GB2168279A

GB2168279A - Feeder made from exothermic heating composition for castings

Info

Publication number: GB2168279A
Application number: GB08530628A
Authority: GB
Inventors: Manfred Scherer; Clemens Schottek
Original assignee: Lungen & Co KG Geb GmbH; Mannesmann Rexroth AG
Current assignee: Lungen & Co KG Geb GmbH; Bosch Rexroth AG
Priority date: 1984-12-12
Filing date: 1985-12-12
Publication date: 1986-06-18
Also published as: SE8505852L; GB2168279B; SE8505852D0; ZA859447B; DE3445209A1; FR2574327B1; FR2574327A1; GB8530628D0

Abstract

A feeder consisting of exothermic heating composition keeps the cast material disposed in the feeder liquid until completion of the shrinkage feed of the casting. To avoid contamination of the moulding sand (2) on separation of the casting (1) due to burnt feeder disintegrating into irregular fragments, the make-up of the heating composition is so chosen that the heating composition during the shrinkage feed sinters together and the burnt feeder is given such a high strength that it can be detached as a whole or in large pieces capable of being sieved out. Various ways in which the constitution of the heating composition is adjusted to achieve sintering at the temperature reached are described with examples of constituents. <IMAGE>

Description

SPECIFICATION Feeder made from exothermic heating composition for castings The invention relates to a feeder made from exothermic heating composition for castings, in particular cast iron, corresponding to the preamble of claim 1.

Such a known feeder (DE-PS 2,010,337) has among others the advantage that the dimensioning of the feeder volume takes account of the shrinkage feed volume of the casting and thus the feeder volume is minimized. During the shrinkage feed operation after pouring in the cast material the cast material level in the feeder drops corresponding to the shrinkage feed requirement of the casting, the cast material disposed in the feeder being kept liquid by the amount of heat liberated by the heating composition. When the sinking or shrinkage feed operation has been completed the cast material level in the feeder has dropped to such an extent that the remaining residual feeder has a relatively small height.

When the mould is emptied the burnt-off feeder disintegrates. The moulding sand is screened and used again. It is frequently found that burnt-off feeder residues cannot be separated from the moulding sand so that when the latter is reused the feeder residues entrapped in the sand cause surface flaws in the castings.

The exothermic heating composition consists in known manner of a thermite mixture with fillers and a binder. A heating composition made up for example of aluminium powder and oxygen carriers such as iron oxide with addition of fillers generates on burning heat which keeps the cast material in the feeder liquid. The reaction temperature and burning time are adjusted by the nature and amount of the fillers. As binder water glass is often used and imparts the necessary initial strength to the feeder body.

To avoid surface flaws on castings due to moulding sand contaminated with feeder fragments or residues, the constitution of the heating composition could be chosen so that after casting it disintegrates completely and is thus so finely distributed that no major casting flaws can occur in the casting. The disadvantage of this proposal is that the complete disintegration of the heating composition cannot be checked or controlled and this possibility is therefore too unreliable.

The problem underlying the invention is therefore to provide a feeder of the type outlined at the beginning which after the casting and subsequent separation of mould and casting can be separated in simple and reliable manner from the moulding sand.

This problem is solved by the features set forth in the characterizing clause of claim 1.

The makeup of the heating composition is so chosen that the feeder during the sinking or shrinkage feed is heated to such a high reaction temperature that a sintering together or caking of the heating composition occurs which gives the burnt feeder such a high strength that after the casting and subsequent separation of mould and casting it does not disintegrate but on the contrary remains in one piece. Since the height of the residual feeder after conclusion of the shrinkage feed or sinking is very low, as already mentioned, the feeder can easily be detached from the casting.

Should in practice a higher mechanical stress take place it is possible for the burnt-off feeder to break into several large pieces which however can also be easily separated from the moulding sand because when the latter is screened for further use these pieces are certainly sifted out in the coarse sieve.

Advantageous further developments and constitutions of the heating composition are characterized in the subsidiary claims. Of decisive importance for the strength of the burnt feeder is the combustion temperature reached during the sinking or shrinkage feed operation. It must be high enough to at least reach the sintering temperature of the heating composition, in particular exceed said temperature. Since the temperature of the cast material is less than the sintering temperature the makeup of the heating composition is chosen so that the combustion temperature of the heating composition is increased at leasttothesintering temperature.

