Thermal transfer printing
View Patent ↗An article for receiving an image by thermal transfer printing includes an image-receiving surface of plastics material coated with a layer of dye-receptive material capable of accepting an image by means of a dye thermal transfer process, wherein the glass transition temperature (Tg) of the dye receptive material is at least 50° C. lower than the softening point of the plastics material of the image-receiving surface. By having a difference between the softening point of the plastics material article surface and the Tg of the material coated thereon of at least 50° C. it is possible to achieve printing in a single step transfer process of high density images without distortion of the plastics material. The invention can be used with three-dimensional articles possibly having compound curves, including casings for mobile telephones made of polymers such as polycarbonate.
1. An article for receiving an image by thermal transfer printing, the article including an image-receiving surface of plastics material coated with a layer of dye-receptive material capable of accepting an image by means of a dye thermal transfer process, wherein the glass transition temperature (Tg) of the dye-receptive material is at least 50° C. lower than the softening point of the plastics material of the image-receiving surface.
2. An article according to claim 1 , wherein the Tg of the dye-receptive material is at least 60° C. lower than the softening point of the plastics material of the article surface.
3. An article according to claim 1 or 2 , wherein the dye-receptive coating material has a Tg of at least 30° C.
4. An article according to claim 3 , wherein the coating material has a Tg in the range 45-50° C.
5. An article according to claim 1 , wherein the dye-receptive coating material is selected from: polyesters, acrylic materials and polyvinyl chloride.
6. An article according to claim 1 , wherein the dye-receptive coating material comprises a cross-linked polymer.
7. An article according to claim 6 , wherein the degree of cross-linking is such that the molecular weight of units between cross-linkable functional groups of the polymer is greater than 900 and less than 10,000.
8. An article according to claim 1 , wherein the dye-receptive coating includes one or more mould releasing agents.
9. An article according to claim 8 , wherein the mould releasing agent is cross-linked with a polymeric dye-receptive coating material.
10. An article according to claim 1 , wherein the dye-receptive coating has a thickness in the range 10 to 50 microns.
11. An article according to claim 1 , wherein the plastics material of the image-receiving surface is a thermoplastic material.
12. An article according to claim 11 , wherein the plastics material is selected from: ABS, blends of ABS with other polymers, acrylic resins, polycarbonates, polysulphones, polyethersulphones and polyether ether ketones.
13. An article according to claim 1 , wherein the article is a three-dimensional article: possibly having complex, curved shapes (concave or convex) including compound curves.
14. An article according to claim 1 , comprising a casing of a mobile telephone.
15. A method of printing an image by dye thermal transfer on an image-receiving surface of an article in accordance with claim 1 , comprising holding a carrier sheet bearing an image to be printed in contact with said surface, and heating the carrier sheet to print the image on said surface.
16. An article bearing a printed image produced by the method of claim 15 .
17. An article according to claim 2 wherein the Tg of the dye-receptive material is at least 65° C. lower that the softening point of the plastics material of the article surface.
18. An article according to claim 3 wherein the dye-receptive coating material has a Tg of at least 45° C.