Metallic finishes in decorated plastic remain popular with both designers and consumers. The reasons for this are many and include the increased perceived value of the finished part, improved performance in the intended application, replacement of actual metal to reduce cost and weight, and taking advantage of the intrinsic properties of plastic substrates while maintaining a metallic appearance.
Industries using metallic decorated plastics include automotive, aerospace, medical, architectural, hardware, telecommunications, home appliance and packaging. Given the range of industries and applications, it is important to understand the intended market and its specific needs and expectations. The cosmetic packaging market is all about image, color and appearance. The automotive market is all about harmony, elegance and performance in the field. Understanding the target market is critical in choosing the correct technology.
The manufacturing techniques available to create metallic appearances on plastic are numerous. Over time, the number of technologies capable of achieving metallic appearances, the quality of the finished appearance and the applications for them have increased. This has been driven by customer expectations, growing design demands, government regulation and continued innovation. This increasing number of options makes it all the more important to understand the alternatives available and choose the correct one for the application. The choice should be based on the end customer preferences, target appearance, performance expectations and expected annual volume.
Technologies in common use
Molded plastic would be the easiest option if it were able to duplicate the desired metallic finishes. Unfortunately, there are many issues, ranging from the inability to match target appearances, the cost of using bulk pigments to achieve a surface effect and the limited geometry that can produce a uniform appearance – without very visible knit lines and flow marks. Limited appearance options and design constraints make this a poor option for most applications.
Coatings provide a wide range of options for metal-like appearances. While not capable of all of the appearances or properties of actual metal surfaces, coatings do offer choices that include bright, matte and satin finishes. Paint is applied with a variety of techniques using different spray gun types to achieve thin layers of uniform thickness. Cure systems can be thermal, UV or dual cure depending upon the type of coating used. To achieve the brightest results, a multi-layer construction, with a base coat, a thin metallic layer (down to 0.1 microns) and a clear coat generally works the best. Matte and satin finishes are easier to achieve.
Metal flake pigments come in a wide variety of sizes, shapes and appearances. Handling and preparation are important issues to consider since metal flakes can be damaged during paint preparation. Care should be taken to not apply the metallic paints too thick; this can allow the pigments to orient in various directions resulting in a less metallic appearance.
Coatings also are used as a clear coat to protect thin metal surfaces produced by other methods, to enhance the secondary decoration process and to provide a color tint as needed.
Inks can produce a very bright metallic appearance. They can be applied directly to the part but are more often used in making the films and appliques used in in-mold decorated or in-mold labeled parts. They are capable of very bright appearances that approach a full metallic appearance using most printing technologies. Inkjet, although it is finding some use in vehicle wraps, packaging and signage, is not as capable as other printing technologies.
In-mold decorating (IMD)
When utilizing an in-mold decorating process to create a metallic surface, multiple layers of printed inks and/or physical vapor deposition (PVD) are applied to a carrier film. This film is then fed into the part mold. After filling the mold with plastic, the inks adhere to the plastic creating a finished part, which is removed from the carrier when opening the mold. The carrier film is advanced, and the process is repeated. The primary advantage of this process is that a decorated part is completed without any secondary processing. In addition to allowing multiple appearances using the same tool, IMD can create antimicrobial surfaces, support backlighting, and produce complex decorative patterns that include metallic appearances.
The disadvantages of IMD are that the part geometry is limited by the ability of the carrier film to conform to the part geometry, and the risk of damage to the relatively thin decoration.
In-mold labeling (IML)
In-mold labeling is similar to IMD but consists of printing or decorating with inks and/or PVD onto a film that is then cut, formed and insert molded. In the case of IML, the film is a part of the finished product. This gives the opportunity for more robust solutions and the ability to create many patterns out of a single tool. The advantages of IML over IMD include a wider range of capabilities, less cost sensitivity due to part production volumes and a more robust decorated part. In addition to allowing multiple outcomes with the same tool, IML can support telecommunications applications by creating a metallic surface that does not interfere with the signal reception and supports backlit appearances and multilayer effects.
The primary issues include design limits due to depth of draw, location of parting lines and, in some cases, metallic ink adhesion.
Electroless plating is useful in applying thin uniform thickness over the entire part. Thicknesses typically range from 1 to 10 microns. Greater thicknesses are possible but can be time-consuming and costly. Like electroplating, it is a wet process and requires first etching and catalyzing the surface. It also can be used to put a final metallic protective coat over a finished part. The primary issues with electroless plating are the difficulty in achieving a thick coating and the waste stream created.
Electroplated plastic is the gold standard for creating a metallic surface in plastic parts. In addition to providing a cool-to-the-touch metallic surface, it provides strength, scratch resistance and is particularly well suited to applications that require superior durability and performance. It should be the obvious choice but faces significant issues relating to regulation, the chemicals used and the waste stream created.