For example, the cast material temperature with cast iron is about 1450 C and the sintering temperature of the heating composition about 1600 to 1700"C.

The necessary high strength of the feeder can be achieved firstly via a considerable increase of the amount of binder in the heating composition. If in known manner water glass is used as binderthe necessary binder amount is of the order of magnitude between 6 and 20% by weight of the composition. The increased addition of inorganic binder such as water glass also has the property of reducing the sintering temperature of the composition due to the high alkali content of the binder.

Binders which have-proved suitable are preferably aluminium phosphate, potassium and sodium water glasses, aluminous cement, magnesium sulphate or colloidal silicic acid. Secondly, the reaction temperature of the heating composition can be increased by a higher proportion of therm ite mixture, i.e. by adding more for example aluminium powder and oxygen carriers.

Preferably, using both measures, i.e. reducing the sintering temperature by increasing the binder proportion and increasing the reaction temperature by a greater proportion of thermite mixture, the combustion temperature of the heating composition is set so that with a cast material temperature of 1450"C (cast iron) a combustion or burning temperature of over 1 600"C and thus the sintering temperature is reached. The quartz constituents of the sand used as filler melt and the aluminium contained in the heating composition oxidizes to aluminium oxide, a composition similar two corundum. The desired high strength is then achieved in the burnt-off feeder.

Thus, tests have shown that with a starting strength of the feeder of 1150 kp/cm2 compressive strength, in the burnt-off state the strength is still 600 kp/cm2. The strength of the burnt feeder is thus at least 40% of the initial strength.

For further reducing the sintering temperature and increasing the strength it is also possible to add high-temperature sintering agent additionally to the heating composition. Such sintering agents are preferably chromium oxide, glass powder or alkali and alkaline earth silicates.

An example of embodiment of a feeder is shown diagrammatically in the single Figure of the drawings in partial longitudinal section.

A sand mould 2 surrounds a casting 1 on which a hood-shaped feeder 3 is placed which consists of an exothermic heating composition. The feeder 3 defines a cavity 4 which in turn defines a feeder volume which corresponds to the shrinkage feed requirement of the casting 1 including a slight amount of cast iron corresponding to a residual feeder 5. At a feeder support face 6 after removal of the feeder 3 the residual feeder 5 is further separated from the casting 1.

The stability of the feeder 3 is increased if at the outer side sharp edges are avoided and the mean wall thickness of the feeder is substantially between 0.5 to 2 times the diameter of the cavity 4. As iilustrated, the outer form of the feeder 3 may correspond to a truncated cone or be appropriately rounded. Due to its strength and stabilitythe burnt feeder can easily be detached from the residual feeder 5 without the feeder disintegrating.

Claims

1. Feeder made from exothermic heating composition for castings, in particular of cast iron, the heating composition consisting of a thermite mixture with fillers and a binder keeping the cast material disposed in the feeder liquid until completion of the shrinkage feed of cast material through the casting area to be fed and the feeder volume corresponding to the magnitude of the shrinkage feed volume plus a safety margin, characterized in that the heating composition is made up of the thermite mixture, the fillers and the binder in a ratio with which during the shrinkage feed operation a temperature corresponding at least to the sintering temperature of the heating composition arises.

2. Feeder according to claim 1, characterized in that the proportion of binder in the heating composition is increased.

3. Feeder according to claim 1 or 2, characterized in that as binder aluminium phosphate, potassium and sodium water glasses, aluminous cement, magnesium sulphate or colloidal silicic acid are used.

4. Feeder according to claim 3 with water glass as binder, characterized in that the proportion is of the order of magnitude of at least 6 to about 20% by weight of the heating composition.

5. Feeder according to any one of claims 1 to 4, characterized in that the proportion of thermite mixture in the heating composition is increased.

6. Feeder according to any one of claims 1 to 5, characterized in that the heating composition additionally contains a high-temperature sintering agent such as chromium oxide, glass powder or alkali and alkaline earth silicates.

7. Feeder according to claim 6, characterized in that the proportion of high-temperature sintering agent is of the order of magnitude of about 0.2 to 10%.

8. Feeder according to any one of claims 1 to 7, characterized in that the outside of the feeder comprises a hood-shaped, inclined and/or rounded form and the centre wall thickness of the feeder is about 0.5 to 2 times the diameter of the cavity forming the feeder volume.

9. Feeder made from exothermic heating composition for castings, substantially as hereinbefore described with reference to the accompanying drawings.