Methods are known for the manufacture of cores for metal casting processes.

One such method is the urethane Cold Box Process which utilises amines as catalyst or curing agent, and which is described in an article in the special issue of Giesserei 78 (1991), volume 11, pages 372 to 374. In this known method for core and mould production a moulding composition is used which contains a benzyl ether resin (ortho-phenol resol) and an isocyanate, and which is cured using a tertiary amine as catalyst. As the catalytic effect of the amine is particularly advantageous in the gaseous phase tests were carried out using trimethylamine (TMA) whose boiling point is about 3 °C, and with which, for this reason, curing is possible in a relatively simple manner with a gaseous amine. The accelerating effect of a tertiary amine by the formation of reactive transitional compounds during the polyurethane reaction is described by the following reaction equation: -RN=C=O + (CH3) 3N-R-N=C-O (-) isocyanate TMA N (cl3) 3

Polyol Polyurethane When making cores by this known method it is disadvantageous that the tertiary amine is fed to the core shooting machine considerably in excess of the required amount. Although it has been shown that the application of trimethylamine makes it possible to advantageously reduce the amount of

amine used by about 50% in comparison to other amines, the application of trimethylamine still has associated disadvantages. One problem is the extreme nuisance created by the odour which makes it essential that all parts of the equipment used are absolutely leak proof. All pipework must there be insulated at relatively high expense.

From the Giesserei article it may further be seen that the reactivity of the different amines increases from"sluggish"to"reactive"in the order TEA, DMIA, DMEA, TMA. Apart from its poor reactivity TEA also suffers from the disadvantage that it forms an aerosol.

A binder for moulding materials for casting light metals is described in DE-A-3017925, which discusses details of Cold Box Process technology. It is shown that for example trimethylamine or triethylamine could be used as the tertiary amine. In the method described the use of a stream of inert gas, such as air, carbon dioxide or nitrogen, containing 0.01 to 30 vol. % of tertiary amine is advantageous.

In G. Engels"Offentliche Diskussion Cold-Box-Verfahren (Open Discussion Cold Box Process) in Stuttgart, Giesserei 58 (1971), No. 9,6 May, pages 249 to 254 the application of triethylamine is described in greater detail. There it is shown that the quantity of triethylamine to be used is dependent inter alia on the size of the core. It is indicated that theoretically one can use as little a quantity of catalyst as from 0.01 to 0.02 vol. %. An addition rate of from 0.05 to 0.1 vol. % should not be exceeded under any circumstances because an excess is disadvantageous not only because of the odour nuisance, but also because it results in a reduction in core quality.

The object of the present invention is to provide a method for the manufacture of cores for metal casting processes in which the above disadvantages are largely avoided, while at the same time providing a method which can be

carried out in a relatively cost effective manner.

According to the invention there is provided a method for the manufacture of sand cores for metal casting processes in which gaseous trimethylamine is conducted through parts of a metering device (3) and then, in a concentration of from 0.01 to 0.12 % wt. % based on the quantity of sand used per core, it is conducted into a core shooting machine (4) where it is brought into contact with the sand, wherein prior to the introduction of the gaseous trimethylamine into the core shooting machine (4) a purging gas is introduced, and wherein for a quantity T of trimethylamine the quantity L of purging gas used is in the ratio of T: L of from 1: 1000 to 1: 10000.

As a rule, the sand which is fed into the core shooting machine has already been mixed with a benzyl ether resin and an isocyanate so that core production can take place according to the principes of the urethane Cold Box Process. General conveying equipment for gases may be used, for example a metering device in which the quantity of trimethylamine is suitably adjusted by means of a control chain or by a regulator. Accordingly the equipment may consist of several parts, such as conveying equipment, measuring apparatus, controllers etc., or it may be manufactured as a single unit. The gaseous trimethylamine is conducted through a part of the metering device, although it need not flow through all of the parts. Thus it is possible for liquid trimethylamine to flow through several parts of the metering device, and then be transformed to the gaseous phase, and flow through the remaining parts of the metering device in the gaseous form. Surprisingly it has been found that when using the method of the invention an extreme odour nuisance can be largely avoided, so that expensive insulation of parts of the core making installation is not necessary. In addition, advantageously, there is no odour nuisance during subsequent storage of the manufactured cores. It is also surprising that an almost stoichiometric conversion of the trimethylamine can be achieved so that the method of the invention can be

carried out cost effectively because of the relatively low amounts of trimethylamine which are used.

