|Vacuum Infusion Project|
The following is a description of building an epoxy/glass/Kevlar/carbon kayak with vacuum infusion, using a cedar strip kayak as a male mold. The intent is that this provides some practical detail to anyone interested in completing a similar project. Questions are welcome.
Vacuum infusion is a process in which the pressure gradient between a volume under vacuum and the surrounding atmospheric pressure is used to flow resin into a fiber reinforcement. Fibre Glast provides a nice introduction to vacuum infusion. There are a number of other terms for the same process, some of which are acronyms for patented systems. For example, SCRIMP is a patented system of vacuum infusion owned by TPI Composites. The ASM Handbook, Volume 21 Composites, has a good section on vacuum infusion with more technical detail than the Fibre Glast introduction. Because there are good descriptions of the process available, the content here will focus on application.
Why vacuum infusion? The technical reasons for using vacuum infusion center on laminate quality, but also include time management. Vacuum infusion allows one person to handle jobs that would be difficult or impossible to accomplish using vacuum consolidation of wet laminates. However, the major drawback to vacuum infusion is that once the infusion process has started – mixed resin is flowing into the laminate – everything must be correct or the entire project can be ruined in a matter minutes.
· Plan to spend as much time or more making test samples as infusing the first boat. If you lack the patience for that, either do something else with your time or be prepared to ruin a lot of material.
· Plan the test samples carefully and keep good notes regarding fiber weight, resin weight, shop temperature, pump-down time, infusion times, and any problems encountered. When the sample is cured, complete your notes with comments about the sample appearance and nominal resin to fiber weight ratio.
· Plan to do at least one “boat unit” sample. A boat unit sample is an 8 to 10 inch long section of the actual mold that includes the greatest distance between the resin feed and the vacuum line. For example, I used a resin feed along the centerline with vacuum lines along the shear, so my boat unit was 10 inches long and spanned the widest part of the boat from shear to shear. Do the boat unit sample when you think everything else is dialed-in.
· Plan to spend time tracking down leaks. No detectable leaks can be tolerated, and if leaks are suspected, keep looking. If you have experience with vacuum consolidation of wet laminates, there might be a temptation to rely on the flow capacity of the pump to overcome minor leaks. Using vacuum infusion , even small leaks can cause entrapped air, or a ruined part. One guideline is a maximum of 5 mbar per minute of vacuum loss with the pump valve closed, prior to infusion [ASM Handbook Volume 21].
· Plan on long pump down times prior to infusion. Hours may be required to remove sufficient air from the reinforcement to prevent porosity. Some recommend actively pumping down overnight. I decided to use a minimum of four hours of pump down on both the hull and deck.
Mold release: Two layers of PVA mold release were applied with a foam brush to the kayak/mold. The boat had been paddled for a year before this, and the only preparation for mold release was to scrub it with soap and water. However, the boat was in good condition.
Reinforcement: The hull laminate consisted of four layers of 9 oz 8-HS S-glass and four layers of 5 oz 4-HS Kevlar. The glass was placed on the outside of the laminate, and a layer of 2 oz E-glass was put on the inside of the Kevlar as a sanding/bonding/abrasion layer. The combination and order of glass and Kevlar was guided by data in the book Boat Builders Manual, Fifth Edition, Edited By Charles Walbridge. The book is out of print, but copies can still be found on-line through used book sellers. The total weight of glass and Kevlar was chosen to provide about the same finished weight as the strip-built hull that was used for the mold. Basic concepts of composite weight fractions were applied in a spreadsheet, so that changes to the cloth weight, number of layers, and assumptions about resin content would predict a finished weight. The same spreadsheet allowed calculation of the nominal resin to fiber ratio that was actually achieved.
Resin: The choice of resin was based on experiments. I started with two resin/hardener combinations left over from other projects, Raka 127/606 and Fibre Glast 2000/2120. I then tried West System ProSet 147LV/229. None of the ProSet, test samples failed to infuse completely, at least not due to issues with the resin and flow media combination. Other resin systems certainly work, especially when coupled with the right flow media, but the ProSet gave me the confidence that any problems would not be due to resin selection.
Release cloth: Release cloth, also called peel-ply, was placed on top of the dry laminate to allow release of flow media and resin/vacuum lines from the cured part. Both Teflon coated glass, and nylon, characterized as having good resin transfer, worked well on this project. Aerospace Composites Products and CST, the Composites Store carry the Teflon coated glass. Fibre Glast carries the Nylon cloth.
