WO1992021514A1

WO1992021514A1 - Method for melt printing dyes on plastic

Info

Publication number: WO1992021514A1
Application number: PCT/US1992/004439
Authority: WO
Inventors: Kenneth E. Neri; Stephen J. Durand
Original assignee: Key-Tech, Inc.
Priority date: 1991-06-03
Filing date: 1992-06-03
Publication date: 1992-12-10
Also published as: AU2146392A

Abstract

A method of printing a meltable disperse dye image onto and into a workpiece, in which an assembly is prepared made up of (1) a workpiece (300) comprising a melt printable plastic substrate or a non-melt printable substrate coated on its surface to be printed with a melt printable plastic coating, (2) a flexible sheet (108) bearing on one side one or more meltable disperse dyes, and (3) a flexible membrane (16) overlaying the surface of the dye-bearing sheet opposite its dye-bearing surface. The space between the flexible dye-bearing sheet and the flexible membrane is then evacuated to cause the membrane to press the dye-bearing sheet tightly against the printable plastic surface. A heat and pressure applying means, e.g., a heated plate (200) having a pressure-applying surface is then pressed against the flexible membrane, while maintaining the vacuum, to cause the meltable disperse dye to melt onto and into the workpiece's printable plastic surface.

Description

— METHOD FOR MELT PRINTING DYES ON PLASTIC—

This application is a continuation-in-part of copending U.S. patent application Serial No. 07/709,229, filed June 3, 1991.

Field of the Invention

The present invention relates to melt printing on large plastic or plastic coated substrates. More particularly, the present invention relates to a melt printing technique for printing dye images on large plastic or plastic coated substrates whose printable surfaces are appreciably greater in area than, for example, that of a key in a telephone, typewriter or computer key array.

BACKGROUND OF THE INVENTION

Melt printing is now well known in the art, and in one commercially important embodiment is typically done by heating meltable disperse dyes to temperatures above their melting points to liquify them and effect the transfer of the melted dye onto and into a plastic surface. The assignee of the present invention holds patents on highly improved techniques of melt printing, using heat and pressure, on plastic surfaces; see Durand U.S. Patent Nos. 4,670,084 ("the '084 patent") and 4,668,239 ("the '239 patent"), and Durand, et al. U.S.

Patent No. 4,587,155 ("the '155 patent"), the disclosures of which are incorporated herein by reference to the extent necessary to complete the present disclosure.

The '239 patent teaches preferred techniques of melt printing which involve imparting images formed by certain preferred meltable disperse dyes to a dye receptive plastic material. Specifically, the '239 patent teaches that a dye forming the mirror image of the image to be printed on a plastic surface is first applied

- l - to a flexible paper sheet. Next, the image-bearing pape sheet is placed on the plastic surface, dye side down, then pressed (at 1-2 psi) ' and heated against the plastic surface. The heat melts the dye while the pressure forces the dye, once melted, to diffuse into the plastic surface. Heat and pressure also cause the image-bearing paper sheet to be permanently deformed to the configuration of the plastic surface on which it is printing, particularly when the sheet is one containing thermosetting polymer intermixed with paper pulp. The preferred printable plastic materials used when practicing this technique are substantially or entirely crystalline polymers, but amorphous polymers and mixture of crystalline and amorphous polymers can also be used. The '084 patent teaches a further improved method of printing dye images on plastic surfaces., A flexible membrane is applied over the flexible paper sheet which bears the dye image. The image-bearing paper sheet is located on positioning pins to hold it accurately in registration with the plastic surface being printed ("the workpiece") , and once the sheet is accurately located, a vacuum is pulled underneath the flexible membrane to pull this membrane tightly against the paper sheet, the latter then being, in turn, pulled against the workpiece. This vacuum technique described in the '084 patent provides close contact between the image-bearing paper sheet and the plastic surface on which printing is to be conducted. Heating is accomplished when practicing the '084 patent's method using heaters which radiate heat while the paper sheet bearing a mirror image, in dye, of the image being printed is being pulled against the printable plastic surface. The combination of heat and vacuum also deforms the image-bearing paper sheet to the shape of the printable plastic surface. The invention disclosed in the '084 patent has proven to be enormously successful commercially. This invention depends in part for its proper operation. however, on a tight vacuum being achievable between the image-bearing paper sheet and the flexible membrane to force the sheet down against the surface of the workpiece. In a preferred embodiment, the '084 patent describes the use of a keyboard as the workpiece.

Typically, telephone, typewriter and computer keys are relatively small, and the spaces between the rows of su keys readily permit a vacuum to be applied evenly over the entire flexible membrane. But as the surface of th workpiece increases in area, it becomes increasingly difficult to exert sufficient continuous pressure acros the flexible membrane and the image-bearing paper sheet by vacuum alone to insure optimum print quality on the workpiece's printable surface. The '084, '239 and '155 patents do not specifically address.printing, on large,plastic or plast coated substrates, e.g., plastic coated metal plates. And while it is believed that the teachings of the '084 patent could be used to print high quality images on su substrates, the present invention improves on the techniques taught in the '084 patent when large plastic or plastic coated objects are the intended printable substrates.

