IMPROVEMENTS TO PRESS PADS

Abstract: A press pad for use in a laminating press and in particular in a high pressure, short cycle press comprises a woven fabric (12) of heat resistant strands wherein at least one of the warp (13) and the weft (14) comprises metal strips (15). Preferably, the warp (13) wholly comprises a series of unannealed copper or brass strips (15) which advantageously have a thickness between 0.1 mm and 0.15 mm inclusive and are between 4.0 mm and 5.0 mm inclusive wide. The weft (14) preferably comprises threads which have a silicone covering (17), which is advantageously at least 0.2 mm in thickness.

Description: IMPROVEMENTS TO PRESS PADS The present invention relates to a press pad for use in a laminating press for the production of laminate sheets, such as decorative laminates, laminated floorboards, and printed circuit boards, using low pressure and high pressure single daylight and multi daylight presses . As shown in Fig. 7 of the drawings appended hereto, in a single daylight laminating press, a laminate sheet 1 to be pressed between two platens 2 of the press is located between two metal caul plates 3 and two press pads 4. The press pads 4 are each located between one of the caul plates 3 and one of the platens 2. The press pads 4 are usually a little larger than the dimensions of the platen 2 to allow for clamping. The purpose of the press pads 4 is to compensate for density variations in the laminate sheet 1 and thereby ensure that an equal pressure is applied to all parts of the sheet 1. In addition, the press pads 4 compensate for any unevenness in the surfaces of the platens 2 of the press itself and any flexure or bowing of the platens 2 when under pressure. Again, this assists in the production of a flat even density laminate. Thus, it is important for a press pad to be resilient and have a natural springiness to permit it to compensate for the aforementioned density variations and the surface unevenness of the press platens but also to allow it to relax after each pressing operation and recover its form to enable it to be used again. The capacity a press pad has to re-form itself after each pressing is an important characteristic to ensure a reasonable working life and to avoid unnecessary downtime of a press whilst the press pads are replaced. However, because the purpose of the press is to apply heat to the laminate sheet whilst it is under pressure, it is important that the press pad also conducts the heat supplied by the press platens to the laminate sheet. Working temperatures for such presses are usually in a range up to 220°C. In a low pressure, single daylight press such as is typically used in the production of decorative laminates, a press pad must withstand pressures ranging between 25 to 35 kg per cm 2 inclusive at temperatures around 200°C. In a high pressure, short cycle, single daylight press such as is typically used in the production of laminated floorboards, a press pad must withstand pressures up to 70 kg per cm 2 at similar temperatures up to 200°C as before. Typically, therefore, a conventional press pad is a densely woven combination of high temperature resistant non-asbestos yarns and metal wire. The metal wire is included to give good heat transmission through the pad to the laminate sheet. In contrast, the non-metal yarn is required to give the pad the springiness and resilience required to enable the pad to relax after each pressing operation. The relative proportion of the two types of material is a consideration when devising a press pad for a particular purpose. Usually a compromise must be reached between the heat transference and the resilience or springiness required in each case. A conventional press pad is described in EP 0 735 949 Al. This describes a woven fabric of heat resistant strands such as copper wires wherein a substantial proportion of either the warp or the weft comprises a silicone elastomer. In particular, the warp or the weft preferably comprises a silicone covered metal wire. As a result of the presence of the silicone, this press pad has a great resilience and springiness whilst the metal wires present ensure that the press pad achieves good heat transference from the platens to the material being pressed. Prior to use, such a press pad has a weave structure as illustrated in Fig. 8 of the drawings appended hereto, which shows to a scale approximately 10 times life size a cross section of such a press pad 5 wherein weft threads 6 are shown in cross section with warp threads 7 interweaving therebetween. The pad 5 is woven in a twill weave wherein the warp threads 7 pass alternatively under and over two weft threads 6. Typically, the warp threads 7 each comprise a stranded metal thread with an overall diameter of around 0.6 mm and the weft threads 6 comprise stranded metal cores 8 covered by a coating of silicone 9 to give an overall diameter of around 1.4 mm. The pad 5 itself has a thickness Tl of around 1.9 mm because the silicone coatings 9 of the weft threads 6 are compressed by the warp threads 7 during weaving . Dependent upon the nature of the laminated sheets being pressed, a conventional press pad of this type can be used for very many pressing cycles of a low pressure, single daylight press, as described above, before it wears out and must be replaced. The pad wears out because in use the weave structure is eventually flattened to such an extent that the press pad is unable to relax after each pressing operation and the pad loses its resilience and springiness. It has been found that the cross-section of such a pad 10 appears as shown in Fig. 9 of the