WO1999025515A1

WO1999025515A1 - Vacuum press for pressing thermoplastic membrane onto an article

Info

Publication number: WO1999025515A1
Application number: PCT/AU1998/000957
Authority: WO
Inventors: Donald John Mccormack; David James Davis
Original assignee: Donald John Mccormack; David James Davis
Priority date: 1997-11-14
Filing date: 1998-11-13
Publication date: 1999-05-27
Also published as: AUPP036997A0

Abstract

A vacuum press of the kind comprising an open topped chamber (7) in which a substrate article (9) may be placed, sealing means to engage a peripheral margin of a thermoplastics lamina (11) so that the lamina extends across the chamber as a sealed closure for the open top thereof, heating means (14) above the lamina to heat and soften the lamina, and vacuum inducing means (15, 18, 19) to evacuate a space below the lamina between it and the substrate article and thereby cause or allow the heated lamina to be pressed into conformation with the substrate article, is characterised in that the chamber is provided with a lid (8) able to seal with a peripheral margin of the lamina, in that the lid is adapted to withstand evacuation of a space above the lamina between it and the lid, in that the vacuum inducing means may operate to withdraw air from the space above and the space below the lamina, and in that vent means (20) are provided to break the vacuum in the space above the lamina when it is to be pressed into conformity with the substrate article. A method is also claimed.

Description

VACUUM PRESS FOR PRESSING THERMOPLASTIC MEMBRANE ONTO AN ARTICLE.

TECHNICAL FIELD

This invention relates to methods and apparatus for causing thermoplastics sheets to conform to at least part of the surface of a substrate article.

More particularly, the invention relates to such methods and apparatus wherein the sheet is held adjacent to, but spaced from, the article, heated to cause it to soften, and the soft sheet is pressed onto the substrate article, usually by atmospheric pressure, following evacuation of the space between them. It is usual for the press to be oriented so that the sheet is initially positioned in a horizontal plane with the article underneath it, and in the following that is assumed to be the case. Thus for descriptive convenience terms such as "above", "below", "upper", "lower", horizontal" and others of like positional or attitudinal intent are used accordingly, although the invention is not limited to presses in any particular orientation

Apparatus of the kind in question are frequently referred to as vacuum presses, less frequently they are called Thermo Transfer Machines or Membrane Presses.

BACKGROUND ART

Prior known vacuum presses have typically comprised an open topped chamber in which the substrate article may be placed, sealing means to engage a peripheral margin of a thermoplastics sheet so that the sheet extends across the chamber as a sealed closure for the open top thereof, heating means above the sheet to heat and soften the sheet and vacuum inducing means to evacuate the space within the chamber, that is to say, the space between the sheet and the substrate article, and thereby cause the sheet to be pressed into conformation with the substrate article by atmospheric pressure. The term "evacuate", as used herein, implies the production of a high degree of vacuum, effective for the purposes described herein, but not necessarily a virtually complete vacuum such as found in outer space or electronic vacuum tubes.

Usually the heating means are supported by a lightly constructed lid covering the chamber. The lid is spaced above the sheet, except at its perimeter where it engages the peripheral margin of the sheet. The perimeter of the lid and the rim of the chamber are furnished with sealing means which prevent substantial in- flow of air into the chamber below the sheet. The heating means are often radiant elements, in which event the lid may function as a reflector. Alternatively the heat may be applied to the film by hot air injection or by contact with a hot surface.

Frequently, a vacuum pressing operation is carried out to enable the sheet to be permanently bonded to the article to constitute an ornamental or protective surface layer thereon. In that event the article may first be coated with an adhesive. Such operations are usually referred to as laminating the article or effecting lamination of the article. In other instances vacuum pressing is carried out to produce a shaped sheet product, in which event the "article", as that term is used herein, may be a reusable mould or pattern and a parting layer or film may be interposed between the article and the thermoplastics sheet. For descriptive convenience the term " substrate article" as used herein includes within its scope multi-component articles, for example articles having a surface layer of adhesive or a parting layer thereon.

Thus articles destined for lamination may be loaded into the press chamber, either on packing pieces or directly onto its floor or deck. The floor or deck is pierced by a plurality of ports connected to a vacuum pump or vacuum vessel or other vacuum inducing means.

A suitable thermoplastic film, membrane, sheet or other impervious web (all of which are embraced by the term "lamina" as used herein), may then be placed across the open top of the chamber. At this point the peripheral margin of the lamina may be lightly clamped by the lid into contact with an air tight seal rimming the open top of the chamber.

Heat is then applied to the thermoplastic lamina.