In light of the melt printing techniques known i the art prior to the present invention, applicants believe that if one skilled in this art were to attempt to melt print an image onto a large substrate, an appliance control display plate, for example, having a printable surface appreciably greater in area than those of workpieces, typically telephone, typewriter or computer keys, to which melt printing techniques have hitherto been applied the technique that would suggest itself would be a hot stamp method such as that discusse in the '239 patent. The problem with using hot melt stamp printing on large substrates, however, would be that the images produced would undoubtedly have an unacceptably high percentage of rejects due to ghosting: the formation of two dye printed images on the workpiece's printable surface, one slightly out of register with the other. '

Ghosts in a melt printed image can result, first of all, from slight movements of the image-bearing paper sheet on the printable substrate, movements that can occur at several points in the melt printing process. For example, ghosts can occur at the moment the pressure applying surface of a hot stamp is lifted from the workpiece, due to the image-bearing paper sheet's tendency to adhere momentarily to the hot stamp's surfac and thus be lifted slightly from the workpiece's printed surface as well. When the paper sheet falls back onto the hot workpiece surface, it can do so in a slightly different orientation than it originally had, thus causing ghosting.- To substantially or entirely avoid this would require an as yet undeveloped technique for tightly holding the paper sheet to the workpiece surface while the hot stamp was being removed. Ghosts can also result from variations in the flatness of the hot stamp's surface, or in the flatness of the printable surface, or both.

A hot stamp, if used by itself to melt print a large printable surface, would have to rely for its proper operation on the pressure applying surface of the stamping structure being precisely the same shape as the surface on which melt printing is to be accomplished. If the printable surface were flat, for example, the pressure applying surface would also have to be flat. More generally, the two surfaces would have to be in registry with one another, meaning that they would both have to be the same precise shape. Since it is of course impossible to obtain perfect registry between any two surfaces, no matter how flat, some kind of approximation would have to be made, and the more time and money one would spend on perfecting such a process, the better the approximation would be. However, applicants have found that as the printable surfaces of workpieces subjected t melt printing increase in area, it becomes even more difficult to bring such surfaces into perfect registry with the pressure-applying surface of a hot stamp. Furthermore, the '239 patent teaches the use of pressures of 1 to 2 psi, and not higher, when using the hot stamping technique to melt print. Higher pressures, if used in operating the hot stamping apparatus disclose in the '239 patent, although they undoubtedly would be conducive to obtaining better melt printed images, could cause increased movement of the image-bearing paper shee in relation to the printable surface.

The present invention provides means of avoiding the problems, summarized above, that would be expected t occur if the previously-known membrane plus_^vacuum metho or hot stamping method were used to melt print images onto large printable plastic surfaces.

SUMMARY OF THE INVENTION In brief, the melt printing method of the presen invention, especially developed for accurate melt printing on large plastic surfaces, comprises:

—providing a large workpiece having a melt printable plastic surface;

—positioning over the plastic surface of the workpiece a flexible sheet, dye side down, bearing one o more meltable disperse dyes in the mirror image of the image which will be melt printed onto and into the plastic surface;

—overlaying the dye-bearing sheet with a flexibl membrane having a first surface and a second surface, whose function it is to further hold the dye-bearing sheet in its proper position relative to the workpiece, so that the first surface of the flexible membrane overlies the dye-bearing sheet; —pulling a vacuum (achieving a negative pressure differential) on the space between the uppermost (non- dye-bearing) surface of the dye-bearing sheet and the underside (first surface) of the flexible membrane sufficient to pull the first surface of the flexible membrane down tightly over the dye-bearing sheet, thus forcing or biasing this sheet against the workpiece and holding it in place for proper image transfer; and

—applying — by means of a heated, preferably relatively flat plate having a pressure-applying surface or other relatively flat heat and pressure transfer surface — heat and pressure to the upper (second) surface of the flexible membrane sufficient to melt the dye and, preferably, somewhat soften the section of the workpiece's melt printable plastic surface which is to receive the dye, thus transferring the dye image onto an into the plastic surface.

To the -best of applicants' knowledge,--melt printing has never been used successfully to transfer images to large plastic or plastic coated substrates, such as appliance control display plates of the type found in convection ovens, washing machines, automatic clothes dryers, microwave ovens, and like devices, or to chainsaw parts, Venetian blind slats, decorative containers and indeed any large surface on which one wishes to print a design for ornamental or utilitarian purposes. The advantages obtainable by now being able to use melt printing for such purposes are numerous. In the case of appliance control display plates, for example, the image-bearing sheet can be reverse printed in any language. Hence, the language in which instructions are given can be changed simply by changing the image- bearing sheet. Heretofore, such plates or panels have been made by first enamel-coating a metal substrate and then printing instructions, logos, etc. over the enamel coating. Aside from being a significantly more expensive method than that of the present invention, this prior art method also involves the use of large quantities of environmentally harmful solvents which must be disposed of in accordance with governmental regulations, if indee they can still be used at all.

