GB2377907A

GB2377907A - Method suitable for forming shaped screens

Info

Publication number: GB2377907A
Application number: GB0217144A
Authority: GB
Inventors: Paul Calver
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 2001-07-24
Filing date: 2002-07-24
Publication date: 2003-01-29
Anticipated expiration: 2022-07-24
Also published as: GB2377907B; GB0117991D0; GB0217144D0

Abstract

A method for forming a laminated glass screen comprises the steps of applying polymeric layer 3 in a ductile state, to glass layer 2 and then applying pressure to urge the polymeric material relative to the glass. The pressure may be applied via a compliant force, eg. by using a compliant material. The method is particularly used to manufacture shaped screens eg. laminated safety visors. A pressure application device 4 is also disclosed comprising two or more hinged sections 6, with a compliant layer 5 (eg of polymeric foam) attached thereto. In use, the hinged sections are gradually opened out as the device is applied, enabling a substantially uniform pressure to be applied across the surface of the laminate.

Description

METHOD SUITABLE FOR FORMING SHAPED SCREENS The present invention relates to a method suitable for producing shaped screens and especially transparent laminated visors.

Producing screens in the form of transparent visors, especially where the visor is required withstand missiles or explosive forces, requires the visor to possess considerable strength. Typically, visors are formed by laminates of glass and polycarbonate layers, with an interlayer to produce the required ballistic resistance and for reasons of safety in the event of breakage.

Traditionally the glass layer is formed by slumping, with the polycarbonate layer being shaped in a separate mould. The resultant layers are then laminated together to form the visor. However, this can give rise to optical distortion in the visor as described below.

Both the glass and polycarbonate layers must be handled carefully during forming, as both are extremely susceptible to marking. Because the glass is formed by slumping there is very little control over the accuracy of its surface form. Unevenness occurs to a lesser extent in the polycarbonate layer if, as is usually the case, insufficient pressure is applied over the surface of a heated blank of the polycarbonate to force the blank to conform precisely to the mould. Areas of maximum optical distortion occur where the surfaces of the glass and polycarbonate converge or diverge in the assembled visor.

One of the main problems currently preventing the use of glass/polycarbonate visors is the optical distortion caused by the slight mismatches between the two layers when the space between them is filled with a transparent interlayer of refractive index = 1. 5.

By their very nature, curved visors which are thermoformed from flat sheet materials and which are capable of defeating fragments at high velocity (the

aim for 1.1 gram fragments is 700+ m/s) are cylindrical lenses with quite high refractive powers. However, provided that the sheet stock from which the flat sheet materials are taken is of good optical quality, i. e. has surfaces which are parallel at any given point, a curved single-element visor of about 120 mm radius creates optical distortions within limits that are acceptable to a user, even if the curvature of the visor varies slightly from place to place. This is because light passing through the visor at an angle to the surface of the visor will exit slightly displaced, but with no net diffraction, i. e. its exit path is parallel to its incident path.

The same is true in the case of a two-layer spaced visor, even at a point where the surfaces of the two layers are not parallel to one another, If, however, the space between layers at such a point is filled with an interlayer formed from a polymeric material with a similar refractive index, the area behaves like a prism with non-parallel sides. In this case, the exit path of light passing through the visor is not parallel to the incident path and the effect is an apparent large displacement of objects viewed through the visor. When such effects occur to differing degrees around a visor, the variations in displacement are seen as gross distortions of the visual field.

Accordingly there is provided a method suitable for forming a laminated shaped screen of a desired screen shape, which method comprises the steps of applying polymeric material in a ductile state to a glass surface, which glass surface has the desired screen shape, and by application of pressure urging the polymeric material relative to the glass to form a laminate.

....

The polymeric material may be ductile enough such that under the action of gravity alone the pressure provided produces an effective laminate. The ductility of the polymeric material can be varied by changing its temperature.

The temperature should be carefully controlled to avoid damage to the surface of the polymeric material because of differential rates of expansion and compression relative to the glass.

It will normally be appropriate to apply pressure to the polymeric material via a compliant force to urge the polymeric material gently into position relative to the glass.

The term'compliant force'as applied herein means any material or influence, such as gas pressure, which can provide a low but substantially consistent pressure across that area of the surface of the polymeric material which corresponds with the interface between the polymeric material and the glass.

The method can be used for producing a shaped screen of transparent materials such that the screen may provide visibility in use. As such, the screen may form the windscreen of a vehicle, or be a face visor providing protection to a wearer.

The ductility of the polymeric material may be enhanced by pre-heating the material. However, heating of the glass and/or polymeric material may be used to assist in the method.

The polymeric material is likely to be selected from polycarbonate and polyurethane. Preferably, the polymeric material is polycarbonate.

Preferably, there is included an interlayer between the polymeric material and the glass for reasons of ballistic resistance and safety. Typically, the interlayer is formed from polyurethane.

