Method for producing a shielding case

Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor

Reexamination Certificate

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C156S244220, C156S578000

Reexamination Certificate

active

06312550

ABSTRACT:

BACKGROUND OF THE INVENTION
Electrically shielding casings for accommodating and electromagnetically shielding units which emit electromagnetic radiation or which are sensitive to electromagnetic radiation have long been known.
While earlier such casings were usually made from metal for obvious reasons, with the onset of the spread thereof on a large scale—for example in relation to mobile telephones or cordless telephones—for cost and weight reasons there has been a change over to making them in particular from plastic material. The prefabricated, in particular injection-moulded casing portions are coated with a conductive material, for example by spraying on conductive lacquer, vapour deposition with aluminium or galvanisation.
Such casings were initially generally provided with prefabricated seals which comprise conductive elastomer and which were fitted upon assembly. DE 38 12 943 A1 shows an internally coated shielding casing of fibre-reinforced plastic material with a prefabricated seal of that kind which was inserted between a groove and a tongue.
Casings of the above-indicated kind are also known from EP 0 629 114 B1and EP 0 654 962 A1.
The casings described therein are composed of two portions which for electrical shielding of the interior of the casing (at least in portion-wise manner) comprise elastically conductive material or are coated with same and in the assembled condition form a Faraday's cage For the purposes of electromagnetic shielding of the casing in the join region between the casing portions which come together, there is provided a shielding seal which comprises an electrically conductive and at the same time elastic material and which adapts to surface tolerances and irregularities so that a very high level of quality in terms of shielding the interior of the casing can be guaranteed even on a mass production scale. This shielding seal is produced directly on at least one of the casing portions while an additional carrier can also be incorporated into the structure.
By virtue of that configuration the casing can be opened for example for maintenance purposes or to replace a power source and then easily closed again while maintaining the sealing and shielding effect.
In production of a material for the shielding seal however it was found that it was difficult to arrive at an optimum compromise between high elasticity, high conductivity and lowest possible material costs. A material with an advantageously high shielding action is a plastic material which is filled with a high proportion of silver powder but this is relatively expensive and has only limitedly satisfactory mechanical properties DE39 34 845 A1 describes the production of a shielding casing with a two-layer shielding profile by the application of an elastic but non-conductive sealing profile to a casing portion and then spraying a conductive coating on to that sealing portion. This method basically makes it possible to save on material costs but it is technologically complicated and expensive and results in seals whose mechanical and in particular electromagnetic properties cannot be completely convincing.
The solution proposed in WO 97/26782 by the applicants also requires if the sealing profile is dispensed on to a casing portion—an additional method step and only makes it possible to produce relatively thin conductive coatings.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method of producing a shielding casing of the kind set forth in the opening part of this specification, which permits simple and inexpensive production of shielding casings with very good mechanical and electromagnetic properties and with a long service life.
The invention includes the technical teaching of forming the shielding profile section in a single application step from (at least) two different. elastically setting components which however are optimised in regard to different functions, in such a way that they are firmly and durably connected together and thus form a reliably dual-functional unit.
Insofar as expulsion from a needle or nozzle with at least a first and second duct or passage is provided as the sole application step, both components can be applied to the casing at a high process speed in a thickness which is adequate for an excellent sealing and shielding effect. As the surfaces of both components, in the initial condition, that is to say before the onset of any cross-linking or hardening are brought into contact with each other, they cross-link to each other or a diffusion interface is formed, which guarantees a practically non-releasable connection between the two profile sections.
The profile portion or section formed from the first material primarily involves the function of filling the gap space in order in spite of mechanical play in respect of the casing portions, which is caused by production tolerances or surface irregularities, to reliably protect the interior of the casing from moisture and dust and possibly prevent relative movements between the casing portions. By virtue of the mechanically optimised properties of the first material it is possible for the internal space in the casing to be durably hermetically sealed off. At the same time the first profile section is naturally a carrier for a section of the electromagnetic shielding and ensures the closure integrity thereof. In regard thereto it is particularly advantageous to adopt a material with a high level of elasticity and/or a configuration for the seal which affords elasticity in respect of shape, on the basis of which upon assembly of the casing portions compression and/or flexion of the sealing element affords a prestressing effect as between the casing portions which for example can prevent a screw or clamping connection from coming loose.
In an advantageous alternative of the invention it is provided that the second material used is a plastic material of the same kind as the first material, with the second material being made conductive by virtue of electrically conductive inclusions. The substantial material uniformity achieved in that way serves to consolidate the above-discussed effect
Production of the shielding profile section is advantageously technologically simplified and rendered less expensive by virtue of the use of a plastic material which hardens in air and at ambient temperature and which in particular has thixotropic properties, more specifically on a silicone basis, but alternatively it is also possible to use heat-hardening or radiation-crosslinking material.
To achieve a shielding effect which satisfies the high test requirements involved, the electrically conductive inclusions added are especially particles comprising a metal or an alloy with a high level of electrical conductivity, in particular silver or a silver-bearing alloy. What is less expensive is the use of a silver-coated powder comprising another metal (nickel, copper or the like) or non-conducting carrier particles (for example of glass). The metal content is typically over 25% by mass, or to achieve very high shielding effects in relation to mobile telephones etc possibly even markedly over 50% by mass, with respect to the mass of the silicone-metal mix.
Besides metal powder, it is also possible in particular to use short metal fibres or metal flakes which in the plastic matrix, if their dimensions are suitably adapted to match the properties of the matrix material and the method characteristics, can advantageously form a kind of metal lattice structure This can impart to the conductive profile portion a high level of conductivity with a relatively small proportion of metal and in conjunction therewith can at the same time afford advantages in terms of relatively low hardness and brittleness.
For situations of use in which adaptation of the degrees of hardness of the first and second materials is desired, it is possible to add to the first material a non-conductive filler, in particular an inexpensive oxidic or ceramic powder (SiO
2
, silicates or the like).
Depending on the specific material properties o

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