It is, therefore, an object of the present invention to provide a new melt printing process. It is also an object of the present invention to provide a new process for melt printing dye images onto and into plastic surfaces.

A further object of the present invention is to provide a new process for melt printing dye images onto and into plastic or plastic coated substrates whose surfaces are appreciably greater in area than those to which melt printing methods have hitherto been applied. These and other objects, as well as the nature, scope and utilization of the present invention, will become readily apparent to those skilled in the art from the following ^description, the drawings,, and the appende claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a perspective view of a printing apparatus which can be used to practice the present invention, with the heat plate raised and the membrane lowered;

FIG. 2 shows a closeup of the table area of the apparatus of FIG. 1 with the membrane raised; FIG. 3 shows a sectional view taken along line 3

3 of FIG. 1;

FIG. 4 shows a sectional view taken along line 4

4 of FIG. l; and

FIGS. 5 and 6 are sectional views showing a plat for applying heat and pressure in contact with the flexible membrane for pressing a dye-bearing sheet against a workpiece. DETAILED DESCRIPTION OF THE INVENTION Preferred substrates to which melt printed image can be applied in accordance with the present invention include, inter alia, planar metal base members, e.g., plates or sheets made of metals such as aluminum, aluminum alloys, steel, brass, bronze, copper, or the like, coated on at least one of their planar surfaces with a melt printable layer of a softenable, dye- permeable, thermoplastic or thermoset material ("plastic"), e.g., thermoplastics such as polyethylene terephthalate, polybutylene terephthalate, or other linear thermoplastic polyesters, polycarbonates, and nylons, such as nylon 6, nylon 6/6, nylon 6/12, and thermosets such as cross-linked, e.g., epoxy cross- linked, urea-formaldehyde, melamine-formaldehyde and phenol-formaldehyde resins,? <epoxy.. resins,, -alkyd .resins and other thermoset polyester resins, and the like, as well as resin blends, e.g., a blend of a polyester with polycarbonate. Bilayer coatings of thermoplastic or thermoset materials can also be used, e.g., a base coat of a high molecular weight epoxy resin cross-linked with a urea-formaldehyde, melamine-formaldehyde or phenol- formaldehyde resin, top coated with a printable oilless alkyd (polyester) resin cross-linked with a melamine- formaldehyde resin. In such bilayer coatings, the base coat can optimally be provided with a pigment and the second layer can comprise a somewhat clear resin.

Since neither the composition nor the thickness of either the substrate (except in the circumstance indicated immediately below) or the printable plastic layer(s) is critical to the practice of this invention, any non-metallic substrate capable per se. or capable above a given thickness, of withstanding, without substantial deformation or other damage, the degree of heat and pressure applied to the workpiece necessary to cause adequate melting and diffusion of the disperse dye onto and into its printable plastic surface, and capable as well of being coated with the plastic layer that will receive the dye image, can also be used. Such substrate include plastics the same as or different than the ones used to coat non-plastic substrates, e.g., an all- polyester substrate, as well as wood, leather, ceramics, and the like.

In those instances where the plastic layer that will receive the dye image adheres poorly, or not at all to a particular substrate, adhesives can be used to provide, in effect, laminated structures whose plastic surfaces can be melt printed in accordance with the present invention.

Typically, substrates having surface areas of from about 3 to about 10 square feet can easily be melt printed by the method of the present invention. Another way to -describe. his advantage provided by the present invention is that plastic surfaces four times or more larger in surface area that those customarily printed by prior art techniques such as those described in the '084 '239 and '155 patents can now be successfully melt printed. Indeed, the only limitations at present on the area of the plastic surface that can be decorated in accordance with this invention are those imposed by the sizes of commercially available flexible membranes, by presently available melt printing machines and their heater controls, and by other such extrinsic limitations In theory, any size plastic surface can be melt printed successfully by the process of this invention.

The sheet bearing the meltable disperse dye imag will usually be a slightly flexible cellulosic paper sheet, preferably one as noted in, for example, the '155 patent at col. 4, 11. 40-56, which contains a thermosetting polymer intermixed with the paper pulp thereof to make the sheet heat deformable, and preferabl one which, as also noted in the '155 patent at from col. 4, 1. 57 to col. 5, 1. 11, is also coated, on the surfac to which the meltable disperse dye is applied, with a cross-linkable polymer to stabilize the sheet against adverse effects caused by humidity. However, "papers" o sheet materials made in part from materials other than cellulosic fibers, such as glass fibers, polymer fibers or fibrids, and the like, can also be used as the dye- bearing sheet.