The method can be used to produce multiple layer laminates.

The compliant material is preferably a foam. Preferably, the foam is made from Confor CF-40 (Yellow) available from Dowty Energy Control Systems, Herefordshire. This foam, believed to be a silicone-modified PVC, has ratedependent and"slow-recovery"properties. At low rates of strain it compresses very easily in a non-Hookean fashion, but has a natural tendency

to recover to its original dimensions. This is identical to the properties required for a slow-recovery foam earplug, and creates an effect similar to a low and even hydrostatic pressure. The CF-40 grade is the softest available.

The compliant material may be used in conjunction with a substantially rigid backing material. The use of the backing material provides support to the compliant material. Preferably, the backing material is polyurethane foam.

For ease of construction, it was envisaged that a prototype core would be created by casting polyurethane foam directly against the inside surface of a layer of the compliant material, the compliant material being a foam. By making the shape of a surface of the backing material substantially similar to the desired screen shape and by applying a consistent thickness of the compliant material across the surface of the backing material, it is possible to produce a composite device which has a shape substantially the same as that of the desired screen shape. The compliant material and the backing material can be co-joined by an adhesive or other suitable means.

In a preferred embodiment, the backing material is present in two or more sections. The two or more sections are moveable relative to each other and in such a manner, that the compliant material acts to apply pressure substantially evenly across the surface of the polymeric material to thereby ensure that the laminated shaped screen formed by the method has the desired screen shape. Preferably, the two or more sections are joined to each other at one or more points to form hinges about which the sections can move. To aid the movement of the sections, it is preferable if plates are attached to the sections to enhance the rigidity of the sections. Movement of the plates displaces the sections in such a manner that the compliant material acts to apply pressure substantially evenly across the surface of the polymeric material. Preferably, the plates are hinged. The plates may be separated by a suitable angle to enable the body that is formed by the two or more sections to be expanded and collapsed.

In a further embodiment, there is provided a pressure application device comprising two or more applicator sections, each applicator section comprising a compliant material and being moveable relative to each other such that, in use, each applicator section may be brought into contact with a surface so as to apply pressure substantially evenly across the surface. The purpose of each applicator section is to apply pressure to the surface. A preferred application for the pressure application device is for use in a method of lamination. Preferably, the applicator sections are joined to each other at one or more points to form hinges about which they can move. It is preferable if plates are attached to each applicator section to enhance the rigidity of each applicator section. Preferably, the compliant material is a foam, such as Confor CF-40. Preferably, the compliant material is co-joined to a substantially rigid backing material, such as polyurethane foam.

The nature of the laminated shaped screen will be a function of the materials used and specific process adopted. Such materials and process are further described herein.

The invention will now be described by way of example only and with reference to the accompanying drawings of which: Figure 1 shows a sectioned view of a pressure application device in accordance with the present invention about to be used in accordance with the method of the present invention to produce a visor, Figure 2 shows the initial stage of formation of the visor in accordance with the method of the present invention, and Figure 3 shows the application of pressure across the surface of the polymeric material to form the visor in a desired shape in accordance with the method of the present invention.

As shown in Figure 1, a support base (1) holds a curved layer of glass (2) with concave side uppermost. A curved layer of polycarbonate (3) of greater radius of curvature than the glass (2) and pre-heated to be in a ductile state sits upon the concave side of the glass (2). An interlayer of polyurethane (not shown) is applied to the concave side of the glass (2). A pressure application device in accordance with the present invention (4) comprises a layer of compliant foam (5) (made from Confor CF-40 (Yellow) from Dowty Energy Control Systems, Herefordshire) of consistent thickness and arcuate form of smaller radius of curvature than the glass (2). The layer of compliant foam (5) is joined by adhesive (not shown) to two substantially rigid foam backing sections (6). The backing sections (6) are attached by adhesive to centre plates (7) and incorporate a hinge attachment (8) which is shown in Figure 3.

As shown in Figure 2, the layer of polycarbonate (3), under the influence of pressure from the pressure application device (4) and gravity, is forced to assume the shape of the glass (2). The centre of the layer of compliant foam (5) first pushes the centre of the layer of polycarbonate (3) against the corresponding centre of the glass (2).

Figure 3 shows the hinging action of the plates (7) which are forced apart along with the backing sections (6), so as to gradually but gently apply pressure to the outer area of the layer of compliant foam (5) so as to urge the layer of polycarbonate (3) against the interlayer and ensure that upon cooling, the polycarbonate layer (3) is maintained in the correct position relative to the interlayer and glass (2) to thereby ensure that a laminate with good optical characteristics is produced. In forcing the plates (7) apart, the effective radius of the backing layer (6) and compliant layer (5) increases to be substantially the same as that of the interlayer and glass (2). To achieve an optimum result, it is necessary to apply light but even pressure over the whole surface of the polycarbonate layer (3) within a few seconds of its removal from an oven (not shown) and application to the concave side of the glass (2). To help prevent freeze-marking of the polycarbonate layer (3), both the glass (2) and

layer of compliant foam (5) are covered with a heat-resistant material with a low thermal capacity. The chosen material is Single Jersey 6S 3S from Cannon Street Jersey. Care is taken to ensure that these layers do not crease during the forming process.

