METHOD OF ENCLOSING AN OBJECT WITHIN A HOMEGENEOUS BLOCK (1)

Abstract: A method of enclosing an object (5), such as an electronic component, within a homogeneous block of microporous ther­ mal insulation material comprises charging a first predeter­ mined amount of a loose microporous thermal insulation mix­ ture (2) into a die (1) and compacting the insulation mixture into a block for supporting the object to be insulated. A recess (4) may be formed in the compacted mixture for receiving the ob­ ject (5). The object (5) is then located on the compacted mix­ ture and a second predetermined amount of loose microporous thermal insulation mixture (6) is charged into the die (1). The loose insulation mixture (6) is compacted against the block of already compacted insulation material (2) which causes the two charges of insulation mixture to merge and to form a homogeneous block enclosing the object.

Description: The present invention relates to a method of enclosing an object such as an electronic component within a homogeneous block of microporous thermal insulation material. When an object to be thermally insulated requires a particularly high performance from the insulation material, care must be taken to ensure that the individual components of the surrounding insulating body are a particularly good fit with each other in order to eliminate as far as possible the passage of heat through the gap which inevitably exists between the individual components of the insulating body. A good fit between the individual components is especially important when the insulation material is a microporous thermal insulation because even still air has a higher thermal conductivity than the insulation material so that even a small gap will act as a path for heat conduction as well as for infra-red radiation. Ideally all gaps and joints should be eliminated so that the object to be thermally insulated is totally enclosed within the insulation material. This can readily be achieved when the insulation material is a mouldable foam material or when it can otherwise be assumed that the object to be insulated will remain undisturbed within the insulation material. Microporous thermal insulation is normally manufactured as a block material by first mixing together the constituents of the insulation material, for example a finely divided insulating powder, an infra-red opacifier and/or a reinforcing fibre, so as to form an intimate mixture. Such a microporous thermal insulation is described, for example, in GB-A-1 350 661. The intimate mixture itself typically has a density of 25 to 100 kgm⁻³ and the manufactured block typically has a density of 200 to 400 kgm⁻³. In practice, during compaction the volume of the mixture is reduced typically to about 10 per cent of its original volume. Heretofore it has been considered impractical to locate an object to be insulated within a homogeneous block of microporous thermal insulation material. This is because the intimate mixture is too fluid to support an object at a predetermined position so that it is not possible, for example, to locate an object at a central point within the intimate mixture and subsequently to apply pressure so as to form the block around the object. It may be possible to provide an external support for the object so as to determine the initial position of the object, but we have found that the final position of the object within the block is still likely to be at a random position. We have found that the handling characteristics of the intimate mixture can vary depending on the storage of the intimate mixture. If the intimate mixture is allowed to stand for only a few minutes it has a relatively low density, but if it is allowed to stand for several hours it has a relatively high density because air is slowly released from the mixture and settling takes place. Irrespective of whether the intimate mixture is allowed to stand for a short or a long time, the mixture is capable of being compacted to produce a satisfactory block of microporous thermal insulation material, but any means used to locate an object to be insulated within the intimate mixture so that the block can be formed around the object must be capable of adapting to different characteristics of the mixture if a large number of objects is required to be insulated in a substantially identical manner. Any such means of locating the object to be insulated within the intimate mixture will inevitably be complicated and eventually may lead to a short heat path through the insulation material. The insulation of small electrical components, for example, has until now been achieved by manufacturing two or more pieces of the insulation material which are designed to fit as closely as possible around the electrical component. A housing is normally provided for the insulation material to give protection during handling of the insulated electrical component. Such an assembly can provide satisfactory thermal performance, but it is expensive to manufacture and some gaps and joints will be present in the insulation material. FR-A-2 406 369 discloses a heating element and its method of manufacture according to which a first layer of refractory material is arranged in a die, an electrical resistance is positioned on the first layer of refractory material, the electrical resistance is covered with a second layer of refractory material, and this assembly is subjected to a pressure between 150 and 500 bar. It is an object of the present invention to provide a method of enclosing an object such as an electrical component within a homogeneous block of microporous thermal insulation material such as that described in GB-A-1 350 661 so as to provide particularly low thermal conductivity in a cost effective manner. According to the present invention there is provided a method of enclosing an object such as an electronic component within a homogeneous block of microporous thermal insulation material, which method comprises the steps of: charging a first predetermined amount of uncompacted microporous thermal insulation mixture into a die; compacting the insulation mixture into a block for supporting an object to be insulated; locating the object on the block; charging a second predetermined amount of uncompacted microporous thermal insulation mixture into the die; and compacting the insulation mixture against the block of compacted insulation material so as to form a homogeneous block enclosing the object. The first predetermined amount of uncompacted microporous thermal insulation material may be compacted into a block having a recess for receiving the object to be insulated. The first predetermined amount of uncompacted microporous thermal insulation mixture may be compacted to a density which is lower than the density of the homogeneous block, for example the density may be about 10 per cent less than the density of the homogeneous block. The homogeneous block may be removed from the die, or the homogeneous block may be retained within the die in order to provide mechanical protection for the block. The die may be provided with a cover so as to enclose the homogeneous block. For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 illustrates a first stage of the method according to the present invention; Figure 2 illustrates a second stage of the method according to the present invention; and Figure 3 shows one embodiment of an object enclosed within a block by the method according to the present invention. The first stage in the process of enclosing an object within a homogeneous block of microporous thermal insulation is illustrated in Fig.1 which shows a lower die 1 which is charged with a predetermined amount of an uncompacted microporous thermal insulation mixture 2. An upper die 3 is provided for compacting the mixture in a manner such that a recess 4 is formed in the compacted mixture. It is known to the skilled person that after compaction the insulation mixture possesses a resilience which causes the material to expand when the lower and upper dies 1, 3 are separated so allowance is made for the expansion when designing the dies 1, 3. The insulation mixture 2 is compacted to a density which may be lower than the density required for the final homogeneous block. An object 5, such as an electronic component, is then located in the recess 4 and a further charge of insulation mixture 6 is added to the lower die as shown in Fig. 2. The second stage of the process is to compact the mixture 6 and to consolidate the insulation mixture around the object 5. When pressure is applied to compact the insulation mixture 6 it will result in further compaction of the insulation mixture 2 because of the resilience of the compacted insulation material. Thus, the object 5 will move as pressure is applied and released. However, the characteristics of the compacted insulation material are predictable and thus the final position of the object 5 can be determined accurately and consistently. In general, we have found that it is desirable to compact the insulation mixture 2 to a density about 10 per cent less than the required density of the homogeneous block. Ultimate compaction of the mixture 2 then occurs when pressure is applied to compact the insulation mixture 6. Much to our surprise, we have found that compaction of the insulation mixture 6 onto the already compacted mixture 2 causes the two charges of mixture to blend at their interface and to form a homogeneous block. It would be expected that the two layers would form separately and at best might be loosely bonded together, but in practice we have found it impossible to detect the interface between the two charges. The homogeneous block is then removed from the lower die 1. By way of example, an electronic component cast into a resin block to provide mechanical protection and having dimensions of 30 x 20 x 10mm may be enclosed within a cylindrical block of microporous thermal insulation material having a diameter of 100mm and a height of 100mm, the electronic component being located at the centre of the block of microporous thermal insulation material. The homogeneous block of insulation may be mechanically protected as shown in fig. 3 if the lower die 1 is not removed from the block. For example, the lower die may be formed as a cup and subsequently may be provided with a lid or cover 7 so that the block is enclosed within a robust skin or container which provides the mechanical protection. It is possible to combine the steps of forming the recess 4 in the charge of insulation mixture 2 and inserting the object 5 into the recess by mounting the object 5 beneath the die 3 in a temporary manner such that the object 5 remains in the compacted insulation mixture 2. It should be noted that the recess 4 is not essential for performing the method according to the present invention. Once the insulation mixture 2 has been compacted its characteristics are predictable and thus the final position of the object 5 can be determined merely by locating the object on the compacted insulation mixture 2. The materials selected for the charges of insulation mixture 2, 6 may be any of those known to provide microporous insulation and typically will include a finely divided microporous powder, an infra-red opacifier, a reinforcing fibre and/or other binding additives.