The meltable disperse dyes used in practicing th present invention are those which, as taught in the '155 patent (col. 3, 11. 45-54), have melting points below and vaporization points above the thermal deflection temperature of the printable plastic surface. An illustrative listing of such dyes and dye types is found in the '155 patent at from col. 6, 1. 22 to col. 7, 1. 5. The preferred resilient flexible membranes used in practicing the present invention are made of silicone rubber of any suitable thickness, e.g. , from about 1/32 to about 1/16 of an inch thick, and suitable degree of flexibility that will provide membranes of adequate durability for repeated industrial use. Particularly suitable silicone rubbers include COHRlastic® silicone rubbers, e.g., COHRlastic® 9235 silicone rubber and the like (Connecticut Hard Rubber Co., New Haven, Connecticut) .

The temperature at which the melt printing process of the present invention can be practiced is not critical, other than as indicated hereinabove and in the •084, '239 and '155 patents with regard to the melting points and vaporization points of the meltable disperse dyes used in relation to the thermal deflection temperature of the plastic surface being printed. The temperature of the surface being printed when printing in accordance with this invention on a polyethylene terephthalate surface typically will range from about 325°F to about 375^βF. Increasing the printing temperature from room temperature (about 25°C) to about 325-375°F customarily takes from about 45-90 seconds, with printing taking place during this time. Preferably, printing will take place at from about 350-360°F, a temperature range reached in about 50-60 seconds, following which the heat and pressure applying means is removed and the workpiece allowed to cool. Likewise, the pressures employed can vary from about 35 psi to about 50 psi, or, in some instances 50 t 100 psi, and preferably the pressure at which printing takes place will be about 40 psi. As also indicated hereinabove, one significant advantage of the present invention is that pressures significantly greater than those taught by the prior art can be applied to ensure the printing of clear, sharp images on the plastic surfaces being coated, with minimal rejects.

Once a vacuum has been pulled between the flexible membrane and the image-bearing sheet, e.g., a vacuum of about 1 atmosphere (at least about 28 inHg) , the assembly of printable substrate, image-bearing sheet and flexible membrane will be subjected to the necessary heat and pressure to accomplish satisfactory melt printing. A preferred method of practicing this step is by bringing a heat and pressure applying means, e.g., a hot plate which can comprise two substantially flat meta plates having an electric heating element sandwiched between them, into contact with the upper surface of the flexible membrane. Printing surfaces and pressure applying means which are somewhat less than substantiall flat, e.g., somewhat concave or convex complementary surfaces, can also be used.

The vacuum pulled between the flexible membrane and the image-bearing sheet serves primarily to secure this sheet in the proper position for printing. The hea and pressure applying means provides the bulk of the pressure actually used, with heat, to accomplish melt printing. Because of the flexible membrane being pulled down by vacuum onto the image-bearing sheet, resulting i the image-bearing sheet's being more tightly and precisely held on the plastic surface being printed, the assembly can be subjected to greater pressures than possible when using prior art melt-printing methods. This too leads to better melt-printed images being obtained than hitherto possible. In a preferred embodiment, a heat plate, attache to an air cylinder arrangement to produce the necessary pressure to force the heat plate into tight registry against the flexible membrane, will be used. Any differences in registry between the heat plate and the assembly are absorbed by the flexible membrane's being depressed by the heat plate.

A preferred apparatus that can be used to practice the method of the present invention will now be described with reference to the drawings. This apparatus is identified as the apparatus 10 in FIG. l.

• The apparatus 10 generally comprises a base 12, a bed assembly 14, a flexible membrane assembly 16 positionable in overlying relationship to the bed assembly 14, a vacuum assembly 18, and a heat and pressure applying means 20. The vacuum assembly 18 serves to evacuate the area between the membrane assembly 16 and the bed assembly 14 when the membrane assembly 16 is positioned in overlying relationship to the bed assembly 14. The bed assembly 14 and the membrane assembly 16 are positionable beneath the heat and pressure applying means 20 so that the latter can be lowered into contact with the membrane 16.

When a workpiece 300 is appropriately positioned on the bed assembly 14, and a sheet 108 bearing a meltable disperse dye in the mirror image of a preselected image (not shown in FIG. 1) is overlaid on the workpiece 300, the apparatus 10 is operable to transfer the dye image from the sheet 108 to the workpiece 300 to produce the preselected image thereon. Specifically, by evacuating the area between the membrane assembly 16 and the bed assembly 14 using the vacuum assembly 18, next positioning the assembly of the membrane 16, the dye image-bearing sheet 108 and the be assembly 14 under the heat and pressure applying means 20, and then energizing and lowering the heat and pressure applying means 20, heat and pressure are simultaneously applied to the dye image-bearing sheet 1 (FIGS. 2, 5 & 6) and the surface of the workpiece to transfer the dye from the sheet 108 to the printable surface of the workpiece on the bed assembly 14.

