Embossing apparatus and method

Abstract: Embossing apparatus has a perforated stainless steel platen (9) which is fixedly disposed vertically above a nip (10) between an embossing roll (11) and a backup roll (12), both of which are designed to rotate about horizontal axes. The platen (9) is curved so as to slidably support a sheet (5) of thermplastic material, as it moves from a generally horizontal position on the platen to a vertical position as it exits the platen downwardly into the nip. Means are provided above (17-20) and below (15) the platen (9) for heating the sheet of thermoplastic material as it slides over the fixed platen (9).

Description: The invention relates to embossing, especially to a device used to preheat a thermoplastic embossable material, prior to contact with an embossing roll. It is well known in the art to pass such material from a heat drum downwardly between a pair of radiant heaters into a nip formed between an embossing roll and a backup or bed roll. It has been found that air or gaseous vapors form under the sheet of material next to the surface of the heat drum, as the material is heated. The accumulation of such vapor forms under the sheet, bubbles which interfere with the uniform heating of the material by the heat drum and radiant heaters causing lower embossing depths which are referred to as embossing streaks. Moreover, once a vinyl-type product is heated to the yield point for embossing, any variation in tension in the sheet can cause distortion of the material. This distortion is particularly bad in geometric patterns that have to have straight lines and configurations. Heat drums have to be large enough to provide necessary dwell time for proper heat transfer to the material. However, use of a larger drum results in more problems of inertia and speed control. Minute speed variations can alter the tension to cause distortion of the material. Then too, the temperature of a heat drum can vary from end-to-end to cause a temperature differential in the material which, in turn, causes a variation in the embossment of the material resulting in an unacceptable appearance and feel of the finished product. U.S. Patent No. 3,950,480 relates to a method of embossing plastic material without the use of a heat drum. Instead, a pair of hot air heaters 14, 15 are used to direct streams of hot air against opposing sides of the material, as the sole method of heating the material for embossing. For some materials, it can be imagined that the heating zone between the heaters would have to be quite lengthy to properly heat the material. Also, the streams of hot air would have to be carefully controlled, so as not to adversely distort the material as it is struck by the streams of air. U.S. Patent No. 4,556,378 relates to a pair of adjacently disposed rotatable discs 2, 3, the first disc 2 to be encountered by the material, carrying a circumferential band which is used to preheat the material, prior to contacting the second, molding disc 3. It can be imagined that the rotational speeds of the two discs must be carefully controlled to prevent distortion of the material. U.S. Patent No. 2,776,451 discloses an embossing system in which a heated, perforated metal conveyor (a rotating drum and optionally also a metal carrier belt) carries the material to the embossing nip. Again, the speed of the conveyor must be carefully controlled to prevent distortion of the material. The invention is directed to the provision of an embossing apparatus in which one or more of these problems are eliminated or substantially reduced. Briefly stated, the invention is in an apparatus and related process for embossing a heated thermoplastic material, such as a scrim or paper backed vinyl. The apparatus comprises a perforated, fixed platen which is curved downwardly towards the nip formed between the embossing roll and backup roll, such that the material will be slidably supported on the platen. Means are provided for heating the platen and, preferably, the outer surface of the material farther spaced from the platen than the underside or backing of the material contacting the platen. A scrim or paper backing of the vinyl sholdnot be exposed to the high temperatures to which the vinyl is exposed, because of the instability of such backing or substrate, when such is used. Description of Drawing The following description of an embodiment will be better understood by having reference to the accompanying drawing which is a schematic drawing of an embossing apparatus and system. Detailed Description of Drawing A sheet 5 of any suitable fabric- or paper-backed thermoplastic material is drawn from a letoff or supply roll 6 by an assembly of drive rolls 7 and passed around a tensioning roll 8 for sliding support on an adjacent, fixed platen 9 which is designed to direct the thermoplastic sheet 5 into the nip 10 formed between an embossing roll 11 and a backup or bed roll 12, having horizontal axes, for embossing and subsequent cooling. The fixed platen 9 is composed of stainless steel and is sized to accomodate the width of the thermoplastic sheet and the length of travel of the sheet 5 necessary to properly heat the sheet 5 for embossment. The platen 9 is perforated in accordance with the need to allow the escape of air or gaseous vapors which are generated during the heating of the sheet 5. The platen 9 curves down in a substantially uniform arc, with inlet end 13 of the platen 9 angularly disposed slightly downwardly from the horizontal, and the discharge end 14 of the platen 9 almost vertical, so as to direct the sheet 5 downwardly into the nip 10 between the embossing roll 11 and the backup roll 12 which is driven to pull the sheet 5 from the platen 9 into the nip 10. The sheet passes directly down into the nip 10 through the short space from the platen end 14. A number of similar strip heaters 15 are secured crosswise to the underside 16 of the platen 9 to heat the platen 9 which, in turn, heats the thermoplastic sheet 5. A plurality of infrared heaters 17-20 are disposed in evenly spaced relation over the platen surface to heat the sheet from above, as it passes between the infrared heaters 17-20 and platen 9. Any suitable infrared heat sensor 21 can be positioned adjacent the discharge end 14 of the platen 9 to monitor the temperature of the sheet 5 and send signals to a panel box 22 which automatically controls operation of the infrared heaters 17-20. The heating can be manually controlled rather than automatically controlled, if desired. Thus, there has been described an embossing apparatus that utilizes a unique perforated stainless steel platen which allows the gaseous products, generated during the heating cycle, to escape, thereby preventing distortion of the thermoplastic material. The relative motion between the platen and material as the material moves across the fixed platen, helps the gaseous products to escape. The shape, position, and slick outer surface of the platen allows the material to move across it with little or no resistance. The platen provides support for the heated material to prevent distortion of the material under its own weight, and is designed to feed the material into the mouth of the nip between the embossing and backup rolls. It can be said that the platen causes the material to move in a downhill direction toward the embossing roll, contrary to U.S. Patent No. 2,776,451 which teaches moving the material in an uphill direction, prior to moving it downhill. The entry and exit portions, as they approach the horizontal and vertical respectively, may reach those alignments or may only curve towards them without reaching them. The heated platen arrangement is particularly well suited for vinyl-type products that lose stability and strength when heated in the embossing process. The vinyl-type products can be heated on the platen to a yielding point for embossment and supported into the nip between the embossing and backup rolls. In this way, tension can be reduced to a minimum to prevent distortion. This is especially true on wallcoverings of vinyl construction.

