TRANSFER PRINTING

Abstract: There is disclosed a method for transfer printing by heating a transfer sheet having a printing ink layer formed on a base sheet with a hot roll and, at the same time, pressing the transfer sheet against a substrate for printing with the hot roll thereby transferring the printing ink layer onto the substrate. The method is characterized by using a biaxially stretched plastics material film as the base sheet and, at the same time, effecting the transfer after the substrate for printing has been preheated. Preferably, the biaxially stretch­ ed plastics material is polyester film or polypropylene film.

Description: This invention relates to the thermal transfer of patterns or letters on substrates such as plastics material containers, for example the plastics material containers disclosed in EP-A-0113160 and EP-A-0163492. For printing onto the curved surfaces of various plastics material containers for cosmetic articles and foodstuffs, the offset method, the silk screen method, and the thermal transfer method are known. The offset method, the silk screen method and the thermal transfer method are suitable for the purpose of printing onto the curved surfaces of, for example plastics material containers. The offset method has the disadvantage that, since it operates by having different colours placed one after another on a rubber plate, the number of colours is limited and the so-called "process picture patterns" using continuously changing colours cannot be printed. The silk screen method has the disadvantages that it suffers from a low printing speed and that it cannot be used to print process picture patterns. The thermal transfer method comprises printing a picture pattern in advance on the transfer sheet by gravure printing and transferring the printed picture pattern by means of heat; therefore, the method is capable of producing clean prints. It nevertheless has the disadvantage that the printing speed is low because the transfer relies for heat and pressure on the hot roll and the transfer sheet is expensive because the base sheet is quality paper. In US-A-3261734, there is disclosed a method for applying indicia to preformed containers made of stiff flexible material which can be softened with heat, the method comprising feeding the containers successively along a predetermined path, at one location along said path applying indicia to surfaces of successive containers, subjecting each container to inflation while the idicia is being applied to it, and in advance of said location heating each container so that said inflation presses outwardly on said surfaces. EP-A-0109313, there is disclosed a process for providing an abrasion-resistant deposit on the surface of a substrate, comprising applying to a transfer carrier an ultra-thin deposit consisting essentially of a non-resinous binder material and mineral abrasive particles, drying said ultra-thin deposit at a temperature of at least 60°, transferring said dried ultra-thin deposit from said carrier to the surface of a substrate under conditions of heat and pressure whereby said deposit becomes adhered to said substrate, and removing said carrier. The present invention has been devised for the purpose of solving the drawbacks suffered by the conventional methods as described above, and is characterized by using a biaxially stretched plastics material film as the base sheet and, at the same time, effecting the thermal transfer after the substrate has been preheated. To be specific, the present invention provides a continuous method for transfer printing on to preheated curved surfaces of a succession of containers or like objects, which which comprises heating by a hot roll a transfer sheet having a printing ink layer formed on a base sheet and at the same time pressing successive portions of said transfer sheet against said surfaces substrate by said hot roll thereby transferring said printing ink layer onto said surfaces, characterized in that said base sheet comprises a biaxially stretched plastics material film. The transfer sheet has strips of a printing ink layer superposed in a regular spacing on a base sheet. Generally, on the base sheet, a peel layer is formed by first applying a peeling agent and, when necessary, superposing a top coat thereon. The peeling agent so used is preferably either of an acrylic type or of a chloride rubber type. At times, the top coat may be formed by using the peeling agent as described above. Generally, on the printing ink layer, an adhesive layer intended to provide required adhesion on the curved surface of a given container is superposed. Of course, the material for the adhesive layer is selected to suit both the material forming the printing ink layer and that forming the outer surface of the container. The method of this invention is characterized by using in the foregoing transfer sheet a film of biaxially stretched plastics material as the base sheet therefor. Although the thickness of the base sheet is variable with the temperature of the hot roll, the preheating temperature of the container, the length of transfer time, etc., it is important that this thickness should at least exceed the minimum required for withstanding the heat used during the course of the transfer. In most cases, the proper thickness falls in the range of 10 to 50 microns. As regards the plastics material film, an unstretched film is unsuitable because the film has a tendency to stretch and deform the printed ink layer; a monoaxially stretched