GB1593346A - Moulding relief patterned transparent sheets - Google Patents

Description

(54) MOULDING RELIEF PATTERNED TRANSPARENT SHEETS (71) We, AKTIEBOLAGET BOFORS of S-690 20, BOFORS, Sweden, a Swedish joint-stock company, acting under the laws of Sweden, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method of producing light transmitting relief patterned sheets and a mould for producing such sheets. These sheets are primarily intended for use in light fittings.

To prevent dazzle and obtain uniform diffusion of light, various methods have been tried such as using pigmented housings and injection moulded housings with various relief patterns. These relief patterns have usually comprised sharp-edged pyramids arranged in regular sequences one after the other in regular patterns intersecting sharp-edged ridges. As a rule, the projections of the relief pattern are on the light-emitting side and the other side is flat.

The pattern has thus consisted of solid prisms, which require a lot of material.

When designing the patterns, it is necessary to take into consideration how the material is to be applied to the mould. The patterns have therefore often been a compromise between production and technical considerations.

The present invention proposes a new and more simple method of producing sheets or housings with a relief pattern for distributing light in predetermined directions which is economical as regards the quantity of material used.

According to this invention therefore we provide a method of producing a relief patterned light transparent sheet from a sheet of thermoplastic material, comprising heating the sheet to a temperature at which it softens, and pressing the sheet against a mould having a plurality of sharp-edged projections and depressions therebetween with a pressure which is insufficient to force the sheet into the depressions but sufficient for the sharp edges of the projections to produce sharp edged deformations in the sheet.

The starting thermoplastic material, such as acrylic plastics sheet, in the form of a flat sheet, is heated to a temperature at which the sheet becomes plastic and is then forced onto a patterned mould surface having projecting elements for forming indentations in the light directing relief pattern desired, and depressions which form the raised parts of the light-directing relief pattern. A specific feature of the patterned mould surface is that it has sharp-edged junctions between the elements of the pattern which have junctions that are reproduced entirely in the moulded product only to the extent these consist of edges or sharp corners facing the sheet, while the depressions of the mould surface are only partly reproduced, as the thermoplastic sheet material, depending on the plasticity of the material, can be forced down into these depressions only to a certain extent.

With a mould surface built up of a plurality of sharp-edged bars resembling gear racks placed beside each other with the teeth of the adjoining bars displaced onehalf tooth in relation to each other, a finished product is thus obtained which on its light emitting side, i.e. the side facing the mould surface, has bands of prism-like projections arranged, with gently rounded ridges, separated from each other by depressions with sharp-edged junctions between the depressions delimiting the flanks.

The starting material is forced to adapt itself to the mould surface by means of a so-called free blowing process. By free blowing is meant that the material is deformed by being acted on by air or other flow jets which force the material into contact with the mould surface. The moulding can also be carried out simply by applying an increase in pressure behind the sheet, or through a reduction in pressure or a or vacuum a between the sheet and the surface.

Housings with bottom surfaces of the kind described above are moulded in the corresponding way in a female mould, the bottom surface of which has been given the pattern described, while the side edges of the mould may he smooth or rifled. Also these rifled side edges can be produced against sharpedged mould surfaces which are only partly reproduced and in the corresponding way give rise to rounded ridges separated by sharp-edged depressions.

As the inner corner edges of the female mould are only partly reproduced, the finished product has rounded junctions along all corner edges.

A preferred embodiment of the invention will now be described in more detail, with reference to the accompanying drawings in which: Figures I and 2 each show a section along the lines I-I and ll-ll of Figure 3.

Figure 3 is a plan view of a small portion of a mould member according to the invention.

Figure 4 is a part section showing how the moulded sheet material adapts itself to the mould surface.

Figure 5 shows a small portion of the mould surface.

Figure 6 shows the moulded sheet material from the light-emitting side, i.e. the side facing the mould surface, and Figure 7 shows the opposite side of the sheet of Figure 6.

A plurality of the gear rack formed parts 1 and 2 shown in Figures 1 and 2 are jointed together so that they form a mould surface of the kind shown in Figure 3, the teeth in the adjoining parts then being displaced in relation to each other. The different parts could be jointed together e.g. through the use of double-adhesive tape against a common carrier plate placed on the rear side.

In the example shown, the angle between the tooth flanks, designated in Figure 2, has been made 90 , as this gives a very good light-directing effect. For the same reason, the tooth width has been made equal to the tooth spacing. Other dimensions and angles are, of course, possible. As will be noted from the figures, all of the edges between the sides of the teeth and the flanks are sharp. This gives rise to sharp creases at the places 4 where the sheet material 5 is deformed over an edge (see Figure 4) while the creases will be considerably more rounded at the places 6 where the sheet material is forced down into depressions in the mould surface by a pressure behind the sheet.

In the finished product, the ridges along the tops of the projections will thus be rounded towards all sides, while the bottoms of the depressions will consist of comparatively sharp creases.

WHAT WE CLAIM IS: 1. A method of producing a relief patterned light transparent sheet from a sheet of thermoplastic material, comprising heating the sheet to a temperature at which it softens, and pressing the sheet against a mould have a plurality of sharp-edged projections and depressions therebetween with a pressure which is insufficient to force the sheet into the depressions but sufficient for the sharp edges of the projections to produce sharp edged deformations in the sheet.

