even staggered such that the application moments of
the different strands can be shifted relative to each
other. Since, furthermore, the device 150 is borne in a
rotating plate 171 in the robot arm 170 the position of
material strands 181 to 184 applied therewith relative to
.each other and to the surface of the casing can be varied
by predetermining the angular position of the rotating
plate 171.
The modules have separate material containers and can com-
prise different nozzle dimensions such that strands of
different material as well as of different shape can be
combined with each other.
Figures 2a to k show examples of different profile cross
sections for casings which may be manufactured in several
applicational steps using the process according to the
invention. It is, however, apparent when using the
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measures according to invention, that the cross sections
may also vary in the longitudinal. direction of the pro-
file, in the geometrical dimensions and material charac-
teristics thereof.
Figures 2a to 2d show a combination of conductive, less
elastic sealing parts (hatched) and non-conductive sealing
parts which are more elastic because of the missing metal
admixture, thereby achieving a combined optimal effect of
sealing and screening.
Figure 2a shows a screening and sealing construction
formed by profiles 81a and 81b with a substantially cir-
cular cross section, arranged side by side in two applica-
tional steps on the surface of a casing part 11. Such a
structure is produced, when the elastic material slightly
wets the surface of the casing.
Figure 2b shows a profile structure produced in three
steps, consisting of a flat-domed, broad conductive pro-
file part 82a and a conductive part 82c "dispensed"
thereon and a non-conductive part 82b on a casing portion
12, the parts 82b and 82c having a substantially circular
cross section.
Such a structure is obtained if a material of the first
profile part 82a wets the surface of the casing to a great
extent and/or was applied with a relatively broad nozzle
instead of the needle 6 shown in Figure 1, whereas the
material of parts 82b and 82c have a mild wetting tendency
with respect to the surface of part 82a.
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Figure 2c shows a structure which is similar to that of
Figure 2b. Here, two approximately semi-circular, conduc-
tive screening profile parts 83b and 83c are arranged on
either side of a nearly semi-circular, non-conductive,
highly elastic sealing profile 83d centrally arranged on a
lower, broad profile part 83a positioned on a casing sur-
face 13.
This last profile shows great stability with respect to
forces acting parallel to the casing surface, but it has a
comparatively lower elasticity. It may, therefore, be
particularly suitable for sliding closures.
By contrast, the profile shown in Figure 2d, which con-
sists of a semi-circular elastic, non-conductive profile
part 84a pressed onto a casing surface 14, and a conduc-
tive coating 84b covering the surface of said profile part
84a, has extremely satisfactory elasticity properties.
A high wetting ability and satisfactory adhesion between
the surfaces of the two materials are necessary for the
manufacture of the profile which is very suitable for
hinged covers, especially if there is a relatively great
play between the closure and casing part, or if they,
themselves, have a certain elasticity.
Figures 2e to 2i show screening profiles which consist
exclusively of conductive material.
Figure 2e shows a specially formed, single-part profile 85
positioned on a casing surface 15 which has two beads 85a
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and 85b connected by a flat path. In the shape as shown or
a similar one, it can especially be built up from two or
three single strands; in the latter case, the central
strand can especially be formed from a material of low
viscosity and/or good wetting properties which will run
relatively flat between the outer, previously initially
cured strands. Such a profile may be suitable for casings
with hinged covers which have shaped edges.
Figure 2f shows a semi-circular screening profile 86 on a
casing surface 16 which consists of a plurality of circul-
ar profile strands of the same, highly conductive mate-
rial, said screening profile, together with said casing
surface, enclosing an air space 86a.
The effect of the profile, in co-operation with said "air
chamber", ensures a high elasticity of the entire profile
despite a comparatively unsatisfactory elasticity of its
components.
Figure 2g shows a lip-shaped screening profile 87 on a
casing surface 17, which consists of a plurality of cir
cular profile strands laid one on top of the other stagge
ring the long axes to each other. It is suitable for hin
ged covers as well.
Figure 2h shows a T-shaped profile 88, positioned on a
casing surface 18 with a rectangular groove 18a, said pro-
file engaging with a broad centre part 88a in the groove
18a and having a planar surface parallel to the casing
surface 18 outside the groove 18a. It can be formed by
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first filling the groove with a first, highly wetting ma-
terial strand (which preferably has been given a corre-
sponding cross-sectional shape by means of a nozzle shaped
corresponding to the groove) and then - prior to curing of
the surface of the first strand - dispensing a second
strand on top of the first strand, the second strand being
as well pre-formed in rectangular shape by an even broader
nozzle opening.
This screening profile has not only a material connection
but also a positive connection with the casing surface
which further increases the stability.
Figure 2i shows a profile structure consisting of a block
89a with an approximately rectangular cross section and
made of conductive, elastic material, and two flat-domed
profile parts 89b and 89c arranged side by side on top of
said block.
Because of its large cross section this profile structure
is especially suitable as a screen with respect to strong
fields, but because of the added-on sealing lips 89b and
89c it is also provided with sufficient elasticity.
It is obvious that other cross sections (almost of any
kind) are possible depending on the requirements to be
met.
For specific applications, a combination of prefabricated,
inserted sealing profiles and of profiles produced
according to the invention may also prove suitable.
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Figure 2k shows a further embodiment of a casing provided
with a sealing according to the invention in the region of
an abutting edge. The casing consists of an upper part 4'
which is provided with a surrounding tongue which engages
in a corresponding surrounding groove 3b of the bottom
part of the casing. The groove and tongue 3b and 3c
taper, thus ensuring a relatively tight closure of the
casing, but the mutual distance of the casing parts may
vary because of manufacturing tolerances. The profile
part 8' according to the invention therefore provides an
additional screen within the region of the edge which,
irrespective of the relative position of the two casing
parts, is highly effective because of its elasticity and
the incorporated conductive materials. Because of the
inclination of its maximum cross sectional extent with
respect to the direction in which the two casing parts
close together, the elasticity is enhanced both by the
compressibility and the flexural deformability of the pro-
file part. In this way, any existing slight inhomoge-
nieties in the sealing of the screen are reliably overcome
because of the fit of the casing, and an excellent
electromagnetic compatibility may, on the whole, be
achieved.
Fig. 3 shows, schematically, the length of a screening
profile 108 along the edge of a casing 101, according to
the invention, which has, by way of example, a rectangular
edge projection lOla and a semi-circular curvature lOlb.
The process according to the invention enables the produc-
tion of any sealing profile, as desired, so that screens
of a high quality may be obtained.
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The above description of casings and parts thereof also
relates to components which act both as electrical and
mechanical carriers and casings, for example a printed
circuit board which is simultaneously used as a casing
part.
Thus, the proposed solution also applies to profiles of
covers in the form of a grid or basket provided in
screening devices or parts thereof.
In its construction, the invention is not restricted to
the above preferred embodiment. On the contrary, many
variants are feasible which make use of the above solution
even if they are basically of a different construction.
*
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