film is also unsuitable because this film has a tendency to fracture. The kinds of containers subjected to printing by the method of this invention are not specifically limited. Typical examples of such containers are various containers having layers of polyethylene, polypropylene, polyethlene terephthalate, vinyl chloride, polycarbonate, and acryl resin as the outermost layers thereof; plastics material handles as for writing instruments; and cans of iron, aluminium, etc. These containers are used for holding beverages, foodstuffs, cosmetic articles, medicines, etc. The temperature of the hot roll preferably does not exceed 240 o C and preferably does not fall below 230 o C. Although this temperature is variable with the materials for the adhesive layer, the peel layer, the printing ink layer, etc., it should exceed 170 o C where the container is used for holding a foodstuff and is therefore destined to be heated in a retort, for example. The preheating temperature is desired to be as high as possible within the range in which the temperature has no adverse effect on the container. In the case of a container made predominantly of polypropylene, the preheating temperature is preferably in the range of 90 o to 130 o C. The type of preheating means used is unimportant. An electric heater, an infrared heater, or a flow of hot air can be utilized effectively. For the purpose of enabling the container to be uniformly preheated, this preheating is usually carried out in a space enclosed within, for example, a box. In the method of this invention, the use of the biaxially stretched plastics material film as a base sheet in the place of quality paper improves the conduction of heat from the hot roll to the printing ink layer and the preheating of the substrate for printing enables the thermal transfer printing to proceed rapidly and produce a clear print. Reference is made herein, by way of example, to the accompanying drawings in which: Fig. 1 is a cross section of a typical transfer sheet used for transfer printing by the method of this invention; Fig. 2 is a plan view of the transfer sheet of Fig. 1; Fig. 3 is a front view schematically illustrating a transfer apparatus; Fig. 4 is a cross section of a typical substrate used for transfer printing by the method of this invention; Fig. 5 is a cross section of another typical substrate; and Fig. 6 is a plan view showing one example of a transfer sheet for use in the present invention. In Figures 1 to 3 and 6, numerals 1 and 1a denote the transfer sheet, numeral 2 the base sheet, numeral 3 the peel layer, numerals 4, 4a, 4b ... the printing ink layer, and numeral 5 the adhesive layer. Referring to Figure 4, a laminated tube for a can body consists sequentially in the outward direction of an unstretched polypropylene layer A which is 70 microns in thickness, a carboxylic acid-grafted polypropylene adhesive layer B which is 7 microns in thickness, an aluminium foil C which is 9 microns in thickness, a urethane type adhesive layer D applied at a rate of 4.5 g/m², an unstretched polypropylene layer E which is 30 microns in thickness, a urethane type adhesive layer F applied at a rate of 4.5 g/m², a superposed sheet 22 of unstretched polypropylene 200 microns in thickness, a coating plastics material layer 23 of a 1:1 mixture of polypropylene and calcium carbonate about 600 microns in thickness, and a coating layer 24 of polypropylene block copolymer 10 to 20 microns in thickness. Transfer printing was carried out with a transfer printing apparatus as schematically illustrated in Fig. 3 under the following conditions: The transfer printing apparatus comprises a feed roll shaft 6 for transfer sheet 1, a hot roll 7, a takeup roll shaft 8, a sensor 9 for issuing commands to start and stop the takeup roll shaft 8, and a mechanism for feeding a laminated tube 10. The transfer sheet 1 is fed from the feed roll shaft 6, passed under the hot roll 7, and wound on the takeup roll shaft 8. The feed roll shaft 6 is constantly urged in the direction opposite the direction of feeding of the transfer sheet 1 so as to preclude any loosening of the transfer sheet. In the meantime, the takeup roll shaft 8 is intermittently driven by the commands from the sensor 9. The hot roll 7 is continuously rotated and, at the same time, intermittently moved vertically and, at the lower reach of the vertical motion thereof, pressed into contact with the laminate tube 10 through the medium of the transfer sheet 1 so as to import rotation to the laminate tube 10 and effect transfer printing. The mechamism for feeding the laminate tube 10 comprises a feed path 11, a box-shaped hot-air heater 12, a rotary plate 14 having a plurality of mandrels 13 rotatably attached thereto, and a discharge path 15. Before the transfer printing is started, this apparatus is arranged such that the hot roll 7 is at the upper reach of the vertical motion thereof and the transfer sheet 1 is at a stop where the centre of a blank space 16 between adjacent printing ink layers 4 (Fig. 2) falls directly below the hot roll 7. A multiplity of laminate tubes 10 are continously fed through the feed path 11 , loosely nipped by the mandrels 13, and advanced by the rotation of the rotary plate 14 to a point directly below the hot roll 7. Then, the hot roll 7 is lowered and pressed into contact with the laminate tube 10 and caused to impart rotation to the laminate tube and effect transfer printing thereon. When the transfer printing is completed, the hot roll 7 is elevated