When the lamina reaches the required temperature, the vacuum inducing means are activated to draw the air from the chamber, and therefore from the space between the lamina and the article. In addition to this, some vacuum presses include means to apply a positive pressure to the upper surface of the lamina to increase the penetration of the lamina into any cavities in the surface of the article, or to increase the contact pressure between the lamina and any adhesive which may be on the article. Some presses use a flexible blanket to transmit both heat and pressure to the lamina.

A major problem encountered with known presses is that before the vacuum inducing means can evacuate substantially all of the air from below the lamina, the growing differential pressure across the lamina may cause it to move onto and around the article, thus trapping air in any such cavities. This leads to a less than faithful reproduction of the shape of the article by the lamina, and voids between the article surface and the plastics lamina applied to it. This problem is exacerbated if the article is non-porous, or, if porous, has a non-porous coating on its underside. Indeed this restricts the usefulness of prior known vacuum presses in that they cannot laminate certain recessed profiles of non-porous articles.

Currently, some vacuum presses apply more positive pressure above the lamina in attempts to overcome this problem, but this complicates the structure and operation of the press and in any event is not fully effective.

Likewise, some prior known vacuum presses have difficulty with objects which are largely porous but are sealed on their underside, such as kitchen cupboard doors which have a melamine (or similar) coating on the opposite side to that which has to be laminated. In such cases some presses require that the sealed surface be pre-drilled before the vacuum forming, so that the vacuum can draw the air through the porous article. Such pre-drilling is unacceptable to many end users.

Figure 1 herewith illustrates the above mentioned deficiency of the prior art. It is a fragmentary sectional view of a wooden cupboard door 1, having a non-porous back surface, in a prior art vacuum press, showing where a thermoplastics lamina 2, has failed fully to enter recesses 3, 4 and 5 in the front surface of the door.

A second problem sometimes encountered in the prior art, is that whilst the thermoplastics lamina is being heated, it distorts and sags. If it touches the article then at the points of contact the lamina is cooled locally, so that when the vacuum is applied a variation in the extension of the lamina is caused. Further, the crest of each "wave" thus produced in the lamina may reach unacceptable temperatures and thus spoil the surface finish.

Some known vacuum presses address the last mentioned problem by supporting the lamina in a frame, which holds the lamina away from the article during the heating process. Such machines are necessarily more complicated and therefore more expensive to build than more usual presses.

A third problem met when using prior art presses, relates to the lamination of porous articles which are coated with hot melt adhesive. The application of such adhesive is a critical operation in that the adhesive layer must be thin enough to allow air to pass through the adhesive into the porous article, or between the lamina and the adhesive on near approach of the lamina to the article, and yet thick enough to provide adequate adhesion.

Another disadvantage of prior art presses arises from the need to ensure that the lamina is hot and pliable when it is pressed against the surface of the article and to ensure that the lamina is hot enough to activate any heat activated adhesive that may be used. In prior known machines it has been necessary to delay commencement of the operation of the vacuum inducing means until the lamina reaches that condition. Thus prior known presses employ very large vacuum pumps or vacuum vessels (vacuum reservoirs) so as to create the vacuum below the heated lamina as quickly as possible, in an attempt to enhance the quality of the contact between the lamina and the article by reducing the time during which the lamina may cool as it moves away from the heating means towards the substrate article. This increases the capital cost of the press.

DISCLOSURE OF THE INVENTION

An object of the present invention is to alleviate the problems and disadvantages of the prior art discussed above. The invention achieves that object primarily by providing a vacuum press having a lid for the chamber which is able to withstand evacuation of the space above the thermoplastics lamina between the lid and the lamina, by providing vacuum means that are able to evacuate both of the spaces above and below the thermoplastic lamina simultaneously, while the lamina is being heated and, venting means to break the vacuum in the space above the lamina to then immediately press the heated lamina into conformity with the substrate article

Therefore, according to one aspect, the invention consists in a vacuum press of the kind comprising an open topped chamber in which a substrate article may be placed, sealing means to engage a peripheral margin of a thermoplastics lamina so that the lamina extends across the chamber as a sealed closure for the open top thereof, heating means above the lamina to heat and soften the lamina and vacuum inducing means to evacuate a space under the lamina between it and the substrate article and thereby cause or allow the heated lamina to be pressed into conformation with the substrate article, characterised: in that said chamber is provided with a lid able to seal with a peripheral margin of the lamina; in that the lid is adapted to withstand evacuation of a space above the lamina between it and the lid; in that the vacuum inducing means operate to withdraw air from the space above and the space below the lamina; and in that vent means are provided to break the vacuum in the space above the lamina when it is to be pressed into conformity with the substrate article.

