GB2310160A

GB2310160A - Loudspeaker Components

Info

Publication number: GB2310160A
Application number: GB9600495A
Authority: GB
Inventors: Nigel Graham Rowley
Original assignee: COLUMBUS PLASTICS Ltd
Current assignee: COLUMBUS PLASTICS Ltd
Priority date: 1996-01-11
Filing date: 1996-01-11
Publication date: 1997-08-20
Also published as: GB9600495D0

Description

Composite-Walled Structural Member Production Methods" This invention relates to methods of producing composite-walled structural members for use in various applications, such as loudspeaker enclosures and horns, for example.

Loudspeaker enclosures are traditionally fabricated from timber or composite board possessing neutral acoustic characteristics and providing an attractive appearance.

However the fabrication of such enclosures in traditional manner is labour-intensive and costly, and there is an increasing trend for loudspeaker enclosures to be formed from moulded plastics material in order to decrease their cost. However existing loudspeaker enclosures formed from plastics material suffer from the fact that sound reproduction quality is severely compromised by the non-neutral acoustic characteristics of the enclosure which will tend to resonate in use resulting in undesirable harmonic resonances being produced.

It is an object of the invention to provide a novel composite-wall structural member which may be used in fabrication of various articles, and which is particularly suitable for the production of loudspeaker enclosures.

According to the present invention there is provided a method of producing a composite-walled structural member comprising forming an outer skin of rigid or semirigid plastics material into a desired shape, subsequent to said forming step placing the formed outer skin in a mould together with an expandable plastics material, and expanding the expandable plastics material within the mould so as to cause the expandable plastics material to fuse with the outer skin to produce said structural member having a composite wall consisting of an inner layer of expanded plastics material surrounded by the outer skin.

The outer skin may be formed by a thermoforming technique, such as vacuum forming, or by injection or blow moulding.

In a preferred method according to the invention the outer skin is formed by a vacuum-forming technique in which a sheet of plastics material is placed over a forming member and a vacuum is applied to one side of the sheet to cause the sheet to be drawn against the forming member by the vacuum to take up the shape of the part of the forming member against which the sheet is drawn.

Most preferably, in this forming step, the sheet of plastics material is heated for a preset time prior to application of the vacuum to enable deformation of the heated sheet into the required shape.

The rigid or semi-rigid plastics material may be high impact polystyrene (HIPS), acrilonitrile-butadiene-styrene (ABS), polyvinyl chloride (PVC) or polycarbonate.

Furthermore the outer skin is preferably an ultra-violet-stable thermoformable sheet.

The mould within which the formed outer skin is placed together with the expandable plastics material is advantageously a two-part mould having a first part within which the formed outer skin is placed in mating contact therewith and a second part which is connectable to the first part after the outer skin has been placed within the first part so as to define a cavity between the outer skin and the second part within which the expandable plastics material is expanded.

The expandable plastics material, which is preferably in granular form, may be expandable polystyrene (EPS), expandable polypropylene (EPP), expandable polyethylene (EPE) or the like.

Furthermore the expandable plastics material, which is preferably partially expanded prior to use in the expanding step, is advantageously expanded in use to a density of the order of 40 to 100g/l.

The invention also provides a composite-walled structural member, such as a loudspeaker enclosure, a loudspeaker horn, a car dashboard, a cool box, a carrying container, a safety helmet or a body protector, which has been produced by the above method.

In order that the invention may be more fully understood, reference will now be made, by way of example, to the accompanying drawings, in which: Figures 1 and 2 are sectional and perspective views of a loudspeaker enclosure produced by a method in accordance with the invention; Figures 34 are front, plan, back and side views of a loudspeaker horn produced by a method in accordance with the invention; and Figures 7 and 8 are sectional views along the lines A-A and B-B in Figure 3 respectively.

Referring to Figures 1 and 2, the loudspeaker enclosure 1 has front and side walls 2 and 3 which are integrally formed by a production method according to the invention so as to have a high impact outer skin 4 and an integral inner layer 5 of black expandable polystyrene (EPS) having good sound absorbing properties. The enclosure 1 is formed by the production method so as to have the required shape with a circular opening 6 in front wall 2 for the sound output from a loudspeaker secured to the inside of the front wall 2 in conventional manner, and with an open back 7 which may be closed off by a detachable back panel (not shown) optionally formed by a similar production method. The outer skin 4, which is preferably formed from a weather-resistant, ultraviolet-stable-high impact polystyrene (HIPS) sheet such as that marketed under the trade mark STYROSUN, can be formed with a simulated wood or other finish to provide an attractive appearance and provides an outer surface which is resistant to denting or other damage in use. Furthermore the composition of the inner layer 5 and the fact that it is fused with the outer skin 4 means that the enclosure 1 provides high acoustic damping and is resistant to acoustic resonance effects in use. Optionally, as shown in broken lines in Figure 1, parts of the front and/or wide walls 2, 3 may be provided with sound deadening inserts 8 placed in positions determined by acoustic considerations. Such inserts 8 may be formed of small sheets (say 10cm x 10cm) of a material, such as polyurethane foam or bitumastic material, of about 1 mum thickness placed in the mould in adhesive contact with the outer skin so as to be trapped between the outer skin and inner layer during moulding.

