What do these have in common? Each represents a significant obstacle to the widespread adoption of vacuum infusion in the marine industry.
Bags: In order for vacuum infusion to become truly economical, marine builders will need to be able to pull many (i. e. many dozens) parts from a single reusable bag. The use of nylon bags is simply too wasteful, too laborious, and too frustrating to satisfy a moderate to high volume builder. What are the elements of a desirable reusable bag? Durability, flexibility, rapid release, and simplicity of construction. Products that have been in the marketplace for some time are not as durable or flexible as builders would like them to be. New products in the marketplace show promise with respect to flexibility, releasability, and ease of use, but are unproven from the standpoint of durability and permeability.
Boils: If you've produced a part with conventional resin in a vaccum infusion process, you've probably noticed what appears to be air bubbles in the finished composite. Most likely, what you've seen isn't simply air from the surrounding atmosphere due to permeability or a leak in the bag...it's vaporized resin! Refer to the "Volatilization" article at http://www.polynovacomposites.com/ for detailed information on this issue; but in short, understanding the relationship between resin selection, ambient temperature, and vacuum pressure is critical to producing a properly cured part via vacuum infusion.
Print-through: Although it has little relationship to the structural integrity of a part, print-through is an important cosmetic issue for any marine builder and a particularly troublesome one for a builder making boats less than 30 feet. People attribute print-through to many things, but in reality, it's a resin shrinkage issue. In large boats (greater than 30 feet), builders may add extra layers of reinforcements in order to diminish the effects of resin shrinkage without posing too significant a weight problem or wildly over-building the hull structure. In small boats, however, print-through is a great challenge to overcome. Certainly, some resins exhibit less shrinkage during cure than others and builders should experiment in order to find the best performer. Many builders will use certain reinforcements known to counteract print-through. Builders beware! In some cases, the reason a reinforcement may reduce print-through is because it effectively suffers shear failure during cure, which has the side-effect of obscuring print-through. Make no mistake, such shear failure is diminishing the structural integrity of the finished/cured part.
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4 comments:
Get a low shrinkage resin system!
Regarding bubbles/boiling, I’ve experimented with vacuum infusion a little and have run parts that end up with bubbles. I was thinking that styrene was causing the problem. Why aren’t you thinking the same?
Thermodynamically the boiling point of a liquid can be thought of as the disruption of an equilibrium state between thermal saturation and a corresponding pressure saturation where an increase in thermal energy or a decrease in pressure results in a phase change to the vapor state. A phase diagrams of this transition can be plotted using the principles of thermodynamics and equilibrium. When we apply these principles to styrene we find that its boiling point pressure at 70 degrees Fahrenheit is ~29.9” Hg. If we look at methyl ethyl ketone peroxide (MEKP), a common initiator, we find that its boiling point pressure at 70 degrees Fahrenheit is ~24” Hg. So, it’s more likely that the MEKP is boiling then the Styrene. Cumene hydroperoxide (CHP), another initiator, does much better then the MEKP at 70 degrees Fahrenheit with a boiling point pressure of 29.5” Hg. Of course you’d need to consult with the resin manufacture before making an initiator switch to understand its effect on the thermosets properties. The thermodynamic principles used to derive these numbers are reviewed in the white paper Volatilization and the Infusion Process - Part 1, which is available on our website. Part 2 of this series we will evaluate an experimental set-up for determining the boiling point and thermal/pressure operating window of a given resin system.
I've found that the exotherm of the resin can greatly affect resin shrinkage. Using CHP definitely helps.
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