End Grain Balsa
structural core material provides exceptionally high shear, tensile and
compressive strengths, and stiffness for its weight. Available as flat
panels, as scored scrim backed drapable sheets and as composite panels
with a choice of skin configurations. Balsa is a high quality sandwich core material. Its end-grain,
micro-honeycomb structure offers exceptional strength and stiffness. It
has a high strength to weight ratio. Balsa sheets are
scored and backed with glass fiber scrim. The grid scoring allows the
core to readily conform to the contours of molds and curved surfaces
without build-up of internal stresses in the laminate.
Divinycell is a semi-rigid PVC foam used as a sandwich core material when strength, stiffness and low weight is desired. It has excellent insulating properties and a closed-cell structure that makes it impervious to water. Widely used in the marine, transportation and aerospace industries.
Crosslinked PVC Foam (Universally Structural)
AIREX® C70 is a lightweight, closed cell foam for universal use in sandwich constructions. Its excellent stiffness and strength to weight ratio and high toughness make it suitable for a large variety of applications. The foam is ideally suited for statically and dynamically loaded structures and is compatible to all resin systems.
Linear PVC Foam (Damage Tolerant Foam)
AIREX® R63 is the only genuine, high quality, linear PVC foam and market leader in impact resistance doubling the values of its next competitor. The ability of a sandwich core to resist failure, even if there is local overloading, determines its mechanical performance. Consequently, for applications in dynamically loaded structures, the closed cell AIREX® R63 is the best of all the structural foams available
Polyetherimide Foam (High Performance Foam)
AIREX® R82 is a thermoplastic foam based on polyetherimide (PEI). Its fire resistance, thermal stability up to 180 °C (356 °F), mechanical damage tolerance, environmental resistance and easy workability, puts AIREX® R82 at the top of the list of high quality core materials for sandwich structures made with high temperature prepregs.
AIREX T90, New
AIREX T90, No other foam core shows as much versatility as AIREX® T90 Easy-Processing structural foam. This closed cell structural foam is designed for use with all resin systems and processing technologies, from hand lay-up to vacuum bagging, and infusion to prepreg processes
Foam Core Materials in the Marine Industry
The first foam material specifically formulated for a marine environment was a poly vinyl chloride (PVC) and isocyanate blend (simply called PVC foam) created in Germany by Dr. Lindemann in the late 1930's and 40's. It has been rumored that this early version of PVC foam was used in the German E-boats and even in the famous 'Bismarck' battleship. After World War II, France acquired the formula as part of its war reparations. From there, the formula was licensed out to companies in Sweden, Switzerland, and Germany, who kept developing the original recipe in their own distinct ways. After many years of different formula offshoots and company consolidations two main suppliers of PVC foam remain, DIAB and Airex/Herex.
based on chemical components other than PVC have also been developed
over the years, including: linear PVC (also originally formulated by Dr. Lindemann), polystyrene (PS), styreneacrylonitrile
The sandwich concept is based on two main ideas: increasing the stiffness in bending of a beam or panel and doing so without adding excessive weight. The general term for bending stiffness is flexural rigidity, which is the product of the material (s) elastic modulus, and the cross section moment of inertia. For a symmetric sandwich beam (both skins have the same thickness and material properties). It is apparent that the core material does not directly contribute to the stiffness of the panel or beam, (at least in lower density cores) but it's the distance between the skins that is the overwhelming factor. Increasing the "d" variable will have a much greater effect on the flexural rigidity than any other component. When dealing with higher density cores (usually > 5 lb/ft 3 ) and thicker skin laminates, the full equation must be used in order to properly predict the stiffness properties. This is due to the high-density core contributing stiffness in the first case, and the thick skins absorbing more shear stress.
