GB2242157A

GB2242157A - Corrugated sheet

Info

Publication number: GB2242157A
Application number: GB9102650A
Authority: GB
Inventors: Robert Jeffrey Mabbutt
Original assignee: DOLEPORT Ltd
Current assignee: DOLEPORT Ltd
Priority date: 1990-03-20
Filing date: 1991-02-07
Publication date: 1991-09-25
Also published as: GB9006215D0; GB9102650D0

Abstract

A corrugated thermoplastics sheet has a main section and two end sections with major corrugations extending along the whole of its length. Minor corrugations are incorporated in one or both of the end sections but do not extend along the main section. The major corrugations are formed by bending preceded by local heating. The minor corrugations are formed by pressing after local heating of the end section. <IMAGE>

Description

CORRUGATED THERMOPLASTICS SHEET AND PROCESS FOR ITS PRODUCTION The invention relates to corrugated thermoplastics sheets and to methods for their manufacture.

It is known to produce corrugated thermoplastics sheet by extrusion or vacuum forming techniques and also to bend a flat sheet by heating the sheet locally and to bend the sheet in the locally softened area or areas. Extrusion processes are expensive to set up and are therefore suitable only for very large production volumes, while vacuum forming techniques involve stretching and consequent thinning of parts of the sheet and also result in unattractive products because of ripples forming in the stretched sheet. Local heating is ideal in many situations but can give rise to problems when the corrugated sheet is required to adjoin and overlap with a further sheet which has a complex shape such as a steel roofing sheet having both major and minor corrugations because the local heating and bending process is not suitable for closely adjacent bends.

Throughout this specification when we refer to ends of the sheet we are treating the direction of the corrugations as the longitudinal direction so that the corrugations run in a direction between the ends. This is regardless of whether the distance from end to end is greater than or less than the transverse direction which is considered as the width of the sheet.

An object of the invention is to provide an improved corrugated thermoplastics sheet or method of manufacturing such a sheet which is suitable for matching with another sheet of relatively complex shape.

According to one aspect of the invention a corrugated thermoplastics sheet has a main section and two end sections having major corrugations extending along the whole length thereof through the main section and the end sections and also incorporates minor corrugations extending parallel to the major corrugations along at least one end section but not along the main section.

Preferably minor corrugations extend along both end sections.

According to a second aspect of the invention, a process for producing corrugated sheet includes the steps of: heating a flat thermoplastics sheet locally along lines of desired bends to soften the sheet along these lines, bending the sheet along these lines to provide major corrugations extending in an end to end direction of the sheet, these corrugations being in the form of flat sections extending between the bends, and supporting the sheet in this bent form while the bends set, heating at least part of at least one end portion of the sheet after forming the major corrugations therein so as to soften the end portion, and forming the softened end portion of the sheet so as to form minor corrugations therein.

Preferably the corrugations are formed in the end section by pressing the end section between two formers.

The resulting sheet has a main section which can be attractive in appearance due to flat undistorted areas between the bends and yet which at its end sections can match the shape of a more complexly bent sheet such as a steel roofing sheet.

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 corresponding drawings in which: Figure 1 is a plan view of one embodiment of a corrugated thermoplastics sheet conforming to and produced by the process according to the present invention; Figure 2 is an end elevation in the direction of the arrow II of Figure 1; Figure 3 is a cross-section along the line III-III of Figure 1; Figure 4 is a diagrammatic cross-section through an apparatus for carrying out part of the process of the invention, showing a thermoplastics sheet undergoing various process steps; and Figure 5 is a diagrammatic cross-section through an apparatus for carrying out a further part of the process of the invention.

The figures show a sheet 10 of thermoplastics material such as polyvinyl chloride. As shown in Figure 4, a flat sheet is heated in a heating station 11, the heating station incorporating four longitudinally extending heating elements 12, 13, 14 and 15 in contact with the sheet 10. Each heating element is constituted by an electrically heated metal blade and extends the whole length of the sheet 10, and is thus capable of heating the sheet along a line extending for the full length of the sheet. As shown in Figure 4, the two outer heating elements 12 and 15 are positioned above the sheet 10 while the inner heating elements 13 and 14 are positioned below the sheet 10. On the opposite side of the sheet 10 from each of the heating elements 12 to 15 there are metal supports 16, 17, 18 and 19 which are arranged to contact the sheet either side of the respective heating element.Each support thus acts as a heat sink which tends to conduct away from the sheet some of the heat which otherwise would be conducted along its surface to regions where heating is not required.

In use, the sheet 10 is positioned as shown and the heaters 12 to 15 are operated simultaneously to provide local lines of heating along the length of the sheet.

When the required extent of heating has been achieved, the heating elements and supports are retracted as indicated by arrows such as 20 to prevent further heating and to free the sheet. The sheet is then indexed so that the part of the sheet which was in the heating station 11 is moved to a bending station 21 where it lies between a male former 22 and a female former 23, both corresponding to the desired shape for a major corrugation in the sheet. Figure 4 shows the situation after the male former 22 has been brought down to the female former 23, bent a major corrugation 24 into the sheet forming all the bends of the corrugation simultaneously, held the sheet accurately in its corrugated form until the sheet has set, and then been retracted.Figure 4 shows that the spacing between the horizontal heating elements 13 and 14 is Y which corresponds to the distance across the base of the former and to the length of a horizontal base section of corrugated sheet as shown. Similarly, the distances X between heating elements 12 and 13 and between heating elements 14 and 15 correspond to the lengths X of inclined ramp portions of the former and the corresponding inclined ramp portions in the completed corrugated sheet. Thus, with the correct indexing distance, the heated and softened sections along the lines of heating are brought into the desired positions for the bends in the sheet necessary to form a major corrugation 24.The degree of heating along the lines is such that the sheet on the outside of the bend is sufficiently soft to stretch plastically while the sheet is bent while the sheet at the inside of the bend retains sufficient rigidity to help to prevent the whole sheet from stretching at the bend.