FIGS. 1 and 2 show that the base 12 comprises a table-like structure having a top 22 and legs 24 supporting the remainder of the apparatus 10. Of cours any other suitable supporting structures can be used as well.

The bed assembly 14 is most clearly illustrated in FIGS. 1, 2 and 3 and comprises a rectangular frame 26 and a support plate portion 28 which.defines a supporti surface for receiving a workpiece or a plurality of workpieces in the apparatus 10 for the application of d images thereto. A mounting assembly 42 is included in the bed assembly 14 for the mounting thereof on the base 12.

The mounting assembly 42 is most clearly illustrated in FIGS. 1 and 2, and comprises a pair of slide rods 44 which are mounted in upwardly spaced rearwardly extending relation on the table top 22 with front mounts 46 and rear mounts (not shown) . Received o the rods 44 are front and rear slide members 48 and 50, respectively; front and rear cross members 52 and 54, respectively, extend between the two front slide member 48 and between the two rear slide members 50, respectively. Front vertical members 56 and rear vertical members (not shown) extend upwardly from the cross members 52 and 54, respectively, and are secured t the frame 26, whereby the frame 26 is slidably mounted o the rods 44.

The membrane assembly 16 is hingedly mounted on the bed assembly 14 along the rear edge thereof as at 57 and comprises an outer frame 58 and a resilient, flexibl membrane 60 which is clamped to the frame 58 by inner frame 62 which, in turn, is secured by several pivot screw/wing nut assemblies 64 and frame support 63, as illustrated in FIGS. 2 and 3. FIG. 3 shows the frame 58 to be of L-shaped cross-sectional configuration, and an inner frame 62 is dimensioned to interfit in the frame 58 so that the membrane 60 is captured therebetween and thereby clamped to the frame 58. Bonded to the outer frame 58 is a silicone rubber "V" seal 34. As will be further seen, the frame 58 is dimensioned so that when the membrane assembly 16 is located in overlying relation on the bed assembly, the V seal 34 is pinched therebetween in order to seal the periphery of the flexible membrane 60. When the membrane assembly 16 is so positioned over the bed assembly and a vacuum is drawn between the membrane assembly and the bed assembly, the relatively greater (atmospheric) pressure on the exterior (open V) side of the seal 34 tends to force the lower arm of the V into a tight sealing relationship with the frame 26.

Conventional telescoping arms 66 extend between the bed assembly 14 and the frame 58 for maintaining the membrane assembly 16 in the upwardly hinged or open disposition, when desired, as illustrated in FIG. 2.

Handles 68 attached to the frame 58 are used to manually close the membrane assembly so that it overlies the bed assembly 14.

The vacuum assembly 18 comprises a vacuum pump 70 which is mounted on the base 12 and which is actuated by a manual switch 72 mounted on the top 22. The vacuum pump 70 is connected through vacuum lines 74 to the vacuum ports 36 on the bed assembly 14 for drawing a vacuum in the area between the membrane assembly 16 and the bed assembly 14 when the membrane assembly 16 is in its lowered or closed position. Vacuum ports 36 are included, but not shown, in the support plate 28 in positions so as to directly underlie the workpiece or workpieces when placed on support plate 28.

The heat and pressure applying means 20 is shown in FIG. 1 mounted to a housing 76 within which a support structure 84 is mounted. The support structure 84 is preferably rigidly mounted to the housing 76 by screws o welding or the like since it will be supporting the high pressure which will be exerted by the heat and pressure applying means. Guide posts 196 and 198 are attached to the housing 76, and to pressure heat plate 200, and accurately guide the movement of pressure heat plate 200 A pneumatic pressure piston and cylinder assembly 80 which receives pressure via pressure line 81 is attached to support structure 84. The operation of the pressure piston moves extending piston rod 82 downward, thereby moving the heat and pressure applying plate 200 downward.

In operation, pressure line 81 and electric line 81a carry pressure and electrical power to the heat and pressure applying plate 200. Specifically, electric lin 81a carries electric current to the plate 200 to energiz and heat the heaters on same. Pressure line 81 is configured such that pressure therein causes movement of piston 80 downwardly which in turn causes the piston rod 82 to extend downward and toward membrane assembly 16 and bed assembly 14. Removal of the pressure on line 81 causes the piston rod 82 to retract towards its uppermos position. This return movement may be accomplished by a suitable spring (not shown) within the piston and cylinder assembly 80, reversed pressures in the cylinder, or the like.

