In-Mold Decorating / In-Mold Labeling
The definition of In-Mold Decorating (IMD) / In-Mold Labeling (IML), in the most general terms, is described as any technology which applies text, patterns, or graphics to a plastic molded part during the molding process.
As In-Mold Decorating / Labeling is a decorating process employed during the molding cycle, it may be of some use to review alternative post-mold decorating techniques such as:
Pad Printing is a printing process that can transfer a 2-D image onto a 3-D object. This is accomplished using an indirect offset (gravure) printing process that involves an image being transferred from the printing plate (cliché) via a silicone pad onto a substrate (surface to be printed). Pad printing is used for printing on otherwise impossible products in many industries including medical, automotive, promotional, apparel, electronics, appliances, sports equipment and toys. It can also be used to deposit functional materials such as conductive inks, adhesives, dyes and lubricants. Physical changes within the ink film both on the cliché and on the pad allow it leave the etched image area in favor of adhering to the pad, and to subsequently release from the pad in favor of adhering to the substrate (material being printed). The unique properties of the silicone pad enable it to pick the image up from a flat plane and transfer it to a variety of surface (i.e. flat, cylindrical, spherical, compound angles, textures, concave surfaces, convex surfaces).
Screen Printing is a wet process where ink is squeezed through a fine mesh material onto the part to be decorated. A film positive is made of the artwork and then laid over a fine mesh material which has been tightly stretched and fastened to a frame. The mesh material has been coated with a light sensitive emulsion and the film and screen are then exposed to a mercury vapor light. This exposure hardens the emulsion on the screen creating a solid barrier. The portion of the screen that was covered by the film positive remains soft and is washed off the screen with a pressurized water spray.
The completed screen now allows ink to pass through this washed out area while preventing ink from passing through the hardened emulsion. This washed out portion of the screen is now an exact duplicate of the artwork to be placed on the part.
The screen is then mounted in the screen printing machine and clamped in place. A fixture is mounted under the screen which is designed to hold the part to be printed in the correct location. The part is then placed in the fixture and ink is added to the screen. The ink is squeezed through the screen with a rubber squeegee while the part is moved under the screen, allowing the ink to be placed on the part.
Water Transfer printing starts with pre-printed high-definition patterns on a water-soluble transfer film. From there, several important steps must be followed to develop a successful print:
- Preparation. Depending on the substrate, some or all of the following steps may be necessary: chemical pretreatment, fill and sanding, masking, application of spray chemical adhesion promoters, plasma treatment, or corona flame treating.
- Tooling and Fixtures. Parts are attached to a specific fixture, which holds the parts during the dip process. Generic fixtures can host many different parts but occasionally, engineers have to design custom fixtures to host unique parts.
- Primer Painting. A coat of primer paint is applied to the part. This paint serves as a bonding agent between the part and the transferred inks. The paint color typically becomes part of the background contrast color as well.
- Ink Transfer or Dipping. The fixtured part is now dipped through the inks, which are floated on water in a specialized processing tank. As the parts are immersed through the inks into the water, displacement of the water carries the inks around the three-dimensional shape of the part.
- Wash and Rinse Process. Parts travel through a hot water wash line to remove excess processing chemicals. The last stage of the wash line dries the part.
- A detailed inspection of the ink transfer and adhesion is completed. On occasion, parts with extremely complex geometry may require manual touch-up using the transfer inks.
- Sealing Top Coat. Based on the customer’s specifications, either a high gloss, satin, or flat top coat of clear Urethane finish is applied to seal the inks. One or more coats may be applied and in some cases, buffing and polishing is required between coats.
Appliqué is a process where a printed, self adhesive label is applied to the finished molded part. This method typically relies on the screen-printing process and typically used in the appliance and automotive industries. It is inexpensive and very easy to use but the label can be peeled off very easily. Additionally, the applied label will not look like part of the molded product and will exhibit “witness” lines where the label ends and the molded part begins.