2. A method as claimed in claim 1 wherein said sheet is pressed against said mould by a plurality of gas jets.

3. A method as claimed in claim 1 wherein said sheet is pressed against said mould by a difference in pressure created by a reduction in pressure at the mould face.

4. A mould when used for carrying out the method claimed in claims 1, 2 or 3 to produce relief patterned light transparent sheets, comprising a plurality of bars each with a plurality of equally spaced triangular teeth forming sharp-edged projections, the bars being mounted side by side with adjacent bars being displaced relative to one another by half-a-tooth width.

5. A mould as claimed in claim 4 wherein the apex angle of each tooth is 90".

6. A mould as claimed in claims 4 or 5, having several mould surfaces joined together to form an internally patterned female mould.

7. A mould as claimed in claim 6, wherein the bottom of the female mould is built up of bars resembling gear racks which are displaced one-half tooth in relation to each other, which in the finished product give rise to prism-like projections arranged in bands, while the side edges of the female mould only have sharp-edged grooves which extend in the vertical direction of the mould.

8. A method of producing a relief patterned light transparent sheet substantially as herein described with reference to the accompanying drawings.

9. A mould substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. contact with the mould surface. The moulding can also be carried out simply by applying an increase in pressure behind the sheet, or through a reduction in pressure or a or vacuum a between the sheet and the surface. Housings with bottom surfaces of the kind described above are moulded in the corresponding way in a female mould, the bottom surface of which has been given the pattern described, while the side edges of the mould may he smooth or rifled. Also these rifled side edges can be produced against sharpedged mould surfaces which are only partly reproduced and in the corresponding way give rise to rounded ridges separated by sharp-edged depressions. As the inner corner edges of the female mould are only partly reproduced, the finished product has rounded junctions along all corner edges. A preferred embodiment of the invention will now be described in more detail, with reference to the accompanying drawings in which: Figures I and 2 each show a section along the lines I-I and ll-ll of Figure 3. Figure 3 is a plan view of a small portion of a mould member according to the invention. Figure 4 is a part section showing how the moulded sheet material adapts itself to the mould surface. Figure 5 shows a small portion of the mould surface. Figure 6 shows the moulded sheet material from the light-emitting side, i.e. the side facing the mould surface, and Figure 7 shows the opposite side of the sheet of Figure 6. A plurality of the gear rack formed parts 1 and 2 shown in Figures 1 and 2 are jointed together so that they form a mould surface of the kind shown in Figure 3, the teeth in the adjoining parts then being displaced in relation to each other. The different parts could be jointed together e.g. through the use of double-adhesive tape against a common carrier plate placed on the rear side. In the example shown, the angle between the tooth flanks, designated in Figure 2, has been made 90 , as this gives a very good light-directing effect. For the same reason, the tooth width has been made equal to the tooth spacing. Other dimensions and angles are, of course, possible. As will be noted from the figures, all of the edges between the sides of the teeth and the flanks are sharp. This gives rise to sharp creases at the places 4 where the sheet material 5 is deformed over an edge (see Figure 4) while the creases will be considerably more rounded at the places 6 where the sheet material is forced down into depressions in the mould surface by a pressure behind the sheet. In the finished product, the ridges along the tops of the projections will thus be rounded towards all sides, while the bottoms of the depressions will consist of comparatively sharp creases. WHAT WE CLAIM IS:

1. A method of producing a relief patterned light transparent sheet from a sheet of thermoplastic material, comprising heating the sheet to a temperature at which it softens, and pressing the sheet against a mould have a plurality of sharp-edged projections and depressions therebetween with a pressure which is insufficient to force the sheet into the depressions but sufficient for the sharp edges of the projections to produce sharp edged deformations in the sheet.

2. A method as claimed in claim 1 wherein said sheet is pressed against said mould by a plurality of gas jets.

3. A method as claimed in claim 1 wherein said sheet is pressed against said mould by a difference in pressure created by a reduction in pressure at the mould face.

4. A mould when used for carrying out the method claimed in claims 1, 2 or 3 to produce relief patterned light transparent sheets, comprising a plurality of bars each with a plurality of equally spaced triangular teeth forming sharp-edged projections, the bars being mounted side by side with adjacent bars being displaced relative to one another by half-a-tooth width.

5. A mould as claimed in claim 4 wherein the apex angle of each tooth is 90".

6. A mould as claimed in claims 4 or 5, having several mould surfaces joined together to form an internally patterned female mould.

7. A mould as claimed in claim 6, wherein the bottom of the female mould is built up of bars resembling gear racks which are displaced one-half tooth in relation to each other, which in the finished product give rise to prism-like projections arranged in bands, while the side edges of the female mould only have sharp-edged grooves which extend in the vertical direction of the mould.

8. A method of producing a relief patterned light transparent sheet substantially as herein described with reference to the accompanying drawings.

9. A mould substantially as herein described with reference to the accompanying drawings.

10. A light transparent sheet produced

according to any one of claims 1 to 3 and claim 8.