FIGS. 4 and 5 show detailed drawings of the heat and pressure applying plate 200. FIG. 4 shows, by a sectional view, the plate 200 to consist of three sandwiched and connected layers. Layer 202 and 204 are upper and lower metal plates. Layer 204 includes a pressure applying surface 206 which is a substantially flat surface in this embodiment, and is adapted to appl pressure and heat to the membrane assembly 16. A pisto rod 82 is suitably connected to the plate 202. Sandwiched between the two metal layers 202, 204 is a resistive heating network 210 which preferably has its electrical connection at the location shown as 212 in FIG. 4. The electrical connection to point 212 causes a electric current to be passed through the resistive heating network 210 thereby effecting a heating thereof. Heat is conducted by the metal layer 204 to the pressure applying surface 206 so that this surface applies pressure due to the downward extension of the piston rod 82 as well as applying the heat caused by the resistive heating in layer 210. The operation of the apparatus described above will now be described in detail. The operation begins b positioning the workpiece on the support plate portion 2 and overlying a sheet 108 bearing a meltable disperse dye, in an image which is the mirror image of the preselected image to be printed on the workpiece, so tha the mirror image is in the desired orientation thereon. The positioning is done while the apparatus is in the open position shown in FIG. 2. The membrane assembly 16 is then moved to its lowered or closed position illustrated in FIG. 1. The switch 72 is then manipulate to actuate the vacuum assembly 18, whereby the membrane 60 is drawn into pressurized communication with the shee 108 overlying the workpiece to effect the pressurized engagement of the sheet with the workpiece. The bed assembly 14 and the membrane assembly 16 are then moved rearwardly in the apparatus 10, so that the membrane 60 is disposed beneath the hood 78 and below pressure/heat assembly 20. The control box 86 actuates the pressure system to increase pressure in line 81. This causes the piston rod 82 to be lowered to move the heated heat and pressure applying plate 200 down with considerable force against the membrane 60, thereby applying pressure and heat to the membrane 60 and accordingly to the image- bearing paper sheet 108 (positioned above the workpiece 300) and the workpiece itself.

Pneumatic pressure and electric power are maintained to energize heating elements 210 and to continue to press the heated pressure transmitting surface of the plate 200 against the workpiece 300. FIGS. 5 and 6 show alternate views of the heat and pressure applying plate 200 as it is pressed against the workpiece 300. FIG. 6 shows the plate 200 with its pressure transmitting surface 206 pressed against the flexible membrane 60, which in turn overlies the meltabl disperse dye bearing sheet 108, which in turn overlies the workpiece 300. Workpiece 300 is shown with two layers, a metal substrate layer 302 and an overlying printable plastic layer 304. The connection of the heat and pressure applying plate 200 to the support guides 198, 196 is also shown, via the support connections 194,192 which attach to the support guides. Various automated control mechanisms, not shown, can also be employed to monitor and regulate operating conditions such as temperature, pressure and vacuum in the system as well as to control the timing of the various operating steps. FIG. 2 shows a frame assembly 86 operable for receiving four workpieces 300. Frame assemblies of this type can be constructed for receiving varying numbers of workpieces as desired. For example, it may be desired t transfer a number of similar or different images onto only one relatively large workpiece, then cut and form individual products from this larger workpiece. Alternatively, individual (relatively smaller) pieces ca be first cut and formed and then have dye images transferred onto and into printable plastic surfaces of the individual smaller workpieces. Frame assembly 86 ca also include workplace receiving pad portions (not shown corresponding approximately to the respective footprints of the workpieces to be processed. These pad portions can, for example, comprise approximately 1/4 inch thick pieces of silicone rubber or similar material bonded to the frame assembly 86. Alternatively, the workpiece receiving pad portion(s) can cover virtually the entire upper (workpiece receiving) surface of the plate portion 28, so long as sufficient vent holes are provided in suc a unitary pad portion to allow the vacuum system to properly evacuate the area between the membrane assembly 16 and the bed assembly 14 when the membrane assembly 16 is positioned in overlying relationship to the bed assembly 14. Such pad portions serve not only to cushio and hold the workpiece(s) , but also insulate each workpiece from temperature increases within the frame assembly 86 which can occur during continuous operation of the apparatus.

Heat transfer from the frame assembly to the workpiece(s) can disadvantageously affect the workpiece's characteristics, particularly those of its printable plastic layer(s). To further aid in avoiding this problem, a heat transfer system (such as water chilling) can optionally be incorporated in the support plate 28 to reduce the temperature of the frame assembly 86 during continuous operations. Or, a heat transfer system can be directly incorporated into the frame assembly 86 to further provide heat removal. By using such features or combinations thereof, the primary source of heat to the workpiece can be made to be the heat and pressure applying plate 200, and thus the heat reaching the workpiece can be specifically controlled to maximize dye transfer and minimize undesired effects.

