GB2474367A

GB2474367A - A stretching machine for a variety of sizes of plastic sheet

Info

Publication number: GB2474367A
Application number: GB1017194A
Authority: GB
Inventors: John Meek Preston
Original assignee: STRETCHFORM ASSOCIATES Ltd
Current assignee: STRETCHFORM ASSOCIATES Ltd
Priority date: 2009-10-12
Filing date: 2010-10-12
Publication date: 2011-04-13
Anticipated expiration: 2030-10-12
Also published as: GB2474367B; EP2488345A2; WO2011045600A3; GB201017194D0; EA201270554A1; WO2011045600A2

Abstract

A stretching machine for stretching a variety of sizes of plastic sheet. A clamping assembly 105 is connected to a primary clamp 111' of an orthogonal clamping assembly 107, the connection 121 being removable to allow connection to a different primary clamp 111 so as to accommodate a different size of plastic sheet. Each clamping assembly may comprise a parking area located beyond the connection means where unused primary clamps 111 ", 111) ' can be retained. The number of discrete primary clamps can be tailored for a particular stretching process. Secondary elongate clamps 113 may be provided to ensure continuous clamping along the edge. Stretching and forming, especially vacuum forming, apparatus employing such clamping assemblies, as well as a method of configuring such machines, are disclosed, each of which provide for increased flexibility.

Description

Clamping Apparatus

Field of the Invention

The present invention relates to an improved clamping arrangement for a stretching machine and an associated method and, in particular, an improved stretching machine and method for configuring such machine for different sizes of plastic sheet.

Background of the Invention

In the field of thermoforming, and vacuum forming in particular, plastic sheets are heated then placed over a mould and clamped in place. A vacuum is then applied between the mould and the plastic sheet forcing the heated sheet to conform to the mould.

Such vacuum forming techniques are employed in the manufacture of various products, for example sanitary ware (e.g. baths, shower trays) and other products ranging from disposable cups and blister packs to vehicle dashboards and aircraft interiors.

The Applicants have found that products formed from pre-stretched thermoplastic sheet, whether stretched as an integral part of the vacuum forming process or having been stretched in a previous stretch-only process, exhibit greater mechanical strength than those formed from an un-stretched sheet of equivalent thickness.

However, stretching and forming apparatuses have to date been limited in the variety of sizes of plastic sheet which they can be used to stretch.

It is therefore an object of at least one embodiment of the present invention to provide a stretching machine for stretching a variety of sizes of plastic sheet.

Moreover, it is an object of at least embodiment of the present invention to provide a stretching and forming apparatus for stretching and forming a greater variety of sizes of plastic sheet.

Summary of the Invention

According to a first aspect of the present invention, there is provided a stretching machine for stretching a plastic sheet, the stretching machine comprising; a first pair of parallel clamping assemblies, and a second pair of parallel clamping assemblies, orthogonal to the first pair of parallel clamping assemblies, each clamping assembly comprising a plurality of primary clamps to clamp the plastic sheet at a corresponding plurality of locations around its edges; connection means to connect one of the first clamping assemblies to a primary clamp of one of the second clamping assemblies such that movement of the said first clamping assembly causes the said primary clamp of the second clamping assembly to move along the said second clamping assembly; wherein the connection means is removable from the said primary clamp of the second clamping assembly such that the said first clamping assembly may be connected to a different one of the primary clamps of the said second clamping assembly so as to accommodate a different size of plastic sheet.

The stretching machine can accommodate different sizes of plastic sheet by allowing the orthogonal clamping assemblies to be connected to a suitable primary clamp for the size of material. This also allows the number of primary clamps, each of which provide a discrete clamping, for a particular stretching process to be tailored.

Most preferably, the stretching machine comprises a plurality of connection means such that each clamping assembly is removably connected to a primary clamp of an orthogonal clamping assembly.

The plurality of connection means provides the flexibility in size around the entire periphery of the plastic sheet to be stretched.

Most preferably, the remaining primary clamps of each clamping assembly are freely moveable along the length thereof.

Free movement of the remaining primary clamps ensures that as the plastic material is stretched, in a direction parallel to the length of the clamping assembly, they are able to move so as to minimise scalloping of the material (while preventing the plastic sheet slipping out as it is stretched in an orthogonal direction).

Preferably, each clamping assembly comprises a parking area located beyond the connection means where unused primary clamps can be retained.

This means that when, for example, a small plastic sheet is to be stretched, one or several primary clamps may be moved along the clamping assembly beyond what will be the starting position of the orthogonal clamping assembly to which the extreme remaining primary clamp will be connected.

