Patrick J. Griffin
Bayer Corp. Pa.
Edited by Sherri L. Singer
Traditional methods for decorating injection-molded parts include painting, screen printing, pad printing, and hot stamping. Recently, however, alternate decoration methods have been developed that require fewer processing steps and environmental controls. These methods include in-mold foils, insert molding using polycarbonate film, and sublimation heat transfers.
films include water-based inks, forming methods and equipment for registering films in the mold, and a high-heat polycarbonate printing ink. These materials and processes provide increased design flexibility, recycling, and lower total costs than traditional decorating methods.
One of the more traditional methods of decorating molded parts is pad printing. It uses an engraved metal or photopolymer plate which is doctorbladed with a printing ink.
For the engraved image, brass or bronze is usually used for long production runs, while photopolymer plates are used for shorter runs. The method works for decorating parts with irregular shapes and surfaces. However, one of the primary disadvantages is that the printed image, similar to a painted surface, wears and abrades.
Adhesive-backed nameplates are another method of decorating molded parts. The pressure-sensitive nameplate or label is affixed to the molded part by removing the release liner on the back of the part and physically placing the nameplate on the molded assembly.
Painting the molded assembly with the desired background color and printing the graphic legend directly on the molded surface using screenprinting techniques can also be used to decorate molded parts. But, because screen printing inks are forms of paints, they are subject to the same wear and abrasion concerns as painted surfaces.
NEW DECORATION METHODS
Several new methods of decorating molded parts are in the works. Designers are looking for new methods to make their products stand out. Also, they want methods that increase productivity and design freedom, and can be recycled.
Two technologies that could achieve these goals are in-mold decoration and insert-mold decoration. Though these terms are often used interchangeably, they are distinctly different methods with their own strengths and limitations.
In-mold decoration places a preprinted carrier, typically a thin polyester film, in an injection mold before resin shoots into the mold. After molding, the carrier is discarded, leaving the decoration on the first surface of the molded part. This process is also referred to as heattransfer molding. More commonly, it is known as the Nisha or Kurtz process after the two firms producing these types of films, but these processes are variants of traditional hot-stamping technology. Instead of being applied to the surface of a molded part, the decoration is transferred using heat from the injectionmolding process.
Another form of transfer decoration is the sublimation ink-transfer process. This involves printing select dyes on special transfer papers. The decorated papers contact the molded parts under heat and pressure. The dyes on the paper vaporize and penetrate into the plastic surface. The dye vapor penetrates the molded surface eight thousandths of an inch.
Multiple colors can be decorated in a single operation, and all the processes offer significant improvements over traditional pad-printing techniques. Manufacturers employing these processes can stockpile printed films or papers rather than molded assemblies, and new design changes involve changing the film or paper transfer rather than discarding completed molded parts.
While these processes offer improvements, they have some critical limitations. For example, the decorated films rely on the injectionmolding resin flow to form the film and force the decoration into proper register in the mold. On parts which have highly defined geometries or which are relatively large, wrinkles develop and make defective or misregistered parts. Also, the decoration transferred by these films is a firstsurface decoration, similar to a painted surface. Films of this type must be evaluated for surface wear and chemical-resistant properties, and may not be suitable for all products or applications.
Insert-mold decoration differs from in-mold decoration in that the decorated film, either formed or flat, is placed into a mold and the resin is injected directly against the film, becoming an integral part of the molded assembly.
Several film constructions are used. One surface-decorated film consists of graphic layers screen printed on the outer surface of a film with a protective hardcoat layer over the printed graphic. The hardcoat layer gives wear and chemical protection to the graphic although the protection is similar to that afforded by paints. The main advantage is that the graphic is protected from the injection-molding resin by the film substrate.
A second film construction also involves a single film layer. The graphic ink layers are screened on the surface of the film contacted by the injectionmolding resin. This technique can be used only when the background color printed on the film matches the base resin color injection molded behind the film. By matching the resin color to the background color of the printed film, ink distortion resulting from the injection-molding resin at the gate locations is masked. This technique works when a limited number of graphic icons are used and the area directly behind the icon can be shut off from contact with the injection- molding resin.
A third film construction involves two film layers. The top layer is decorated on either the first or second surface. The second layer is then bonded to the first using a heat-activated adhesive. The second film layer protects the second surface graphic from the melt of the injection-molding resin.
One major benefit is that these techniques eliminate pressure-sensitive adhesives. While adhesives adhere appliqués to molded assemblies, they also can ooze during shipment, transferring the adhesive to the face of the appliqué which must be removed after assembly. Also, the adhesive can continue to ooze to the edges of the appliqué, accumulating dirt and dust. The appliqué must be removed before recycling the molded assembly because the adhesive is a low-temperature polymer, incompatible with the injection-molding resin when reground and recompounded.
Designs can incorporate 3D contours, integrated backlighting, and surface textures which match other components of the assembly. Fewer process and assembly steps result in a lower final product cost.
The insert-molding process supports many different film and resin combinations. Unlike the in-mold processes, the injection-molding resin is not limited by the temperature resistance of the decorated film. Typically, the insert-molding process incorporates polycarbonate or polycarbonate- blend film substrates.
These substrates allow the most costeffective injection-molding resins to be used including polycarbonate, polycarbonate/ABS blends, PBT, PETG, and PC/PETG blends.
