CS214259B1 - Mixture for foundry molds and cores - Google Patents

Abstract

The invention relates to a mixture for the production of foundry molds and cores, the solidification of which occurs by drying or drying at higher temperatures, which replaces the previously used oil mixtures for drying or mixtures for drying containing a resin binder. The purpose of the mixture according to the invention is to eliminate the disadvantages of the previous mixtures, which consist in the necessity of using additional additives of weighting substances, in the necessity of using long drying times. The mixture is composed of sand blasting, carboxymethyl cellulose and water and according to the invention contains .0.01 to 4 parts by weight of urea. The mixture prepared in this way provides the advantages of shortening the drying time and improving work hygiene and improving disintegration after casting. The mixture has good binding and sufficient strength even for the production of complex cores. It can be used advantageously in the casting of light alloys, where excessively high disintegration is required with simultaneously high quality of the casting surface.

Description

The invention relates to a mixture for the production of foundry molds and cores, the solidification of which occurs by drying or drying at higher temperatures.

Foundry molding and core mixtures contain, in addition to the granular filler, an inorganic or organic binder that enables the hardening of the manufactured molds and cores. Hardening occurs at laboratory temperature by reaction with a hardener present in the mixture (self-hardening mixtures), by reaction with a hardener introduced into the mixture from the outside (controlled hardening mixtures) or at elevated temperatures by inducing polymerization or polycondensation of the binder or by removing the solvent - mixtures cured by the heat of the model device and mixtures for drying. Each technology characterized by the method of hardening has its advantages and disadvantages. For example, mixtures cured by the heat of the model device are suitable only for serial production of cores, self-hardening mixtures require special equipment, such as mixers, arrangement of core holders on lines, etc. Inorganic binders provide cores and molds with impaired disintegration, organic binders ensure good disintegration, mixtures are usually more expensive.

For the piece and small-scale production of smaller and shape-demanding cores, drying mixtures are used in less mechanized foundries. Their advantage is low requirements for machinery, almost unlimited service life of the mixture, the preparation of which is very simple. The mixtures have high strengths even after drying, allowing the manufacture of even complicated cores. Usually, drying or semi-drying oils are used as a binder, which are cheap and give the cores very good disintegration after casting.

Oil mixtures are not heavy, to make them heavy it is necessary to add bentonite or dextrin, which will slightly reduce the strength and increase the evolution of gases during casting.

The disadvantage of oil mixtures is the high energy consumption during drying, as the kernels must be dried at a temperature of at least 200 °C for 1 to 2 hours or more, depending on the size and shape of the kernel.

Another disadvantage is the fluctuation of the quality of the binder, manifested in changes in the surface resistance and gas formation of the cores. The use of oils brings about hygiene problems, during drying of the cores and during casting, foul-smelling and irritating fumes and vapors are developed, which greatly worsen the working environment.

Resins and other polymers or polycondensates are often used as binders instead of oils. Their use does not solve the issue of hygiene, and usually other irritating and carcinogenic products are released into the air, such as phenols, formaldehyde and ammonia. These binders often contain unsuitable solvents, which further worsens work hygiene. The hardening of resin binders is difficult, accompanied by a significant reduction in the strength of the cores. The mixtures are prone to sticking to hands and model equipment.

For the special cases of thin-walled castings cast from alloys with a low melting point, most of these binders have too high thermal stability and thus impaired disintegration, which leads to the use of more expensive special binders.

One way to solve the issues of occupational hygiene and disintegration after casting is to use water-soluble cellulose derivatives, i.e. carboxymethylcellulose, hydroxyethylcellulose, etc. Mixtures with these binders have good and easily controllable binding, are hygienically harmless and have excellent disintegration.

The disadvantage is the lower strength after drying, which is further reduced by aging the mixture or by standing the cores, which tend to get wet. This leads to the need to increase the binder and water content in the mixture. The result is a deterioration in the compactability, an increase in the stickiness of the mixture, a lengthening of the drying time and an increase in the gas formation of the molds and cores during casting. The use of mixtures with lower strengths after drying increases the abrasion of the cores during handling and casting, which results in a deterioration in the surface and internal quality of the casting.

