[extruded sheet of acrylonitrile butadiene styrene (ABS),
thermoplastic olefin (TPO), or polycarbonate (PC)] that
facilitates handling during manufacturing. The film is
thermoformed and placed into an injection mold followed by injection of the material that comprises the
part to be coated. The backing layer and the part material
are melt-bonded during the injection molding process.
Film laminates are typically single-layer PVC calendered films produced in a variety of gauges based on
the intended end-use application. These films can be
printed and embossed to provide a wide range of decorative effects. Usually, a high performance coating is
applied during manufacture to enhance durability and
prevent staining. The film is applied to a metal substrate
through a high-temperature flat-lamination process
along with an adhesive.
Some laminates are applied using a process called
in-color thick-sheet thermoforming, which eliminates
the injection molding step. In this case, the film is
laminated to a thick sheet that actually provides the
structure of both the part and the film. From this thick
sheet, parts are thermoformed. Due to the limitations of
the thermoforming process, this technology is limited to
relatively flat parts and does not allow for production of
parts with features (e.g., ribs, bosses) on the back side of
the part.
Coextruded multilayer sheets have also been developed as a coating alternative. These materials have a
clear layer, a color layer, and at least one backing layer
but are produced by extruding the different resins at one
time through a multilayer extrusion die. The films can
be cut into sheets or wound on a roll. These thick films
are then thermoformed much like thick-film laminates.
Since coextrusion is not a printing process, its biggest
disadvantage is color and graphic limitations.
In-mold coatings, dry paint films, and laminates
present several advantages from both a cost and environmental perspective, according to Michael D. Brown,
president of StrategyMark, Inc., a consulting firm based
in Yorklyn, DE. “In all of these processes, essentially
100% transfer efficiency is achieved when compared to
spray-applied coatings. The number of defective parts
is often reduced, too, because contamination from airborne materials is not an issue.”
In addition, significant reductions in VOC emission
are possible because of the nature of the application
processes, he notes. Laminates, for example, are often
produced in a printing operation where VOCs can easily
be captured, and in-mold coatings are typically formulated as 100% solids.
On the performance side, laminates and dry paint
films allow printing of graphics and patterns such as
wood grains that are not possible with traditional coat-
ings. The clear-layer component of films and laminates
is often a fluoropolymer, urethane, or acrylic resin that
imparts increased durability. The thicker film of a laminate, according to Brown, can also prevent the denting
of underlying metal in larger parts, thus providing additional protection.
Finally, Brown notes that in many cases the capital
investment (and subsequent footprint) required by the
user to apply these alternate decorating techniques is
substantially less than for traditional coatings.
These alternative coating technologies find their widest use in end-use markets where there is a prevalence
of plastic parts. Thus, the automotive, electronics, and
sporting goods industries are the largest end users. “In
the automotive sector, in-mold coatings and dry paint
films are used predominantly for interior parts, where
special effects like wood grains are designed and exterior
trim parts where the relatively flat dimensions allow
more easy accommodation of laminates and thick films.
Also, they are used in operations where the need for
high-speed production requirements is in conflict with
the curing requirements (dry time and baking time) of
traditional coatings,” explains Brown.
Laminates are more widely used in coil metals and
engineered wood (where they are referred to as “foils”),
where the substrate is flat and can more easily be applied in-line with rollers, according to Brown. Here
again, special effects like wood grain are made possible
by the foils. In coil metal, the laminates are often a very
good alternative for prevention of denting and wear.
“In-mold coatings act to fill porosity, reduce sinks,
and furnish a primer-like or topcoat-like coating, thus
upgrading the part surface to automotive standards,”
states Elliott Straus, manager of technical services for the
Functional Coatings business of OMNOVA Solutions
Inc. There is no orange peel associated with these decorated parts as is often the case with wet primered parts.
High and low spots (amplitudes of 0.1 mm) are leveled
out using the in-mold coating technology and the distinctness of image (DOI) is enhanced. This method also
offers the option of providing a surface which can be
conductive or color-matched to suit specific finish coating needs. As a consequence, in-mold coating is generally an integral part of the molding cycle when producing compression-molded SMC exterior automotive
body panels.
The automotive, transportation, and marine markets
in North America have embraced the use of in-mold
coating as a primer across a broad product line, according to Straus. The use of this technique as a functional coating beyond a primer on SMC is in place in
some niche markets. OMNOVA offers a wide range of
in-mold coating products utilized as conductive and
nonconductive primers to impart superior adhesion to
both substrates and conventional topcoat paint systems.
