Rapid Automated Induction Lamination (RAIL) of carbon fiber
weave and thermoplastic film

Abstract: This work is a continuation of the RAIL - Rapid Automated Induction
Lamination - process developed by S. Yarlagadda et al. at the Center for
Composite Materials, University of Delaware, USA.
The RAIL process was developed and optimized for laminating prepregs, i.e.
preimpregnated carbon fibers that are oriented parallel to each other and
then formed as a sheet 12” x 36”. These sheets are stacked so that there is
always an angle between the fibers of two sheets that are in contact with
each other. If the angle is zero degrees there will be no heat generated in
the induction heating zone due to the fact that conductive loops are
required for heating to occur.

This project’s primary goal was to test an alternative combination of carbon
fiber and thermoplastic polymer and run it in the existing RAIL
machine. Carbon fiber weave would be stacked with thermoplastic film and the
result would be a flexible and strong composite. Both raw materials could be
delivered to the machine separately, on rolls, and the need for pre-
impregnation would be eliminated. This would reduce the raw material cost
compared to prepregs. A requirement was that the quality of the final
composite had to be at least as good as the same composite manufactured in a
hot press.

The secondary goal was a continuation of the primary: it consisted of
modifications of the RAIL process stages, allowing manufacture of non-flat
laminates. For this purpose, aluminum molding dies were designed and
manufactured and the process would then be a combination of lamination and a
variant of pultrusion.

The experiments that were carried out showed that it would be possible to
replace prepregs with stacks of carbon fiber weave and thermoplastic film in
the RAIL process. All experiments were done with Nylon™ as resin. Optical
microscopy revealed that the process can heat the fiber weave enough to melt
the resin and then evacuate the air in the stack at the consolidation stage.
The cooling seemed sufficient as no voids were created.
The preheat zone in the RAIL process was developed because impregnated
carbon fibers were used at the time and it was necessary to melt the resin
so that contact between the fibers could be achieved. Tests with carbon
fiber weave and Nylon™ film indicated that the preheat station was not
needed in order to reach the processing temperature because the majority of
the heating occurs by induction heating inside each layer of the carbon
fiber weave. However, preheating was still used for the evaluated laminates
because the temperature distribution across the laminate turned out to be
much easier to control with the preheating station active.

The resulting laminate is very flexible and can easily be cut if desired.
Molding dies were designed and manufactured. The purpose was to give the
laminate a profile-type shape directly after the lamination stage. The
cooling rollers would then have been replaced by the stationary molding dies.