By Tom Mucenski, Klöckner Pentaplast of America,
Inc.
Proper core stock selection is relatively easy once you understand
some basic PVC characteristics and several key processing parameters
within the card-manufacturing environment. To best match finished card
performance, ease of processing, and economical purchase of core material,
one needs to gather basic information on the specific print process
used; type of card construction; and the lamination parameters.
The first assumption that needs to be made is that the film supplier
is offering substrates that meet ISO card standards as outlined in ISO
7810 and tested to methods described in ISO 10373. Quality film suppliers
will have internal data confirmed at an outside test laboratory. The
most known test facility in this hemisphere is Eclipse Labs in Bloomington,
MN, USA.
Printing processes (UV, conventional litho, or silk screen) are dictated
by the end customer's graphics requirements and the print equipment
at the card manufacturer's facility. The type of card construction (solid
core or split core) is also dictated by printing press capability, specifically
the substrate thickness range of the press. The end use of the card
and/or the lamination settings determine the type of material to be
used - homopolymer versus copolymer film. Conversely, the material chosen
by the card manufacturer (homopolymer or copolymer) will determine the
lamination settings required to attain the proper core-to-core bond.
Homopolymer core films require hotter and sometimes longer lamination
settings particularly for split core applications. Copolymer core films
can be laminated at lower temperatures and some times shorter cycle
times. Lamination of copolymer core films at lower temperatures can
also help minimize "ink color shift" or in "burn out."
The film supplier then takes the above card process information and
translates them into the most cost effective use of PVC film through
the proper selection of the film's properties to include VICAT points,
opacity (color), surface roughness, and specific gravity. These factors
become the foundation for "Core Selection 101."
VICAT
VICAT is the temperature at which plastic begins to soften. Productivity
and card performance can be greatly affected by the VICAT of core film.
Printing presses often limit the thickness of film that can be processed.
Thus when print presses have limitations of 400 microns thickness or
less, split core construction will be used to produce finished card
product. This means printing the front and back of the card on two separate
opaque white sheets. The sheets are typically 12 to 13.5 mils each and
are solidified during a subsequent lamination process that normally
includes the application of a clear overlay film (1.6 to 3 mils) on
each side to protect the printed graphics. The critical VICAT temperature
for split core cards is that of the two core sheets, because the plastic
must soften all the way to the center of each sheet or approximately
15 mils. The VICAT of the overlay is less critical.
Lamination is often a bottleneck work center in the card manufacturing
process. In order to maximize productivity in a split core card construction
environment, it is best to choose a core material that has a sufficiently
low VICAT temperature so that the core materials will soften and bond
together more quickly. Typically, a high ratio copolymer formulation
film is preferred. Such a film requires the least amount of lamination
processing time, as well as minimizing discoloration or card flowing
that may appear in the card once bonded. On the negative side, copolymer
films are higher priced than blended or homopolymer films due to higher
raw material cost.
In a solid core construction, the front and back graphics are printed
on the same sheet. Typically, solid core film construction uses 24 to
26.5 mil opaque white center with the graphics on each side being protected
by 1.6 to 3.0 mil overlay film. Since the only bonding required is between
the overlay and core/ink surface, which is typically no deeper than
3 mils on either side of the card, the critical VICAT is that of the
overlay which must bond to the solid core material. Therefore, a higher
VICAT core material can be used. Typically, this core material is a
full homopolymer film. Homopolymer material is the most cost effective
film to use in card construction and will allow for a quick lamination
cycle to enhance productivity in a solid core construction environment.
Low VICAT blended copolymer materials are basically a hybrid between
the two extremes of the high ratio copolymer film and a full homopolymer
film. The film supplier is attempting to supply the desirable characteristics
of a high ratio copolymer film without having the customer absorb the
full cost of such a film. The price point is usually between copolymer
and homopolymer.
