Film Insert Molding: Combining Form and Function
Another option for manufacturers looking to streamline decorative and functional printing steps.
Film insert molding (FIM) can be an excellent method for integrating printed graphics into plastic parts. Such parts are used in many applications, including automotive, consumer electronics, industrial, white goods, and medical devices. The greatest benefit of FIM, however, may lie in using it as a platform for integrating printed electronics and decorative printing into a single molded part that is both functional and durable. Manufacturers are beginning to incorporate circuitry created with conductive inks and encapsulate discrete semiconductor chips within molded, decorated parts.
The automotive industry represents an excellent opportunity for this technology, as auto makers seek to differentiate themselves with distinctive dashboard designs and displays that include both decorative and functional features. Auto makers are reaching out to companies in the FIM supply chain who can help them incorporate advanced technologies into their vehicles.
FIM is compatible with a wide variety of coatings and inks. Designers have complete freedom in color and design for decoration, since images are screen printed onto a flat film before the product is formed into a 3D shape. It is possible to include special effects such as metallic, mirror, or high-gloss black or white finishes.
In contrast, standard injection-molded 3D plastic parts are formed from a single color of resin. They are typically pad printed for decoration, or sometimes labeled using screen or inkjet printing if the surface is sufficiently flat. But printed images on the surface of the part tend to wear off after repeated use. FIM creates a durable image that cannot be rubbed off. The process can incorporate coatings with various desirable properties, including hardness, chemical resistance, and light reflection (or the lack thereof).
The automotive industry is especially demanding in its desire for coatings that are durable, scratch-resistant, and fingerprint-resistant with embedded graphics that will last the lifetime of the vehicle. Different surfaces – center consoles, dashboards, steering wheels, and interactive display screens – need different types of coatings and finishes. FIM provides a solution that can address all these needs, but it is important to optimize materials and process parameters to get the best results.
Processes and Materials for FIM
FIM is a multistep process that includes, at its core, screen printing, forming, and molding steps. Additional optional steps may be needed depending on product design and selected materials. The chart below shows a possible process flow.
Films, inks, and resins need to be chosen carefully to meet the demands of the end application. Such demands include the geometry of the design, the environment in which it will be used, and any relevant regulations. In the case of the automotive industry, materials and processes must meet stringent specifications that may vary by auto manufacturer. Medical devices come with their own set of regulations depending on the setting in which they will be used.
Films for FIM can be made from various thermoplastic polymers, including polycarbonate, acrylic (PMMA), acrylonitrile butadiene styrene (ABS), polyester (PET), and thermoplastic polyurethane (TPU).
Polycarbonate is probably the most common FIM film since it is durable and easily formable. The material is, however, susceptible to scratching and sensitive to chemical exposure, so suppliers usually incorporate a hard coating on the front surface to create a more durable product. Although the printed design is still protected, damage to the polycarbonate surface can affect the look of the part. The harder the coating, the greater the scratch resistance, but hard coatings are also less flexible and therefore not as well-suited to products that need to be formed into deep 3D shapes with tight radii of curvature. For relatively flat products, however, coatings made from glass or similar materials can be desirable.
Some suppliers have come up with creative solutions. MacDermid, for example, makes a polycarbonate with a hard coating that can undergo printing and forming while partially cured. The film remains pliant enough to be compatible with deep forming processes. A final UV cure after forming creates a coating with sufficient hardness to meet automotive industry specifications and a high-gloss finish.
Surface finish requirements vary by application. Some products need high-gloss coatings, while others look better with matte surfaces. Textured coatings with embedded nanoparticles reflect light in multiple directions, creating antiglare surfaces. Antireflective coatings combine layers with differing indices of refraction to reduce reflection without increasing haze. Fingerprint-resistant coatings need to repel both water- and oil-based contaminants, posing an additional challenge for coating manufacturers.
Inks for FIM need to be formable and heat-resistant to survive the elevated temperatures during the molding process. Standard screen printing inks cannot meet this requirement. Inks also need to adhere well to the film surface and therefore must be chosen with the substrate material in mind. Improper ink selection can lead to delamination, cracking, or melting of the printed design during FIM processing. Washout, where the heat and pressure of injection molding cause localized film melting, can be avoided through optimized product design and materials selection.
The FIM industry is mature enough that many suitable inks are commercially available. These inks are heat-resistant and flexible, and are designed with FIM in mind. Several suppliers produce solvent-based FIM inks that are free from toxic solvents such as benzene and toluene and do not have any intentionally added halogens (chlorine or bromine). Most FIM inks require heat curing, but some are UV-curable.
Inks may be both decorative and functional. Possible functions include selective light transmission, phosphorescence, antibacterial properties, or electrical conductivity. The availability of electrically conductive inks that are compatible with FIM has enabled integration of FIM with printed electronics.
As digital displays replace analog ones in vehicles and interactive touch surfaces become the norm, the automotive industry is primed to benefit from FIM of parts that incorporate electronic functions. The ability to incorporate printed electronics and rigid silicon chips into the FIM process promises reduced weight and thickness, greater freedom in design, and a more streamlined production process. FIM enables designs that replace mechanical buttons and dials with virtual ones, creating a single surface that is easier to keep clean and dust-free, while removing mechanical parts that may break down over time. The automotive industry is taking advantage of these benefits by incorporating electronics into FIM in commercial vehicles that are either on the road now or will be soon.
Example of plastic surface combining FIM with electronically controlled buttons, switches, and lighting. Photo courtesy of TactoTek.