Laser Etching is most often used in conjunction with spray painting to etch graphics on plastic or elastomers. The laser etching process can be used to mark a variety of materials that would otherwise be difficult, if not impossible, to mark mechanically. It is ideal for products requiring intricate decorative work however, it is a slow process and not recommended for high volume applications.
Hot Stamping is a dry printing process. In hot stamping, metallic or color pigmented hot stamping foil is applied to the part by trapping the material between a heated die and the part. Heat and pressure is applied and the foil is released from the carrier and is permanently bonded to the part. Hot stamping machines are specific to certain types of applications. Flat hot stamping machines are designed to be used exclusively on flat parts or flat areas of a part. Roll hot stamping machines are designed to rotate a round or oval part under the die to print on these types of surfaces allowing for 360 degree printing on round cylindrical shapes and front or back printing on oval shapes.
Heat Transfer is a dry printing process like hot stamping where a preprinted graphic image is trapped between a heated plate and the part. Heat and pressure is applied to the part and the graphic is then permanently bonded to the part. The advantage over the Heat Transfer process is that multi-color graphics can be applied to a part in one pass; therefore the customer can have the benefit of a multi-colored print while only having to pay the cost of a single application. Although this process has many advantages, it is usually only cost effective in higher volume applications because of the cost associated with setting up and screening the transfers.
Insert Molded In-Mold Decorating (IM-IMD)/ In-Mold Labeling (IM-IML)
For over 25 years, CPX Inc. has been on the cutting edge of development in Insert Molded In-Mold Decorating (IM-IMD) / In-Mold Labeling (IM-IML).
In its simplest form, Inert Molded IMD involves a flat screen printed label that is positioned in an injection mold and bonded in place during the molding process. In its most advanced form, Insert Molded IMD / IML utilizes a hard-coated, formable film which is screen printed on the second (interior) surface. A selected texture is then applied to the first (outer) surface, the film deep formed (thermoformed) to fit the mold cavity, and placed in the mold cavity. During the molding process, the film forms a complete skin over the surface of the part complete with graphics, multiple colors, and textures that are completely encapsulated by the molded plastic.
A flowchart depicting a typical insert molded decorating process is depicted below:
Thermoform or cold form the film
Cut and trim to size
Position and secure film in the mold
Design tool for IMD > Injection mold the part < Select resin
Eject finished part
Insert Molded In-Mold decorating / labeling offers significant advantages over alternative decorating techniques such as:
- Capable of deep formed applications – true 3D applications.
- Can be used to apply graphics in close proximity / register to the mold profile – within +/- 0.2mm positional tolerance or better.
- Images or graphics can be applied to the second surface (or interior) where the printing or graphics become completely encapsulated during the molding process; rendering the graphics virtually indestructible.
- Selective first surface (or outer) printing can be used to apply texture or modify the gloss level of the finished product.
There are several design factors that must be addressed when utilizing In-Mold Decorating such as the choice of inks, the image or graphic design, film consistency and clarity, ink drying characteristics, substrate resin selection, and mold design.
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.
Some conventional inks are adequate for In-Mold Decorating / Labeling applications as long as the graphics are far removed from the gates. In areas close to the gates however, very few conventional inks can stand up to the high temperatures of the molten resin. Here, temperature resistant inks need to be employed and fall into three general categories: thermal-cure inks, UV curable inks, and high melt resistant inks.
Thermal cure inks resist high melt temperatures well but require long curing cycles. However, these inks tend to have excellent adhesion to the molten polymer resin. UV curable inks offer good temperature resistance but any residual monomers must be baked out to prevent issues during molding. Melt resistant inks are difficult to apply and tend to be very expensive.
Proper design of the graphic image can help overcome some of the limitations of the ink. At the gate area for instance, extending the print area into the gate region can help to minimize ink washout. Placement of the graphic is also critical as they should not be placed too close to sharp edges as this will make registration of the graphic more difficult. Deep draw details where the width is less than or equal to the detail height are not recommended. Symbols should be located in the flat regions where distortion will be kept to a minimum.
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 caviy. Venting of the tool must be from the back of the tool and reverse ejection should be used if at all possible.