by Nick Strauss, director of corporate development, Inno-Flex

There are a number of reasons manufacturers decorate plastic. Aesthetics, instructions, information, safety and branding are among the most common. One often overlooked reason for decorating plastics is to create an effective user interface.

As products become smarter and more connected, the need to incorporate an intuitive user interface also increases. The human machine interface (HMI) is a very important aspect of the overall user experience and opens the door for enhanced features. When using plastic parts, there are a few ways in which designers can incorporate their user interface into a cohesive and integrated product.

By leveraging these methods, injection molders and electronics manufacturing service (EMS) providers can expand their businesses, provide more value and ultimately better serve the changing needs of their customers.

Overlays and membrane switches

One of the simplest methods of enhancing a plastic part is to add a simple adhesive-backed overlay. Graphics are applied to a plastic film, which is attached to a plastic part using adhesive. Typically, overlays are placed over electro-mechanical buttons and often utilize cantilever or spring switches incorporated into the plastic part. The graphic overlay provides instructions and locations for the cantilevered switch, which makes contact with an electromechanical switch typically surface-mounted to the control board. This form of decorating a plastic case allows for vivid graphics and the ability to have multiple designs or brands with the same plastic part and control board simply by changing the overlay graphics.

A similar method is to apply a membrane switch to a plastic part. By adding circuitry to the graphic overlay, functional switches can be integrated into the design. Using printed circuitry allows buttons to be distributed away from the control board, thus reducing printed circuit board (PCB) size and cost. Membrane switches provide a durable, reliable and low-profile method of electrical switching. Both of these methods provide an easy way to add value to a plastic part and enhance its value.

Capacitive touch

In addition to overlays and membrane switches, many customers are leveraging capacitive touch circuitry for enhancing their plastic housings and user interfaces. By incorporating graphical overlays with capacitive touch circuitry, injection molders can economically add to their plastic parts a control interface that utilizes capacitive touch features such as buttons, sliders, proximity and scroll wheels. Additionally, capacitive touch circuitry can be implemented without a graphical overlay and utilize molded features as the indicators by using adhesive to attach the capacitive touch circuitry to the backside of the part.

Injection molders can create landing pad areas, raised buttons and scroll areas by adding those features to the mold and placing printed capacitive touch circuitry behind the part. This allows the manufacturer to distribute the capacitive touch control away from the control board to reduce PCB size and cost. With continued improvements in capacitive sensing technology, using the plastic part as a capacitive touch user interface is gaining popularity with designers and engineers.

In-mold decorating

Another method of plastics decorating that is often utilized for control and user interfaces is in-mold decorating (IMD). The process involves placing a flat or formed printed film into a mold and injection molding a plastic part that fuses with the film. IMD is used in everything from cellphone cases to automotive dashboards to appliance consoles. It provides an integrated part that is durable, functional and sealed.

Left: In-mold decorated user interface using embossed keys and LED/display windows. Right: In-mold decorated part using thermoplastic elastomer as a more durable alternative to silicone keypad.

This sealed design not only protects electronics from water immersion but also eliminates the issue of exposed adhesive around the perimeter of overlays that attracts dirt. Many customers in the appliance and medical markets choose to implement IMD instead of traditional membrane switches or overlays to avoid this contamination issue and to take advantage of the sealed nature of the part to stand up to humid environments and frequent wipe downs.

In addition to graphics, LEDs, displays and buttons can be incorporated into the in-mold decorated part.  As manufacturers leverage displays and touchscreens in their products, IMD can be used to accent and customize the bezel to create a seamless implementation of the screen into the design. Integrating capacitive touch into in-mold decorated parts is also a very popular method. Graphics and LED windows are printed onto an applique that is decorated using IMD, creating a plastic control panel. A control board with capacitive touch circuitry, surface mount LEDs and even LCD displays is mounted on the backside of the part. For applications that require electromechanical switches, a formed applique is used.

