Supporting Bed for Sheet Material Cutting Machine and Method of Manufacture

The present invention relates generally to pattern cutting machines for sheet materials, and more particularly, concerns a bed for supporting and retaining the sheet material during the cutting operation and a method for manufacturing the same.

Sheet material, such as fabric, is commonly cut into patterns on electronically guided machines comprising an elongated table over which a cutting tool is moved in a desired pattern by means of an precision positional control mechanism. Such tables are typically provided with a perforated top, below which a vacuum is applied for the purpose of drawing a multiple ply stack of the sheet material against the tabletop, thereby retaining it in position while it is being cut. Should the multiple layers of sheet material be retained effectively, a consistent relationship can be maintained between the cutting tool and the stack, enabling sheets with accurately cut patterns to be obtained reliably. On the other hand, should the flawed pattern is cut into the sheets, resulting in excessive waste of material. The efficacy of the vacuum-operated sheet retention system therefore has a direct bearing on the economics of the entire cutting process.

To assure that the lower layers of the stack are cut properly, the cutting blade must be permitted to pass below the lowest layer. In order to avoid damage to the surface of the table, it is the common practice to provide a supporting bed between the tabletop and stack of material being cut. This supporting bed must have certain physical properties, in order to serve its purpose effectively. First of all, it must provide a firm, relatively unyielding support beneath the stack of material being cut, to avoid undesirable stack movement beneath the blade and resultant pattern errors in or damage to the cut sheet material. Secondly, the supporting bed must not impede the vacuum which is applied beneath the tabletop. It must therefore be capable of having a substantial volume of airflow through it. Third, it should have a relatively high coefficient of friction and should present the largest possible surface area to the bottom sheet of the stack, in order to avoid slipping of the stack relative to the tabletop. Finally, the supporting bed must have an upper surface which resists the gouging action of the cutting blade, in order to maintain the uniformity of its surface and to minimize the frequency of replacement of the supporting bed.

Various materials have been utilized for the supporting bed. Most commonly, it is made of a sheet of polyethylene foam which is approximately one inch thick. Polyethylene foam provides a rather firm support for the stack of sheet material. However, being a closed cell foam it is impervious to air. Accordingly, it is the common practice to punch or drill interspersed vertical holes through the polyethylene foam sheet, and a substantial number of such holes is required (per unit of sheet surface area), in order to provide the vacuum at the surface of the polyethylene sheet. Typically, for a one inch thick sheet, the holes would be about 5/16 of an inch in diameter and would be at a center-to-center spacing of about 1.5 inches. However, such a density of holes substantially reduces the firmness and surface area of the supporting bed, and the expense involved in forming the holes substantially increases the cost of the supporting bed.

In addition, such a perforated supporting bed holds the fabric effectively only at the holes. Between the holes, there may be wrinkling or bunching of the fabric, and the fabric above the holes may be stretched or frayed when the blade passes into the hole. Both of these effects result in cutting errors or damage to the fabric. The use of a perforated polyethylene foam supporting bed therefore represents, at best, a compromise, which results in a serious limitation upon the height to which the sheet material may be stacked and, even then, a certain amount of undesirable movement of the stack and damage to the sheet material will occur during cutting. As a result, some portion of the sheets cut by the machine will be unacceptable and must be discarded.

It has also been suggested that the supporting bed be made of upright bristles. Although such a construction provides a substantial airflow, it hardly provides an adequately firm supporting surface, particularly when a relatively heavy sheet material is being cut. Furthermore, this relatively weak support deteriorates rapidly, as the bristles are damaged by the cutting blade, after repeated use, and the supporting surface they provide becomes uneven.

