Faster cycles produce more parts for
the same time and overhead. "Piece-part price is what it is all
about". Randy does this with composite metal tooling. This
he accomplishes with high heat transfer metals and surface hardening.
Composite Metal Tooling (CMT) is designed to do for a mold what no
single material of construction can do. It features a core of a
proprietary metal called Heat Transfer Metal (HTM), which reportedly is
easily machined or formed to a desired shape, and combines it with salt
bath heat treating of a through-hardened steel. The result is a tough,
hard, and slick mold with a high degree of thermal conductivity, which
speeds cycles and improves the consistency of either thermoset or
thermoplastic molded parts.
The CMT approach also can be used to make screw and barrel assemblies.
An agreement has been reached with a major screw and barrel
manufacturer to begin the development of this application. In addition,
HTM can be used in channels where a mold's heat transfer medium
flows--it is not subject to calcium plate-out or oxidation--or anywhere
optimum heat control is a must.
"When we consider that molds are nothing but high-pressure heat
exchangers, we understand that materials other than tool steel would be
able to provide better heat transfer, but for the problems of strength
and wear," says Randy Lewis, president of P.R. Lewis Enterprises (Lake
Wylie, S.C.) and the inventor of the CMT process. "The molding industry
has come to accept very poor heat transfer as the price for tool life."
"Basically," he continues, "this new composite method can
through-harden an H-13 or S-7 cavity to 50 to 52 Rockwell C and passes
the cavity through a heat treatment process that takes the surface
hardness to 72 Rockwell C up to .020 inch deep. All the steel in the
cavity not needed for strength is removed and is replaced with HTM,
which improves heat transfer characteristics by two to five times."
Balanced Heat Flow
says his HTM mystery metal functions very much like a
heat pipe. Both are not in direct contact with the melt and both are
in steel. However, unlike heat pipes, he says HTM never needs replacing
if the mold is accidentally overheated. Also, HTM can easily be
or bent to fit intricate shapes. Heat pipes cannot.
He stresses that HTM is not meant to replace heat pipes. Heat pipes
move more heat over a longer distance, but he says HTM can reduce the
amount of steel through which the heat has to be moved.
"The ultimate goal in using HTM is not just to move heat faster, but to
balance the heat flow in the mold so that cooling or heating is not
limited by the properties of solid steel. Removing portions of the tool
steel and replacing it with HTM can accomplish this."
Though easily shaped, HTM is strong enough to support 44 percent of
what H-13 tool steel will bear. If, for example, 44 percent of the
steel mass of a mold is replaced with HTM, Lewis says that the CMT mold
be just as thermally conductive as one made completely from beryllium
copper. HTM has a coefficient of thermal conductivity that is
reportedly more than nine times that of steel and twice that of
Bathing in Salt
hardening methods were considered to increase the wear
resistance and service life of the tooling, while reducing the
of friction where the plastic contacts steel. Lewis chose through
with surface hardening. He says surface hardening steel that is not
hardened only increases the hardness of the steel by a few thousandths
of an inch. Below this, the metal is at its original state.
"Conventional surface hardening leaves an extremely hard but brittle
area in contact with the plastic," he says. If the mold is struck hard
enough to dent the soft base steel, the hardened layer cracks and chips
However, through hardening imparts hardening properties all the way
the mass of certain types of steel. Though Lewis admits that it can't
as great a hardening of the steel surface as surface hardening, it is
subject to surface chipping.
Through-hardened steel is often plated to achieve a desired surface
hardness. But plating--hard chrome plating, for instance--is subject to
chipping and other maintenance troubles as well. That's why Lewis says
he opted for salt bath surface heat treatment. In this process,
through-hardened steel is placed in a heated vat of salt under vacuum
and the temperature is raised. The steel's surface is molecularly
altered to a depth of .020 inch. There is nothing to chip off, as it is
a part of the steel.
Worth Its Wait
bathing raises surface hardness by 10 Rockwell C and increases
the toughness of the base metal. Surfaces also become more
so than hard chrome--for easier mold release. And Lewis says they are
chemically resistant than stainless steel surfaces. HTM can be placed
the .020-inch hardened surface.
The cost of using CMT during initial construction or retrofitting
depends on its application. But Lewis contends that the price is small
when compared to the increased profitability generated by more
"In many cases, just the reduction in heat-up time can pay for the
additional cost," he said.