By
Hiram P. Ball, JR.
The competitive nature of
the flexible moldmaking market in the last few years has spurred the development
of significant new products and techniques. These developments have had
a terrific impact on the quality of reproduced parts, and on the economics
of the casting business in total.
The customer is the ultimate beneficiary of these
technical advances, from the first steps of the casting process to the
finished product that is viewed by the public. This is true of the very
small cast figurine as well as the 20-foot, larger-than-life feature character
at Disney World, Las Vegas, or the newest theme park.
The procedure to make flexible molds has not
really changed over the years, with one exception, which I will discuss
shortly. There are just more products to choose from so that the user
can be sure of getting the best material for the project.
If anything, there are too many choices of materials
available in the marketplace, which makes for more decisions. This variety
requires the moldmaker to really think through every step and aspect of
the specific process: the composition of the master; the quality required
of the cast piece; the moldmaking technique; the material to be cast into
the mold; the number of repetitions; the quality of the labor doing the
work; and finally, the economics of the entire process taken as a whole.
I find it extremely difficult to get company
owners to grasp the idea that by spending more money on a particular material,
in the end they will have a cheaper mold and, very possibly, reduced breakage.
Obviously, the major issue is the quantity and quality of labor required
to make the mold. The newest technique is the capability of spraying a
specially formulated polyurethane via a fairly simple and inexpensive
machine. In addition to the spraying of the flexible mold, the rigid support
shell can also be sprayed with the same equipment by simply swapping out
the buckets. The spraying of silicones has been possible for several years,
but the cost of the proper equipment, about $20,000, has ruled this investment
out for all but the major players doing theme park work.
The original motivation to develop the equipment
to spray urethanes was to have a simple system that could go into the
field to make molds of architectural or terra cotta details quickly and
with a minimum of hassle. Specifically, by reducing the time involved,
this equipment and material has drastically lowered the cost of making
field molds of original architectural elements that are still in place
four stories up the side of a building. As a result, the quality of the
field mold has also improved, so that a minimum of shop work is required
to get a master mold for the production mold.
From using the equipment in the field, it quickly
became apparent that the spray machine and the urethane rubber had a fit
in the day-to-day operations of mold shops in the GFRC, GRG, and ornamental
precast industries. Spray application of mold rubber is growing.
While spray rubber produces molds surprisingly
free of bubbles, other methods to reduce trapped bubbles involve selecting
a low-viscosity mold rubber with good flow and/or use of vacuum or pressure
to aid bubble removal. Often a good low-viscosity rubber will produce
a perfect mold if mixed and poured with appropriate care.
Finally, one of the most important things to
understand when using flexible moldmaking materials is how to calculate
the cost of the various rubbers on the market. To make this economic decision
on an educated basis, see "Can You Save 30% on Your Foundry Mold Rubber?"
on page 4.
Flexible moldmaking materials are a growing
and innovative market today. Putting these observations into practice
will allow you to take full advantage of the new developments. |