While the core keeps the skins an equal distance apart from each other thereby increasing the stiffness, it also bears most of the shear loading. In bending, the lower skin is in tension, while the upper (or inner) skin is in compression thereby putting the core in shear (See Figure 1). In order for the sandwich to function correctly the adhesive layers between the skins and the core must be able to transfer the loads, and thereby be as least as strong as the core material. Without a proper bond, the three entities work as separate beams/plates and the stiffness is lost. This is why proper core/skin bonding is so critical.
Sandwich vs. Single Skin Fiberglass
Exactly how much stiffer is a sandwich structure versus a single skin laminate, and what are the weight savings? As noted above, the flexural rigidity of a structure is dependent on two factors: the material(s) stiffness or modulus, and the cross sectional geometry or moment of inertia. The material properties are often difficult to change (and sometimes expensive), so a change in the geometry can be done to increase stiffness while not compromising on strength or other properties of a single skin laminate. Figure 2 shows. the difference in stiffness, strength, and weight when a core material is placed between the plies of a single skin laminate (all attributes are approximately normalized).
From just increasing the cross sectional geometry, the stiffness increased 48 times, while the flexural strength increased 6 times, and all with a marginal increase in weight. The increase in strength and stiffness allows builders to use less skin materials, resulting in considerably lower weight structures. Decreased weight helps to increase top speed and acceleration, increases cargo capacity, and reduces fuel consumption. A sandwich construction is compared to a single skin laminate with relatively the same flexural rigidity in Figure 3:
Other advantages of the sandwich construction include: greater insulation, better impact/damage resistance, sound attenuation, and reduced labor. The core material, which is usually cellular in construction, provides a much lower thermal conductivity and higher R-value than a comparable single skin laminate. Labor is reduced since less plies of material are being used and the greater stiffness of the sandwich reduces the number of needed stiffeners. With less stiffeners, and consequently larger panel sizes, impact energy is dissipated more readily. The cellular core materials also reduce the "drum head" effect, thereby reducing noise, resulting in a quieter ride.
Some downsides to sandwich construction include the core material cost and the employee learning curve. Core materials are generally more expensive than the resin and glass that it is replacing, and in some cases the labor savings will not offset the cored laminate price. More care and attention needs to be taken when processing cored laminates. Employees need to be aware of all the possible problems that could occur if the core materials are not handled or bonded to the skins correctly.
Foam Core Properties and Applications
While core shear stiffness and strength are paramount in sandwich construction, other properties are also important. Compression strength is needed to withstand localized loads such as dropping an anchor on a deck or resting a craft on trailer mounts. Graphs in Figures 4 and 5 compare different foam core materials in shear and compressive strength.
CL PVC Foam: This foam is based on a thermoplastic PVC (Poly Vinyl Chloride) and crosslinked (or highly bonded) thermoset polyurea, or for short, a crosslinked PVC (ex. Divinycell, Klegecell, Airlite). This foam provides higher strength/stiffness in both static and dynamic situations, good temperature stability up to 180 0 F, good fatigue resistance, and a closed cell structure for low resin/water absorption. One problem encountered when using crosslinked PVC's is some grades are not compatible with some epoxy prepregs, and if the foam is not specifically treated, some prepregs will not sufficiently bond. Crosslinked PVC's are typically used in decks, superstructures, hull bottoms and sides, bulkheads, and transoms.
SAN and L PVC Foams: These foams are based on the thermoplastic Styrene Acrylo Nitrile (ex. Corecell) and a non crosslinked or "linear" PVC (ex. Airex R63), respectively. Typical reasons for using these materials are: high toughness, good impact resistance/energy absorption, good fatigue resistance, and a closed cell structure. Some drawbacks to using these foams are: comparatively lower strength/stiffness, high temperature problems, and is susceptible to styrene attack (the styrene in the resin may seep through the foam, leaving the resin uncured and the foam degraded). These foams are typically used in areas where high impact is prevalent, such as hull bottoms and sides.