Claims

CLAIMS What we claim is: 1) A method suitable for forming a laminated shaped screen of a desired screen shape, which method comprises the steps of applying polymeric material in a ductile state to a glass surface, which glass surface has the desired screen shape, and by application of pressure urging the polymeric material relative to the glass to form a laminate.
2) A method as claimed in claim 1, characterised in that the application of pressure is via a compliant force which provides a low but substantially consistent pressure across that area of the surface of the polymeric material which corresponds with the interface between the polymeric material and the glass.
3) A method as claimed in claim 2, characterised in that the compliant force is provided via a compliant material.
4) A method as claimed in claim 3, characterised in that the compliant material is a foam.
5) A method as claimed in claim 4, characterised in that the foam is Confor CF-40.
6) A method as claimed in any one of claims 3 to 5, characterised in that the compliant material is used in conjunction with a substantially rigid backing material to provide support to the compliant material.
7) A method as claimed in claim 6, characterised in that the backing material is polyurethane foam.
8) A method as claimed in either of claims 6 or 7, characterised in that the shape of a surface of the backing material is substantially similar to the desired screen shape and a consistent thickness of the compliant material is applied across the surface of the backing material so as to produce a composite device which has a shape substantially the same as that of the desired screen shape.

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9) A method as claimed in any one of claims 6 to 8, characterised in that the compliant material and backing material are co-joined by an adhesive.
10) A method as claimed in any one of claims 6 to 9, characterised in that the backing material is present in two or more sections that are moveable relative to each other and in such a manner that the compliant material acts to apply pressure substantially evenly across the surface of the polymeric material.
11) A method as claimed in claim 10, characterised in that the sections are joined to each other at one or more points to form hinges about which the sections can move.
12) A method as claimed in either of claims 10 or 11, characterised in that plates are attached to the sections to enhance the rigidity of the sections.
13) A method as claimed in claim 12, characterised in that the plates may be separated by a suitable angle to enable the body that is formed by the two or more sections to be expanded and collapsed.
14) A method as claimed in any one of claims 1 to 13, characterised in that heat is applied to at least one of the glass and polymeric material to assist in the method.
15) A method as claimed in any one of claims 1 to 14, characterised in that the ductility of the polymeric material is enhanced by pre-heating the polymeric material.
16) A method as claimed in any one of claims 1 to 15, characterised in that the polymeric material is selected from polycarbonate and polyurethane.
17) A method as claimed in any one of claims 1 to 16, characterised in that an interlayer is included between the polymeric material and the glass.
18) A method as claimed in claim 17, characterised in that the interlayer is polyurethane.

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19) A method as claimed in any one of claims 4 to 18, characterised in that the glass and foam are covered with a heat-resistant material with a low thermal capacity.
20) A method as claimed in claim 19, characterised in that the heat- resistant material is Single Jersey 6S 3S.
21) A method substantially as described herein with reference to the accompanying drawings.
22) A product produced in accordance with any one of claims 1 to 21.
23) A product produced by the method as claimed in any one of claims 1 to 21, characterised in that the product is a shaped screen that provides visibility in use.
24) A product as claimed in claim 23, characterised in that the screen forms the windscreen of a vehicle.
25) A product as claimed in claim 23, characterised in that the screen forms a face visor providing protection to a wearer.
26) A pressure application device comprising two or more applicator sections, each applicator section comprising a compliant material and being moveable relative to each other such that, in use, each applicator section may be brought into contact with a surface so as to apply pressure substantially evenly across the surface.
27) A pressure application device as claimed in claim 26, characterised in that the applicator sections are joined to each other at one or more points to form hinges about which they can move.
28) A pressure application device as claimed in either of claims 26 or 27, characterised in that plates are attached to each applicator section to enhance the rigidity of each applicator section.
29) A pressure application device as claimed in any one of claims 26 to 28, characterised in that the compliant material is a foam.
30) A pressure application device as claimed in claim 29, characterised in that the foam is Confor CF-40.

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31) A pressure application device as claimed in any one of claims 26 to 30, characterised in that the compliant material is co-joined to a substantially rigid backing material.
32) A pressure application device as claimed in claim 31, characterised in that the backing material is polyurethane foam.
33) A pressure application device substantially as described herein with reference to the accompanying drawings.
34) A laminated shaped screen substantially as described herein with reference to the accompanying drawings.