drawings appended hereto. The pad 10 initially had a structure identical to that of the pad 5 shown in Fig. 8. However, it can be seen that in use the weave structure becomes compressed so that the warp threads 7 bend closely around the weft threads 6 and eventually form rigid bridges 11 over the weft threads 6. In addition, the weft threads 6 themselves become considerably distorted so that eventually the silicone coatings 9 between the metal cores 8 and the warp threads 7 is squeezed out and trapped beneath the bridges 11. When this occurs, there is no longer any springiness between the warp and the weft of the pad 10 because the metal cores 6 contact the metal warp threads 7 directly . The number of pressing cycles which has to occur before a conventional pad 5 similar to that described above becomes worn out and similar in structure to the pad 10 shown in Fig. 9, with a thickness T2 is dependent to a very large degree upon the nature of the laminated sheets being pressed. Decorative laminates and medium and low density fibreboards have an inherent springiness and resilience so that during a pressing operation they also assist in providing the compensation required. However, laminated floorboards and high density fibreboards have very little natural springiness and it has been found that conventional press pads as described above wear out relatively quickly when used for pressing these types of laminates. It is an object of the present invention to provide a press pad which will overcome or substantially mitigate the aforementioned problem by providing a press pad which will retain its springiness and compensation ability for a greater number of pressing cycles than a conventional press pad when used, in particular, for pressing high density laminated sheets such as laminated floorboards and high density fibreboard, without any loss of heat transfer ability. It is also a secondary object of the invention to provide in some embodiments thereof a press pad with an improved heat transfer ability over a conventional press pad. According to the present invention there is provided a press pad for use in a laminate press comprising a woven fabric of heat resistant strands, characterised in that at least one of the warp and the weft comprises metal strips. By use of the term "strips" herein and in the claims it is meant that the threads of the weave which comprise the strips have relatively flat upper and lower surfaces and have a narrow thickness in contrast to their width, unlike conventional yarns or metal wires which have a predominantly circular cross-sectional shape. Preferably, the warp comprises a series of metal strips and the weft comprises threads which have a silicone covering. The metal strips of the warp preferably comprise metal ribbons but alternatively may comprise flat plaited metal straps or braided, flat rolled metal straps. Preferably also, the strips have a thickness between 0.05 mm and 0.2 mm inclusive. Advantageously, the strips have a thickness between 0.1 mm and 0.15 mm inclusive. Preferably also, the strips have a width of least 2.0 mm. Advantageously, the strips have a width of between 4.0 mm and 5.00 mm inclusive. Preferably also, the weft comprises at least one of silicone covered metal wire, silicone covered stranded metal wire, silicone covered aramide yarns, silicone covered glass threads or filaments, solid silicone threads, aromatic polyamide yarn, wire wrapped polyamide yarn, and glass threads. Preferably also, the metal strips are made from copper or a copper alloy. Advantageously, the strips are made from unannealed copper or brass. The present invention will now be described by way of example with reference to the accompanying drawings, in which: - Fig. 1 is a plan view of a portion of a press pad according to the invention woven in a plain weave; Fig. 2 is a view similar to Fig. 1 but of a press pad woven in a twill weave; Figs. 3 and 4 are diagrammatic cross-sections of the weaves shown in Figs. 1 and 2 respectively; Fig. 5 is a cross sectional view, to an increased scale, of the press pad shown in Fig. 1 prior to any use; Fig. 6 is a view similar to Fig. 5 but after use of the press pad; Fig. 7 is a diagram showing the conventional arrangement of a laminate sheet and press pads in a single daylight press; Fig. 8 is a cross sectional view, to an increased scale of a conventional press pad prior to any use; and Fig. 9 is a view similar to Fig. 8 of the same conventional press pad after it has been worn out through use . As shown in Figs. 1 and 2, a press pad 12 for use in a laminate press comprises a woven fabric which is made of materials that are heat resistant to at least 220 C C and substantially resistant to pressures ranging between 20 and 100 kg per cm 2 inclusive. In particular, the press pad 12 is suitable for use in high pressure, short cycle presses which are used for producing high density laminates such as laminated floorboards where the pressures used are up to 70 kg per cm 2 at temperatures around 200°C. However, the press pad 12 is also suitable for use in low pressure single daylight and raulti daylight presses. The press pad 12 comprises warp threads 13 and the weft threads 14, at least one of which comprises metal strips 15. Although the warp threads 13, the weft threads 14 or both may comprise the metal strips 15, in the examples shown in the drawings it is the warp threads 13 which wholly comprise the metal strips 15. As explained below, it is expected that in practice it will always be the warp threads 13 which comprise the strips 15. However, it will be appreciated that the strips 15 could be