The production method used in forming such a loudspeaker enclosure 1 is substantially the same as that used in the production of a loudspeaker horn 10 shown in Figures 3-6 (except that an aluminium insert is provided in the latter case) and accordingly this production method will be readily understood from the description given below of the method of production of the loudspeaker horn.

Referring to Figures 3-6 the loudspeaker horn 10 has a concave horn part 11 (as best seen in the front view of Figure 3) having a composite wall consisting of a high impact outer skin 12 and an expanded polystyrene inner layer as already described with reference to the loudspeaker enclosure 1 of Figures 1 and 2, as well as an outer flange 13 provided with screw holes 14 by means of which the horn 10 may be secured within a loudspeaker enclosure. Figures 7 and 8 show sections along the lines A-A and B-B in Figure 3 and show in particular the shape of the inner layer 15 of expanded polystyrene beneath the outer skin 12 of the horn part 11. As may be seen in these figures, the horn part 11 includes an aluminium insert 16 in the form of an apertured plate, and the inner layer 15 is shaped to define the rear part 17 of the horn.

Furthermore the horn part 11 is formed with a central aperture 18 and four blind bores 19, as well as four holes 20 by means of which screw threaded connections may be made to the aluminium insert 16.

The multi-stage production method by means of which the horn 10 is fabricated will now be described. In a first vacuum forming step a sheet of Styrosun u.v. stable thermoformable HIPS (or other HIPS material including non-u.v. stable and re-grind HIPS) of 1-6mm thickness is placed within the open clamp frame of a vacuum forming machine so as to cover the aperture of the machine, and the clamp frame is then closed down ensuring a secure fit of the sheet against the aperture. An automatic preprogramme cycle is then run so as to heat the sheet for a preset time. The drape table of the machine, to which vacuum forming resin has been applied, is then raised, and, when it has been fully raised, the vacuum is applied for a preset time so as to draw the thermoformable sheet under vacuum pressure against a forming member to take up the shape of the part of the forming member against which the sheet is drawn. At the end of the vacuum time a small quantity of air is injected through the inflate system to break the vacuum, and the drape table is then lowered and the clamp frame opened so as to release the thermoformed component.

In a subsequent EPS moulding step, the thermoformed component is inserted into a mould together with a quantity of pre-formed EPS granules which may optionally be loaded with carbon or ceramic fibres or other metallic or non-metallic fillers to increase sound deadening. The mould is provided with an aluminium casting tool shaped so that, when the thermoformed component is inserted in the mould, it will accurately mate with the tool surface. Furthermore a closure plate of the mould has a shape corresponding to the required inner profile of the EPS inner layer to be produced in this step. Four Allen screws are positioned on the closure plate and are used to secure the aluminium insert within the mould. The closure plate is then clamped in position to close the mould.

The mould is then lowered by means of lifting tackle into a process tank filled with hot water at a predetermined temperature of 98.1000 Celsius, in order to partially expand the EPS granules to a typical density of 60.70gII. After opening of the mould, including removal of the Allen screws and the stainless steel cylindrical clamping bars, the moulding is removed from the mould and placed in a drying room for a predetermined drying time in order to remove excess moisture and stabilise the product dimensionally, the temperature being accurately controlled to achieve optimum strength and surface finish. After a set period of time determined by a process timer, the mould is removed from the process tank and cooled in a cold water tank for a predetermined cooling time in order to prevent further expansion and permit easy release of the moulding.

In a subsequent assembly step the flange on the moulding is accurately trimmed, and four lengths of closed cell high density foam are applied to the flange. The screw holes in the flange are then drilled to complete the assembly.

Various modifications to the above production method are possible within the scope of the invention. For example, instead of utilising HIPS in the vacuum forming step, other materials, such as ABS, PVC and polycarbonate, may be used. Furthermore, instead of utilising black EPS in the moulding step, other grades of EPS (e.g. white flame retardant, oil resistant or other coloured material including modified EPS grades), EPP, EPE or similar materials may be used. Furthermore special metallic or nonmetallic inserts can be included in the moulding for fastenings, threaded inserts, metal fixing points and special acoustic damping pads to reduce resonance.

The above-described production method is particularly useful in the applications described above as it enables intricate shapes to be produced in a straightforward manner whilst providing good acousting damping characteristics. Apart from such acoustic applications, a variety of other applications of the production method of the invention and ofthe composite-walled structural member formed by such a method are possible. For example the material so formed may be used for the production of car dashboards, spoilers and other car accessories, as well as for various types of personal protective equipment, such as body protection equipment and safety helmets, and specialised carrying containers, military equipment protective packs, etc. Because of the thermal insulation properties of the material, it may also be used for cool boxes, picnic containers and for other thermal applications, such a refrigerators etc.