Preferably, the machine further comprises a plurality of elongate clamps arranged to clamp the plastic sheet around its perimeter.

The elongate clamps allow the plastic sheet to be clamped entirely around the perimeter, rather than just the discrete clamping provided by the primary clamps. This may be necessary, or simply preferred, for subsequent processing steps such as vacuum forming or cooling.

Preferably, each of the clamping assemblies comprises a secondary clamp, the secondary clamp being elongate such that when moved into contact with the plastic sheet a continuous clamping is provided along the edge thereof.

Providing the elongate clamps on the clamping assemblies ensures that regardless of the starting or finishing size of the plastic sheet the continuous clamping is provided in the appropriate location.

Optionally, the machine further comprises cooling means to cool the plastic sheet prior to releasing the clamps.

This allows the plastic sheet to be set, in its stretched condition, once the desired material size is achieved.

According to a second aspect of the present invention, there is provided a stretching and forming apparatus for stretching and vacuum forming a plastic sheet, the stretching and forming apparatus comprising: a stretching machine in accordance with the first aspect; and a vacuum forming machine configured to cooperate with the stretching apparatus such that a plastic sheet can be stretched and then vacuum formed in a single process.

Most preferably, the stretching and forming apparatus comprises elongate clamps to hold the stretched plastic sheet around its perimeter during vacuum forming.

Preferably, the stretching machine comprises the elongate clamps.

According to a third aspect of the present invention, there is provided a stretching apparatus for stretching a plastic sheet, the stretching apparatus comprising: a stretching machine in accordance with the first aspect; and an oven to heat the plastic sheet prior to stretching.

Optionally, the stretching apparatus further comprises cooling means to cool the plastic sheet prior to releasing the primary clamps.

According to a fourth aspect of the present invention, there is provided a stretching and forming apparatus for stretching and vacuum forming a plastic sheet, the stretching and forming apparatus comprising: a stretching machine in accordance with the first aspect; an oven to heat the plastic sheet prior to stretching; and a vacuum forming machine configured to cooperate with the stretching apparatus such that a plastic sheet can be heated, stretched and then vacuum formed in a single process.

According to a fifth aspect of the present invention, there is provided a method of configuring a machine or apparatus of any of the first to fourth aspects so as to accommodate a particular size of plastic sheet, comprising the steps of: (a) removing the connection means from the said primary clamp; (b) selecting a different primary clamp of the said second clamping assembly dependent on the size of the plastic sheet; and (c) connecting the said first clamping assembly to the said different primary clamp.

Optionally, the step of selecting a different primary clamp includes moving the said primary clamp beyond the connection means to a parking area of the said second clamping assembly.

Alternatively, the step of selecting a different primary clamp includes moving the said different primary clamp from a parking area of the said second clamping assembly to the connection means.

Brief Description of the Figures

The present invention will now be described by way of example only and with reference to the accompanying figures in which: Figure 1(a) to Figure 1 (j) illustrates in schematic form the process by which an apparatus, in accordance with at least one embodiment of the present invention, stretches and forms a plastic sheet; Figure 2 illustrates in schematic form the relative arrangement of clamping assemblies, in accordance with at least one embodiment of the present invention; Figure 3 presents a first schematic view of a clamping assembly of the apparatus shown in Figures 1(a) to (j) in accordance with at least one embodiment of the present invention; Figure 4 presents a second schematic view of the clamping assembly, showing a corner section of the apparatus shown in Figures 1(a) to (j), in accordance with at least one embodiment of the present invention; Figure 5 presents a third schematic view of the clamping assembly, more clearly showing the arrangement of the secondary clamp or clamp bar in relation to the primary clamps, in accordance with at least one embodiment of the present invention; Figure 6 presents a fourth, enlarged, schematic view of the clamping assembly, showing in more detail the connection piece in relation to the primary clamps, in accordance with at least one embodiment of the present invention; Figure 7(a) and (b) presents alternative schematic views of a clamping assembly showing a number of primary clamps that have been parked beyond the connection piece to allow the machine to accommodate a smaller plastic sheet; (a) before; and (b) after, a stretching step; and Figure 8 illustrates in schematic form an elongate arrangement of primary clamps along a clamping assembly, in accordance with at least one embodiment of the present invention.

