Hallmark's new moulded composite walls are much more than fiberglass panels glued to a wood or aluminum frame. The walls are actually free standing exterior frames with end grain balsa wood sandwiched inside marine grade gel coat and hand laid fiberglass. This technology is very strong and used in 2008 corvette's, high end yachts, race cars, and military vehicles. Over two years were spent with C.F. Maier building these one of a kind four season pop-up walls.
The gel-coated composite wall panels are structurally robust and are constructed of a fiberglass sandwich with a structural end grain balsa core employing the same techniques used to construct many high-end yachts. End grain balsa is a renewable resource that imparts impressive strength and stiffness to the sandwich panel. The end grain configuration of balsa provides high resistance to crushing, and is very difficult to tear apart. End grain balsa cored panels also have the ability to handle excessive dynamic loads with high resistance to fatigue and are used in products like military aircraft, high speed trains, and luxury yachts.
Click the "C.F. Maier" link below to find out more about true composite construction and our composite manufacturing partner:
C.F. Maier / click here to find out more
Standard for 2008 is hallmark's exclusive one piece carbon truss roof. With hallmark's new technology you can forget about roof maintenance or failure. Carbon trusses not only make a fiberglass roof a good idea, it makes a roof that is sure to change the reputation of pop-up truck campers forever. Click below to find out more.
hallmark's carbon truss roof / click here to find out more
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MATERIALS USED IN COMPOSITE CONSTRUCTION
A word about hallmark’s choice for composite construction. You will note, hallmark DOES NOT USE a vacuum press OR any glues in the construction of our fiberglass composite walls. “Aluminum frame, vacuum-bonded composite walls” are nothing more than traditional glued walls and have been used by hallmark in the past and many others for over 20 years. Hallmark uses epoxy composite sandwich construction, including end grain balsa wood as a core material and hand laid fiberglass and or carbon as a reinforcement material. Read more below to educate yourself about true composite fiberglass construction, only available on hallmark pop-ups.
Core Materials
Choosing the proper core material is critical to the overall composite’s performance. The first example is one piece of material with its respective weight and strength being shown as 1.0. When we insert a core material doubling the thickness of the composite notice that the strength increases to 3.5 the stiffness to 7.0 but the weight only increases by 3%. Further strength is noted by increasing the thickness 4 times. Observe even in this case the weight only increases by 6%.
Lightweight core materials include wood, foam, and honeycomb. Wood has obviously been around for a long time. It serves as a good core material for many composite designs. It is stiff, strong, and has high shear properties. However, its variations in density and physical properties along with the difficulty in fabricating limit its application.
Foam is usually the choice of material for the custom aircraft builder. Foams are easy to shape and reasonable in cost. Three types of foam are generally used within our industry. Polystyrene foam is the first. It is blue in color and is supplied in large billets. Polystyrene foam is often used to construct boat docks. This type of foam can be easily shaped using a "hot-wire" technique described later in this article. Polystyrene foam is the type used in several popular composite airplanes in the wings and control surfaces. It does have the disadvantage of being softened by exposure to gasoline and several other solvents. This type of foam cannot be used with polyester or vinyl ester resins both of which will be discussed later.
Polyurethane foam is basically a low-density insulating type foam also used for the construction of surf boards. Polyurethane foams are often used within a fuselage structure or for parts requiring detailed shaping. This type of foam is impervious to most solvents. Its color is usually tan or green. Polyurethane foam has certain hazards. It emits a poisonous gas when burned. DO NOT USE A HOT WIRE DEVICE TO CUT POLYURETHANE FOAMS. You also do not want to burn any scraps of this type foam. Carving and cutting should be accomplished using a knife, saw, or other cutting tools.
Polyvinyl chloride foams (PVC) are based on the same chemistry used in common PVC water pipe material. Divinycell and Klegecell are tradenames for this type of foam. Both of these are suited for structural cores. This material is resistant to most solvents and it can withstand a high temperature.
The last type of core material is honeycomb. This material has an appearance much the same as honeycomb found in a beehive. The sheet material used to form honeycomb can be woven fabric, metal, or paper. Honeycomb cores are used very extensively in the aerospace industry. Varying thicknesses are available along with a wide variety of materials. Honeycomb is usually supplied in 4 feet by eight feet sheets. Honeycomb materials offer exceptional strength to weight ratios but reliable bonding to outer skins is more difficult to achieve.
Reinforcement Materials
Many types of reinforcement materials are available for composite use. Three types are used most often to build custom composite structures. These are fiberglass, carbon fiber, and Kevlar.
