Mould and core production

Moulding consists of making a mould in which the molten metal will be poured. Some moulds may need to have special properties to produce high quality castings, which, for example  

Just as the mould defines the outer shape of the casting, the core defines the inner one, or at least the parts not directly attainable by moulding. 

The cores used for ferrous castings are practically always made of sand. The choice of binder technology used depends on factors such as the size of the casting, the production rate, the metal poured, the shake-out properties, etc. 

For sand moulding, the mould may be produced by manual or mechanical ramming actions, such as by jolt, squeeze, air impact, vibration, etc.. When the mould has sufficient strength it is released from the model, which can then be used to produce a new mould. 

Generally, cores are produced by the same techniques as moulds, but small or medium sized cores are often blown or shot into wooden, plastic or metallic core-boxes. 

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Each of these processes has had wide application in mould and core production as alternatives to the cement-sand and conventional carbon dioxide processes. Another alternative is the thermosetting resin process known as shell moulding. Because of its importance it is properly considered in the next section. 

Dust and particles releases are a general issue in all stages of the foundry process, and for all processes used. Dust is generated in the production and processing of sand moulds and cores, as well as in the finishing of the castings (both from lost moulds and permanent moulds). 

In these processes, curing takes place by injecting a catalyst or a hardener in a gaseous form. The curing speed can be very high, which allows high production rates to be achieved. They are suitable for moulds and cores of limited size, in medium batch or mass production. Their use has been constantly expanding over the last few years. 

The use of chemical binders results in the production of various compounds during mixing, mould- and core-making, storage, pouring and cooling, as indicated in Table 3.35. Some indicative emission factors for several processes are given in Table 3.40 and Table 3.41. 

Alkyd-oil During production of the moulds and cores, there are no emission problems, unless they are cured by heat, in which case odour problems may occur Ester silicate This process does not generate any emission problems. 

Modem mould and core-making machines allow the production parameters of the various product types to be saved in an electronic database. This allows easy change-over to new products, without the loss of time and materials from finding the right parameters by simply using trial and error. For new products, the settings for similar products may be used to shorten the optimisation time. 

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 . 

Collation trays, as used in supermarkets to transport and display products, remain one possibility for mechanical recycling but this depends on a system to identify the polymer, linked to a suitable collection system. One outlet for this material is a foam layer in coextruded cladding (439). PVC water bottle scrap has also been incorporated into the foam core of coextruded pipe. Another outlet is moulding of rainwater fittings. 

Whatever binder applied, the physical and chemical properties of the refractory material used to make the moulds or cores affects their characteristics and their behaviour during pouring. This is not surprising as these materials make up 95 to 99 % of the products used. 

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. 

Many similarities exist between SM and SF mouldings. Briefly, both techniques are based on the injection moulding process and the products consist of solid skins with low-density cores. However, two important differences exist between the techniques ... 

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