Detailed Description of the Invention

With reference to Figures 1(a) to (j), and with further reference to Figures 2 to 7, there is presented an apparatus 101 that functions to stretch and form a plastic sheet 103 as described in detail below. The apparatus comprises a rectangular stretching frame comprising first 105 and second 107 orthogonal pairs of opposed elongate clamping assemblies arranged around the periphery of the apparatus 101.

A drawboard 109, upon which the plastic sheet is placed, is central to the apparatus 101. The drawboard 109 is heated to prevent any unwanted cooling of the plastic sheet 103, which will generally have been pre-heated in an oven prior to placement on the drawboard 109. The plastic sheet 103 lies on the drawboard 109 while clamping takes place so as to avoid sagging or indeed to prevent the plastic sheet 103 from falling through the stretching frame (105,107) onto the floor or any components arranged underneath.

The clamping assemblies 105,107 are arranged such that one end of each assembly abuts an edge of an orthogonal assembly, effectively forming a ring around the drawboard 109 which can be resized by relative movement of each clamping assembly 105,107 along said orthogonal assembly edge. This is illustrated schematically in Figure 2 which shows the position of the clamping assemblies 105,107 before (shaded) and after (hatched) an exemplary stretch takes place (stretching plastic sheet from original size 103 to stretched size 103'). The arrows indicate the relative movement of the clamping assemblies 105,107 during such a stretch (in practice, as indicated, one clamping assembly 1 07a is usually fixed in place).

Each clamping assembly 105,107 consists of a primary clamping mechanism and a secondary clamping mechanism, effected by a plurality of discrete primary clamps 111 and an elongate secondary clamp 113 (examples of which are indicated in Figure la and Figure lj). An actuator, in the form of a piston (an example of which is indicated in Figure 1 b), acts on each clamping assembly 105,107 which is hinged via the primary clamps 111. Figure 3, for example, illustrates the relationship between the primary 111 and secondary clamps 113 and the opposing faces 111 a,1 11 b of the primary clamps 111 between which the plastic sheet is held. In this example, the lower face 111 b comprises a raised, generally key-shaped portion 112; which engages against the upper face 111 b of the primary clamp between which the plastic sheet is gripped when the upper and lower faces are brought together. This arrangement is one of several arrangements that provide efficient clamping of the plastic sheet.

Additionally, the secondary clamp 113 can be seen to comprise a flat, elongate edge. This edge may be urged into contact with the elongate surface 114 situated in front of the primary clamps 111 SO as to provide, in contrast to the primary clamps 111, a continuous clamping along a whole edge of a thermoplastic sheet.

Figures 1(a) through (d) illustrate how the primary clamps 111 engage the plastic sheet 103. Pistons 115 associated with an opposing pair of clamping assemblies 107 move the primary clamps 111 from an open position (Figure 1(a)) to a closed position (Figure 1(b)), whereupon the primary clamps 111 clamp opposing edges of the plastic sheet 103 at a number of discrete locations along its length. Once the first pair of clamping assemblies 107 has been rotated into place, the second, orthogonal pair of clamping assemblies 105 is similarly rotated into place (Figures 1(c) and (d)). At this stage, the plastic sheet 103 is now clamped at a number of discrete locations around its perimeter.

Figure 1(e) shows the plastic sheet being stretched by relative movement of the clamping assemblies 105,107 outwards from the starting position.

In the illustrated example, a 700mm by 700mm plastic sheet is stretched to a size of 1000mm by 1000mm. The primary clamps 111 are slideably mounted in the clamping frame and move independently of one another.

This independent movement combined with the discrete nature of the individual clamps 111 allows the sheet to stretch uniformly and without significant scalloping as the clamping assemblies 105,107 are urged apart; the primary clamps 111 moving along the length of the respective clamping assembly 105,107 by the expansion along the edge of the plastic sheet 103 as it is stretched.

Once the desired sheet size is reached, actuators move the secondary clamps 113 from their starting/open position to a closed position (Figure 1(f)) to effectively lock the plastic sheet 103 in its stretched configuration.

As stated above, the clamping effected by the secondary clamp 113 is a continuous clamping around the entire perimeter of the plastic sheet 103, and ensures the stretched sheet 103 does not come free during the following forming step.

At this stage, the drawboard 109 is lowered away from the stretched sheet, and a vacuum forming mould moved into place under the plastic sheet. The mould is then raised up into the sheet until it reaches a desired height (Figure 1(g)). At this stage an optional bubble or pre-stretch step (not illustrated) can be employed, whereby air is blown from the mould creating a bubble which stretches the plastic sheet 103 and ensures an even thickness distribution across the sheet 103 during forming. The bubble step is particularly applicable to deep draw forms, not so much to shallower forms such as the shower tray illustrated.

