Foam Core Materials in the Marine Industry
by Trevor Gundberg, Composite Materials Engineer with DIAB Inc. Continued from Page 2
Foam Core Processing
Machining Foam Cores: Most foam cores are easily machined and formed using standard wood working tools, such as band saws, lathes, drills, sandpaper, and routers. Higher density foams may require lower feed rates during cutting (due to the material's low thermal conductivity) or else the material may burn and char. Before machining any foam core, be sure to consult with the manufacturer, since each foam chemistry has its own unique properties.
Special Contours: Many foam core suppliers contour, or process the foam sheets is different ways for specific applications. The most familiar form of contouring is the grid scored pattern (See Figure 8). The foam sheet first has a lightweight fiberglass scrim attached to one side, and then the foam is cut into 1" x 1" squares. The scrim holds the squares together, allowing the foam sheet to conform to complex curves. Another type of contouring is known simply as a "cut" (Double cut, triple cut, etc. See Figure 9). The foam is cut, again into 1" x 1" squares, but only about 2/3rds of the way through the thickness. This allows the sheet to have some flexibility, but the cut is mainly used as a flow media for resin infusion processes, or as air escape channels when utilizing hand layup. Some suppliers also produce grooved core (See Figure 10). These grooves are usually 0.120" thick and deep, and are used as flow channels in vacuum infusion processes with relatively thick skins. Other processing types available from suppliers include perforating and surface scoring, both of which are used in resin infusion processes or as air escape channels. Another process recently available from suppliers is pre-cut kits. For high production runs, the foam is pre-cut to customer's specifications, thereby eliminating waste and reducing labor.
Manufacturing with Foam Cores
Material Preparation: When using foam cores in manufacturing, there are some strict guidelines that must be followed to produce a mechanically sound part. The foam materials must be stored and prepared correctly, or else the bonds will be highly suspect. First of all, the foam must be stored in a clean dry place. Any dust or moisture allowed to settle on the core may produce a disbond after laminating. If dust is present on the foam surface remove with a vacuum, or at worst blow it off with an air nozzle (with a DRY air source). Do not use solvent to wipe the surface; this only spreads the dust/contaminants around the foam surface and into the open surface cells. A strong solvent, such as acetone, might also degrade the foam on the surface, leaving a weak core/skin bond. With a properly clean surface, little else can go wrong when bonding on the skins.
Processing with FRP: Foam cores can be used in almost all forms of fiberglass/advanced composite fabrication. The three main processes that utilize sandwich construction, hand layup/spray up, vacuum bagging/infusion, and prepreg/autoclave can all produce durable end products if they are executed correctly.
In hand layup/spray up applications, the core/skin bond has the chance to be compromised the most of all the processes. For aesthetic reasons, many fabricators lay up a skin coat or print barrier over the gel coat and let it cure before the core is installed. To bed the core into the skin coat, either a core bonding adhesive (CBA, ex. Divilette, Corebond, Baltekbond, etc.) or a resin rich chopped strand mat (CSM) is used. To bed the core with CSM a minimum of ¾ oz/ft 2 material is needed with an approximate resin to glass weight ratio of 75:25. Once the CSM is saturated with the resin, be sure to prime the foam surface with the laminating resin to fill the open surface cells and bedded into the CSM. Some of the SAN and linear PVC foams require a "hot coat" or highly catalyzed (around 2% by weight) resin priming coat to reduce the effects of styrene attack and increase bond strength (which, consequently also works for the other foams). When using a CBA, the catalyzed adhesive is applied to the skin coat by either applying by hand and leveling out to the proper thickness using a hand trowel, or by spraying with an air driven putty spray gun. The core is then primed and bedded into the CBA. Using CBA's instead of resin rich CSM has some distinct advantages such as lower weight, less core profiling, and lower exotherm. When using grid scored or other forms of contoured core, just pressing the core into the bedding layer still leaves the cuts open and.susceptible to water infiltration. One way to fill these cuts, or kerfs, as they are more commonly known, is to use a vibrating roller on the backside of a bedded core. The vibration allows the resin or CBA to fill the open kerfs and consolidate the sandwich structure. Another way to fill the kerfs completely is to employ a vacuum and draw the resin/CBA up through the cuts, which is done in vacuum bagging.
