What is Vacuum?

Atmospheric pressure, the force per unit area exerted against a surface by the weight of the air molecules above that surface, can be measured and expressed in a number of ways. At sea level the standard pressure is 14.7 psia or 29.92" of mercury (Hg) or 760 mm of mercury (Torr). Because the atmospheric pressure varies with weather and altitude, the sea level pressures are used as a reference point. The term "vacuum" describes pressure that is below atmospheric pressure.

nches of mercury ("Hg) are a common measure of vacuum and are expressed in two different ways. One way is in "Hg gauge ("HgV), where the scale starts at 0"Hg (atmospheric pressure) and goes to 29.92" Hg, or full vacuum. The other way is in "Hg absolute ("HgA), where the scale is reversed such that the gauge reads 29.92" Hg at atmospheric pressure and 0" Hg at full vacuum. To relate the two methods 24" Hg gauge pressure at sea level would be 29.92 - 24 = 5.92"Hg absolute pressure.

Polybeam enabled cabin top infusion

On February 5^th 2008 the Landing School of Boat Building and Design infused a cabin top for a small powerboat. Polybeam703 was utilized as an infusion flow reinforcement to allow rapid and uniform resin flow. Polybeam703 was layed directly against the plain balsa core to act as both a structural reinforcement and flow media. The z axis fibers improve damage tolerance and shear strength, while the smooth surface profile allows for excellent bond strength between the core and skins.

The laminate schedule consisted of the following materials and resin.

Vectorply E-LTM 1808
Polynova Polybeam703
1/2" Balsa
Polynova Polybeam703
Vectorply E-LTM 1808

Resin: Pro-Set epoxy 500-700 cps viscosity

Seven resin feeds were setup using Enkafusion strips as the resin carrier. Rope was used to facilitate the vacuum around the perimeter of the tool. The total infusion time was approximately 30 minutes.

Part prior to infusion

Finished part

Have a question about composites? Ask here

Predictive Analysis and Flex Testing

Recently, Liquid Access approached Polynova Composites with the task of engineering a stiffer version of their Rocket competition slalom ski. While the current version performed well for skiers up to 135lbs., heavier skiers were requesting an increase in the ski’s stiffness for improved turning responsiveness. Since stiffness of a panel is dependent not only on the material’s flexural modulus, but is also a function of the cube of the thickness of the panel, increasing the core thickness would be the simplest solution. However, the desire to utilize the current tooling and manufacturing method excluded this option. To solve the problem, Polynova selected predictive finite elemental analysis as an aid in developing a new laminate. The analysis was then verified with three point bend testing of skis built with both the original and the proposed laminates.

The original Rocket Sit-Ski is infused with a vinyl ester resin and is comprised of a gel coat on the ski bottom, unidirectional carbon, a plain sheet of ½” foam core enveloped with Polybeam® 703 – a spacer fabric based IFR™, biaxial fiberglass, and a gel coat on the ski top. Vinyl ester resin was chosen because of its increased mechanical properties, particularly elongation to failure.

In manufacturing the gel coat is first sprayed into a standard female mold with a two-inch flange. Unidirectional carbon fiber is laid into the mold, followed by the Polybeam®. A plain sheet of ½” foam core precut to fit the mold is then laid in, followed by another ply of the Polybeam®. The biaxial fiberglass lamina and a semi rigid gel coated floating counter tool (float tool) are placed over the ply stack. The term floating here refers to the counter tools free positioning within the vacuum envelope. The Polybeam® extends beyond the float tool to accommodate the resin feed and vacuum port lines. The resin feed and vacuum ports are then appropriately placed, and the infusion proceeds. The ski is released at the end of the cycle, and trimmed for delivery.

Liquid Access set a flexural modulus multiple of two to three over the original ski as the design parameter for the new ski. Theoretical analysis of the original and candidate laminate schedules was made possible by VectorLam™ and Strand7 software. VectorLam™ accurately predicts mechanical properties of laminates built to various processes

including vacuum infusion. VectorLam’s™ ability to rapidly build and compare multiple laminate scenarios significantly decreased the time required in the initial design phase. The individual ply data from VectorLam™ was then imported into Strand7 FEA where the laminates were reconstructed. Strand7 is a general-purpose finite element analysis system consisting of pre-processor, solvers and post-processor. The laminated composites module provides fully interactive analysis of both symmetric and unsymmetrical composite laminates. The laminate engineering properties and the characteristic matrices are calculated based on standard laminate theory.

A major challenge was to accurately simulate the core under loading in conjunction with the laminate skins. Plate elements representing the laminate skins and brick elements representing the core accurately portrayed displacement but there was a gray area in the first mode of failure. Building the entire laminate utilizing Quad8 plate elements, which contain 8 equally spaced nodes around the perimeter of the plate, allowed for accurate representation of the interaction between core and skins without compromising displacement prediction. The core was assigned on the mid-plane and the laminate skins were offset by half the thickness of the core to accurately replicate the relationship between core and skins.

Utilizing additional unidirectional carbon on the top and bottom of the ski proved to be the most effective solution to meeting the design goal (2X to 3X flexural modulus). The added carbon unidirectional along the length of the ski increased the modulus which in turn decreased overall displacement of the ski under load.

To verify the predictive model, flexural testing of skis built with the original and proposed laminate was performed at the National Composites Center.

To learn more and read the complete article, please visit our website.

National Renewable Energy Laboratory study

The following statistics represent failures of wind turbine prototypes.

National Renewable Energy Laboratory study;

45 failures of prototypes;

20% lightning related;

16% foreign object impact;

13% tip deflection/tower contact;

20% adhesive bond failure;

18% voids in skin core;

13% improper cure of materials.

Above is an excerpt from North American Windpower, May 2006, p24.

It’s clear there are still major processing issues in composite wind turbine manufacturing. The Polybeam703 and HiFlux90 can help provide answers to those issues.

Polybeam703 is a three dimensional reinforcement which also acts as a resin flow channel. The z axis fiber increases overall laminate shear strength and provides uniform resin flow. The smooth surface profile of the x and y axis fibers provide excellent bond strength.

Key Benefits Include:

Reduces waste associated with other infusion techniques;

Improves infusion rates and uniformity;

Improves damage tolerance, core interface bond, and shear properties;

Highly conformable with an excellent surface profile;

HiFlux90 is a bilateral infusion flow reinforcement designed to maximize permeability and minimize set up time. From woven roving and chopped strand mat to high yield carbon the HiFlux90 provides uniform laminate wet out and superior infusion speed.

Key Benefits Include:

Enhanced mechanical properties;

Improve infusion rates and uniformity;

Reduce waste associated with other infusion techniques;

Highly conformable;

Permitted Foolishness

The Federal Clean Air Act mandates permitting as a Title V source for any stationary source that emits (i) more than 100 tons of any pollutant per year, (ii) more than 10 tons per year of any hazardous pollutant, or (iii) more than 25 tons per year of a combination of hazardous pollutants.

Progressive technologies, such as vacuum infusion, offer the opportunity to dramatically reduce emissions of cancer-causing agents like styrene, while also positively impacting production by reducing labor costs, reducing cost associated with waste disposal, and increasing unit through-put per production hour. Considering the relative inefficiency of open-mold processing, the exposure risk it poses to laborers, and the resulting environmental emissions, its unfortunate that manufacturers continue to file for Title V status as their production capacity increases, rather than choosing to adopt advanced processes that pose long-term economic gains and reduce pollution.

Bags, Boils, and Print-Through

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.