Polyurethane foam has been suggested as a covering material for the surface of a supporting bed, because it exhibits the property of "healing" or recovering instantaneously from surface nicks inflicted by a sharp implement. Polyurethane foams may be either of the open or "tight" cell variety. In polyurethane foams, the individual cells are formed from a 3-dimensional skeletal structure comprising interconnected strands. Membranes or windows are attached to the strands and serve to divide or partition individual cells. In general the skeletal structure is substantially thicker than the windows or membranes. In so called "open cell" foams, a substantial number of the windows or membranes are broken or ruptured (even though they are still attached at their peripheral edges to the skeletal strands). Some small percentage of the windows may not be attached to the strands at the edges, or may be missing altogether, and this permits a limited air flow through the foam mass. Tight cell urethane foams have essentially all of the cellular windows or membranes intact (unbroken) and attached to skeletal structure of the foam. The use of polyurethane has been substantially limited, however, for essentially the same reasons as polyethylene.

"Reticulated" materials are also known to the art. Such materials have the cell membranes or windows partially or totally destroyed. These reticulated materials are prepared from the cellular materials of the prior art. Reticulated foam materials generally permit the passage of substantially greater volumes of air, in comparison to open or tight foam materials. Such reticulated foams generally have higher porosity than comparable "open" or "tight" cell foams. Thus, in these reticulated materials, the primary support is supplied by the skeletal structure, since the cell membranes have been partially or totally eliminated. Examples of such reticulated materials extensively used by the prior art are the membrane destroyed or reticulated polyurethane foams which are employed in various filtering and detraining applications and as garment liners. Such reticulated foam materials and their process of manufacture are disclosed, for example, in U.S. Pat. Nos. 3,175,025 and 3,175,030 granted to Henry C. Geen on Mar. 23, 1965.

Reticulated materials of the flexible polyurethane type, have been in use for some time, owing to their porosity and softness as compared to non-reticulated flexible polyurethane cellular materials. However, attempts to use such materials in the supporting bed of a cutting machine have proven unsuccessful, because such materials offer virtually no support to the stack of sheet material while it is being cut and because the reticulated foam tends to collapse when the vacuum is applied.

In copending patent application serial number 825,811 filed Feb. 4, 1986 there is disclosed a supporting bed which is manufactured from a sheet of reticulated foam material that has been compressed under heat and pressure so as to be permanently reduced to approximately 10-35% of its initial thickness. The degree of compression, the temperature and compression time, and the porosity of the reticulated foam starting material are selected to provide particular airflow and firmness characteristics for the finished supporting bed.

Reticulated foam sheets of the type used in this copending patent application are manufactured from blocks or "buns" of foam material, from which each individual sheet is cut as a layer. Often, after all the sheets have been cut from the bun, the last remaining sheet will be too thin to use in the manufacture of a supporting bed. Until now, such thin sheets of reticulated foam have not been useful and have been treated as a waste material. As a result, there has been a substantial amount of waste, often in excess of 10%, in the manufacture of such supporting beds.

It is an object of the present invention to reduce substantially or eliminate this waste, thereby providing significant economies in the manufacture of supporting beds.

In accordance with the present invention, the starting material for the supporting bed comprises a composite structure including a plurality of relatively thin reticulated foam layers which are stacked in superposed relationship, with at least one layer of a hot melt adhesive web being interposed between the reticulated foam layers. The thickness of the individual foam layers is selected to give a cumulative thickness which is the same as when a single sheet is used to manufacture a supporting bed, and similar pressure and heat are utilized to compress the starting material. In the process of forming the supporting bed, the adhesive webs melt and bond together the individual layers of the composite structure. The resulting supporting bed is comparable in strength, surface firmness and permeability to a supporting bed made from a single sheet of reticulated foam starting material.

The foregoing brief description, as well as further objects, features and advantages of the present invention will be more completely understood from the following detailed description of presently preferred, but nonetheless illustrative, embodiments of the present invention, with reference being had to the accompanying drawing, in which:

FIG. 1 is an elevational view taken from the front end of the cutting machine, with portions being shown in section, to illustrate certain details of the table top;

FIG. 2 is a sectional view taken along contour 2--2 in FIG. 1 and looking in the direction of the arrows; and

FIG. 3 is a schematic representation of the manufacture of a supporting bed in accordance with the present invention.