In addition to printing the graphics, button areas are formed to create a 3D domed button that has the ability to actuate and engage a mechanical switch. The process of IMD is the same, except the mold closes around the perimeter of the button area, creating a void of plastic resin to allow for the applique to move – much like an overlay button. A control board is still mounted on the backside of the part, but, in this case, surface mount mechanical switches are used underneath the formed button area.

By doing this, one still can achieve the benefits of having a sealed and durable IMD part, yet also incorporate the functionality and tactile feel of a mechanical switch. This is especially important when capacitive touch is not an option or the power requirements of the switch necessitate a mechanical design.

Along with a diverse set of control options, IMD can be implemented with a diverse set of plastic resins.  Polycarbonate, acrylonitrile butadiene styrene (ABS) and polypropylene are most common, but IMD also can be used with thermoplastic elastomers like thermoplastic polyurethanes (TPUs) and thermoplastic polyolefin (TPOs).

Using thermoplastic elastomers can provide a rubberized surface that can mimic the appearance of a silicone keypad, but is much more durable. This eliminates the typical failure modes of silicone keypads, such as ink-based graphic wear and button tearing, while still maintaining the sealed nature of the keypad.   

In-mold electronics

Plastic part utilizing in-mold electronics technology

In-mold electronics (IME) is the newest technology leveraging plastics decorating to create user interfaces. IME is the combination of printed electronics used in capacitive circuits and in-mold decorating. By printing the electrical circuitry and graphics on the plastic film applique and injection molding the part with an electrical connector insert, it is possible to create a part that is aesthetically pleasing, electrically functional and very durable. This saves space and cost and eliminates secondary parts when compared with traditional interface design.

By moving the capacitive touch circuitry to the plastic decorated part, designers are able to shrink the size of the PCB. This is especially important in the appliance and sports equipment markets, as the buttons and displays are typically distributed over a wide area, normally requiring a large and expensive control board that covers the entire area, or potentially using two separate control boards connected by a wiring harness.

Another major benefit of IME is the ability to integrate capacitive touch sensing on curved surfaces. Traditionally, manufacturers were limited to flat PCBs with capacitive touch circuitry that was fastened to plastic parts. The other option was to use adhesive to attach a flat printed capacitive touch circuit to the back of a curved plastic part. In both instances the solution was bulky, cumbersome or required extra components.

With IME, this situation can be avoided, which allows designers to create smart surfaces with troughs, dials, custom raised surfaces and compound curves that all have built-in capacitive touch capability within a single, completely sealed part.

Furthermore, other printed electronic features can be created along with touch circuitry and LEDS.  Near field communication (NFC) antennas, EMI/RFI shielding, proximity sensing and even electrical heating can all be incorporated into IME. The applications for this technology are expanding rapidly while also anticipating the customer’s future needs.

For instance, as cellphone NFC technology expands and is more commonly incorporated into products to augment the user experience, IME provides the ability to incorporate antennas directly into a plastic part. Heated headlamps are already using in-mold electronics technology to incorporate heated conductive traces into clear plastic lenses. These heated traces are used to defrost the headlamp, which is especially important for LED bulbs that lack the melting power of more traditional headlamps. As the widespread adoption of this technology increases, the number of different ways to apply the technology also will expand as engineers explore new ways to implement it.     


The options for utilizing plastics decorating techniques in user interface applications are very diverse.  Traditionally, the field of plastics decorating was primarily focused upon branding and aesthetics that often utilized pad printing and decals. Pushing beyond that mindset shows there are numerous applications for using plastics decorating to develop advanced user interfaces that are still cost effective. This wide range of methods and tools is very often only limited by the designer’s imagination.

Nick Strauss
Nick Strauss

Nick Strauss is director of corporate development for Inno-Flex. Inno-Flex is a manufacturer of custom in-mold decorating, printed electronics, decorative/functional overlays and membrane switches. For nearly 50 years, Inno-Flex has been developing innovative solutions and delivering high quality products for its customers in the medical, appliance, automotive, consumer and industrial markets. For more information, visit