Composite Materials Research Progress

Major Trends in Polymeric Composites Technology 121
matrix composite this could yield a prepreg ply about two orders of magnitude less thickness
with the same strength and stiffness as conventional prepreg offering benefits in weight
critical structure.
4. Processing Technologies
Out-of-Autoclave Processing: Although the autoclave has served the composite industry well
providing structural integrity as well as thermal curing, there is a growing demand for a leaner
means of curing parts. Being able to cure parts in a continuous stream like a pizza oven is the
Lean Manufacturing teams dream. But even a common oven can provide leaner flow.
The economic advantage of an oven process for structural composites is large.
Autoclaves are an order of magnitude more expensive than ovens with the same temperature
uniformity. The process flow and batch constraints of autoclaves can potentially be
eliminated with ovens. And liquid nitrogen systems used to prevent fires would not be
needed. Also there is the possibility of part growth that may limit the autoclave usefulness.
Such an example is seen in the windmill blades in which designs are growing faster than an
autoclave can be depreciated. Blades of necessity have been hot bag cured for many years
and as they surpassed the 38-meter length the designs have been using carbon fiber in place of
glass fiber composites. There is also the issue of large-scale complex parts such as racing
sailboat hulls that would require an autoclave of the scale that not many groups can afford.
These also have been “cooked in a tent” for many years.
In the past there have been attempts to utilize UV curing, e-beam curing, X-ray curing,
oven curing, tent curing, hot press curing, continuous pultrusion curing, many forms of resin
infusion, etc., all with various degrees of success or the lack of significant applications. The
key to success is the material. If they can be developed to have no voids in the uncured
laminate and no volatile components in the resin system then the pressure element becomes
moot. The concerns include matching the fiber volume associated with autoclave cures and
processing variable that can impact design allowables. One of the noted successes in this
trend is oven cured epoxy carbon fiber systems approved by the FAA for structural
applications on civil aircraft (Agate Program allowables database is FAA approved) [20].
Some of these systems are improved with high (>30 in Hg). Generally if a void free
uncured laminate can be achieved either through hot debulking, ultrasonic compaction or
other means, vacuum bag pressure in an oven will be sufficient to achieve a structural part.
There are many new methods that involve either single or double diaphragms to hold the
part while being vacuum-cured. One of these is the double diaphragm resin infusion process
RIDFT of Florida State University High Performance Materials Institute which should offer
lower tooling costs and shorter cycle times. Another novel approach is that of Quickstep®
developed jointly with the Australian research organization CSIRO, it is based on a liquid
filled container in which a lightweight mold floats on one of the flexible faces of the pressure
chambers [21]. The container is filled with a heat transfer fluid which is circulated through
the chamber to rapidly heat and cool the mold. The process enables the cure cycle to be
stopped and restarted at any time and parts of the laminate to be left uncured. Parts can be
consolidated and formed and then final cured in-situ at a later time. It is being developed
further with several universities and the National center for Composites.