Foams structural

With the structural foam (SF) construction, large and complicated parts usually require more critical structural evaluation to allow better prediction of their load-bearing capabilities under both static and dynamic conditions. Thus, predictions require careful analysis of the structural foam s cross-section. 

This increased rigidity allows large structural parts to be designed with only minimal distortion and deflection when stressed within the recommended values for a particular foamable plastic. Depending on the required analysis, the moment of inertia can be evaluated three ways. 

First approach is where the cross-section is considered to be solid material (without a core). Moment of inertia (7 ) is then equal to  

This commonly used approach provides acceptable accuracy when the load-bearing requirements are minimal. An example is the case of simple stresses or when time and cost constraints prevent analysis that is more exact. 

This formula results in conservative accuracy, since the core does not contribute to the stress-absorbing fimction. It also adds a built-in safety factor to a loaded beam or plate element when safety is a concern. 

Structural foams are usually produced as fabricated articles in injection molding or extrusion processes they vary in properties (see Table 10.2). Again, the most important structural variables are polymer composition, density, and cell size and shape. Structural foams have relatively high densities (more than 320 kg/m3), and cell structures are primarily comprised of holes, in contrast to the pentagonal dodecahedral structure characteristic of low-density plastic foams. Because structural foams are generally not uniform in cell structure, they exhibit a considerable variation in properties with particle geometry (see Table 10.5) [24] these relations can provide valuable guidance. 

High-pressure structural foam parts have generally been found to require httle or no 

When an engineering plastic resin is used with the structural foam process, the material produced exhibits behavior that is predictable over a large range of temperatures. Its stress-strain curve shows a significantly linearly elastic region like other Hookean materials, up to its proportional limit. 

However, since thermoplastics are viscoelastic in nature, their properties are dependent on time, temperature, and the strain rate. The ratio of stress and strain is linear at low strain levels of 1 to 2 percent, and standard elastic design principles can thus be applied up to the elastic transition point. 

The composite cross-section of a structural foam part contains an ideal distribution of material, with a solid skin and a foamed core. The manufacturing process distributes a thick, almost impervious solid skin that is in the range of 25 percent of the overall wall thickness at the extreme locations from the neutral axis (see Fig. 5-59a). These are the regions where the maximum compressive and tensile stresses occur in bending. 

Thermoplastics can be foamed, giving useful structural properties while also saving in material weight and cost, with improved thermal and acoustic insulation. There are two basic technologies direct gassing and the use of 

Most blowing agents conform to international health and food regulations, and are classified as GRAS (Generally Recognized as Safe) under FDA regulations 

Blowing agents will, typically, comprise a range of grades which commence decomposition at 135-240°C, with recommended processing temperatures ranging from 170-215°C to 285-310 C. Typical processing temperatures for thermoplastics are shown in Table 15.3. 

It is important to note that the thermal conductivity is dependent on the mean temperature involved in the test. The relationship may be illustrated by quoting results obtained from a commercial material of density 1 Ib/ft (0.016 g/cm ) (Table 16.11). 

Tensile strength Flexural strength Compression strength Water absorption 

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. 

Moulding systems are usually divided into low-pressure and high-pressure systems. 

In the low-pressure systems a shot of material is injected into the mould which, if it did not expand, would give a short shot. However, the expanding gas causes the polymer to fill the mould cavity. One important form of the low-pressure process is the Union Carbide process in which the polymer is fed to and melted in an extruder. It is blended with nitrogen which is fed directly into the extruder. The extruder then feeds the polymer melt into an accumulator which holds it under pressure (14-35 MPa) to prevent premature expansion until a predetermined shot builds up. When this has been obtained a valve opens and the accumulator plunger rams the melt into the mould. At this point the mould is only partially filled but the pressurised gas within the melt allows it to expand. 