and the sensor 9 is actuated to rotate the takeup roll shaft 8 by an angle equivalent to one pitch of the transfer sheet 1. In the meantime, the next laminate tube is advanced by the rotation of the rotary plate 14 to the point directly below the hot roll, with the result that the apparatus is made ready for the next round of transfer printing. The laminate tube 10 which has undergone the transfer printing and has been brought to the discharge path 15 is released via the discharge path 15 and transferred to the next step. The transfer sheet 1 used in this working example has a construction whose cross section is as shown in Fig. 1 and whose plan view is as shown in Fig. 2. This transfer sheet 1 comprises a base sheet 2 of biaxially stretched polypropylene film 20 microns in thickness, a peel layer 3, a printing ink layer 4, and an adhesive layer 5. The printing ink layer 4 consists of a plurality of strips regularly spaced by blank spaces 16 as illustrated in Fig. 2. When the transfer printing described above was made onto the outer surface of the laminate tube for use as a can body the transfer of print occurred satisfactorily and the printed surface was clean. The speed of printing in this operation was high. Referring to Fig. 5, a laminated tube for a can body consisting sequentially in the outward direction of a low density polyethylene layer which is 60 microns in thickness, an aliphatic urethane type adhesive layer b applied at a rate of 4.5 g/m², an aluminium foil c which is 9 microns in thickness, a low-density polyethylene layer d which is 25 microns in thickness, a vinyl acetate type adhesive layer 25, a sheet layer 26 of paper 0.2 mm in thickness, a vinyl acetate type adhesive layer 27, an intermediate paper layer 28 which is 0.3 mm in thickness having the lateral edges spirally coiled in an abutted state, a polypropylene 29 of MI = 50 to 100 intended to aid in adhesion, an upper paper layer 30 which is 0.3 mm in thickness having abutted portions spirally coiled and staggered from the abutted protions of the aforementioned intermediate paper layer 28, and a polyethylene layer 31 of MI = 50 to 100 having a thickness of 10 to 20 microns. Transfer printing was effected with a transfer printing apparatus schematically illustrated in Fig. 3 under the following conditions: When the transfer printing was made as described above on the outer surface of the laminate tube for can body the transfer of print occurred satisfactorily and the printed surface was clean. The speed of printing in this operation was high. In the method of the present invention, the printing cost can be lowered by using the base sheet of inexpensive biaxially stretched plastic material film in the transfer sheet and the speed of printing can be increased by having the substrate for printing heated in advance. A further transfer sheet 1 for use in the invention consists, for example as shown in Fig. 6, of a base sheet 2 of continuous length made of polyethylene terephthalate, on which printing ink layers 4a, 4b, 4c, ... are coated at equal intervals, leaving a blank space 16 between each. The blank spaces 16 formed between the printing layers 4a, 4b, ... of the transfer sheet are provided for the purpose of avoiding deformation of figures and patterns on the transfer sheet due to any stretching thereof near the hot roll, caused by the heat. The substrates are not limited to a particular type but may include any type of containers or like objects having a curved surface on which transfer can be effected with a hot roll. Plastics material receptacles having circular surfaces are suitable objects for this process. According to the present invention as described above, when each heat transfer operation is complete, the blank space alone is located immediately below the hot roll so that the heat of the hot roll does not affect the printing ink layer. Accordingly, it is possible for the dimension of the blank space to be reduced. Thus, the number of the transfer cycles in a unitary length of the transfer sheet can be increased, while the unit cost of the transfer can be reduced.

Claims: A continuous method for transfer printing on to preheated curved surfaces of a succession of containers or like objects which comprises heating by a hot roll (7) a transfer sheet (1) having a printing ink layer (4) formed on a base sheet (2) and at the same time pressing successive portions of said transfer sheet (1) against said surfaces by said hot roll (7) thereby transferring said printing ink layer (4) onto said surfaces, characterized in that said base sheet (2) comprises a biaxially stretched plastics material film. A method according to claim 1 for thermal transfer printing, which comprises: (a) heating, with a hot roll (7), a transfer sheet (1) comprising a printing ink layer (4) formed on a base sheet (2) with a peel layer (3) therebetween, and with an adhesive layer (5) on the outer side of the printing ink layer (4), wherein said base sheet (2) is a biaxially stretched plastics material film; and (b) concurrently pressing, with said hot roll (7), said transfer sheet (1) against said surfaces for printing, with a pressure sufficient to thereby transfer said printing ink layer (4) to said surfaces. A method according to claim 1 or 2, wherein said biaxially stretched plastics material film is polyester film or polypropylene film.

Inventors: IGOTA, SHOJI, SASAKI, HITOSHI
Applicants: AJINOMOTO CO., INC.
Application year: 1985
Main classification: B41F16
IPC codes: B41F16, B41M5