Flow media: After a series of test samples, I decided to use EnkaFusion nylon mat, purchased from Fibre Glast, which is placed on top of the release cloth. The mat is formed by a random pattern of nylon monofilament, which allows good resin flow over the top of the laminate. As the resin front progresses, infusion takes place primarily through the thickness. I also experimented with Lantor Soric, purchased from Fibre Glast, which is a polyester mat having hexagonal flow channels for the resin. The Soric is typically placed between layers in the laminate and provides bulk as well as resin flow. It appears to be the same as Corecell, with embossed channels. I decided to use the EnkFusion mat primarily because I wanted a solid laminate. The Soric also had the drawback, based on my test samples, of creating a surface that needed more fairing than with the EnkaFusion mat. An experiment with tulle from the local fabric store suggested that a couple of layers could be used as an inexpensive substitute for EnkaFusion mat on small parts, but resin flow was not as good. Test panels without flow media failed to infuse completely.
|EnkaFusion mat (left) and Lantor Soric (right) flow media. Both provide good resin flow to the laminate. I used EnkaFusion mat on top of the release cloth. The Soric is typically used between lamina, and adds thickness to the laminate. The hexagonal cells of the Soric maintain an open structure, while the flow channels are resin rich.|
|Resin feed: EnkaFusion filter jacket, from Fibre Glast, was used to feed resin. It consists of a crimped version of the EnkaFusion mat encased in a porous jacket. The crimped mat creates channels within the jacket for fast resin feed. I cut the four-inch wide jacket in half for test panels.
Vacuum lines: Within the bag, .25 inch polyethylene spiral wrap was used to maintain open vacuum along the edge of the laminate. The spiral wrap was purchased from Newark Electronics.
Bag film: I used nylon bag film because it is more puncture resistant than polyethylene, which is a nice hedge against leaks, particularly along cut edges of the EnkaFusion mat. The nylon bag film is also good quality with respect to pinholes and thin spots. While polyethylene sheet from the hardware store is an inexpensive film for vacuum bagging wet lay-ups, I had some trouble with leaks when using it on test panels. I have had polyethylene film react with polyester resin, but nylon was unaffected. So if you use polyester or vinyl ester resin, make sure the bag film is compatible.
Seal tape: I found it was convenient to have two types of seal tape. The gray tape is good for long or difficult sections where it may be nice to adjust the tape occasionally before final sealing and it comes off the mold easier than the yellow. The yellow tape has higher tack and is softer, which is nice for sealing around fittings or fixing leaks.
|Seal tape, sometimes called mastic, or tacky -tape, used to seal the bagging film to the mold. The yellow tape is softer and has higher tack. The gray tape is easier to adjust, and comes off the mold after cure with less effort.|
|Tubing: Thick-wall vacuum tubing is preferred over standard wall flexible tubing (Tygon) on the vacuum side of the system since the standard wall will tend to collapse. I used .25 inch, inside diameter. On the resin feed side, standard wall is preferred so that it is easy to seal the ends with a spring clamp or zip-tie. I used .5 inch for the resin line on the hull, and .25 inch for test samples. McMaster-Carr carries both styles.
Connectors: Barb fittings work well and allow easy hose changes. I used brass fittings on the pump and resin trap, and nylon to connect to the vacuum bag. Check McMaster-Carr or the local hardware store.
|Thick-wall .25 in ID vacuum tubing and standard wall .5 in ID tubing for resin feed. The nylon 'T' and straight barb connectors were used for bag attachments.|
|Vacuum system: My primary pump was a Dayton Speedaire oiless rotary vane. A smaller Gast diaphragm pump was also on-hand as a backup, and was used for some of the test samples. Considering the cost of materials in a kayak hull or deck, the back-up pump seems like reasonable insurance to me, especially when buying used equipment. Both pumps were purchased off e-bay. A resin trap is a must for vacuum infusion. I made a couple of different traps out of 4 inch diameter ABS pipe. The end caps can be drilled and tapped with pipe threads for the inlet/outlet fittings. If enough resin is drawn into the traps, the heat generated during cure can soften the ABS enough to collapse, so keep the trap in a bucket of water for larger infusions. I installed a vacuum switch and solenoid valve on one trap to eliminate continuous running of the pump, particularly during cure. The valve I bought off e-bay, and the switch came from Omega Engineering. Both resin traps have vacuum gages and bleed valves threaded into the cap. A brass ball valve on the pump allows quick isolation from the rest of the system, and allows shutting off the pump while checking for leaks.|
|The complete vacuum system is shown on the left. The ABS pipe resin trap also serves as a vacuum reservoir, and was kept in a bucket of water to keep it from over heating. The rotary vane pump with isolation valve is shown in the upper right. The top of the resin trap is shown in the lower right. The Parker solenoid valve (1) is a normally closed model, which means it is closed when the vacuum switch (2) is open. When the vacuum level reaches the adjustable set point of the switch, it opens, cutting power to the pump and solenoid valve. The Bleed valve (3) allows venting the vacuum and a simple vacuum gage (4) shows the pressure at the trap.|
|Balance: I would not want to play with infusion without a balance, both for weighing test sample cloth and resin prior to infusion and for checking the resin to fiber ratio of test samples after cure. A balance is also a good way to meter the resin and hardener prior to mixing. I bought a basic three-beam balance off e-bay that works fine.|
|Basic three beam balance with total capacity of 2.6 kg|
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