The process consists of first molding a plateable plastic, mounting it in a rack and then moving it through a number of wet chemistry processes and rinses. The first step is to etch the surface and then apply an electroless metal. For a chrome finish, this is followed by several baths to apply copper, nickel and chrome in various thicknesses. Final plating thicknesses can range from 3 microns to 100 microns. Typical automotive interior chrome is 18 microns, and a typical automotive exterior chrome is 40 microns.
The geometry of the part has a significant impact on plating thickness uniformity. Standards and Guidelines for Electroplated Plastics, by the American Society of Electroplated Plastics, although dated, gives good advice on part design to achieve an acceptable appearance. Plating companies and their chemical suppliers also have useful guidance. Not all plastics are plateable and the molding process parameters have a significant impact on plating outcomes. Advances in technology allow the plating of additive manufactured parts and replication of textured surfaces not previously possible.
Historically, hexavalent chrome has been used in electroplating. This is changing, due to regulation and concern for the environment. There is a push to quickly change to trivalent chrome. Unfortunately, trivalent chrome does not match hex chrome in terms of deposit performance, ease of application and production cost.
Physical vapor deposition
PVD is a great option and has a wide range of capabilities, alone, or when combined with other decorating technologies. It is most likely the best option when sustainability is an issue.
PVD, rather than one technology, describes a variety of techniques that allow the deposition of thin metal films on surfaces. The two most commonly used in plastic decoration are evaporated metal and magnetron sputtering. Typically, applied thicknesses are less than 0.1 microns. With PVD, a wide variety of metals and alloys can be deposited. Because the metal is deposited by line of sight from the target there are some limits on part geometry, for example recesses with a high aspect ratio are not always possible to coat. The primary constraints are the high cost of the equipment and the slow build time needed to prevent overheating of the plastic part.
Thermal transfers (hot stamping foils)
A cost-efficient alternative to electroplating and PVD is thermal transfers or commonly referred to as hot stamping foils. The metallic that appears on a hot stamping foil is created through vacuum metallizing an extremely thin layer of aluminum onto the polyester film carrier. The image – in this case, a metallic appearance – is then transferred from a carrier to the part by means of a heated metal die or silicone die or roller. A wide range of custom metallic, bright chrome, satin chrome and tinted appearances are available, as well as specific metallic colors. Recent advances have enabled hot stamping foils to meet the standards set for automotive exterior applications, such as grills. The primary limit is the required geometry; relatively flat parts, or at most 2.5D parts, are the best that can be achieved.
Real metal, plastic combinations can take the form of both insert-molded and assembled plastic parts. The most common approach is to insert mold or apply an aluminum sheet. The aluminum sheet can be machined to create a texture, be anodized for color, or coated to improve appearance, pad printability and tactile properties. Hard coats can be used to improve the damage resistance and micro perforations can be used to create a hidden-until-lit backlighting. Primary applications for this technology are in the automotive, consumer products and appliance markets.
A significant challenge for any real metal application is the differing expansion coefficients between plastics and metals. Validation testing should include temperature cycling and thermal shock.
More than just a metallic appearance
Metallic finishes can be multifunctional, providing additional capabilities beyond appearance or their inherent physical properties. Obvious applications include electromagnetic and electrostatic shielding. One recent trend is the inclusion of lighting or backlit graphics through the metallic decoration. This effect goes by various names, including shy technology, dead front and hidden-until-lit.
On the functional side, several technologies allow the integration of anti-microbial capabilities. In automotive applications, bright chrome finishes have been developed to hide vehicle radar systems, and in telecommunications applications, metallic finishes that do not attenuate signals are in production.
Cost is always important and many factors influence the piece cost, including regulation, material costs, the expected production volumes and capital investment. In choosing a process it is important to understand the technologies design limits. The answer to the question “which is the most expensive process?” is any process that is operated outside of its capabilities. Design standards exist for a reason. Violating them usually leads to unpleasant results such as low yields, failure to produce any usable parts, or a failure to meet durability standards.
Although in many applications the decorative part is complete when the metallic decoration is in place, there are many more applications where additional secondary operations are used to add value or complete the part. Examples include modifying the color with an overcoat, application of patterns or graphics to the metallic surface (as in badging), and laser ablation to create backlighting.
New, integrated modular lines have been developed that allow various decorating processes to be applied sequentially. These systems offer a significant advantage in high-volume applications and when all parts that run on the line are dimensionally similar.
The above has been, of necessity, a brief introduction to some of the many techniques to create metallic appearances in decorated plastics.
Make sure to understand the application, customers’ expectations, and the limits and capabilities of each technology before choosing what path to take. Take the time to understand the design constraints for each technology and recent advances. By combining technologies, a wide range of colors and appearances are possible.
Finally, sustainability is becoming a larger issue. When choosing a process, make sure to understand both the impact of the process and end-of-life constraints for the part.
Paul Uglum has 43 years of experience in various aspects of plastic materials, plastic decoration, joining and failure analysis. He owns Uglum Consulting, LLC, working in the areas of plastic decoration and optical bonding. For more information, send comments and questions to [email protected].