interspersed with warp threads such are used in conventional press pads, for example metal wire, aromatic polyamide yarns and the like. Figs. 1 and 2 show two examples of weaves suitable for use in the manufacture of a press pad according to the invention. However, it will be appreciated that many other types of weave could be used. The weave cross-sections corresponding to these examples are shown in a conventional manner in Figs. 3 and 4 respectively. Here it is the weft A which is shown in cross section with the warp B interweaving therebetween. In Figs. 1 and 3, the weave comprises a single ply plain or linen weave. Here, the warp threads B, namely the strips 15, pass over and under a single weft thread A at a time. However, in a 2/2 twill weave, as shown in Figs. 2 and 4, the warp threads B in the form of the strips 15 pass alternatively under and over two weft threads A and the woven fabric has the same amount of warp material on both faces of the fabric, which thus presents the same appearance on both sides. The twill weave of Figs 2 and 4 provides a press pad with surfaces wherein the metal strips 15 run for a longer distance over the weft threads 14 than in the linen weave of Figs 1 and 3. This provides a pad which has a smoother metal surface with a metal herringbone pattern formed thereon. This may be advantageous in some applications. In order to include the strips 15 in the weave of a press pad it is necessary to alter the structure of a conventional loom. In a loom, the warp threads are wound around a warp beam, pass over a back rest and through the eyes of healds or heddles before passing through the interstices in the reed behind which the shuttle plies to and fro carrying the weft thread. The healds or heddles form part of the shedding harness and comprise shafts carrying steel wire eyes which are normally circular to accommodate warp threads that are of a predominantly circular cross section. It will be appreciated that to accommodate strips as proposed in the present invention the shape of the eyes in the healds or heddles will have to be altered in order to accommodate the strips. As shown in Figs. 1 and 2, the strips 15 comprise flat metal ribbons. However, they could also comprise flat plaited metal straps or braided, flat rolled metal straps, both of which are made from plaited or braided metal filaments or thin wires. The strips 15 preferably have a thickness between 0.05 mm and 0.2 mm inclusive and advantageously a thickness between 0.1 mm and 0.15 mm inclusive. If the strips 15 are too thin, then they will not be strong enough to withstand the pressures they will encounter in use and may split. However, if the strips are too thick they cannot be woven successfully. In particular as will be described below with reference to Fig. 5, it is important for the strips 15 to bend or crimp around the threads of the weft 14 during weaving in order to minimize shrinkage of the press pad lengthwise during use. Shrinkage occurs because when the pad is subjected to high pressure within a press, the pressure causes the metal strips 15 to be castellated around and into close contact with the weft threads 14. If the warp threads 13 are woven with an insufficient contact around the weft threads 14 initially, then in use these threads 13 are forced into a greater contact with the weft threads 14 which causes them to be pulled into the pad, thus causing it to shrink lengthwise along the warp. Shrinkage of the order of 10% and greater can occur in these cases. Preferably, the strips 15 are at least 2.00 mm wide but advantageously from the point of view of heat transfer through the pad, the strips 15 have a width of between 4.00 mm and 5.00 mm inclusive. Again in order to maximize heat transference through the pad, the strips 15 are made from a metal which has high heat transference properties. Preferably, therefore, the strips 15 are made from copper or a copper alloy. In order to reduce metal fatigue in the strips 15, particularly if the weft threads 14 comprise silicone, the strips 15 comprise either unannealed copper or brass. The weft threads 14 of the press pad and any warp threads 13 which do not comprise the metal strips 15 preferably comprise one or more of the following:- silicone covered metal wire; silicone covered stranded metal wire; silicone covered aramide yarns: silicone covered glass threads or filaments; solid silicone threads; aromatic polyamide yarn; wire wrapped polyamide yarn; glass thread. In addition, metal strands such as copper or stainless steel strands may be wrapped with an aromatic polyamide yarn in a conventional manner and stainless steel strands may also be used to braid the exterior of silicone covered copper wires. Advantageously, in order to maximize the resilience and springiness of the pad, the weft threads 14 comprise silicone covered threads or wires, such as copper wire, as shown in Fig. 5. Here, each warp thread 13 comprises a metal strip 15 and each weft thread 14 comprises a stranded copper wire core 16 with an extruded silicone elastomeric outer covering 17. Preferably, the specific gravity of the silicone elastomeric covering 17 falls within the range of 1.1 g/cm 3 to 1.4 g/cm 3 inclusive in order to secure the best properties wherein it is neither sufficiently springy under compression nor too brittle. The wall thickness dl of the covering 17 is preferably at least 0.2 mm and the overall outside diameter d2 of the silicone covered wire is at least 1.0 mm and preferably around 1.4 mm. Preferably also, the metal core 16 comprises stranded or braided wire which comprises at least 7 wire strands and