Structural foams cellular core

Tbe term structural foam was originally coined by Union Carbide to describe an injection moulded thermoplastic cellular material with a core of relatively low density and a high-density skin. The term has also been used to describe rigid foams that are load bearing. Today it is commonly taken to imply both of the above requirements, i.e. it should be load bearing and with a core of lower density than the skin. In this section the broader load-bearing definition will be used. Whilst structural foams are frequently made from polymers other than polystyrene, this polymer is strongly associated with such products and it is convenient to deal with the topic here. 

They range from structural foam molded products (which come from the mold as completed molded products) incorporating low density cores and high density skins of the same materials to products vacuum formed of a plastics material, the core of which becomes cellular during the heating process (Chapter 8). RP translucent structural panels for curtain wall building construction using... 

Structural foams are made of a cellular core with a dense skin. The technique is used for industrial and aesthetic goods for the automotive, electronics, household appliance and aeronautics sectors, such as housings of machines, TV cabinets, computer housings, roofs for caravans or ships, hard tops of 4WD (Jeep), luggage boxes, parts for washing machines. Structural foams are processed by thermoplastic injection moulding using ... 

The method of structural foam moulding permits the manufacture of foams with a compact skin and a cellular core. The properties of such mouldings depend, among other parameters, on the structure of the foam. This article reports on a study of the influence of density, specimen thickness, orientation and ambient temp, on mechanical properties and chemical resistance of injection monlded structural foams made from PP. Results are discussed with reference to relationships between structnre and properties. 5 refs. 

Thermoplastic structural foams with bulk densities not less than 50% of the solid resin densities are considered. Cellular morphology, uniform-density cell behaviour, the I-beam concept in designing, core-density profile and the role of the skin, mechanical properties, and ductile-brittle transitions are discussed. 63 refs. 

Polyurethane rigid structural foam was one of the earliest applications for RIM. Lightweight and rigidity characterize the material. It consists of a solid skin and a lower density cellular core. Use includes equipment housings, furniture, building components, fancy tires,266 and a variety of industrial and consumer applications (Table 12.1). 

Polymer foams may be homogeneous with a uniform cellular morphology throughout or they may be structurally anisotropic. They may have an integral solid polymer skin or they may be multicomponent in which the polymer skin is of different composition to the polymeric cellular core. Schematic representations of the different physical forms of cellular polymers are given in Figure 2.58. Some... 

FIGURE 2.58 Schematic representations of section through different types of cellular polymer, (a) Low-density open-cell foam, (b) High-density closed-cell foam, (c) Single-component structural foam with cellular core and integral solid skin, (d) Multicomponent structural foam, (e) Fiber-reinforced closed-cell foam, (f) Syntactic foam. 

Structural foam is the term usually used for foam produced in an injection molding press and made of almost many thermoplastic resin. Structural foam is always produced with a hard integral skin on the outer surfaces and a cellular core in the interior, and is used almost exclusively for production of molded parts. The process is thus ideally suited for fabrication of parts such as business machine housings (commonly for ABS), and similar parts or components in which lightweight and stiffness are required. 

Some special types of foams are (1) structural foams (2) syntactic foams and multifoams and (3) reinforced foams. Structural foams (Figure 2.58c and d), which possess full-density skins and cellular cores, are similar to structural sandwich constructions or to human bones, which have solid surfaces but cellular cores. Structural foams may be manufactured by high pressure processes or by low-pressure processes (Figure 2.61). The first one may provide denser, smoother skins with greater fidelity to fine detail in the mold than maybe true of low-pressure processes. Fine wood detail, for example, is used for simulated wood furniture and simulated wood beams. Surfaces made by low-pressure processes may, however, show swirl or other textures, not necessarily detracting from their usefulness. Almost any thermoplastic or thermosetting polymer can be formulated into a structural foam. 

The term structural foams designates components possessing full density skins and cellular cores, similar to structural sandwich constructions, or to bones, whose surfaces are solid but whose cores are cellular. For structural purposes, they have favourable strength and stlffness-to-welgh ratios, because of their sandwich type configuration. Frequently, they can provide the necessary structural performance at a reduced cost of materials. 

Several processes are employed for the manufacture of structural foams. In the high-pressure process, the first step is to fill a mould solidly with the resin under pressure. While it is still soft, the mould is expanded or a core retracted, which provides an interior space to be foam-filled by expansion and foaming inward of the still-soft resin or by injecting foam into the Interior space. In either case, the result is a dense skin surrounding a cellular core. In the low-pressure process, a mould is partially filled and the molten resin expands to fill the mould, forming a skin upon contact with the walls of the mould. 

The term structural foam refers to materials which consist of a cellular core surrounded by a solid skin. The core and skin are made from the same material and the composition is characterised by a dramatic increase in flexural stiffness compared with similar products - weight for weight -produced from solid polymer. The term sandwich moulding also refers to solid skin/cellular core materials but in this case two or more polymer melts are injected from separate chambers into the mould cavity the SF process involves injecting a single material into a mould cavity from one screw. In contrast to SF the sandwich construction very often comprises different polymer types for skin and core, as we shall see later. 

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