Claims: Embossing apparatus comprising rotatable embossing and backup rolls (11,12) forming a nip (10), and means for driving a sheet of thermoplastic material past heating means and into the nip (10), characterised in that the heating means comprise a curved heatable perforated platen (9) disposed above the nip (10), fixed to support a sheet of said material slidably in use and, from a generally upright exit portion (14) above the nip (10), curving upwardly and away from the upright to an entry portion (13) approaching horizontal. Apparatus according to claim 1 in which the heating means further comprise at least one heater (17-20) spaced over the platen surface for supplying additional heat to a sheet of material moving along that surface. Apparatus according to claim 2 in which the at least one heater above the platen (9) comprise one or more infrared heaters (17-20). Apparatus according to any one of claims 1 to 3 in which the platen (9) is heatable by a series of electric strip heaters (15) at the underside (16) thereof. Apparatus according to any one of the preceding claims in which the platen (9) is of stainless steel. Apparatus according to any one of the preceding claims in which the means for driving a sheet of material comprise means for driving the backup roll (12) in rotation. Apparatus according to any one of the preceding claims in which the means for driving a sheet of material comprise drive rolls (7) disposed above the platen (9). Apparatus according to any one of the preceding claims, comprising a tensioning roll (8) for engaging a said sheet of thermoplastic material upstream of the entry (13) of the platen (9). Apparatus according to any one of the preceding claims in which the sliding surface of the platen (9) forms a substantially continuous arc from entry (13) to exit (14). A method of embossing a sheet (5) comprising thermoplastic material, in which the sheet (5) is heated and passed continuously through a nip (10) formed by an embossing roll (11) and a backup roll (12), characterised in that the sheet (5) is heated by sliding it over the upper surface of a fixed, heated platen (9) which supports the sheet (5) as it slides, curves smoothly downwardly from one side of the nip (10) to a generally downward exit portion (14) above the nip (10), and has perforations for gas to escape from between the platen (9) and sheet (5). A method according to claim 10 in which the sheet (5) initially meets the platen (9) at a generally horizontal entry portion (13) thereof. A method according to claim 10 or claim 11 in which the thermoplastic material is a vinyl-type thermoplastic material. A method according to claim 12 in which the sheet (5) has a scrim or paper backing. A method according to claim 12 or claim 13, comprising making a wallcovering of the sheet (5). A method according to any one of claims 10 to 14, comprising directing heat onto the sheet (5) from at least one heater (17-20) spaced above the surface of the platen (9). A method according to any one of claims 10 to 15 in which the sheet (5) is continuously driven by means of drive rolls (7) disposed above the platen (9).

Inventors: GODWIN JAMES E, GODWIN, JAMES E.
Applicants: GENCORP INC, GENCORP INC.
Application year: 1991
Main classification: B29B13
IPC codes: B29C35, B29C59, B29B13, B29L9, B29K105