Other typical properties for a 1 Ib/ft (0.016 g/cm ) expanded polystyrene material are 

Plastics in Architecture A Guide to Acrylic and Polycarbonate, Ralph Montella 

Metal-Filled Polymers Properties and Applications, edited by Swapan K. Bhattacharya 

Injection and Compression Molding Fundamentals, edited by Avraam I. Isayev 

High Modulus Polymers Approaches to Design and Development, edited by Anagnostis E. Zachariades and Roger S. Porter 

Corrosion-Resistant Plastic Composites in Chemical Plant Design, John H. Mallinson 

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Table 3. Typical Physical Properties of Commercial Structural Foams...

Structural Foams. Stmctural foams are usually produced as fabricated articles in injection mol ding or extmsion processes. The optimum product and process match differs for each fabricated article, so there are no standard commercial products for one to characterize. Rather there are a number of foams with varying properties. The properties of typical stmctural foams of different compositions are reported in Table 3. 

Although such products do not have a high-quality finish they do exhibit two typical characteristics of structural foams ... 

Perhaps, however, the greatest virtue of structural foams is the ability to increase the ratio of part rigidity/weight. A foam of half the density of a solid material only requires a 25% increase in wall thickness to maintain the rigidity. 

Amongst the special grades that should be mentioned are those containing blowing agents for use in the manufacture of structural foams (see Chapter 16). 

The introduction of self-extinguishing, glass-reinforced and structural foam grades has led to steady increase in the use of these materials in five main application areas. These are ... 

Glass-reinforced grades have widely replaced metals in pumps and other functional parts in washing equipment and central heating systems. In the manufacture of business machine and computer housings structural foam materials have found some use. Mouldings weighing as much as 50 kg have been reported. 

Foam density is largely a function of the concentration of blowing agents. There has been a strong development towards the use of less expanded, i.e. higher density rigid cellular polyurethanes. This includes not only the so-called structural foams for simulated wood but also unexpanded solid materials used for brush handles and gun stocks. This range is clearly indicated in Table 27.4. ... 

Polycarbonate, polypropylene and modified PPO are popular materials for structural foam moulding. One of the main application areas is housings for business equipment and domestic appliances because the number of component parts can be kept to the absolute minimum due to integral moulding of wall panels, support brackets, etc. Other components include vehicle body panels and furniture. 

Structural foam mouldings may also include fibres to enhance further the mechanical properties of the material. Typical performance data for foamed polypropylene relative to other materials is given in Table 1.1. 

Comparison of structural foams based on various grades of polypropylene with some traditional... 

Throne, J.L. Mechanical properties of thermoplastic structural foams, in Wendle, B.C. (ed.) Engineering Guide to Structural Foams, Technomic, Lancaster, PA, USA (1976) pp. 91 -114. 

Property Steel Solid Plastic Structural Foam Ribbed Solid0... 

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... 

Injection molded Structural foam molded Compression molded... 

Conventional machining operations are used preferably from the same plastic to be used in the product (Chapter 8, SECONDARY EQUIPMENT). Different casting techniques are used that provide low cost even though they are usually labor intensive. The casting of unfilled or filled/reinforced plastic used include TS polyurethane, epoxy, structural foam, and RTV silicone. Also used are die cast metals. 

Using several materials such as PP, glass-filled PS, and PS molded structural foam that is a natural sandwich panel material, the design procedure follows to determine the deflection and stress limitations of the material in each of the several designs. 

In this case the designer has freedom of choice of both form and dimension as well as in the selection of the materials. Given this freedom, it would be desirable to examine several of the alternatives to see which would provide the best seating at the lowest cost. Obviously, there is no point in doing all of the possibilities so a selection should be made on the basis of anticipated use as well as style requirements. Three types will be analyzed. They are the single curve sheet cantilever mounted from the back, the molded pan supported on four legs, and the structural foam molding which is front supported. 

In order to simplify the analytical exercise, a particular material was selected for each. The single curved sheet is made of TS polyester fiber glass molded to the shape. The corner supported pan is molded from ABS plastics. The structural foam unit is molded from PP with glass fiber filler. 

The following review concerns structural foams (SFs). Review Chapter 8, FOAMING regarding the different TP and TS types of foam available. Most of the foamed plastics... 

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