According to preferred embodiments the press is further characterised in that control means are provided which: ensure that the degree of vacuum in the space above the lamina is not less than that in the space below the lamina to keep the lamina clear of the substrate article as heating of the lamina is effected; and break the vacuum in the space above the lamina when it reaches a predetermined temperature, to then cause the lamina to be pressed against the substrate article

According to still further preferred embodiments of vacuum presses according to the invention the control means include sensor means responsive to the position of the lamina and the control means control the degrees of vacuum above and below the lamina to keep it substantially in a predetermined position relative to the substrate article as heating proceeds.

According to another aspect of the invention it consists in a method of conforming a thermoplastics lamina to a surface of a substrate article comprising the steps of ; holding a peripheral margin of the lamina so that the lamina extends over the surface of the article and is spaced therefrom; heating the lamina to a predetermined softening temperature; evacuating the spaces above and below the lamina while the heating proceeds; and breaking the vacuum in the space above the lamina when the heating has been completed to cause the heated lamina then to be pressed into conformity with the surface of the article.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, an embodiment of the above described invention is described more fully hereinafter with reference to the accompanying drawings.

Figure 1 is a part sectional view of an article undergoing lamination in a prior art press, showing a deficiency therein.

Figure 2 is an end elevation of a vacuum press according to the invention, shown with its lid in the open position.

Figure 3 is a front elevation of the press of figure 2.

Figure 4 is a diagrammatic simplified perspective view of the press of figure 1 with some external walls and other external parts removed to show essential internal components more clearly.

BEST MODE OF CARRYING OUT THE INVENTION

The illustrated embodiment of the invention comprises a base cabinet 6 housing vacuum inducing means and control means, described hereinafter, and supporting an open topped chamber 7 and a swing mounted lid 8 therefore.

A substrate article 9 to be laminated may be placed within the chamber 7. In this illustrative example the article is a wooden cupboard door of the kind shown in prior art figure 1, with an impervious rear surface, profiled edges and an ornamental endless groove 10. Thus this article displays the recesses 3, 4 and 5 of the figure 1 door and could not be properly laminated by prior art presses. Furthermore the article 9 may be liberally coated with hot melt or thermosetting adhesive before being placed in the chamber 7 without deleterious effect on the finished laminated product.

A thermoplastic lamina 11 , for example a film, may then be extended over the open top of the chamber 7 and the lid 8 swung closed to clamp the peripheral margin of the lamina between the lid and the chamber. If desired the lamina 11 may be coated with hot melt or therm-setting adhesive on that part of its lower side in register with the article 9 without detriment to the operation of the illustrated press.

For preference the rims of the lid 8 and chamber 7 contacting the lamina 11 are provided with resilient sealing strips (not shown) or other conventional seals to produce closed spaces above and below the lamina 11.

The lid 8 may be open and closed by means of a double acting linear actuator, for example a so called hydraulic cylinder 12, and a linkage mechanism 13. It will be appreciated that the mounting arrangements for the lid and the mechanism for opening and closing it are not essentials of the invention. In what is probably the simplest arrangement the lid may be put in place and lifted off by hand.

The lid 8 is sufficiently robust to withstand evacuation of the spaced between it and the lamina 11. Nevertheless the lid 8 may perform the functions of prior art lids. In particular, it may have a reflective inner surface and be adapted to support lamina heating means such as, for example, infra- red radiant elements 14.

Vacuum inducing means are provided to evacuate the respective spaces within the lid 8 and chamber 7 above and below the lamina 11. Those means may comprise a motorised vacuum pump 15, duct connected to the respective spaces by hoses 16 and 17 and respective on-off valves 18 and 19.

The lid space above the lamina 11 may be sealed or selectively vented to atmosphere by electrically operable vent valves 20.

The operation of the various functional components described above is dictated by control means comprising a programmable logic controller (PLC) 21, an infra-red sensor 22 connected to a temperature controller 23, which in turn sends input signals to the PLC 21, a laser emitter 26 and a dual photo-electric transducer 24 comprising separately responsive upper and lower elements connected to send input signals to the PLC 21, and positioned to receive light from emitter 26 through sealed windows 25 in the lid 8.

The operation of the illustrated embodiment may now be described.

The article 9 and lamina 11 are put in position. If need be either or both of those items may be pre-coated with adhesive. I say "if need be" because it may well be that the nature of the lamina is such that it becomes tacky when heated and is thus self adhering to the article.