The frame assembly 86 typically is positioned over one or more vacuum ports 36 on the plate portion 28 and also includes holes or slots (not shown) extending through the assembly 86 to facilitate drawing a vacuum around or near the periphery of each workpiece on the frame assembly 86. A sheet 108 bearing dye images 100 and having positioning slots 112 therein is receivable o the frame 86 so that the positioning pins 106 are received in the slots 1121 Rounded bars 114 are receivable on the portions of the pins 106 which protrud through the sheet 108 so that the pins 106 do not ruptur the membrane 60 when it is drawn downwardly with the evacuating assembly 18. The frame assembly 86 typically remains on the plate portion 28 during continuous printing operations, with the individual workpieces bein placed onto and removed from their respective positions on the assembly 86 with each printing cycle. However, the frame assembly 86 can also be removed from the plate portion 28 with each printing cycle if desired or necessary. After sufficient heat and pressure have been applied to transfer the dye onto and into the plastic surface of the workpiece, the heat plate is lifted so that the workpiece 300 can be cooled. The vacuum is maintained for a certain amount of time in order to prevent the paper from moving before cooling, to prevent ghosting. The vacuum is subsequently released after the danger of ghosting is passed, the flexible membrane is lifted and the spent image-bearing sheet is removed. The above discussion of this invention is directed primarily to preferred embodiments and practice thereof. It will be readily apparent to those skilled i the art that further changes and modifications in the actual implementation of the concepts described herein can easily be made.without departing from the spirit and scope of the invention as defined by the following claims.

Claims

WE CLAIM:

1. A method of printing a meltable disperse dye image onto and into a plastic surface on a workpiece, comprising: providing a workpiece which includes a plastic surface which can be disperse dye melt printed, the area of said surface being appreciably greater than that of a key in a telephone, typewriter or computer key array; positioning over the plastic surface a flexible sheet, dye side down, bearing one or more meltable disperse dyes in the mirror image of the image which will be melt printed onto and into the plastic surface; overlaying the dye-bearing sheet with a flexible membrane so that a first surface of the membrane overlies the sheet; pulling a vacuum on the space between the first surface of the flexible membrane and the non-dye-bearing side of the dye-bearing sheet to pull the flexible membrane into pressure contact with the dye-bearing sheet and bias the dye-bearing sheet into contact with the plastic surface of the workpiece and, while maintaining the vacuum; applying, by means of a heated plate having a pressure-applying surface, heat and pressure to the upper surface of the flexible membrane sufficient to melt the disperse dye and thereby transfer the dye image onto and into the plastic surface of the workpiece.

2. A method as recited in claim 1, wherein the workpiece comprises a planar base member coated on at least one of its planar surfaces with a coating comprising a melt printable layer of a softenable, dye- permeable thermoplastic or thermoset material.

3. A method as recited in claim 2, wherein the planar base member comprises a metal plate or sheet.

4. A method as recited in claim 3, wherein the metal is aluminum, an aluminum alloy, steel, brass, bronze or copper.

5. A method as recited in claim 2, wherein the planar base member comprises a non-metallic substrate.

6. A method as recited in claim 5, wherein the non-metallic substrate comprises a plastic, wood, leathe or ceramic substrate.

7. A method as recited in claim 2, wherein the melt printable layer comprises a linear thermoplastic polyester.

8. A method as recited in claim 5, wherein the polyester is polyethylene terephthalate or polybutylene terephthalate.

9. A method as recited in claim 1, wherein the melt printable layer comprises a base coat of a high molecular weight epoxy resin cross-linked with a urea- formaldehyde, melamine-formaldehyde or phenol- formaldehyde resin and topcoated with a printable oilles alkyd resin cross-linked with a melamine-formaldehyde resin.

10. A method as recited in claim 1, wherein the plastic surface of the workpiece being melt printed has printable surface area of from about 3 to about 10 squar feet.

11. A method as recited in claim 1, wherein the meltable disperse dye has a melting point below and a vaporization point above the thermal deflection temperature of the printable plastic surface.

12. A method as recited in claim 1, wherein the method is practiced using a workpiece receiving pad positioned beneath the workpiece being printed.

13. A method as recited in claim 10, wherein th workpiece receiving pad comprises a silicone rubber pad.

14. A method as recited in claim 2, wherein the heat and pressure applied to the upper surface of the flexible membrane range from about 325^βF to about 375°F and from about 35 psi to about 50 psi, respectively.

15. A plastic surfaced workpiece printed with a disperse dye image by the method of any one of claims 1- 14, inclusive.

16. A method of melt printing on a large workpiece, comprising the steps of: providing a backing support structure which includes a large plate of a material which will not accept melt printing; forming a coating of a material which will accept melt printing on a first surface of said backing support structure to form a coated plate workpiece; disposing a dye-bearing sheet, which has an image thereon formed in a dye of a type for melt printing, over the coating on said coated plate workpiece; applying a negative pressure differential to said dye bearing sheet to pull it into pressure contact with the coating on said coated plate workpiece; and transferring said image from said dye bearing sheet to said coating of said coated plate workpiece.

17. A method as in claim 16 wherein said transferring step includes the step of applying a heated plate which has a pressure applying surface of approximately the same outline as said surface of said coated plate workpiece thereagainst at a high pressure while applying said negative pressure differential to said dye bearing sheet to heat said dye under pressure t thereby melt said dye and transfer it to said coating of said coated plate workpiece.

