The second generation moulding methods are based on chemicalization of the core and mould production. Although, the moulding aggregate must be also compacted as in the first generation, a lower intensity of compaction is sufficient, and the ultimate strength is the result of chemical reaction of the binder during the process of hardening. The benefits of the second generation moulding methods are: high strength of moulds and cores, higher accuracy and better surface quality. Disadvantages of their usage are: higher price, lower production rate (60–100 moulds per hour) and the moulding aggregate cannot be reused without further modification – reclamation. Compliance with the ever-increasing requirements of environmental and hygiene regulations is also difficult.
According to the binder system employed, all moulding methods can be divided into two classes:
- The ones using organic binder system, and
- The ones using inorganic binder system.
The use of chemical binders, particularly those based on organic compounds, has significant environmental and health implications, with the need for close control of reagents and of evolved vapours, gases and fume generated during mould production and while casting. Safety risks to operatives can arise from the skin or eye contact, by ingestion as well as inhalation. Inorganic binders pose fewer problems in this respect, and have in some cases been preferred for this reason. The moulding aggregates are, however, less readily reclaimed than some of those using organic binders. Inorganic binders are odourless and non-flammable; the smoke and fumes are reduced during pouring and shakeout. A typical drawback of the inorganic binder system is the high residual strength of the moulds and cores after casting and poor shakeout properties. On the other hand, the organic binders as a group offer superior breakdown qualities under the influence of heat from the casting. The collapsibility is particularly advantageous in the removal of confined cores.
Methods of the chemical bonding system hardening are divided into the following groups:
- Hot processes – using thermal cured binder systems:
- oven bake curing;
- curing by contact with heated pattern equipment.
- Cold processes – using chemical hardening by the liquid or gaseous reagents:
- controlled hardening – binder is cured by a vapour or gas catalyst that is applied after the mould or core is compacted;
- self-setting aggregates (No-bake) – the binder hardens in certain time after the binder components have been combined.
The oven bake curing was used for oil-based binders, often in combination with water-activated cereal to give sufficient green strength to the mass of the aggregate, so that it can be immediately removed from the core box. The uncured plastic-like cores were generally placed on a dryer plate (or core plate), a supporting structure to maintain the shape of the core, for oven curing. The cores were then hardened by baking at 200 to 260 °C for 1 h or longer, depending on the section thickness. Owing to lengthy and expensive heating, low productivity and poor dimensional accuracy, this method of curing has been gradually replaced by other methods.
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