GB2040295A

GB2040295A - Molding sand mixture for the manufacture of molds and cores

Info

Publication number: GB2040295A
Application number: GB7846001A
Authority: GB
Original assignee: V NI I PI TEKHNOL KHIM I NEFTY
Current assignee: V NI I PI TEKHNOL KHIM I NEFTY
Priority date: 1978-11-24
Filing date: 1978-11-24
Publication date: 1980-08-28
Also published as: GB2040295B

Abstract

A molding sand mixture comprises a filler, a binder in the form of a solution of hydrolyzed ethylsilicate and a curing agent which is an organic solution of a polyorganosilazane of the formula: {(R2SiNH)n[R'Si(NH)1.5]m}k, wherein R is CH3, R' is CH3 or C6H5, n=1, k=4 to 10, and m=1 to 3, the components being taken in the following proportions in percent by weight: solution of hydrolyzed ethylsilicate 10 to 35 organic solution of polyorganosilazane 0.2 to 2.0 filler the balance.

Description

SPECIFICATION Molding sand mixture for the manufacture of molds and cores The present invention relates to a molding sand mixture for the manufacture of molds and cores, which may be most advantageously used for the production of precision castings with a high surface finish, but which may also be used for other types of casting.

The present invention provides a molding sand mixture for the manufacture of molds and cores, comprising a filler, a binder in the form of a solution of hydrolyzed ethylsilicate, and a curing agent causing gelling of ethyl-silicate, wherein the said curing agent is an organic solution of a polyorganosilazane of the formula {(R2SiNH)rMlsi(NH)1i1m}k 1 wherein R is CH3, R' is CH3 or C6H5, n = 1, k = 4 to 10 and m = 1 to 3, the components being present in the following proportions, in per cent by weight: solution of hydrolyzed ethylsilicate 10 to 35 organic solution of polyorganosilazane 0.2 to 2.0 filler the balance The use of a polyorganosilazane in the molding sand mixture according to the invention is based on two re-actions successively occurring in the molding sand mixture.

First of all, due to instability of the bond = Si-N=, there occurs hydrolysis of this curing agent with the formation of siloxane bonds (1) ~ Si - N - Si+ H2O )SiNH2 + HOSi H (2) SiNH2 + H2O -=SiOH + NH3 (3) 2SiOH ) - Si - 0 - Si - + H20 I (4) -SiNH2 + HOSi < - Si - O - Si - + NH3.

In this hydrolysis of the polyorganosilazane in the medium of hydrolyzed ethylsilicate there is consumed the co-ordination-bound and adsorbed humidity of the filler, which makes it possible to use previously harmful "ballast" humidity for the formation of additional siloxane bonds and increase of adhesion.

Secondly, ammonia formed at the surface of the filler particles according to reactions (2) and (4) above facilitates the formation ofsiloxane bonds (i.e. gelling of ethylsilicate).

Therefore, in contrast to previously used curing agents, polyorganosilazane, being less toxic acts as a binder as well as a curing agent for hydrolyzed ethyl-silicate and as an agent removing so-called ballast water.

The bending strength of the cured molding sand mixture produced with the use of a polyorganosilazane is as high as 20 to 25kg/cm2, which is 1.5-1.75 times higher than that of a molding sand mixture produced with the use of triethanolamine. Due to the fact that the above-mentioned reactions involve two stages, it is possible to uniformly distribute polyorganosilazane over the molding sand mixture volume, and to ensure even and uniform hardening of the molding sand mixture while eliminating agglomeration and cracking and while increasing mechanical strength.

The solvent for the polyorganosilazane may be suitablytoluene, benzene, kerosene and other hydrocarbons.

The above-specified limits of the content of the hydrolyzed ethylsilicate solution in the molding sand mixture are due to the fact that on the one hand when the ethylsilicate solution is present in an amount of below 10% by weight the mixture lacks plasticity (it becomes thick) and cannot be poured into a-model, and on the other hand when the content of the ethylsilicate solution is above 35% by weight the mixture has too high a fluidity and sedimentation of the filler occurs therein; after curing, a network of cracks appears on the surface of the final mold or core which impairs the quality of the surface and lowers mechanical strength.

The above-specified range of the content of the polyorganosilazane is selected due to the fact that on the one hand when it is present in an amount of below 0.1% by weight the time required for hardening of the molding sand mixture is increased so that segregation (sedimentation) of the ingredients occurs which results in the production of defective molds, while on the other hand when the polyorganosilazane is contained in an amount exceeding 2.0% by weight the curing time or lifetime of the molding sand mixture is shortened to 0.5 minutes or less, as a result of which the operator is unable to pour the sand mixture into the model.

The amount of the polyorganosilazane in the organic solution is not strictly limited, though with the use of low-concentration solutions the durability of the final molds and cores is reduced due to the incorporation into the molding sand mixture of a large amount of organic solvent which does not impart strength and which therefore comprises ballast. Furthermore, the curing time of the molding sand mixture is increased.