While the previously heated part of the sheet is being corrugated and is setting in the bending station 21, the heating elements 12 to 15 are caused to heat the sheet along four further lines which will correspond to the positions of subsequent bends. The heating should be so controlled that by the time the new lines for bending have been heated to a sufficiently soft condition the previously established corrugation 24 has set at least sufficiently to allow gentle handling without deformation.

Figures 1 and 2 show that an end section 27 of the sheet also incorporates minor corrugations 26 which are smaller than the major corrugations 24 and extend parallel to the major corrugations over the end section only and not along the main section of the sheet. The minor corrugations are in two different forms, some having three angles and others having four angles.

The minor corrugations are formed in the sheet by the apparatus shown in Figure 5. Figure 5 shows an upper former 28 and a lower former 29 which are shaped to conform to the required shape of the sheet as a whole including both the major and the minor corrugations. In order to form the minor corrugations in the end section of the sheet, this end section is heated sufficiently to soften it. The end section is then placed between formers 28 and 29 which are then closed on to the sheet.

The formers are unheated so the sheet cools by heat transfer to the formers and sets with a shape incorporating the minor corrugations as well as the major corrugations. The additional length of sheet required to produce the minor corrugations is produced by local stretching. The formers are then separated and the sheet is removed. Generally, the whole of the end section of the sheet is heated in which case minor adjustments to the shape of the major corrugations may also occur.

However, it would be possible to heat the sheet more locally in the region of the minor corrugations but not in the region of the major corrugations but in this case, measures should be taken to ensure that the formers do not damage the sheet in the region of the major corrugations if they are not in exact conformity with the formers.

For a typical sheet length of 1000 to 2000 mm the formed end section should have a length of about 50 to 300 mm.

The formers themselves may incorporate transitional shapes to help a smooth transition from the minor corrugations in the end section to corresponding flat parts of sheet in the main section but particularly with small corrugations, a relatively smooth transition may occur naturally by some stretching of the sheet and deformation a short distance beyond the formers.

Typically the sheet is transparent and is used as a roof light in a roof which is clad primarily in corrugated steel sheet. The steel sheet has minor corrugations as well as major corrugations for stiffness and strength.

In a typical pitched roof, a transparent sheet in accordance with the invention intended as a roof light will have its upper end lying under an end of a corrugated steel sheet and its lower end laying over an end of another corrugated steel sheet. Minor corrugations are necessary when the roof light lies over the steel so that the roof light can bed down over the steel sheet and not be held proud by its minor corrugations. Where the roof light is under the steel sheet, minor corrugations in the roof light are not so important and they may be omitted.

The thickness of the thermoplastics sheet tends to be much greater than that of steel sheet with which it is used and because of this, some adjustment may be desirable to the size of the major corrugations so that one size is used for fitting over steel sheet and another size is used for fitting under steel sheet. Compensation to this degree can be incorporated by modifying the major corrugations at the same time as the minor corrugations are formed.

Roof lights provided by sheets as described above have large flat sections which allow light to pass through without distortion and also have end sections which conform accurately for matching with other types of sheet.

Claims

1. A corrugated thermoplastics sheet having a main section and two end sections and having major corrugations extending along the whole length thereof through the main section and the end sections and also incorporating minor corrugations extending parallel to the major corrugations along at least one end section but not along the main section.

2. A corrugated thermoplastics sheet as claimed in Claim 1 and having minor corrugations extending along both end sections.

3. A corrugated thermopltstics sheet substantially as described with reference to and as illustrated by Figures 1 and 2 of the accompanying drawings.

4. A process for producing corrugated sheet including the steps of: heating a flat thermoplastics sheet locally along lines of desired bends to soften the sheet along these lines, bending the sheet along these lines to provide major corrugations extending in an end to end direction of the sheet, these corrugations being in the form of flat sections extending between the bends, and supporting the sheet in this bent form while the bends set, heating at least part of at least one end portion of the sheet after forming the major corrugations therein so as to soften the end portions, and forming the softened end portion of the sheet so as to form minor corrugations therein.

5. A process according to Claim 4 wherein the minor corrugations are formed by pressing the end section between two formers.

6. A process as claimed in Claim 4 or Claim 5 wherein all the bends constituting one major corrugation are effected simultaneously following simultaneous heating along all the bend lines required for that major corrugation.

7. A process according to and one of Claims 4 to 6 wherein simultaneously with forming the minor corrugations the shape of the end portions of the major corrugations is adjusted in the pressing step.

8. A process according to any one of Claims 4 to 7 wherein minor corrugations are formed at both end portions of the sheet.

9. A process for producing corrugated sheet substantially as hereinbefore described with reference to substantially as hereinbefore described with reference to the accompanying drawings.