Claims

1. A method of producing a composite-walled structural member comprising forming an outer skin of rigid or semi-rigid plastics material into a desired shape, subsequent to said forming step placing the formed outer skin in a mould together with an expandable plastics material, and expanding the expandable plastics material within the mould so as to cause the expandable plastics material to fuse with the outer skin to produce said structural member having a composite wall consisting of an inner layer of expanded plastics material surrounded by the outer skin.

2. A method according to claim 1, wherein the outer skin is formed by a vacuumforming technique in which a sheet of plastics material is placed over a forming member and a vacuum is applied to one side of the sheet to cause the sheet to be drawn against the forming member by the vacuum to take up the shape of the part of the forming member against which the sheet is drawn.

3. A method according to claim 2, wherein the sheet of plastics material is heated for a preset time prior to application of the vacuum to enable deformation of the heated sheet into the required shape.

4. A method according to claim 1, 2 or 3, wherein the rigid or semi-rigid plastics material is high impact polystyrene (HIPS), acrilonitrile-butadiene-styrene (ABS), polyvinyl chloride (PVC) or polycarbonate.

5. A method according to any preceding claim, wherein the outer skin is an ultraviolet-stable thermoformable sheet.

6. A method according to any preceding claim, wherein the mould within which the formed outer skin is placed together with the expandable plastics material is a two-part mould having a first part within which the formed outer skin is placed in mating contact therewith and a second part which is connectable to the first part after the outer skin has been placed within the first part so as to define a cavity between the outer skin and the second part within which the expandable plastics material is expanded.

7. A method according to any preceding claim, wherein the expandable plastics material is expandable polystyrene (EPS), expandable polypropylene (EPP), or expandable polyethylene (EPE).

8. A method according to any preceding claim, wherein the expandable plastics material is partially expanded prior to use in the expanding step.

9. A method according to any preceding claim, wherein the expandable plastics material is expanded in use to a density of the order of 40 to 100 girl.

10. A method of producing a composite-walled structural member, substantially as hereinbefore described with reference to the accompanying drawings.

11. A composite-walled structural member, such as a loudspeaker enclosure, a loudspeaker horn, a car dashboard, a cool box, a carrying container, a safety helmet or a body protector, which has been produced by a method according to any preceding claim.

Amendments to the claims have been filed as follows 1. A loudspeaker component comprising an outer skin of rigid or semi-rigid plastics material formed into a desired shape and having an inside surface, and an inner layer of expanded plastics material moulded onto the inside surface of the outer skin so as to produce a composite walled component in which the inner layer is fused with the outer skin and in which the inner layer has a moulded inner surface of defined shape.

2. A loudspeaker component according to claim l, which is at least part of a loudspeaker horn.

3. A loudspeaker component according to claim I, which is at least part of a loudspeaker enclosure.

4. A method according to claim I, 2 or 3, wherein the rigid or semi-rigid plastics material is high impact polystyrene (HIPS), acrilonitrile-butadiene-styrene (ABS), polyvinyl chloride (PVC) or polycarbonate.

6. A method according to any preceding claim, wherein the expandable plastics material is expandable polystyrene (EPS), expandable polypropylene (EPP), or expandable polyethylene (EPE).

7. A method of producing a loudspeaker component comprising forming an outer skin of rigid or serni-rigid plastics matenal into a desired shape, subsequent to said forming step placing the formed outer skin in a mould together with an expandable plastics material, and expanding the expandable plastics material to fill the mould so as to cause the expandable plastics material to fuse with the outer skin to produce a composite wall in which of an inner layer of expanded plastics material is fused with the outer skin and in which the inner layer has a moulded inner surface of defined shape.

8. A method according to claim 7, wherein the outer skin is formed by a vacuumforming technique in which a sheet of plastics material is placed over a forming member and a vacuum is applied to one side of the sheet to cause the sheet to be drawn against the forming member by the vacuum to take up the shape of the part of the forming member against which the sheet is drawn.

9. A method according to claim 8, wherein the sheet of plastics material is heated for a preset time prior to application of the vacuum to enable deformation of the heated sheet into the required shape.

10. A method according to claim 7, 8 or 9, wherein the mould within which the formed outer skin is placed together with the expandable plastics material is a two-part mould having a first part within which the formed outer skin is placed in mating contact therewith and a second part which is connectable to the first part after the outer skin has been placed within the first part so as to define a cavity between the outer skin and the second part within which the expandable plastics material is expanded.

11. A method according to any one of claims 7 to 10, wherein the expandable plastics material is partially expanded prior to use in the expanding step.

12. A method according to any one of claims 7 to 11, wherein the expandable plastics material is expanded in use to a density of the order of 40 to 100 g/l.

13. A loudspeaker component substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 3 to 8 of the accompanying drawings.

14. A method of producing a loudspeaker component substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 3 to 8 of the accompanying drawings.