PUR and PIR Foams: The polyurethane (PUR) and polyisocyanurate (PIR) foams have good compression strength and moderate physical properties at higher densities, but have a tendency to be friable, or deteriorate with time. Due to this, these foams are typically used in non-stressed acoustical and insulation panels. Higher density less friable versions of these foams are used extensively in transoms (due to their high compressive strength), while the lower density materials are used as formers or in stringers.
There are also other versions of these foams tailored for specific purposes, such as high temperature CL PVC's and SAN's for use with prepregs. Other foams such as PMI (ex. Rohacell) and PEI (Airex R82) are generally used in aircraft due to their relatively high cost and marine environment issues (e.g. water absorption)..
Foam vs. Balsa Wood and Honeycomb
Balsa wood and more commonly plywood have been used extensively in boat construction for many years. While these materials provide excellent compression and stiffness properties for a relatively low cost, they can be heavy, susceptible to water absorption, and will eventually rot. Foam cores can be much lighter, fungi resistant, and do not absorb water or any other fluids encountered in a marine environment. There is also evidence that foam cores have better fatigue resistance than balsa wood. Laminates made with foam cores can last longer and weigh less than wood cored laminates, while producing adequate physical properties. Wood is still a viable material to be used in areas where highly localized compression loads or through fittings are present (such as engine mounts and around cleats), where the appropriate high-density foam core may be too expensive.
Honeycomb materials such as Nomex (aramid paper and phenolic resin) and aluminum honeycomb are staples in the aerospace field due to their high strengths, high temperature stability, and low weight. While these properties work well for aerospace applications, honeycomb does have some drawbacks in a marine environment. Honeycombs have a relatively small area for the skins to bond to, producing weak core/skin bonds and poor fatigue resistance. The open cell structure of honeycombs is susceptible to water infiltration and bond degradation. Processing honeycomb materials also requires much higher end processing equipment and materials, such as autoclaves and high temperature epoxy prepregs.
The following figures compare the shear and compressive strengths of some foam cores with balsa wood and various honeycombs. NOTE: The 1/8" and 3/16'" designations for the honeycombs represent the cell size.
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
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.
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.
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.
It is difficult. if not impossible to know the laminate core structure of every vessel you survey. I have checked with some manufacturers who claim they do not know, or will not want to disclose the core material of an older vessel. This next example of a laminate failure occurred in 2007.
The vessel I was surveying was a 1988 Albin 28. Albins have a good reputation and are expensive. A very nervous broker was present during the survey. I make it a practice to invite the buyer to attend the survey, but when the seller or broker want to be there it is usually for a reason. Remember, whenever someone is constantly talking to you during your inspection, he may be trying to distract you.
After the initial walk around and inspection of the freeboard section I sounded the bottom. To my surprise there was a large section (at least five feet long by about three feet wide) on the port side that sounded very punky. The surface was smooth and clean. There were two through hull fittings in the area. I did not report this to the broker, and even though he was over my shoulder, he did not seem to be aware of the problem. To make a long story short I finished the bottom, and the rest of the survey and went on my way. The report to the buyer indicated my findings along with a suggestion that the bottom be repaired before going any further with the sea trial.
A few days went by and I received a call from the local fiberglass shop. They could not find a problem with the bottom and wanted me to come to the shop and show them the problem. I spoke with the buyer and he sent me a check for my visit to the shop as he still had great interest in the vessel. I asked the manager of the fiberglass shop to remove the larger through hull fitting before I made the trip to the vessel. He agreed, and when I got back to the vessel a couple of days later he said he did not need me. After removing the through hull fitting he discovered a soft brown material between the laminates of glass. The shop had soon cut away a few square feet of outer glass and still did not reach solid laminate. I demonstrated, with a metal mallet, the procedure for sounding a laminate. The shop manager then marked off the area we thought was bad with chalk. As he picked up a large piece of cork, partially adhered to the outer skin, he said, "How do you fix this one?" My suggestion was a new solid build up of the structure and to forget about replacing the cork. I took a few photos and reported back to the buyer. The vessel is still for sale.