Of course, the card industry has continually evolved since the early
'60's and today specific card requirements include high stress environment
material for GSM card and similar products that are met by films with
various ratios of ABS and PVC to meet industry standards or customer
requirements for VICAT minimums. The future will certainly see the evolution
and development of hybrid materials and cards for high-end use as well
as increased use of polystyrene and polypropylene on low-end card products.
Opacity
Opacity, not color, is of greatest interest in controlling a card manufacturer's
cost. As the chart summarizes, the degree of opacity is directly related
to the amount of TiO2 in the formulation. It is also true that the higher
the concentration of TiO2 the higher the film cost. Thus, it is in the
best interest of the card manufacturer not to over specify the requirements
in this area. While card construction may have equipment limitations
forcing the card manufacturer to seek a more costly split core construction,
opacity is within a card manufacturer's total control for proper selection.
Higher concentrations of TiO2 lead to higher raw material costs and
an increase in specific gravity and thus a higher weight/cost per sheet.
From a costing perspective the lower the levels of TiO2, the more attractive
the cost can be. While different levels in TiO2 do represent up to $
.10 per pound differences in costs, even a slight price advantage of
$ .01 per pound would be significant when multiplied by a consumption
rate of a million pounds or more of core stock in any given year.
It also should be noted here that opacity requirements on credit card
product is for the finished card design and not for the blank opaque
card body that will be used in satisfying an order. Most credit cards
today are full bleed front and back with extensive use of 4-color process
and/or silk-screen inks. Thus, by the very nature of the card design
itself, will meet opacity requirements via ink coverage and not blank
card opacity.
Surface
Surface roughness is the third parameter that requires consideration.
Surface roughness helps ensure that the film can be successfully printed
on at the card manufacturer's facility. As shown at right, typical surface
roughness is designed either for a UV print process environment or a
conventional litho print environment. UV ink is more expensive but less
ink is used on a smoother surface. UV ink is also partially cured either
between printing cylinders or at the end of the printing press with
the aid of UV lamps. Therefore, it makes sense for a smoother surface
to be utilized in UV print process.
Where conventional litho printing is used, a rougher surface is desired
to help accelerate the cure time. Often sheets are stacked to a minimal
height and allowed to "air-dry" for several hours to several
days prior to printing the next pass.
The differences in matte film finishes are controlled as part of the
film manufacturer's process. Even though only two types of finishes
are noted above, several smoother or rougher finishes can be obtained
if required by the end customer. The skilled print pressman at select
manufacturing sites may prefer these surfaces. Increased film costs,
due to set-up time at the calender, can be expected unless significant
volumes accompany each special surface requirement.
Specific Gravity
The higher the specific gravity of core stock film used the lower the
yield or sheets/pound. Despite this knowledge, it is amazing how many
card companies purchase purely on cost per pound basis. Anytime a company
purchases in one unit of measure ($/lb.) and issues inventory in another
unit (sheets), yield, impacting cost per sheet, becomes a strategic
factor.
In the world of card film manufacturing, product can be produced with
the introduction of fillers such as CaCO and/or talc. While this may
give the film manufacturer ease of production, it has no or minimal
benefit to the card manufacturer. In fact, the use of fillers may result
in both a higher specific gravity for the film and lower performance
characteristics in card testing such as flex and impact. While the test
results may still meet ISO requirements, the costs may actually be higher
due to lower yield (fewer sheets/lb).
Fillers, when used, and the over specification of opacity can both
significantly increase sheet weight. Non-filled films with proper product
formulation and color selection can generate a 1% to 10% yield advantage
over its filled counterpart. In effect, a non-filled product that is
less opaque color may actually have a higher unit price ($/pound) but
may be your best value when calculating yield on a cost/sheet basis.
So buyers beware; your best value may not always be your lowest $/lb
price.
Conclusion
Proper core stock selection should be a joint decision between card
manufacturer and film supplier. An experienced film supplier can aid
in providing the most cost-effective selection of film once the final
use of product and internal manufacturing parameters/limitations are
understood.