has an overall diameter d3 of around 0.6 mm. It can be seen in Fig. 5, that the metal strips 15 are bent or crimped around the weft threads 14 during weaving with only a minimal distortion or compression of the silicone covering 17. With warp and weft threads of the dimensions indicated above, this produces a press pad with an overall thickness of around 1.6 mm prior to use. This is the case because of the wide bearing surface of the strips 15 which lie over the surface of the weft threads 14 without significantly pressing into the silicone covering in contrast to the much more severe distortion and compression caused in a conventional press pad 5 by the metal warp threads 7 as shown in Fig. 7. Hence, after and during use when the surfaces of the pad 12 are flattened and the metal strips 15 bent more closely around the surface of the weft threads 14 as shown in Fig. 6, the metal strips 15 of the warp are less likely to cut into the silicone covering 17 and therefore much less likely to come into direct contact with the metal cores 16 of the weft threads 14, in contrast to the used conventional pad shown in Fig. 9. The relative thicknesses of the metal strips 15 and the silicone covering 17 are also relevant here and the relative thinness of the strips 15 helps to retain a significant thickness of silicone between the strips 15 and the metal cores 16 of the weft even when subjected to high pressures. This means that a degree of resilience, provided by the presence of the silicone covering between the metal cores 16 of the weft and the metal strips 15 of the warp, will always remain in the pad 12, even when it has been flattened to the same degree as the press pad 10 shown in Fig. 9. Thus, it is expected that the life of the pad 12 will be greatly increased over a conventional pad, particularly when used for pressing high density laminates such as laminated floorboards and high density fibreboard and the like which have very little inherent resilience of their own. In addition to the foregoing, it is also expected that the heat transfer capability of the press pad as shown in Fig. 5 will also be increased over that of the conventional press pad shown in Fig. 8. There are two main reasons for this. First, the metal strips 15 exhibit a greatly increased area which will contact the platens of a press over the metal wires of the warp 7 of the conventional pad 5, thus increasing the speed of heat conduction through the pad. Second, the weight of metal in a press pad according to the invention as shown in Fig. 5 is approximately 50% lower than that of the conventional pad 5. This means that the new pad will heat up significantly faster than the conventional pad and therefore be faster to transmit the required temperature through to the laminate sheet being pressed. As a result, pressing cycles can be kept to a minimum, making the pad more efficient in use than a conventional pad. The reduction in metal content in the new press pad over the conventional pad 5 also has the additional advantage of making the pad lighter in weight, facilitating the handling and manipulation of large press pads by press operatives .

Claims: CLAIMS 1. A press pad for use in a laminate press comprising a woven fabric (12) of heat resistant strands, characterised in that at least one of the warp (13) and the weft (14) comprises metal strips (15). 2. A press pad as claimed in Claim 1, characterised in that the warp (13) comprises a series of metal strips (15) and the weft (14) comprises threads which have a silicone covering (17) . 3. A press pad as claimed in Claim 1 or Claim 2, characterised in that the strips (15) have a thickness between 0.05 mm and 0.2 mm inclusive. 4. A press pad as claimed in any one of Claims 1 to 3 , characterised in that the strips (15) have a thickness between 0.1 mm and 0.15 mm inclusive. 5. A press pad as claimed in any one of Claims 1 to 4 , characterised in that the strips (15) have a width of least 2.00 mm. 6. A press pad as claimed in any one of Claims 1 to 5 , characterised in that the strips (15) have a width of between 4.00 mm and 5.00 mm inclusive. 7. A press pad as claimed in any one of Claims 1 to 6 , characterised in that the metal strips (15) are made from copper or a copper alloy. 8. A press pad as claimed in any one of Claims 1 to Claim 7, characterised in that the warp (13) wholly comprises a series of metal strips (15) and the weft (14) comprises at least one of silicone covered metal wire, silicone covered stranded metal wire, silicone covered aramide yarns, silicone covered glass threads or filaments, solid silicone threads, aromatic polyamide yarn, wire wrapped polyamide yarn, and glass threads. 9. A press pad as claimed in any one of Claims 1 to 8 , characterised in that the weft (14) comprises metal wire (16) with a silicone covering (17). 10. A press pad as claimed in Claim 2 or Claim 9, characterised in that the silicone covering (17) has a specific gravity within the range of 1.1 g/cm 3 to 1.4 g/cm 3 inclusive. 11. A press pad as claimed in Claim 10, characterised in that the thickness of the silicone covering (17) is at least 0.2 mm, and the overall outside diameter of the silicone covered weft (14) is at least 1.0 mm. 12. A press pad as claimed in any one of Claims 9 to 11, characterised in that the metal wire (16) comprises a stranded or braided copper wire wherein the diameter of each wire strand is at least 0.05 mm.

Inventors: DOUGLAS MELVYN, DOUGLAS, MELVYN
Applicants: DOUGLAS MELVYN, MARATHON BELTING LTD, DOUGLAS, MELVYN, MARATHON BELTING LIMITED
Application year: 1999
Main classification: B30B15
IPC codes: B30B15, H05K3