Turning now to the details of the drawing, FIGS. 1 and 2 illustrate a cutting machine 10 for sheet material, which incorporates a supporting bed 20 in accordance with the present invention. The cutting machine includes a support table 30, which is provided with an air permeable top surface member 32 (shown diagrammatically as a grating). The supporting bed 20 rests upon the top 32 and is retained in position by means of an upright frame 34. A stack 36 of sheet material to be cut is supported directly upon supporting bed 20. Below the table 30, there is provided a vacuum pump 40, which is appropriately coupled to a vacuum chamber 38 underneath the table top 32.

A cutting tool 50 is borne by a sub-carriage 52 which is, in turn, borne on a carriage assembly 54, which is mounted for precisely controlled movement along the length (i.e. perpendicular to the plane of FIG. 1) of the table 30. The subcarriage 52 is mounted for precisely controlled movement along the carriage 54 and therefore moves across the table 30 (i.e. to the left and right in FIG. 1). Appropriate motors and control mechanisms are provided to achieve the precisely controlled cutting action of cutting tool 50 through a pre-programmed cutting pattern.

In operation, air flow produced by pump 40, is drawn through supporting bed 20 and table top 32 into vacuum chamber 38 (illustrated by curved arrows in FIG. 1). As a result, ambient air pressure forces the stack of sheet material downwardly and retains it against the supporting bed 20. By design, the supporting bed 20 is firm, yet provides uniform air permeability over its entire area. As a result, not only is the sheet material held downwardly, but it is drawn into a very flat position, so as to avoid any wrinkling or bunching of the sheet material. Also, the firm support provided by supporting bed 20 assures that the fabric will not move downwardly as a result of the pressure provided by cutting blade 51, thereby assuring accurate cuts.

As can be seen in FIG. 1, the cutting blade 51 must extend below the bottom sheet of stack 36, in order to assure that the sheet is completely cut. Consequently, blade 51 will also cut into the top surface of supporting bed 20. As a result of its polyurethane foam composition, supporting bed 20 exhibits the property that the blade cuts "heal" or close up directly behind the blade. This avoids the need for frequent changes of the supporting bed and guarantees the continued durability and flatness of the bed.

Foamed or cellular polyurethane products are made, in a manner well-known in the art, by reacting an organic isocyanate, such as an aromatic di-isocyanate (e.g. toluene di-isocyanate), with a polyether polyol or a polyester resin, along with various other ingredients (e.g. catalysts, blowing agents, stabilzers and the like). A gas or vapor is usually generated (along with heat) in situ while the reaction mixture remains in the plastic or fluid state. The generation of this gas results in the formation of bubbles, approximately spherical in form, in the plastic material. As these bubbles expand, cells are formed and the resulting structure of the cooled foam material is comprised of a skeletal structure and cell membranes.

In accordance with the invention of copending patent application Ser. No. 825,811, supporting bed 20 is manufactured from a sheet of reticulated foam material which has been compressed under heat and pressure in a conventional heated press so as to be permanently reduced to approximately 10-35% of its initial thickness. The degree of compression, the temperature and compression time, and the porosity of the reticulated foam starting material are selected to provide particular airflow and firmness characteristics for the finished supporting bed. Preferably, a 1-inch thick sheet of the support bed should permit an airflow of at least 1.5 cfm through an area 4 inches square, with a pressure drop between the surfaces of the sheet material corresponding to 1/2 inch of water, and it should be sufficiently firm so that compressing a 1-inch thick sheet by one quarter of its thickness requires a pressure in excess of 1.5 psi. With reticulated polyurethane foams, this is typically achieved by compressing the foam at 300.degree.-450.degree. F. for a time period between 8 minutes and 2 hours.