Any air between the plastic sheet 103 and the mould is then evacuated using a vacuum pump connected to the mould and which extracts air via a number of holes in the mould. This is the vacuum forming step and causes the plastic sheet 103 to conform to the shape of the mould (Figure 1(h)). High speed fans then blow directly onto the surface of the plastic sheet 103, by way of pipes/ducting (e.g. 117), so as to rapidly cool to a temperature at which it can be released from the mould without damage or deformation. Air is then blown back through the mould, to free the formed sheet from the mould 103. The mould is then moved down and away from the plastic sheet 103 and the clamping assemblies 105,107 are released (Figures 1(i) and (j)).

Typically, the thermoplastic material used (particularly for bathroom furnishings) is acrylic, however the method and apparatus will work equally well with a variety of other thermoplastic materials such as ABS, PTFE and other fluoroplastics, TEFLON® and KYDEX®, to name but a small sample.

With reference to Figure 6, a connection piece 121 is shown which connects the extreme primary clamp (e.g. 111') of one clamping assembly 107 to an orthogonal clamping assembly 105. Thus as said orthogonal clarriping assembly 105 is urged outwards (in the direction of arrow 123), the extreme primary clamp 111', which will clamp on or near a corner of the plastic sheet (and in this example provides a clamping point in line with the primary clamps of the orthogonal clamping assembly 105) will cause the plastic sheet to stretch in the direction of movement thereof (as indicated by arrow 123').

However, the primary clamps 111 are of a particular size (in this example, the primary clamps are approximately 40mm wide) which means that for a machine having a set number of clamps there is a minimum practical limit on the size of plastic sheet that can be stretched. By way of example, it is recommended to use one clamp per 60mm of plastic sheet on the illustrated machine. Therefore a machine stretching a 600mm plastic sheet will have 11 primary clamps. However, with a clamp size of 40mm, the smallest possible starting sheet size would be around 400mm (although would necessarily be slightly larger to account for manufacturing tolerances etc.).

To accommodate smaller sheet sizes one might suggest using less clamps per unit distance; however this will potentially compromise the benefits of using the greater number of primary clamps in terms of reducing scalloping and ensuring the plastic sheet undergoes a uniform stretch.

Instead, the connection piece 121 connecting the extreme primary clamp 111' to the orthogonal clamping assembly 105 is removeable from the extreme primary clamp such that the extreme primary clamp, and as many more primary clamps as are required, can be slid beyond the orthogonal clamping assembly 105 and the connection piece 121 used to join one of the remaining primary clamps 111 to the orthogonal clamping assembly.

See Figure 7(a) and 7(b) which illustrate the situation where two primary clamps 111 ",l 11" (top left of Figure 7(a)) have been parked at one end of the clamping assembly so as to accommodate a smaller sheet of material.

Likewise, the orthogonal clamping assembly has a number of primary clamps (top right, indicated by reference numerals 112) which have been similarly parked.

With reference to Figure 6 again, the connection piece 121 comprises a shaped portion 131 adapted to engage a correspondingly-shaped indentation 132 on the extreme primary clamp 111' (or indeed any other primary clamp 111 as required); thus outwards movement of the orthogonal clamping assembly 105 (i.e. in a direction 123 parallel to the direction of movement of the primary clamp 111,111'), in use, causes the corner of the plastic sheet clamped by the extreme primary clamp 111' to move a corresponding distance in the same direction. Thus, the plastic sheet is stretched. The same arrangement applies for the other clamping assemblies 105,107.

Should a smaller plastic sheet be used, it may be desirable to park the extreme primary clamp and use the neighbouring primary clamp instead, in which case the connection piece 121 is removed from the extreme primary clamp. This is achieved for example by rotating or lifting the connection piece 121 away such that the shaped portion 131 exits the indentation 132 on the primary clamp. Once freed, the primary clamp is slid into the "parking area", the next primary clamp slid up and the connection piece can then be rotated or lowered back into position such that the shaped portion 131 engages the indentation 132 of the new extreme primary clamp.

The connection provided by the shaped portion 131 and the corresponding indentation 132 may be sufficiently secure, particularly if the downward force holding the primary clamp closed is also able to hold the connection.

However, it is envisaged that said connection may be further secured by clips, bolts, screws, pins or any other of a host of known securing means that require manual intervention to remove them. Alternatively, it is envisaged that a remotely controlled securing means, under the control of a computer operably connected thereto, might ensure the connection means can only be removed and/or put in place when one or more operational checks have been passed.

