Low-pressure structural foam

Table 10.5 Design criteria for low-pressure structural foam...

Shallenberg, D. Low pressure structural foam molding carry the load with style. Presented at Society of Manufacturing Engineers Annual Meeting, 1999 Vol. CM99-193, 1-13. 

Molded-in stresses are minimal. The thick but hollow sections provide rigidity and do not create sink or warpage problems. The cycle time is reduced because the thick sections are hollow. As the gas is not mixed with the melt, there is no surface splay, which is typical of low-pressure structural foam molding (1). Gas injection is being used with commodity and engineering resins. 

Recently, many papers have been published on fiber catalysts and foam structures (Figure 9.2). Although, strictly speaking, fibers and foams might not be considered as structured systems, beds of such catalysts exhibit typical features of structured catalysts, namely, low pressure drop, uniform fiow, a good and uniform access to the catalytic surface, and they are definitely nonrandom. Therefore, we have included them in this chapter. 

PP-structural foam mouldings were produced on an injection moulding machine in a pre-pressurised mould cavity by the classical low-pressure process and an alternative low-pressure process. Melt temperature, injection direction and sprae diameter were varied. Cross-sections cut from the middle of the small cylinder in longitudenal orientation were investigated by site-resolved X-ray scattering. Morphological properties were investigated. 4 refs. 

The optimization of the catalyst formulation is relevant not only to the active species but also to the structure of the support. Indeed, structured catalysts in the form of monolith or foam offer great advantages over pellet catalysts, the most important one being the low pressure drop associated with the high flow rates that are common in environmental applications. 

Because of low injection pressure, some cost savings are possible in mold and press construction. Molding cycles are somewhat longer than for injection molding. The part must be cooled in the mold long enough to be able to resist swelling from internal gas pressure. In structural foam parts there is almost a total absence of sink marks, even in the case of unequal section thickness. Structural foam has replaced wood, concrete, solid plastics, and metals in a variety of applications. 

Common uses for polyester resins are reinforced plastics automotive parts boat hulls foams encapsulation of electrical equipment protective coatings ducts flues and other structural applications low pressure laminates magnetic tapes piping bottles non-woven disposable filters and low-temperature mortars. 

The process of mould filling should not be considered as completely understood and controlled 98>, though some important features of gas-filled system flow were established by Fridman and co-workers 99 100) who have explained the formation of laminated structures in low-pressure moulding of foaming melts and found the correlation between moulding conditions and morphologic macro-structure and properties of foam articles. 

Ultem PEI resins are amber and amorphous, with heat-distortion temperatures similar to polyethersulfone resins. Ultem resins exhibit high modulus and are stiff yet ductile. Light transmission is low. In spite of the high use temperature, they are processible by injection molding, structural foam molding, or extrusion techniques at moderate pressures between 340 and 425°C. They are inherently flame retardant and generate little smoke dimensional stabilities are excellent. Large flat parts such as circuit boards or hard disks for computers can be injection-molded to maintain critical dimensions. 

ABS structural foam can be processed by injection molding, through conventional or low pressure injection machines (Chapter 4) by expansion casting in rotational molding machines (Chapter 13) or conveyorized oven systems or it can be extruded into profiles through conventional extruders (Chapter 5). 

Structural-web molding is a low pressure foam molding method. It is the phrase usually used to identify the gap between structural foam (SF) molding and injection molding. Its surface does not have the usual SF characteristic swirl pattern. It can produce very large, lightweight parts with smooth surfaces like conventional injection molded parts. 

The high reaction rates and the high selectivities that are desired require not only short residence times but also short diffusion lengths (diameters of catalyst particle as small as possible), the absence of significant gradients in temperature, and low-pressure drops. It is obvious that a structured reactor should be used. There are three candidates monoliths, gauzes, and foams. 

In the low-pressure process, a resin containing a blowing agent is forced into the mold, where it expands to fill the mold under pressures of 690-4100 kPa (100-600 psi). This produces structural foam products with a characteristic surface-swirl pattern caused by the collapse of cells on the surface of molded articles. 

There are a number of different systems and machines available for producing structural foams. The most important categories are low-pressure molding and high-pressure molding. Low-pressure molding... 

Reticulation is an after-treatment that removes residual cell membranes to produce a foam with a skeletal rib structure. Reticulated foams are very effective filters for the removal of dust and fibers from air and other gases. They allow high flow rates and low pressure gradients (i.e., minimal energy consumption). Methods of reticulation include chemical hydrolysis and the use of an explosion flame front to melt the membranes.f ... 

A foam is a dispersion of gas bubbles in a relatively small volume of a liquid or solid continuous phase. Liquid foams consist of gas bubbles separated by thin liquid films. It is not possible to make a foam from pure water the bubbles disappear as soon as they are created. However, if surface active molecules, such as soap, emulsifiers or certain proteins, are present they adsorb to the gas-liquid interfaces and stabilize the bubbles. Solid foams, e.g. bread, sponge cake or lava, have solid walls between the gas bubbles. Liquid foams have unusual macroscopic properties that arise from the physical chemistry of bubble interfaces and the structure formed by the packing of the gas bubbles. For small, gentle deformations they behave like an elastic solid and, when deformed more, they can flow like a liquid. When the pressure or temperature is changed, their volume changes approximately according to the ideal gas law (PF/r= constant). Thus, foams exhibit features of all three fundamental states of matter. In ice cream, the gas phase volume is relatively low for a foam (about 50%), so the bubbles do not come into contact, and therefore are spherical. Some foams, for example bubble bath. 

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