When choosing a material, consider the end-use performance requirements. Factors include heat and/or chemical resistance, environment, impact resistance requirements, and aesthetics. The materials must be compatible with the decoration process and the graphic construction must be compatible with the injection-molding resin considered. When not considered, cost increases for tool modifications and products.
The choice of ink systems is another critical factor. Because the majority of film assemblies are preformed prior to insertion in the injection-molding tool, the ink must form without cracking. Also, the ink pigments have to withstand the injection- molding process without changing color. This requires higher temperature pigments than typically used to decorate either adhesivebacked appliqués or in-molded decorated foils. Depending on the film construction, the resin for the ink system must bond to the injectionmolding resin. High levels of silicone flow agents in many inks can cause adhesion problems between the injection- molding resin and the ink screened on the film substrate.
Preforming the graphic appliqué must match the forming process. Appliqués have to fit the injection-molding tool for adequate registration during the molding process. Also, the fit of the appliqué to the mold cavity assures thermal transfer through the decorated film layer, preventing ink distortion from the injection-molding resin blowing through the decorated appliqué at gates.
Several forming methods for insert molding include thermoforming, high-pressure forming (HPF), hydroforming, and matched metal dies. A forming process depends on the complexity of the part.
Thermoforming consists of a film held in a rigid frame and heated to its plastic flow point. A combination of vacuum, air, and mechanics forces the film into the correct shape. Depending on the film substrate, it may be necessary to predry the film prior to forming to eliminate bubbles. The material is heated to its plastic flow point to assure proper forming with as little internal stress as possible. Also, the film must not overheat because the wall thickness can vary and the graphic appliqué can misregister. Tool temperatures, control using air and vacuum, and tool design are critical factors for successful thermoforming.
This process achieves deep draws on the formed appliqué. The decorated film can be drawn to depths of 4 to 6 in. depending on the ink used for decoration. However, this process suffers from registration problems on the graphic appliqué. This can be a problem for critical graphic registration.
Another method uses high-pressure air to form the graphic film into the forming tool. To accurately register, the decorated film is prepunched with holes before the graphic applies to the film. These holes are used in subsequent forming and die-cutting operations. When the film is formed, the sheet is placed on registration pins on the forming tool. A pressure vessel mounts over the film and the tool locks into place. High-pressure air injects into the pressure vessel, forcing the film into the geometry defined by the tool. The pressure releases and the vessel lifts to remove the film. Benefits of this process include minimal stress in the part because of the uniform force applied to the film. Also, the inks may be heated prior to forming, letting the graphic form without cracking the ink. The front surface of the film contacts only the air injected into the tool, reducing rejects from contaminants which can contact the front film surfaces formed in other processes.
Hydroforming is similar to highpressure forming except the forming mechanism is a hydrostatic bladder. The graphic film sits on pins machined in the forming tool. The tool moves into the press and hydraulic pressure on the back surface of the bladder forces film into the tool. Like high-pressure forming, hydroforming provides good registration of the graphics to the molded part. However, hydroforming does not let the film’s heat elongate the graphic inks. It allows for fast tool setup and changeover, producing a formed part with relatively low stress. But, cycle time is considerably longer than in high-pressure forming.
Matched metal dies are not usually used for production applications. As with the other processes described, the graphic film is placed on pins built into the tool. Hydraulic pressure applied to the top half of the tooling mechanically forces the film into the lower tool cavity. When the tool halves are separated, the formed film is removed and the next film mounts on the pins. The tool halves can be heated to improve ink elongation during forming. However, one limitation is that metal tooling contacts the front surface of the appliqué, producing gloss variations and forcing contaminants into the film surface.
Once the appliqué forms, the insert is trimmed before placing in the injection- molding tool cavity. Three methods can pretrim inserts prior to molding including die cutting, laser cutting, and hard tooling.
Die cutting offers high-quality, low-cost trimming for medium to long production runs which do not allow for expensive long-run hard tools. Laser cutting is a low-volume prototype method for trimming parts. Edges on the formed appliqué do not rival die-cut or hard-tooled parts, but give the accuracy needed for insert molding. Hard-tooling is the choice for high-quality long-run parts. Punches and cutting blades are permanently built into the bed of the cutting tool base, providing accurate registration and long tool life. But, high initial costs of tooling limit hard tooling to the appliance and automotive industries.
In addition to forming, trimming, and materials, several other key parameters affect the insert-molding process. These concern the design of the injection-molding tool, the location of the film appliqué in the mold, and the process conditions related to molding the final assembly.
With the injection-molding tool, consider the types and locations of the gates and gating system. Gate locations must let the film press against the mold cavity for adequate thermal transfer through the film layer. Molds for insert molding consider the film insert. Perform mold-flow and filling analysis on the mold before cutting steel. Also, model a mold-cooling analysis to minimize hot spots in the mold. Finally, how the films are placed and located in the mold is critical and should be designed into the mold. Too often molds not meant for protoinsert molding are adapted without these factors being considered.
The film insert acts as an insulator on half of the mold. Adjust mold temperatures to reduce the stress this insulating layer provides. This is particularly critical for thin-walled assemblies where the film insert accounts for a major portion of the wall thickness. Also, the texture of the tool can affect appearance. Depending on the initial texture of the appliqué, it may be necessary to texture the tool to minimize gloss variations in the final part. Finally, though it is important to control as many parameters as possible, what works for one resin may not work for another with different processing parameters. However, when these different parameters are taken into account, the end product is cosmetically attractive and can be assembled at a lower cost.