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Semipermeables: Next trend in infusion? | CompositesWorld

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These liquid barrier films can ensure that molders have better performance, reduce waste and reduce risk.

The semi-permeable membrane technology has had a real impact: the sea racing team of the 2012 Miss Geico World Championship began testing thin film infusion (MTI) hoses from DD-Compound of Germany DD-Compound (see the figure and chart below). Carbon fiber/epoxy parts were injected into the boat in 2013, including the engine mounting structure, the instrument panel structure (see the picture below) and the canopy, which can achieve zero failure even after bearing an impact load of up to 12G. Source: Miss AMF-Geico Racing Team

The carbon fiber/epoxy parts injected by Miss Geico's team on the ship in 2013 included this instrument panel structure. Source: Miss AMF-Geico Racing Team

European Aviation Defense and Space Company (EADS, Amsterdam, Netherlands) has obtained a patent for vacuum assisted process (VAP), which uses a semi-permeable membrane to separate the vacuum outlet from the surface of the part, thereby forming a vacuum gradient and proceeding in-situ on the entire surface of the part Degas instead of degassing only at the edge of the part. Source (above): Andrew George from the Aircraft Design Institute (IFB) of the University of Stuttgart. Source (below): "VARTM Variability and Materialization", Joint Advanced Materials and Structures (JAMS) Center of Excellence Technology Review, University of Delaware D. Heider, C. Newton, JW Gillespie, 2006.

MTI has the function of a non-woven fabric layer and a breathable membrane. It is reported that it can optimize the air discharge process during the infusion process, eliminating the need for a water collection basin and preventing dry spots. Source: DD-Compound

In the conventional infusion process, the vacuum system ideally sucks in the resin and exhausts the air to reduce the occurrence of surface porosity and laminate voids. However, despite the use of vent holes and vent holes, it is inevitable that resin will accompany the discharged air. This wastes resin, blocks the vacuum line, and must prevent damage to the vacuum pump by using a collecting basin. The vacuum line must be flushed with solvent or discarded. In either case, the processing cost will be magnified. One promising solution is semipermeable membrane (SPM), a thin, flexible film or fabric usually made of polytetrafluoroethylene (PTFE) or thermoplastic polyurethane (TPU). SPM can permeate gas, but not liquid.

SPM is usually used in the reverse osmosis process in water filtration (for example, see "Designing pressure vessels for desalination plants" under "Editor's Choice" in the upper right corner) and in the medical industry. Gore-Tex is a PTFE-based porous fabric produced by WL Gore & Associates (Newark, Delaware), which is a breathable but waterproof component of a functional jacket.

In the composites industry, SPM is touted by proponents as a potential route for easy, repeatable, high-quality infusion. Supporters made various suggestions, suggesting that SPM be used as an additional layer in all or part of the material pile, or just as a filter cloth wrapped around the exhaust hose.

The vacuum assisted process (VAP), patented by the European Aviation Defense and Space Company (EADS, Amsterdam, Netherlands), can significantly improve the infusion process by using SPM on the entire surface of the entire laminate. Andrew George described this in his 2011 PhD (PhD) dissertation in Engineering Sciences at the Institute of Aircraft Design (or IFB) at the University of Stuttgart:

"VAP is a patented variant of resin infusion, in which a semi-permeable membrane separates the vacuum outlet from the surface of the part. This forms a complete vacuum gradient and continues to degas on the entire surface of the part. This is only in the traditional resin infusion. The end edges of the parts are opposite. Theoretically, the full vacuum gradient can lead to fewer voids and reduced thickness gradients...and thus have higher mechanical performance and repeatability."

George evaluated Gore-Tex and various similar semi-permeable materials from Saertex (Salbeck, Germany), Pil Membranes (King Lynn in Norfolk, England) and Airtech (Huntington Beach, California). Other sources include Trans-Textil GmbH (Freilassing, Germany), which has now become an authorized supplier of SPM products for the EADS VAP method.

VAP proponents believe that using SPM can improve the consistency of the flow front and remove dry spots without reducing the vacuum. George pointed out that in the process of removing dry spots in the VAP, the placement of the ventilation holes is not optimized. Citing the research results of the University of Delaware published in 2004

George's article "Process and Performance Evaluation of Vacuum Assisted Process" also pointed out that without any pre-injection and degassing, the porosity was reduced from 1.64% to only 0.37%, and the thickness gradient was reduced by 77%. However, he cited some shortcomings:

George also warned: “In order for the membrane to work properly, it must not be saturated with resin during the infusion to full curing. Some membranes can use a specific set of process parameters and cannot be used with other membranes.” He concluded. , The wettability of the membrane is a key parameter.

