For first surface (outer) printing, the choice of ink is straightforward as long as the ink employed is formable. However, since the graphics will be on the outer surface of the finished part and subjected to wear, some sort of secondary protective coating will, most likely, need to be employed. Second surface (interior) printing will require inks that can survive the extreme temperatures and pressures common with injection molding – remember that the ink will come into direct contact with the molten resin during the molding cycle. The main areas of concern are around the “gate” areas where the pressures and temperatures are at their extremes.
Typically the film is either a polycarbonate or polycarbonate alloy. It is essential that the film has consistent gage thickness, good clarity, and is free of internal stresses.
In-Mold Decorating / In-Mold Labeling requires that the inks used for the graphics are completely cured and are free of any residual solvent. This is a critical factor as any residual solvent present will be trapped by the film and will vaporize during the molding cycle – resulting in poor ink adhesion and in the most extreme cases, trapped bubbles.
High temperature molding resins are typically required to achieve a good bond with the graphics film. Polycarbonate, Acrylonitrile Butadiene Styrene (ABS), and blends of the two are used. Ideally both the film and resin substrate should be sourced from the same material supplier to eliminate or minimize compatibility issues and to draw upon any technical resources of the material supplier if needed.
Probably the most critical factor in the success of your In-Mold Decorating / In-Mold Labeling program is the correct tool design. The tool should be designed for In-Mold Decorating / In Mold Labeling from the onset of the program. Although modifying an existing tool is possible, the best results will be achieved with a tool specifically designed for IMD / IML. Wall thickness is critical especially near the gate(s). Too thin of a wall section will result in excessive shearing of the material during injection. This will cause the melt temperature to increase resulting in damage to the film and/or ink. The gates should be positioned as far away as possible from the graphics. If this is not possible, the gates should be designed to minimize shearing. Turbulent flow or jetting of the material during the molding cycle will damage the film – design the gating to balance the flow throughout the mold cavity. Venting of the tool must be from the back of the tool and reverse ejection should be used if at all possible.