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Point-of-Use Resin Heating Reduces Cost and Improves

Eficiency in Pre-Impregnated Composites
by Mac Larsen

Figure 1 Figure 2 Figure 3
Figure 4

HIGH PERFORMANCE COMPOSITES CONSIST OF CARBON, Kevlar, or glass ibers impregnated drum contents remains at

with a resin system, and cured into a shape using various molding methods. hey are used exten- ambient temperatures, and
sively within the aerospace industry, as well as in sporting goods, automobiles, and wind power gen- safe from advancing or ag-
eration equipment. One widely used resin-iber intermediate is called “prepregs” or pre-impregnated ing. (Figures 2 & 3)
iber sheets. hese sheets are formed when a thermoset, heat-curable resin is combined with carbon Once the drum is emp-

iber and partially cured in a continuous ilming impregnation process. Prepregs are produced in tied, a second or tandem
factories under controlled conditions to ensure the carbon iber-to-resin ratio is tightly managed and platen heating system can
the resin formulations have consistent quality. Prepregs have been used extensively for decades to be in place to continue an
make composite parts for the most demanding and critical applications.
uninterrupted supply of hot

As demand for high performance composites grows, traditional prepreg processes must be trans- resin to the batch mixing
formed from ineicient batch operations into more eicient, and higher-capacity automated process- process. his point-of-use
es. Many resins used in high performance composites have molecular makeups which result in their heating process eliminates
being thick or semisolid at room temperature. As a result, they need to be heated to be mixed into the need for large ovens

formulations that can be processed into prepreg intermediates. One area of focus which addresses and extended wait times
Figure 5
the need for increased capacity is speeding up the resin preheating process. he beneits of a faster involved when bringing an
heating process are many: high quality end products and improved prepreg production logistics, and entire container up to temperature.
reduced resin waste, which is normally attributed to lengthy heating and reheating cycles. Heating

only what is needed, when it is needed, is the primary goal of point-of-use resin heating. Point-of-use Improving yield by temperature controlled premix delivery
resin heating is a continuous heating process that ofers a high-volume, cost-saving alternative to If premix temperatures vary when they are put into the coating head of the ilmer, coating thickness
traditional bulk heating methods. In addition to saving costs, point-of-use heating enables new for- will vary, ultimately afecting the resin-iber ratio of the prepreg product. (Figure 5) his efect is due

mulation technologies and faster, more continuous prepreg manufacturing concepts.
to viscous shear. Viscous shear is the friction that occurs between boundary layers (knife and roll) at
a certain low velocity (coating line speed). As viscosity changes so does shear, and as shear changes
Prepreg production process
so does the efective deposit of coating thickness. he coating head is a doctoring blade or roll that
he prepreg resin and carbon iber are actually a formulation of diferent resins, modiiers, and cura- deposits the right amount of resin on a release paper in a continuous in-line fashion. Tightly moni-

tives, which are mixed together and poured or pumped by hand onto a ilmer or coating machine, toring resin ilm thickness is key to producing a consistent resin-iber ratio of the inal prepreg.
sometimes with the assistance of a transfer pump. he mixed resin formulation, called a premix, can If the resin temperature increases, the viscosity decreases, resulting in reduced viscous shear
either be frozen to suspend its reactivity for ilming at a later time, or delivered to the ilming process through the coating slot. When shear diminishes, more resin can low through the gap (under the
while it’s still hot. As the resin mix is coated onto a base release paper, a continuous supply of dry knife), resulting in greater resin deposition on the release paper. Assuming a constant carbon iber

carbon iber strands are pressed into the resin ilm, producing prepreg. As the prepreg continues feed down the line will result in a resin-rich prepreg.
down the coating line it is cooled, and either cut into sheets, or rolled up. When the carbon iber and Inversely, as premix temperature decreases, viscosity increases, creating more coating head shear,
resin mix cools, the resin solidiies and enters what is known as the “b-stage”. his is an intermediate resulting in reduced resin thickness. hese variations in thickness, if large enough, will translate ul-
stage where the resin is partially reacted and stable for a short period of time. To ensure the highest- timately into higher labor costs to constantly monitor and adjust coating gaps and, if not managed,

quality prepreg product, with the correct carbon iber-to-resin ratio, limited volumes of the resin pre- rejected prepreg materials. To counter this temperature and shear variation, point-of-use heating
mixes are created several times a day and delivered to the prepreg ilming process. (Figure 1)
pumps can be used to create a constant resin temperature delivered to the coating head, leading to a
more controlled prepreg manufacturing process and ultimately higher yields.
Preparing the resins for batch mixing

Resins are usually supplied in ive- to 55-gallon (20 to 200 liter) drums, and heated using an external Point of Use Heating is a Technology Enabler
heating source, such as an oven, or a clamshell drum heater. he heating process for a 55-gallon drum Another major advantage for inline or point-of-use heating techniques is the enabling of inline resin
can take as long as 24-48 hours. Once the preheat process begins in the drum, higher performance metering and mixing processes. Inline meter-mix concepts leverage continuous drum platen heating
multifunctional resins will begin to advance, or self-polymerize, due to their autocatalytic or reactive by supplying accurate resin low to a continuous in-line mixing process, eliminating batch mixing of

nature. If these resins are kept hot for any extended period of time, they will age, resulting in prop- reactive materials and the associated cleaning reactive materials from expensive batch mixing vessels.
erties which are out-of-speciication, making them no longer useable. If a 55-gallon drum of resin
Further, by eliminating batch mixing, faster reacting resin systems can be investigated to increase
is heated, but only partially used, the remaining material might age beyond its useable life and may prepreg production throughput by curing faster in the inal molding process. Figure 4 shows a typi-
need to be disposed.
cal inline meter mix system supplied by a point-of-use drum heater. he reactive hardener is metered

Even in high-volume prepreg production operations, where waste is less of an issue, bulk heating accurately with the hot resin and the two components are mixed in line using a static mixer. his pro-
of drums can be a logistical problem. he time-consuming nature of bulk heating means production vides a constant temperature low of freshly mixed prepreg resin to the ilmer with the advantages
demands must be carefully planned and monitored to ensure a constant supply of appropriately described above.
heated resin. In some cases, there is simply not enough heating space to stage as many drums of ma- With the reduced waste, improved logistics, and enabling of next generation processes, point-of-use

terial as are needed for a full day of production. his is when point-of-use heating becomes not only a resin heating ofers great promise as the logical composite industry standard resin heating method.
high value, but a necessity.
When using point-of-use drum heaters, the drums of ambient temperature resins can be moved Mac Larsen is director of application development at Graco Inc.

directly from ambient temperature storage shelves, and placed one at a time into a platen heating
system and the resin is melted and pumped “on-the-ly” out of the drum. his technology heats and Graco, Inc. | www.graco.com
transfers only the resin that is in close proximity with the machine’s heated platen. he rest of the
Kevlar is a registered trademark of E. I. du Pont de Nemours and Company

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