Printing Directly on the Mold’s Surface
A newly developed process allows plastic surfaces to be decorated with ink directly during injection molding. A pad printing machine next to the injection molding machine transfers the printed image directly to the surface of the mold. The ink is then overmolded by the plastic melt from behind. In contrast to known methods of in-mold labeling (IML) and in-mold decoration (IMD), no carrier film is required.
To transfer the printed image to the plastic part without any flaws, the adhesion mechanisms must be adjusted precisely. First of all, the ink must adhere to the mold surface very well to prevent shifting or blurring of the printed image during injection of the plastic melt. Next, the ink must be transferred completely to the plastic part so that no ink residue remains on the mold’s surface. This means that the bond between the printed image and plastic must be stronger than the adhesion of the printed image to the mold’s surface.
By selecting suitable printing inks and injection molding parameters, the Institute for Conveying Technology and Plastics at the University of Chemnitz has succeeded in printing polycarbonate plaques with very high image accuracy. No ink residue was found on the surface of the mold even after numerous cycles.
Finally, the adhesion characteristics and scratch hardness of the ink layers were investigated. This confirmed the durability of the plaques printed in the mold. The researchers are currently investigating application of the process to polyolefins. With appropriate surface modification during the injection molding process, it is possible to achieve even better ink adhesion with this process than when printing in a postmolding operation.