High-concentration solutions of the polyorganosilazane have a viscosity of above 150 cSt, which hinders their use. It has been found that the concentration of the solution of the polyorganosilazane is preferably within the range of from 20 to 40%.

The best results are obtained with the following content of the components of the mixture, in per cent by weight: solution of hydrolyzed ethylsilicate 15 to 25 2040% solution of polyorganosilazane 0.3 to 1.0 (more preferably 0.5 to 1.0) filler the balance.

The invention will be further described with reference to the following illustrative Examples.

Reference will also be made to the accompanying drawings, in which Figure la shows the surface of a final core produced with the use of a previously known molding sand mixture, at 5-fold magnification; and Figure lb shows the surface of a final core produced with the use of a molding sand mixture according to the present invention, also at 5-fold magnification.

The preparation of a molding sand mixture according to the present invention may be performed by any conventional method for preparing a liquid molding sand mixture which generally comprises incorporation, into hydrolyzed ethylsilicate, of a filler and then a curing agent under continuous stirring.

EXAMPLE 1 Preparation of a molding sand mixture for the manufacture of cores for steel castings of working wheels of centrifugal pumps.

Quartz sand and dust-like quartz were introduced into a solution of hydrolyzed ethylsilicate under stirring thereof with a stirrer rotating at a speed of 2,000 r.p.m. After homogenization of the mixture, a solution of a poly-organosilazane in toluene was added thereinto and stirring was continued for 1-2 minutes. For the preparation of the molding sand mixture, the following composition of the components was used, in per cent by weight: quartz sand with a fineness of 0.08 to 0.4 mm 30.00 dust-like quartz with a fineness of 0.01-0.06 mm 45.00 solution of hydrolyzed ethylsilicate 24.25 30% solution of polyorganosilazane in toluene 0.75 Then the intermixed mixture was poured into a core box, wherein it was cured within 5 minutes. Then the solid core was put into a drier and placed, at normal temperature, into a liquid fixation medium for 2 to 6 hours. The core was then calcined at a temperature of from 900 to 1,100O for 1-2 hours at a heating rate of from 100 to 150'O/hr. As a result of these operations, a molding sand mixture having a bending strength of from 30 to 80 kg/cm2 was obtained.

EXAMPLE 2 Preparation of a molding sand mixture for castings from refractory alloys.

The molding sand mixture was prepared as described in Example 1, except that the following components were employed, in per cent by weight: zirconium silicate with a fineness of 0.1-0.3 mm - 44.8 zirconium silicate with a fineness of 0.01-0.04 mm 45.0 solution of hydrolyzed ethylsilicate 10.0 20% solution of polyorganosilazane 0.2.

The curing time of the composition was 15-30 minutes. The bending strength after heat-treatment was 25-30 kg/cm2.

EXAMPLE 3 Preparation ofa molding sand mixture for small decorative castings from non-ferrous metals.

The molding sand mixture was prepared as described in Example 1, except that the following components were employed, in per cent by weight: quartz sand with a fineness of 0.08-0.4 mm 17.0 dust-like quartz with a fineness of 0.01-0.06 mm 24.0 alumina with a fineness of 0.005-0.03 mm 22.0 solution of hydrolyzed ethylsilicate 35.0 40% solution of polyorganosilazane 2.0 The curing time of the resulting molding sand mixture was 0.8 to 1.5 minutes. The bending strength after heat-treatment at a temperature of 800"C was 15-20 kg/cm2.

The surface of the cured core produced from the molding sand mixture according to Example 3 developed no cracks and has a very smooth surface (see Figure 1b), which gives castings having a high surface finish.

On the contrary, the surface of a core produced from a previously known molding sand mixture is, as a rule, covered by a network of developed cracks (Figure la), which impairs the quality of the casting surface.

Claims

1. A molding sand mixture for the manufacture of molds and cores, comprising a filler, a binder in the form of a solution of hydrolyzed ethylsilicate, and a curing agent causing gelling of ethylsilicate, wherein the said curing agent is an organic solution of a poly-organosilazane of the formula {(R2SiNH)n[R'Si(NH)1.5]m} k, wherein R is CH3, R' is CH3 or C6Hs, n = 1, k = 4 to 10 and m = 1 to 3, the components being present in the following proportions, in per cent by weight: solution of hydrolyzed ethylsilicate 10 to 35 organic solution of polyorganosilazane 0.2 to 2.0 filler the balance.

2. A molding sand mixture as claimed in Claim 1, wherein the polyorganosilazane is added in the form of a 20-40% solution in an organic solvent.

3. A molding sand mixture as claimed in Claim 2, wherein the components are employed in the following proportions, in per cent by weight: solution of hydrolyzed ethylsilicate 15 to 25 20-40% solution of polyorganosilazane 0.5 to 1.0 filler the balance

4. A molding sand mixture according to Claim 1 for the manufacture of molds and cores, substantially as herein described in any of the foregoing Examples.