Claims: 1. A method of enclosing an object (5) such as an electronic component within a homogeneous block of microporous thermal insulation material (2, 6) characterised by the steps of: charging a first predetermined amount of uncompacted microporous thermal insulation mixture (2) into a die (1); compacting the insulation mixture into a block for supporting an object (5) to be insulated; locating the object (5) on the block; charging a second predetermined amount of uncompacted microporous thermal insulation mixture (6) into the die; and compacting the insulation mixture (6) against the block of compacted insulation material (2) so as to form a homogeneous block enclosing the object (5). 2. A method according to claim 1, characterised in that the first predetermined amount of uncompacted microporous thermal insulation material (2) is compacted into a block having a recess (4) for receiving the object (5) to be insulated. 3. A method according to claim 1 or 2, characterised in that the first predetermined amount of uncompacted microporous thermal insulation mixture (2) is compacted to a density which is lower than the density of the homogeneous block. 4. A method according to claim 3, characterised in that the first predetermined amount of uncompacted microporous thermal insulation mixture (2) is compacted to a density about 10 per cent less than the density of the homogeneous block. 5. A method according to any one of claims 1 to 4, characterised in that the homogeneous block is removed from the die (1). 6. A method according to any one of claims 1 to 4, characterised in that the homogeneous block is retained within the die (1) in order to provide mechanical protection for the block. 7. A method according to claim 6, characterised in that the die (1) is provided with a cover (7) so as to enclose the homogeneous block.

Inventors: HUGHES, JOHN THOMAS
Applicants: MICROPORE INTERNATIONAL LIMITED
Application year: 1987
Main classification: B29C67
IPC codes: H01C1, B29C43, B29K105, F16L59, B29C70