In the copending patent application, five specific examples were given, and the reticulated polyurethane foams which were used as the starting material in the examples were all commercially available under the trademark Filtercrest from Crest-Foam Corp. of Moonachie, N.J. These foams were reticulated by the process described in U.S. Pat. No. 3,175,025. This process involves providing a combustible mixture of an oxidizer material and an oxidizable material within and about a block of the foam material and igniting the mixture, so that the shock waves produced by the ignition destroy substantially all the windows within the block of material. However, this was merely illustrative of one type of starting material that could have been used.

As illustrated in FIG. 3, a supporting bed 20 in accordance with the present invention is manufactured from a structure comprising a plurality of sheets or layers 60, 60 of reticulated foam material which are stacked in superimposed relationship. Between each pair of foam layers 60, 60, there is provided a hot melt adhesive web 62. A stack of such layers of foam material and adhesive webs is calculated to have the same total thickness as would be used if there were a single sheet of reticulated foam starting material. This composite conventional heated press so as to be permanently reduced to less than about 35%, and preferably to approximately 10-35 percent of its initial thickness. The dwell time Within the press is selected to be long enough so that the adhesive web 62 is totally melted, bonding the individual foam layers 60, 60 together. In addition, the degree of compression, the temperature and compression time, and the porosity of the reticulated foam starting material are selected to provide particular air flow and firmness characteristics for the finished supporting bed.

Preferably, a one-inch thick sheet of the supporting bed should permit an air flow of at least 1.5 cfm through an area 4 inches square, with a pressure drop between the surfaces of the sheet material corresponding to one-half inch of water, and it should be sufficiently firm so that compressing a one-inch thick sheet by one-quarter of its thickness requires a pressure in excess of 1.5 psi. As in the case of a single sheet of foam material, this is typically achieved by compressing the foam at a temperature of 300.degree.-450.degree. F. for a time period between 8 minutes and 2 hours. The supporting bed manufactured from the composite structure exhibits air flow and firmness characteristics comparable to those obtained with a supporting bed manufactured from a single sheet of reticulated foam material.

In the examples described below, the reticulated polyester foam utilized as the starting material is commercially available under the trademark Filtercrest S-15M foam from Crestfoam Corp. of Moonachie, N.J. This material has a density of about 1.4 pounds per cubic foot and a porosity of about 15 pores per inch, and an airflow of about 20 cfm through a four square inch area, with a pressure difference corresponding to half an inch of water between its surfaces. This foam was reticulated by the process described in U.S. Pat. No. 3,175,025, referred to above. However, those skilled in the art will appreciate that materials reticulated by any other process, as well as other types of foams will work equally well in the invention.

The starting material was selected as a 7-inch thickness of reticulated S-15M foam. The sheet was compressed to a thickness of one inch and maintained at a temperature of about 400.degree. F. for about 10 minutes. The resulting sheet material retained a thickness of one inch when pressure was removed, but exhibited substantially improved firmness, while permitting a substantial air flow: compressing the new sheet material by one-quarter of an inch required 6.3 psi and an air flow of 2.9 cfm through a 4 square inch area, the pressure difference corresponding to an half-inch of water between its surfaces.

This supporting bed was then used as a control sample for comparing the characteristics of supporting beds made from stacks of superposed sheets of reticulated foam material. In each case, two sheets, each 3.5 inches thick, were stacked. However, any combination of thicknesses totalling 7 inches could have been used equally well. For the three different examples, the composite structures were provided, respectively, with 1, 2, and 3 layers of a hot melt adhesive web between pairs of foam sheets. The particular web utilized is commercially available from Sharnet Corporation of Ward Hill, Mass. under the designation SHAR-NET SH151. In each case, the composite structure was compressed to a thickness of one-inch and maintained at a temperature of about 400.degree. F. for about 25 minutes. The resulting sheet material retained a thickness of one inch when the pressure was removed. Table I below indicates the characteristics of the controlled sample and the three examples. In all cases, the firmness was that pressure (in psi) required to compress the one-inch thick supporting bed by one-quarter of an inch.

Although preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications and substitutions are possible, without departing from the scope and spirit of the invention as defined in the accompanying claims.