EP0219607A2 - Method of mixing foundry moulding materials - Google Patents

Abstract

In a process for mixing wet molding materials, in particular wet molding sands, for foundry purposes, the dry components are premixed in a container with a free surface, the wet components are added after the dry mixing and then homogenized with the dry components by further mixing to give the wet molding material. To reduce the mixing time, to improve the mixing result and to avoid contamination above the free surface of parts of the mixer, the wet components are added below the free surface into the dry mix under excess pressure.

Description

The invention relates to a method for mixing wet molding materials, in particular wet molding sands, for foundry purposes, by premixing the dry components in a container with a free surface, adding the wet components after dry mixing and then homogenizing them with the dry components by further mixing to give the wet molding material.

Mixing processes for foundry molding materials of the aforementioned type are carried out in different mixer designs. Common to these mixers is always a standing or revolving, trough-like container, which is provided with a discharge opening on the circumferential or bottom side and engages in different types of mixing tools from above. Filling with dry components and adding the moist components is done from above. On the one hand, the dry components are composed of granular, free-flowing bulk materials, such as new sand and processed old sand, as well as dusty, likewise almost dry bulk materials, such as coal dust, etc. The primary binder used is sodium bentonite, which is derived from mineral Layer particles exist that are activated and swell when water is added. The process is due to an incorporation of the water molecules in the layered mineral. Furthermore, water glass is sometimes used as a binder, which dries under the influence of air and creates the bond. The binders or the binder components, like the dry components, are applied to the free surface from above.

In the known mixing process described, the wet molding material is produced in batches, with particular emphasis being placed on short mixing times in order to be able to continuously provide the wet molding material on the molding machine. The entire mixing cycle, which is essentially determined by the type of mixing tools and the circulation speed within the mixer, is at least two minutes.

The object of the invention is to reduce the mixing time for a given mixer design and to achieve a better mixing result.

This object is achieved in that the wet components are added below the free surface in the dry mix under pressure.

In contrast to the known mixing processes with a free surface, in the process according to the invention the moist components are introduced into the premixed dry components below the free surface. It has been found that the mixing time in the area of the wet mixing cycle can be reduced by up to half a minute, regardless of whether bentonite- or water glass-bonded molding materials are used. It has also been shown that it is much less difficult to achieve an absolutely homogeneous mixture. The following considerations can be considered as the cause for this:

When the wet components are added to the free surface, agglomerates are formed very quickly, which consist primarily of the wet components and the dust-like particles. The damp components are thus surrounded by a largely dry coat. In extreme cases, this leads to the formation of tubers, the tubers having a high moisture content on the inside. These tubers are difficult to mix in and difficult to destroy, so that on the one hand they lead to inhomogeneities in the mix, and on the other hand an overdosage of moist components is necessary. This is particularly critical in the case of bentonite-bound molding materials, since these tubers not only bind the binding water, but above all also bind the dust-like bentonite particles. These bulbs lead to form errors and casting errors when pouring. By adding the moist components below the free surface, the formation of pearls and tubers, which is caused by the rolling motion of the wet components on the free surface, is completely prevented, since the wet components penetrating into the mixed material under excess pressure find a type of capillary system in which they separate from one another Distribute the location of your task evenly.

In the case of bentonite-bonded molding materials, the process according to the invention can be used not only to reduce the mixing time, but - with the same mixing time - to improve the mixing result. Studies have shown that the swelling capacity of concrete is time-dependent. About 70% of the swelling capacity is reached within 90 seconds from the beginning of the wetting of the bentonite particles, while 100% swelling only took place after about 20 to 30 minutes. If the shortening of the mixing time, which is possible per se, is used to allow the swelling process more time, a better binding is achieved with the method according to the invention.

The process according to the invention achieves a further advantage which is important for the mixer operation and the mixing result: when the moist components are added to the free surface of the material to be mixed, the parts of the mixer lying above the surface inevitably also occur. As a result, when the dry components are added and in the dry mixing phase, caking occurs in the areas of the mixer wetted in the previous batch, which will grow steadily over a longer operating period. These cakes gradually dry out from the contact surface and break off from time to time. They are then transported under the mix and are no longer dissolved there, so that there are inhomogeneities, which in turn lead to shape and casting errors. These caking occurs to a particular degree in water-glass-bonded molding materials, where they are also particularly critical because the caking becomes hard as glass and can no longer be comminuted in the mixer. With the method according to the invention, this problem is also largely eliminated, so that on the one hand the inhomogeneities mentioned do not arise in the mix, and on the other hand the cleaning measures necessary from time to time in the known mixing method are no longer necessary or can only be carried out at substantially longer intervals.

The wet components can be injected or injected into the dry mix, the overpressure, of course, being chosen such that the wet components cannot, or can only, penetrate to the surface.

It is also advantageous if the moist components are added to the dry mix at a number of locations, preferably evenly distributed.

Claims (3)

1. Process for mixing wet molding materials, in particular wet molding sands, for foundry purposes, by premixing the dry components in a container with a free surface, adding the wet components after dry mixing and then homogenizing them with the dry components by further mixing to give the wet molding material,
characterized,
that the wet components are added below the free surface into the dry mix under pressure. 2. The method according to claim 1, characterized in that the wet components are injected into the dry mix. 3. The method according to claim 1 or 2, characterized in that the moist components are added to the dry mix at several, preferably evenly arranged locations.