Screen Printing - April/May 2017 - 29
PROCESS FLOW FOR STANDARD FILM INSERT MOLDING
1. Screen print decorative
design onto underside
of flat film
2. Heat and form film into
desired 3D shape
3. UV cure inks and
4. Cut holes into parts
for switches and
5. Insert film into mold and
inject thermoplastic resin
on back side of part
Courtesy of MacDermid Enthone.
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.
Example of plastic surface combining FIM with electronically controlled
buttons, switches, and lighting. Photos courtesy of TactoTek.
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