The introduction of the purging gas may take place before or after the metering device. It is advantageous that the sand in the core shooting machine is contacted simultaneously by the trimethylamine and the purging gas so that an extreme odour nuisance can also be completely avoided during core storage, because the core is purged by the purging gas at the same time that it is exposed to the trimethylamine.

In a preferred embodiment of the invention the purging gas is introduced directly into the metering device. By this measure the amount of conduits or piping can be reduced, and this is particularly advantageous when space for installation of the core making equipment is limite.

In a preferred embodiment of the invention the trimethylamine is supplied in liquid form to metering bellows forming part of a metering device, and thereafter it is conducted in gaseous form through a measuring, controlling or regulating unit which also forms part of the metering device. Trimethylamine is normally available commercially in liquid form. However, before the triethylamine comes into contact with the sand, which has already been mixed with a benzyl ether resin and an isocyanate, in the core shooting machine, it is advantageously converted to the gaseous phase. This makes possible relatively simple homogeneous mixing of the trimethylamine with sand, and at the same time increases reactivity of the trimethylamine. Surprisingly it has been shown that the conversion of the trimethylamine form the liquid phase to the gaseous phase can be achieved advantageously by introducing the trimethylamine into metering bellows. Although heat is absorbed from the surroundings during conversion of the trimethylamine from the liquid to the gaseous phase, the metering bellows remains able to function better than other conveying equipment. Subsequently the gaseous trimethylamine is

passed through a measuring, controlling or regulating unit, which is able to dispense the desired specified quantity of trimethylamine which is to be used.

This information is transferred directly to the metering bellows by means of a circuit, and is there changed into the corresponding setting for the stroke of the metering bellows. It is particularly advantageous that the metering bellows can take a relatively simple form because it can readily be adapted for use on existing core making equipment.

The invention is illustrated by way of example in the accompanying drawings in which : Figure 1 is a simplified, schematic process flow diagram of a method for the manufacture of cores for metal casting processes according to the invention and Figure 2 is a simplified, schematic process flow diagram of another embodiment of the method of the invention.

Referring to Figure 1, sand, which has already been mixed with a benzyl ether resin and an isocyanate, is taken out of sand mixer (5) and transported to a core shooting machine (4) by means of conduit (10). A cylinder (1) contains liquid trimethylamine and is warmed with the aid of a water bath (8).

Heating is achieved by means of a heating coil (7) which is connected directly to a heating apparatus (6). Conversion of the trimethylamine from the liquid phase to the gaseous phase results from the application of heat. The gaseous trimethylamine is passed through conduit (2) to a metering device (3). In the metering device (3) the amount of trimethylamine is measured, and is adjusted to the desired amount according to a given specified value.

Subsequently the trimethylamine is passed through conduit (9) into the core shooting machine (4) in a concentration of 0.01 to 0.12 wt. % based on the weight of sand being used per core, and makes contact with the sand there.

To avoid causing an extreme nuisance due to the odour of the trimethylamine, purging air is introduced to the conduit (2) through conduit (11) and valve (12) so that during introduction of the trimethylamine through the conduit (9) into the core shooting machine (4) air purging of the sand in the core shooting machine (4) takes place at the same time.

Referring to Figure 2. metering bellows (3a) and measuring, controlling and regulating unit (3b) together form a metering device corresponding to the metering device (3) of Figure 2. Trimethylamine is present as a liquid in cylinder (1). When valve (15) is closed liquid trimethylamine is conducted into the metering bellows (3a) via open valve (13). Thereby the trimethylamine expands and is largely converted to the gaseous phase. The setting of the stroke of the metering bellows (3a) is made by means of circuit (14) according to the given specified value of the measuring, controlling or regulating unit (3b). Subsequently, when the valve (13) is closed, the gaseous trimethylamine is conducted via valve (15), which been opened, into the measuring, controlling or regulating unit (3b). Addition of purging air is achieved by means of conduit (11) and valve (12). Since the metering bellows (3a) and the measuring, controlling or regulating unit (3b) together correspond to the metering device (3) of Figure 1, the addition of purging air in the Figure 2 embodiment also takes place directly into the metering device.