It is to be noted that at least two types of machine are envisaged which may take advantage of the repositionable, or parkable, primary clamps.

Firstly, a stretching and forming machine which is capable of stretching and vacuum forming a plastic sheet, and secondly a stretching machine (i.e. no vacuum forming) which is used for stretching only. It will be readily apparent that a stretch only machine would work without the need for secondary clamps and so the present invention should not be considered as being so limited. An oven, for heating the plastic sheet prior to stretching, may also be included so as to provide an apparatus that can heat and subsequently stretch a plastic sheet, and optionally vacuum form the stretched sheet, in a single process. Furthermore, cooling means (for example fans or blowers) may be provided to cool the plastic sheet after stretching (whether a forming step is performed or not).

Throughout the specification, unless the context demands otherwise, the terms comprise' or include', or variations such as comprises' or comprising', includes' or including' will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

Further modifications and improvements may be added without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS: 1. A stretching machine for stretching a plastic sheet, the stretching machine comprising; a first pair of parallel clamping assemblies, and a second pair of parallel clamping assemblies, orthogonal to the first pair of parallel clamping assemblies, each clamping assembly comprising a plurality of primary clamps to clamp the plastic sheet at a corresponding plurality of locations around its edges; and connection means to connect one of first clamping assemblies to a primary clamp of one of the second clamping assemblies such that movement of the said first clamping assembly causes the said primary clamp of the second clamping assembly to move along the said second clamping assembly; wherein the connection means is removable from the said primary clamp of the second clamping assembly such that the said first clamping assembly may be connected to a different one of the primary clamps of the said second clamping assembly so as to accommodate a different size of plastic sheet.
2. The stretching machine of claim 1, further comprising a plurality of connection means such that each clamping assembly is removably connected to a primary clamp of an orthogonal clamping assembly.
3. The stretching machine of claim I or claim 2, wherein the remaining primary clamps of each clamping assembly are freely moveable along the length thereof.
4. The stretching machine of any of claims 1 to 3, wherein each clamping assembly comprises a parking area located beyond the connection means where unused primary clamps can be retained.
5. The stretching machine of any preceding claim, wherein the machine further comprises a plurality of elongate clamps arranged to clamp the plastic sheet around its perimeter.
6. The stretching machine of any of claims 1 to 4, wherein each of the clamping assemblies comprises a secondary clamp, the secondary clamp being elongate such that when moved into contact with the plastic sheet a continuous clamping is provided along the edge thereof.
7. The stretching machine of any preceding claim, wherein the machine further comprises cooling means to cool the plastic sheet prior to releasing the clamps.
8. A stretching and forming apparatus for stretching and vacuum forming a plastic sheet, the stretching and forming apparatus comprising: a stretching machine in accordance with any of claims 1 to 7; and a vacuum forming machine configured to cooperate with the stretching apparatus such that a plastic sheet can be stretched and then vacuum formed in a single process.
9. The stretching and forming apparatus of claim 8 when dependent on claim 5 or claim 6, wherein the elongate clamps are configured to clamp the stretched plastic sheet around its perimeter during vacuum forming.
10. A stretching apparatus for stretching a plastic sheet, the stretching apparatus comprising: a stretching machine in accordance with any of claims 1 to 7; and an oven to heat the plastic sheet prior to stretching.
11. The stretching apparatus of claim 11, further comprising cooling means to cool the plastic sheet prior to releasing the primary clamps.
12. A stretching and forming apparatus for stretching and vacuum forming a plastic sheet, the stretching and forming apparatus comprising: a stretching machine in accordance with any of claims 1 to 7; an oven to heat the plastic sheet prior to stretching; and a vacuum forming machine configured to cooperate with the stretching apparatus such that a plastic sheet can be heated, stretched and then vacuum formed in a single process.
13. A method of configuring a machine or apparatus according to any preceding claim to accommodate a particular size of plastic sheet, comprising the steps of: (a) removing the connection means from the said primary clamp; (b) selecting a different primary clamp of the said second clamping assembly dependent on the size of the plastic sheet; and (c) connecting the said first clamping assembly to the said different primary clamp.
14. The method of claim 13, wherein the step of selecting a different primary clamp includes moving the said primary clamp beyond the connection means to a parking area of the said second clamping assembly.
15. The method of claim 13, wherein the step of selecting a different primary clamp includes moving the said different primary clamp from a parking area of the said second clamping assembly to the connection means.