In a vacuum bagging process, the foam core is bedded into the CSM or CBA and a thin flexible bag material is sealed around the perimeter of the part and a vacuum is pulled. The vacuum allows atmospheric pressure to be evenly applied over the part, pressing the foam into the bedding layer and pulling the resin/CBA into the open kerfs. In most vacuum bagging operations, first the skin coat is first allowed to cure, then the bedding layer is applied, the foam is placed on to of the bedding layer and bedded in using the vacuum bag, and finally the top skin in hand laid on the core. One important feature of vacuum bagging process is the utilization of breather and peel plies. Before the vacuum bag is applied, a thin nylon sheet, or peel ply is placed over the foam. This layer acts as a release layer for the breather ply, which allows the vacuum to evacuate all the air from under the bag. Without a breather ply the vacuum could be "pinched off" in some areas, therefore not allowing the resin/CBA to fill the cuts.
Another way of ensuring all the kerfs or cuts are filled is to use a resin infusion process. In this process, all the glass and core materials are laid in the mold dry and a vacuum bag in sealed around the perimeter. Once the vacuum is pulled, resin is allowed into the bag and is "pulled" through the fiber and core materials. One way to distribute the resin across the part is to use the cuts and/or grooves in the core as flow channels. Sandwich constructions made using this process have high fiber to resin ratios, almost unlimited setup time, and no handling of sticky resins.
Another high-end process in which foam core can be used is prepreg/autoclave lamination. Prepreg lamination usually involves higher temperatures (> 200 0 F) and pressures (> 15 psi) than the more traditional manufacturing processes. In order to withstand the elevated temperatures and pressures, the proper foam cores must be used. Many foam core suppliers produce high temperature versions such as Divinycell HT, Klegecell TR, and Airlite/Herex C71. These foams are more dimensionally stable up to 250 F than normal foams, and are stabilized to reduce the amount of overpressure in the foam cells. Without this stabilization, the pressurized gases in the cells will be released during the high temperature processing and inhibit a bond from forming with the prepreg skins. While foams can be used in this process, there are some limitations. A general rule is to not use the foams in temperatures over 250° F at pressures over 25 psi. Any additional heat or pressure will cause the foam to become dimensionally unstable, and eventually shrink in thickness, and/or degrade. There are also some compatibility issues with some types of epoxy prepreg and various grades of foam. If you plan on using an epoxy prepreg with foam, contact your foam supplier for compatibility information.
Joining Foam Laminates: When it comes to joining foam core and foam core laminates, most adhesives used in composites work rather well. For adhering foam to foam, to maybe increase the cross sectional thickness, most epoxy, urethane, or acrylate adhesives will work. For the bond to work correctly, the adhesive must be stronger than the foam material it is joining (shear and tensile). The essential part of joining foam to foam is to fill the surface cells with the adhesive. Most bonds to foam are mechanical, and it is imperative that all the open cells are filled with the adhesive, on both substrates. When bedding multiple sheets of foam core into a laminate, simply butting the edges together is adequate, as long as the skins across the joint are continuous. For added protection against water intrusion, laminating resin can be used as an adhesive between the sheets. For joining foam core laminates, again any typical composite adhesive will work in a variety of joint configurations. Probably the most recommended joint is the lap, with the core components butting up together and the skins on either side creating the lap. This creates a continuous skin over the core joint, thereby not creating one weak point in the laminate. Other favorable joining methods would include scarf and stepped joints. Again, the adhesive must be stronger than the foam core, and the skins must be continuous over the core joint.
Foam core is becoming more and more prevalent in boat building, with new formulations and applications being created every day. The different grades and densities all have different properties, uses, and price tags. If you plan on using foam in your boat, be sure to contact the supplier's technical service providers for what exact foam and laminate is right for your application.
We would like to thank Trevor Gundberg, Composite Materials Engineer with DIAB Inc.for writing this article for us to feature on Boat Design Net. For more information on using foam core materials in your next boatbuilding project, we recommend you direct questions and inquiries to DIAB, Inc.