These concerns emphasize the importance of modeling processes and performing actual tests (as often suggested by infusion experts) to ensure the required resin flow when introducing any new variables into the infusion device (see "Editor’s Fine Select "Infusion for application" in ""). When SPM is used in combination with a distribution medium (DM), George's test shows the lowest porosity. When DM is used, the void content of PTFE is lower than that of TPU. However, PTFE is much more expensive, so George believes that TPU is worth considering, but it should first be tested in specific infusion applications.

In contrast, Jay Carpenter, a technical instructor at Abaris Training Resources (Reno, Nevada), used semi-permeable materials and found that they can solve problematic flow areas well. "We put the material on the top of the area and ventilate it," Carpenter said. "This solves the problem of dryness."

He added: "This makes sense, because now the air can be exhausted vertically instead of having to draw the entire laminate from the vacuum inlet. This is an added step and expense, but it is a relatively easy solution," he Pointed out. "The downside is that because the semi-permeable membrane is opaque, you will never see water anymore."

Importantly, George reports that the origin of VAP is the use of PTFE, just to protect the vacuum port from resin intrusion. The Membrane Infusion (MTI) hose was introduced in 2011 and was designed specifically for this purpose. According to the description of its proponents, MTI is described as an evacuation hose surrounded by a non-woven fabric layer and a breathable membrane, and it also optimizes the evacuation process during infusion. MTI was developed and patented by DD-Compound (available through German Advanced Composites in Miami, Florida, USA), thereby eliminating the need for collecting basins and reducing resin consumption, but it is reportedly also Reduced dry spots and porosity.

Developer Dominik Dierkes said: "The MTI hose can be used anywhere a spiral tube or suction medium is used." When the hose is placed around the die lip, it is reported that the resin will stop when it reaches the diaphragm, but it will continue to flow. Pass the rest of the drying fabric, eliminating the need for complex calculations on the resin injection point and ensuring complete wetting of the drying fabric.

when. . . when

The offshore cooperation team met with Dierkes in October 2013 and agreed to try the MTI hose. The team partner and crew leader Gary Stray admitted: "At first I was very skeptical because I saw many examples of this when using composite materials. .Because the team has been using standard vacuum bag wet-laid technology and post-curing technology to build a 50-foot/15.2m carbon fiber/epoxy catamaran, Stray is also full of doubts about the conversion from wholesale to infusion.

"We have a variety of material suppliers coming in and telling us that infusion is the only way to go, but I am not fully confident about it. You must always close the valve, and always observe and control the flow of resin to completely lubricate. Wet parts but not dry points," he recalled. "This is not easy and it is not repeatable." In addition, these ships are powered by 3,500 hp twin engines, each weighing 1,500 lb / 680.4 kg. They can drive at 200 mph/322 kmh in flat seas, often face 4 ft/1.2m waves, and withstand continuous shock loads from 4G to 12G. Therefore, there are great risks in decision-making. Stray said: "Therefore, we conducted thorough tests on MTI hoses and infusion fluids, manufactured a large number of test panels, and analyzed the performance of pure material samples."

In the end, Stray and his team were impressed: “With the MTI hose, the resin flow during the infusion will take care of itself.” He explained, “It moves until it reaches the MTI line and then stops. If If you have a dry patch, there is only one way to remove the resin, and the vacuum keeps pulling until the resin has penetrated everything."

Having said that, the team didn't turn to infusion, but chose specific components on the ship, including the structure bolted to the engine, the dashboard and the canopy. Stray said: "All of these are key components, but they also benefit from superior performance and compact laminates."

For example, even if the previous instrument panels are built into the ship, they will vibrate and move when they hit the waves. Stray said: "Right now, with these injected structures, we haven't seen or felt any movement," he added. The analysis of the cross-section and the finished laminate showed: "There is no air in the parts we pulled out, and no Pinholes, in the past, we had to do a lot of post-curing treatments.” As a result, after four months of racing, the team’s 2013 ships did not have any failures, and the team made a plan to inject the entire ship in early 2014. .

Long-term infusion expert André Cocquyt (ACSM, Harpswell, Maine) agrees that, in particular, MTI hoses "will almost eliminate resin-filled vacuum lines and in many applications do not require a water collection bucket", adding that "the membrane must It will become an important asset in our infusion toolbox." However, he recommends proceeding cautiously to prevent molders from repeating the process covered by the EADS patent. He asserted: "Users need to review existing patents to avoid infringement, and instead, they recommend that they" use these materials as a basis for [their own] process innovation. "

Although the application is not as demanding as fuselage composites, the requirements are still demanding-passenger safety is the key.

There are many ways to make composite parts. Therefore, the choice of method for a particular part will depend on the material, part design, and end use or application. This is a selection guide.

Compared with traditional materials such as steel, aluminum, iron and titanium, composite materials are still aging. Until now, design and manufacturing engineers have a better understanding of composite materials. However, the physical properties of composite materials coupled with unparalleled lightness undoubtedly make them attractive. 

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