The temperature controller 23 is set to suit the lamina material and thickness, emitter 26 is energised, the lid 8 is closed and a start signal is sent by the operator to the PLC. The PLC closes vent valves 20, opens lid valve 18, closes chamber valve 19, energises the heating elements 14 and starts the vacuum pump 15, all more or less simultaneously. As the lamina softens and the pressure above it falls it rises until it interrupts the laser beam from the emitter 26 to the lower element of the dual transducer 24. The transducer 24 signals the PLC accordingly which then opens chamber valve 19. As the lamina rises still further it interrupts the laser beam from the emitter 26 to the upper element of the dual transducer 24, which signals the PLC accordingly and it closes valve 18 until the upper element of the dual transducer 24 signals it can see the emitter 26 once more, whereupon the PLC opens valve 18. The PLC may open and close valve 18 repeatedly as heating and evacuation continues to ensure that throughout those operations, the high point of the lamina remains at or closely below the line of sight between emitter 26 and the upper element of the transducer 24.

When the lamina reaches the desired temperature, and the degree of vacuum in both spaces is substantial, the lid valve 18 is closed and the chamber valve 19 is opened regardless of the signal then being received by the PLC 21 from the receiver 24, and the vent valves 20 are opened to break the vacuum in the space above the lamina. This causes the heated lamina to be immediately pressed into conformity with the substrate article.

The heating means may remain on, for a time predetermined by the initial program settings input by the operator beforehand, to suit the lamina and adhesive being used. The PLC 21 then switches the heaters 14 off but maintains the vacuum in the chamber to ensure that the lamina maintains close contact with substrate article while the lamina and adhesive cool.

The PLC then activates the opening of the lid 8, switches off the vacuum pump 15 and closes chamber valve 19, in readiness for the next operation.

In other embodiments of the invention, the means for detecting the lamina position may differ from the line of sight emitter/ receiver combination described above. For example , one may use a retro-reflective sensors rather than the through beam type, or a proximity sensor, radar, intersecting light beams, sound ranging equipment, or mechanical means. The height may also be extrapolated by controlling the pressure difference across the lamina in accordance with a pre-set program, or by predetermining the rate of removal of the air from each side of the lamina to give the desired lift without sensing the lamina position. The height to which the lamina lifts may be limited mechanically, and therefore require no detection devices, for example when the lamina is drawn upward into contact with a heating plate.

Then again if the lamina is well spaced from the substrate article to commence with, it may be possible to rely on equal degrees of vacuum above and below the lamina, for example, by having a substantial by-pass joining the space above the lamina to the space below the lamina during heating and evacuation, a vacuum can be formed in both spaces with the lamina remaining slack. That by-pass would be closed immediately before breaking the vacuum in the upper space.

Rather than employing one vacuum pump fitted with two valves to direct the vacuum above or below the lamina, a plurality of independently controlled pumps could be used.

Furthermore the method of the invention could be effected manually using manually operable valves and power supply switches.

Claims

1. A vacuum press of the kind comprising an open topped chamber in which a substrate article may be placed, sealing means to engage a peripheral margin of a thermoplastics lamina so that the lamina extends across the chamber as a sealed closure for the open top thereof, heating means above the lamina to heat and soften the lamina, and vacuum inducing means to evacuate a space below the lamina between it and the substrate article and thereby cause or allow the heated lamina to be pressed into conformation with the substrate article, characterised: in that said chamber is provided with a lid able to seal with a peripheral margin of the lamina; in that the lid is adapted to withstand evacuation of a space above the lamina between it and the lid; in that the vacuum inducing means operate to withdraw air from the space above and the space below the lamina; and in that vent means are provided to break the vacuum in the space above the lamina when it is to be pressed into conformity with the substrate article.

2. A vacuum press according to claim 1 further characterised in that control means are provided which: ensure that the degree of vacuum in the space above the lamina is never less than that in the space below the lamina to keep the lamina clear of the substrate article as heating of the lamina is effected; and break the vacuum in the space above the lamina when the lamina reaches a predetermined temperature, to then cause the lamina to be pressed against the substrate article

3. A vacuum press according to claim 2 wherein the control means include sensor means responsive to the position of the lamina and the control means control the degrees of vacuum above and below the lamina to keep it substantially in a predetermined position relative to the substrate article as heating proceeds.

4. A vacuum press according to claim 2 wherein the heating means comprise a hot plate and wherein the control means ensure that the degree of vacuum in the space above the lamina exceeds that in the space below the lamina to an extent that ensures the lamina contacts and remains in contact with the hot plate as heating proceeds.

5. A vacuum press according to claim 1 including means to pressurise the space above the lamina after the vacuum in that space has been broken.

6. A method of conforming a thermoplastics lamina to a surface of a substrate article comprising the steps of ; holding a peripheral margin of the lamina so that the lamina extends over the surface of the article and is spaced therefrom; heating the lamina to a predetermined softening temperature; evacuating the spaces above and below the lamina while the heating proceeds; and breaking the vacuum in the space above the lamina when the heating has been completed to cause or allow the heated lamina then to be pressed into conformity with the surface of the article.