A variety of functions can be added to parts formed using FIM. Options include antennas, sensors, LEDs, and integrated circuits. FIM can incorporate a variety of sensors that may detect proximity to illuminate a display or activate gesture-based controls, detect ambient light to make controls visible in dim or bright conditions, or add capacitive touch to control virtual switches or sliders. Once the part is injection-molded, any discrete electronic components are encapsulated and therefore mechanically and chemically protected from the environment.
Creating printed antennas is a relatively straightforward addition to FIM, as printed antennas are common in RFID tags. Even though such antennas are designed for flat substrates, it’s not a big stretch to incorporate them into FIM parts. Silver-based inks are printed into circular patterns to create an antenna. The design need only be modified to accommodate the bending that occurs during the forming step. Recent developments include a trend toward using aqueous rather than solvent-based conductive inks for a more environmentally friendly process. These inks, however, may not be sufficiently conductive or flexible for FIM applications.
Capacitive touch sensing is an especially compelling application for FIM. Sensors are located very close to the surface of the part, making it more sensitive. Performance can even be enhanced sufficiently that the features work while the user is wearing gloves. Canatu makes innovative films for capacitive touch, aimed primarily at the automotive market. The films combine two forms of carbon: carbon nanotubes and fullerenes, 60-atom, soccer ball-shaped molecules also known as buckyballs. The films have the high electrical and thermal conductivity of CNTs and the chemical reactivity of fullerenes, allowing them to bond to other surfaces. They’re placed in between the decorative printing layer and conductive printed traces in the FIM process.
Photo courtesy of Chesky / shutterstock.com.
One example of capacitive touch sensing is Canatu’s FIM-based transparent touch sensor that controls both power windows and seat heating in a single panel. Canatu developed this product in partnership with automotive interior system manufacturer Faurecia.
Design and Process Considerations
While there are clear motivations for combining printed electronics with FIM, the process is not without complications.
The FIM process flow is slightly more complex when adding printed electronics since the printed circuitry is produced as a separate layer from the decorative printing. The added complexity is offset by eliminating the need for a separate rigid printed circuit board (PCB) and other layers. This reduces the overall thickness of the part from 25 millimeters to 2 to 4 millimeters, which conveys a significant advantage for design flexibility.
Incorporating printed electronics also removes the need to cut holes in the formed part to allow for switches and dials, eliminating a process step. These functions are instead integrated into the single molded part. The process flow for FIM with electronics follows a path similar to that of standard FIM, as shown below.
In contrast, traditional designs require many more layers. The FIM layer includes only the decorative printing. A rigid PCB needs to be mounted on a backing plate, and designs with lighting require light pipes overlaid to allow the light from LEDs to shine through in the correct location. Capacitive touch features require still more layers.
The same challenges that FIM encounters, such as ink cracking and washout, are also present when electronics are added to the process. A decade ago, the biggest challenge may have been finding suitable inks that retain their conductivity during the forming and molding process. Cracking affects not only the look of the product, but also its ability to function. Inks, therefore, need to be extremely flexible. It’s also important to optimize their drying rate. Today, multiple ink formulations are available with properties that are compatible with FIM processes, so that is no longer the bottleneck.
Component mounting is probably the next big challenge for FIM with electronics. The method to add lighting, for example, typically involves attaching LED chips directly to the formed film. Such chips are very small and thin, but attaching them reliably is where the challenges come in. Adhesives need to be able to withstand the heat and pressure of injection molding.
Not only is the film heated to an elevated temperature inside the mold, but there is also a significant temperature difference between the mold and the injected liquid thermoplastic. The greater this difference, the greater the likelihood of warping and blistering. Such defects can be fatal to electronic components, causing them to detach from the film. Choices in component location and temperature profile can minimize this risk. It’s important to conduct thorough testing to identify any defects that may affect the function of the part.
Capacitive touch sensors are an exciting new application for FIM technology. The sensors are created by placing a carbon-based capacitive film between the decorative printing layer and the circuit traces in the FIM process. Parts are molded so that the sensors will be very close to the surface. Markets include automotive, home appliances, and more. Photos courtesy of Canatu.
Washout can be especially detrimental to FIM parts with printed electronics. When thermoplastic is injected into the mold cavity, any spots that experience localized concentration of heat or pressure can experience voids in the film and defects in the inks. For inks that are purely decorative, it may be possible to ignore defects that are too small to see with the naked eye. But the pressure of injection molding may affect the distribution of metallic particles within a conductive ink, leading to defects that affect performance, if not appearance, and render the part unusable.
Adhesives for attaching components may be cured before the molding step using either UV radiation or heat. Alternatively, a correctly chosen epoxy adhesive could be cured during injection molding, streamlining production. This approach can, however, be risky, since the pressure of injection molding may dislodge discrete components if the adhesive is not sufficiently strong.
One approach to solving the challenges inherent in securely attaching discrete components is to avoid using them. Instead of attaching LED chips, it is possible to print light-generating sources on the same layer as conductive traces. Inventors from startup Rohinni, which makes printable ink infused with microscopic LED particles, have been granted a patent for their process of incorporating these particles into FIM.
Despite potential challenges, integrating electronics into FIM promises significant advantages. Whether in vehicles, home appliances, or medical instruments and devices, the lure of doing away with mechanical buttons and switches and creating one seamless, aesthetically appealing user interface is too compelling to ignore.
Read more from Screen Printing's April/May 2017 issue.