Glass fiber or fiberglass is the most widely used reinforcing material. Fiberglass is manufactured with varying physical characteristics and cost. One of the most widely used is termed E-glass. This type of glass fiber has the best physical characteristics at the lowest price. One other type with limited use in our area is S-glass that is about 30% stronger than E-glass but the cost is often 2-3 times higher. Fiberglass is also offered in various weaves. The terms unidirectional and bi-directional are used. Unidirectional simply means all of the glass fibers are running in one direction-lengthwise. They are held together with threads running parallel to the glass fibers. Bi-directional fabric means the same number of fibers go across the material as found lengthwise. The type of weave is then defined. Several weaves are available such as plain, basket, satin, twill, etc. Fiberglass also is available in varying weights from less than 1 ounce per square yard to over 10 ounces per square yard.
Carbon fiber or graphite is a very strong reinforcement material. It is used on sail boat masts, golf clubs, bike frames, etc. Carbon fibers combine low weight, high strength, and high stiffness. In the custom pop-up camper area, carbon is used in critical areas such as hallmark’s roof, and can be ordered in the new composite walls as well. Working with carbon fiber is somewhat difficult and when it fails it will snap like a carrot snapping in two. Of course, the failure point where this occurs is extremely high.
Kevlar is a product of the DuPont Corporation. It is a very tough material with a high strength. It is used in making bulletproof vests. Kevlar is very effective in applications requiring resistance to abrasion and puncture. However, its use in primary structures is often limited by the relatively low compression strength and difficulty in handling.
Resin Matrix
The resin component in a composite serves to maintain fiber orientation, transfer loads, and to protect the structure against the environment. While a composite’s stiffness, flexibility, and tensile strength are more affected by the reinforcement material, its heat resistance, shear and compressive strength are more dependent on the resin system. Three types of resin systems are available: (1) polyesters, (2) vinyl esters, and (3) epoxies. All three require the user to mix a specific amount of hardener with a base chemical. The chemicals involved are shipped separately and combined only when the builder is ready to use the resin.
Polyesters are most widely used for industrial applications and within the boat industry. They are cheap and they set up fast. A typical polyester is Bondo. Polyesters are easy to mix with the amount of hardener added only affecting the time needed to develop full strength. Polyesters are not suitable for applications requiring high strength. They also will shrink over a period of time. You may have noticed an automobile fender repair where the paint cracked over a period of time. Chances are Bondo was used as a filler and since it is a polyester it cracked under the paint. In a few words, polyesters are the least capable resin for structural use.
Vinyl esters are used extensively throughout the industry. Vinyl esters are a crossbreed between polyesters and epoxies. They are much more capable than polyesters in strength and bonding. Vinyl esters are low in viscosity making them easy to use. The cure time can also be easily affected by adding more hardener thus speeding up the cure time. Despite the cure time, hardened vinyl ester usually exhibits consistent properties of strength and flexibility. Vinyl esters are not subject to moisture problems during application and are also lower in price than epoxies. One of the disadvantages found in using vinyl esters is in the mixing of the chemicals. Vinyl ester resin is usually "awakened" from its dormant state with cobalt napthenate (CONAP) prior to use. Just before using the system dimethyl aniline (DMA) is added as an accelerator that determines how quickly the mix will cure and, in addition, methyl ethyl ketone peroxide (MEKP) is added as the hardener that actually starts the curing process. Mixing of these chemicals can be somewhat complicated in addition to being hazardous. MEKP mixed directly with DMA or CONAP , apart from the base resin, can be explosive. Overall, vinyl esters provide an easy to use, inexpensive resin system. Proper care certainly must be taken during the mixing process.
Epoxies have come to dominate the aerospace industry and are the basic resins used in most amateur-built aircraft. Epoxies differ from polyesters and vinyl esters in that they harden through a process termed "crosslinking." Epoxies are essentially long chains of molecules that intertwine when hardened to form a strong matrix of crosslinked chains. This provides an inner structural strength to the resin. When combined with the proper reinforcement material, composite structures using epoxies are unmatched in strength and lightness. Epoxies are packaged in two parts: a resin and a hardener. Unlike polyesters and vinyl esters, the resin to hardener mixture must be strictly followed. Adding more hardener will not accelerate the cure time, in fact, it may seriously impede the drying and strength of the cured resin. Epoxies are offered with different characteristics including strength, curing time, etc.. Care must be taken to follow the manufacturer’s recommendation regarding the type to use. Most epoxy cures at room temperature. Working time with epoxies can be much longer than polyester and vinyl-ester because you can use specific hardeners that have custom working times some as short as 4 minutes others over 24 hours at 70 degrees. This makes removing excess resin that may accumulate much less of a problem.
