Sandwich-core foam

PVC foams used as sandwich cores generally have ... 

It is interesting to note that foamed composites are suitable sandwich core materials for FRP boats (8). Cross-ties (or sleepers) for railroads, especially for use in tunnels and subways, are very promising applications. Japanese National raUways carried out a long-term test in a tunnel, and the results were excellent. 

Foamed carbon has 600 times the thermal insulation capacity of the same weight of firebrick and is therefore used as a self-supporting thermal insulation material in furnaces or in fire-retarding sandwich-constructions. Foamed carbon cores are used in foundry technology, because they are easily produced and can be easily removed by combustion from inaccessible cavities. Furthermore foamed carbon is utilized as a filter material for the filtration of corrosive materials, as sintered plugs in gas inlets, as an electrode material and as a catalyst support. 

Table 3.4 Properties of PVC foam cores (Divinycell H Sandwich Core Material, reference 3.6).

Wood is the most appropriate material for a solid sandwich core and has been used extensively for many applications, such as doors and partitions. For lightweight constructions flat or end grain balsa is still used, although this is being superseded by synthetic materials. Bonded microspheres of inorganic materials, glass, ceramics, etc. to form syntactic foams are useful core materials. 

For obtaining a low -weight, high-bending-stiffness structure, sandwich constructions are a conunon choice for composite components. To make a sandwich, low-density materials are inserted as sandwich cores between two faces of the structural material itself (so in this case between two stacks of prepreg plies). Commonly used core materials are plastic foams (for example, made from PVC, PS, or PET) and balsa wood. Examples of core materials for more sophisticated, structured sandwich cores are honeycombs (made from aluminium, or resin-impregnated paper sheets), or fibre-reinforced foams. 

Gupta N and Maharaa R (2005) Enhancement of energy absorption in syntactic foams by nanoclay incorporation for sandwich core applications, Appl Compos Mater 12 247-261. 

Adhesives for sandwich bonding progressed similarly. Early low-flow adhesives were used with plastic foam cores and end-grain balsa to manufacture sandwich as.semblies. The development of less dense honeycomb cores necessitated a change in adhesive. Development of good bond strength to honeycomb core requires relatively high flow so that an adhesive fillet forms on the core walls (Fig. 6). 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

Example 2.8 A polypropylene sandwich moulding is 12 mm thick and consists of a foamed core sandwiched between solid skin layers 2 mm thick. A beam 12 mm wide is cut from the moulding and is subjected to a point load, IV, at mid-span when it is simply supported over a length of 200 mm. Estimate the depth of a solid beam of the same width which would have the same stiffness when loaded in the same way. Calculate also the weight saving by using the foam moulding. The density of the solid polypropylene is 909 kg/m and the density of the foamed core is 6(X) kg/m. ... 

Compare the flexural stiffness to weight ratios for the following three plastic beams, (a) a solid beam of depth 12 nun, (b) a beam of foamed material 12 mm thick and (c) a composite beam consisting of an 8 mm thick foamed core sandwiched between two solid skin layers 2 mm thick. The ratio of densities of the solid and foamed material is 1.5. (hint consider unit width and unit length of beam). 

This type of analysis could also be used for a sandwich structure with solid skins and a foamed core. It is simply a matter of using the appropriate values of E, G 2,Ei,v 2 for the skin and core material. This is illustrated in the following Example. 

Example 3.15 A sandwich moulding is made up of solid skins with a foamed plastic core. The skins and core may be regarded as isotropic with the following the properties ... 

Foamed polyurethane. The basic chemicals are mixed in the liquid state with foaming agents, and swell into a low-density foam which sets by polymerization into a rigid mass. As the swelling material will expand into any shape required, it is ideal for the core of sandwich panels, and the sheet material skins may be flat or profiled. When the panels are manufactured the mixture is injected between the inner and outer skins and expands to the thickness required, adhering to the lining materials. 

The plastic insulants are rigid, homogeneous materials, suitable as the core of sandwich panels. Such a method of fahrication is facilitated when using foamed rigid polyurethane, since the liquids can he made to foam between the inner and outer panel skins and have a good natural adhesion, so making a stiff structural component [40]. 

In addition to developing solid RP structures, work has been conducted on sandwich structures such as filament-wound plastic skins with low-density foamed core or a plastic honeycomb core to develop more efficient strength-to-weight structures. Sandwich structures using a syntactic core have been successfully tested so that failures occurred at prescribed high-hydrostatic pressures of 28 MPa (4,000 psi). 

The chassis is made from a 1/4 in. sheet of either ABS or TPO vacuum formed into a tub and reinforced with reinforced pultruded glass fiber-TS polyester plastic tubing. The hood and other products are being made from a 20 mm thick sandwich of thermo-formed PPO-alloy skins, glass fabric infused with thermosetting vinyl ester, and a urethane foam core. The bumper and front fascia is thermoformed from a polyolefin elastomer sheet with an UV-resistant cap layer of DuPont s Tediar PVF film. The dash and... 

Since the stiffness of a laminate varies as a cubic relationship with the thickness, there are alternate methods employed to achieve thickness - and hence stiffness - than by using multiple layers of fiberglass and resin. Lightweight core materials, such as end-grained balsa wood, high-density polyurethane foam, PVC foam, and honeycombed materials, are available. These materials are sandwiched in between layers of unsaturated polyester resin to achieve increased laminate stiffness. The common terminology used for this technique is sandwich construction . 

Certain incinerator chimneys are in hybrid stainless steel with an inner lining in sandwich resin/glass fibres with a core in foamed polyurethane. 

Frontages in sandwich made of PVC foam core and skins of fire-resistant glass fibre reinforced polyester. The panels are attached onto a steel framework. 

Rohaceir, a closed-cell PMI foam developed by Rohm of Germany to be used as cores in lightweight sandwich composites for transport, has specific advantages ... 

In a sandwich structure, the foam is used as the core with two skins of reinforced resin sheets firmly stuck on the foam to obtain high rigidity. The sandwich composite behaves as an I-beam see Figure 6.11. 

Figure 6.11. Schematic principle of a sandwich composite with a foamed core...

Seats made of sandwich composite with foamed core and thermoplastic skins. 

This rigid linear foam with closed cells was developed by ATC Chemicals Company to be used as the core in lightweight structural sandwich composites particularly intended for boatbuilding. SAN foams have ... 

The Ultratect foam from BASF is used as the core with glass fibre reinforced polypropylene skins for the sandwich rear seat backrest of the M3 CSL sports car by BMW. The weight saving is 50% versus a metal part. 

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