18. A method as in claim 17, wherein said applying a negative pressure differential step includes the steps of overlying said dye-bearing sheet with flexible membrane so that a first surface of said flexible,-membrane overlies .said dye., earing sheet and applying said negative pressure differential to said first surface of said flexible membrane to pull said flexible membrane into pressure with said dye bearing sheet so that said first surface biases said dye bearing sheet into contact with said coating of said coated plat workpiece.

19. A method as in claim 16 wherein said backin support structure is metal, and said coating is plastic.

20. An apparatus for melt printing on a large workpiece, which has a first surface which will accept melt printing thereon, comprising: means for holding said workpiece with said first surface facing in a first direction; means for holding a dye-bearing sheet, which has an image thereon formed in a dye of a type for melt printing, over said first surface of said workpiece; flexible membrane which is movable into a first position in which a first surface of said flexible membrane overlies said dye bearing sheet; evacuating means for applying a negative pressure differential to said first surface of said flexible membrane when positioned over said dye bearing sheet, to bias said flexible membrane against said dye bearing sheet so that said first surface of said flexible membrane pressurizes said dye bearing sheet into contact with said first surface of said workpiece; and a heat/pressure assembly, which has a pressure applying surface of approximately corresponding contour as said first surface of said workpiece, which includes a heater for heating said pressure applying surface, and means for positioning said pressure applying surface in pressure contact with said flexible membrane and dye- bearing sheet against said metal member to heat said dye under pressure to thereby melt said dye and transfer it to said surface of said workpiece.

21. An apparatus as in claim 20, wherein said workpiece is a metal sheet, which has a coating thereon, said coating formed of a material which will accept melt printing.

22. An apparatus as in claim 21, wherein said locating means of said heat pressure assembly includes a air piston, operable to press said pressure applying surface against said flexible membrane, said dye bearing member and said metal plate.

23. An apparatus as in claim 22 wherein said heat pressure assembly includes a plate which includes a first surface attached to said air piston, said pressure applying surface, and a heater assembly between said first surface and said pressure applying surface.

24. An apparatus as in claim 21, wherein said holding means includes a frame with positioning pins tha extend upwards, and said dye bearing sheet includes positioning slots which mate with said positioning pins.

25. An apparatus as in claim 24, further comprising means, receivable over said positioning pins for protecting said flexible membrane against puncture thereby.

26. An apparatus for melt printing, comprising at least one workpiece, which has a backing support structure of a material which will not accept melt printing thereon, and a first surface, coated on said backing support structure, which will accept melt printing thereon; means for holding said workpiece with said first surface facing in a first direction; means for holding a dye-bearing sheet, which has an image thereon formed in a dye of a type for melt printing, over said first surface of said workpiece; evacuating means for applying a negative pressur differential to said dye bearing sheet, to pull it into pressure contact with said first surface; and means for transferring said image from said dye bearing sheet to said first surface of said workpiece.

27. An apparatus as in claim 26, wherein said transferring means includes a heat/pressure assembly, which has a pressure applying surface of approximately a same contour as said first surface of said workpiece, which includes a heater for heating said pressure applying surface, and means for locating said pressure applying surface into pressure contact with said flexible membrane and dye-bearing sheet against said metal member to heat said dye under pressure to thereby transfer said dye to said workpiece.

28. An apparatus as in claim 27, wherein said evacuating means comprises a flexible membrane which is movable into a first position in which a first surface of said flexible membrane overlies said dye bearing sheet, said flexible membrane including means for vacuum sealing an area underneath said flexible membrane, including said workpiece, and a vacuum pump which evacuates an area under said flexible membrane.

29. An apparatus for melt printing on a workpiece, which has a first surface which will accept melt printing thereon, said first surface being of a first shape, comprising: means for holding said workpiece with said first surface facing in a first direction; means for providing a dye of a type for melt printing over said first surface of said workpiece; a flexible membrane which is movable into a firs position in which a first surface of said flexible membrane overlies said dye providing means, said flexibl membrane having a property of compressibility; a pressure assembly, which has a pressure applying surface of approximately a mating shape to said first shape of said first surface of said workpiece; and means for locating said pressure applying surfac into pressure contact with said flexible membrane and compressing said flexible membrane such that any shape variations between said pressure applying surface and said first surface of said workpiece are absorbed by sai flexible membrane and transferring said die to said workpiece under said pressurized communication.

30. An apparatus as in claim 19 further comprising evacuating means for applying a negative pressur differential to said first surface of said flexible membrane when positioned over said dye bearing sheet, to bias said flexible membrane against said dye bearing sheet so that said first surface of said flexible membrane pressurizes said dye bearing sheet into contact with said first surface of said workpiece.

31. An apparatus as in claim 30, wherein said pressure assembly includes a heater for heating said pressure applying surface.