Structural Foam Injection Moulding

Foamed thermoplastic articles have a cellular core with a relatively dense (solid) skin. The foam effect is achieved by the dispersion of inert gas throughout the molten resin directly before moulding. Introduction of the gas is usually carried out either by pre-blending the resin with a chemical blowing agent which releases gas when heated or by direct injection of the gas (usually nitrogen). 

When the compressed gas/resin mixture is rapidly injected into the mould cavity, the gas expands explosively and forces the material into all parts of the mould. 

Foamed plastic articles may be produced with good results using normal screw-type injection moulding machines (see Fig. 4.43(a)). However, the limitations on shot size, injection speed and platen area imposed by conventional 

Wall sections in foam moulding are thicker than in solid material. Longer cycle times can therefore be expected due to both the wall thickness and the low thermal conductivity of the cellular material. In contrast, however, the injection pressures in foam moulding are low when compared with conventional injection moulding. This means that less clamping force is needed per unit area of moulding and mould costs are less because lower strength mould materials may be used. 

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Olefins or alkenes are defined as unsaturated aliphatic hydrocarbons. Ethylene and propylene are the main monomers for polyolefin foams, but dienes such as polyisoprene should also be included. The copolymers of ethylene and propylene (PP) will be included, but not polyvinyl chloride (PVC), which is usually treated as a separate polymer class. The majority of these foams have densities <100 kg m, and their microstructure consists of closed, polygonal cells with thin faces (Figure la). The review will not consider structural foam injection mouldings of PP, which have solid skins and cores of density in the range 400 to 700 kg m, and have distinct production methods and properties (456). The microstructure of these foams consists of isolated gas bubbles, often elongated by the flow of thermoplastic. However, elastomeric and microcellular foams of relative density in the range 0.3 to 0.5, which also have isolated spherical bubbles (Figure lb), will be included. The relative density of a foam is defined as the foam density divided by the polymer density. It is the inverse of the expansion ratio . 

Figure 4.5 Layered structures with high bending stiffness, (a) Glass-fibre-reinforced skins on an aluminium honeycomb core (b) structural foam injection moulding with maximum density at the skins.

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. 

All the molten-state methods are usable extrusion, injection, compression, blown film, blow moulding, thermoforming, structural foam, co-extrusion, machining for high hardness grades, welding. 

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 ... 

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. 

INJECTION MOULDED STRUCTURAL FOAM MADE FROM POLYPROPYLENE... 

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. 

Gas-assist injection moulding is becoming a frequently-used process for forming hollow, stiff parts. Foamed material, sandwich structures with compatible core and face sheets, and sandwich constructions with recycled material are other examples of how intelligent material utilisation can lead to source reduction. 

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. 

Polystyrene is an aromatic polymer made from styrene, an aromatic monomer which is commercially manufactured from petroleum. Polystyrene is commonly injection moulded or extruded while expanded polystyrene is either extruded or moulded in a special process. Solid polystyrene is used in disposable cutlery, plastic models, CD and DVD cases, etc. Foamed polystyrene is mainly used for packing materials, insulation, foam drink cups, etc. Polystyrene foams are good thermal insulators and therefore used as building insulation materials such as in structural insulated panel building systems. They are also used for non-weight-bearing architectural structures. The information on OPF-polystyrene composites is limited. 

In the next group of studies the structural foam (SF) and sandwich moulding (SM) techniques are featured. PST 2 deals with the principles of these processes and the design possibilities they offer, and there is also a comparison with the conventional process, known as compact injection moulding, or CIM. 

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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