The above disadvantages of the drying mixtures used are eliminated by a mixture which, in addition to 81 to 99.7 parts by weight of grinding stone, 0.3 to 5 parts by weight of water-soluble carboxymethylcellulose and 0.3 to 10 parts by weight of water, according to the invention, contains an additive of 0.01 to 4 parts by weight of urea.

The advantage of the mixture with the composition according to the invention is primarily that, with the same binder and moisture content, the strength of the cores after drying increases strongly, almost twice, and the surface quality of the cores improves significantly. It is therefore possible to use a lower binder and water content, which significantly reduces energy consumption during drying and shortens the drying time.

The mixtures are non-sticky, hygienically safe and sufficiently binding without the need to add another weighting agent; the cores can be pressed very well. The shelf life of the cores is extended.

As a result of using a low binder content, gas evolution during casting is reduced, the disintegration of cores is improved; the result of using the mixture according to the invention, in addition to improving hygiene and working conditions, is increased casting quality, reduced gas defects and improved surface smoothness in connection with increased core surface quality.

The preparation of the mixtures is very simple, it consists of mixing the dry ingredients in the ratio according to the invention and then moistening the dry mixture with water, after which it is mixed for 5-15 minutes, depending on the type of mixer, so that the binder dissolves. Therefore, pre-dissolving the carboxylmethylcellulose in water is not necessary.

The mixture has an almost unlimited shelf life, i.e. several days, the dried mixture can be re-moistened to the required humidity. Carboxymethylcelluloses with different degrees of substitution and different molecular weights can be used as a binder. No difference was found between samples with different viscosities in the range of 0.005 - 0.022 PaS (5 - 22 cP). Carboxymethylcellulose can be used both in the form of flour and in the form of flakes, in a dried state or partially moistened. Urea can be used in the form of a technical product.

The content of carboxylmethylcellulose in the mixture is selected primarily according to the requirements for the strength of the cores after drying, which is determined by their type and size. The binding is a secondary criterion, as it can be adjusted by possible other additives, such as calcium hydroxide.

Usually, a content of 1 to 2% carboxymethylcellulose is sufficient.

The water content is determined by the susceptibility of the core shape to sticking. In more demanding cases, a water content in the lower range is used. The binder and water content is also determined by the type and grain size of the sand used; for cleaner sands, the binder content can be reduced, for finer grain sizes, the water content increases.

The cores are dried at a temperature of 150 - 220 °C, at the upper limit of temperatures it is necessary not to exceed the optimal drying times, as this would cause partial destruction of the binder. Drying times are approximately half that of oil mixtures.

The cores can also be hardened by simply air drying for a period of time sufficient to remove moisture.

The cores can be surface-treated in various ways.

The composition of the mixture according to the invention is evident from the following examples, which also show the properties of mixtures of known composition.

Example 1 mixture known mixture according to the invention

sand with a leachable content of 1% 94.3 94.8 mass parts carboxymethylcellulose mass parts 1.5 1.5 urea mass parts 0.5 0 water mass parts 3.7 3.7 seriousness kPa 6 6 Strength after drying in compression at 150 °C 1 hour MPa 6.6 3.0

To achieve the same strength, the mixture according to the invention contains 1% carboxymethylcellulose compared to the known mixture, i.e. the content of carboxymethylcellulose is 1/3 lower.

Example 2 mixture known mixture according to the invention sand with a leachable substance content of 0.2%

mass parts 93.7 94.0 carboxymethylcellulose mass parts 1.0 1.0 urea mass parts 0.3 0 water mass parts 5.0 5.0 seriousness kPa 6 5 Strength after drying in compression at 150 °C 1 hour MPa 5.5 2.9

SUBJECT MATTER OF THE INVENTION

Claims (1)

Mixture for the manufacture of foundry molds and cores, drying by drying or drying at elevated temperatures, containing 81 to 99.7 parts by weight of granulated slag, 0.3 to 5 parts by weight of carboxymethylcellulose and 6 to 10 parts by weight water, characterized in that it further contains 0.01 to 4 parts by weight of urea.