CA1073606A

CA1073606A - Mold spray

Info

Publication number: CA1073606A
Application number: CA264,225A
Authority: CA
Inventors: Virgil L. Houser
Original assignee: Amsted Industries Inc
Current assignee: Amsted Industries Inc
Priority date: 1975-11-03
Filing date: 1976-10-26
Publication date: 1980-03-18
Also published as: ZA766433B; MX154236A; AU1925476A; BR7607294A; IT1066741B; IN144969B

Abstract

ABSTRACT OF THE DISCLOSURE

A method of producing castings in a graphite mold having casting surfaces defining a mold cavity, comprising the steps of: coating the casting surfaces of the graphite mold with a slurry of water and fused silica, the fused silica having an average particle size in the range of 5 to 30 microns, while holding the mold at a temperature in the range of about 177°C to 232°C, and then filling the mold cavity with molten steel.

Description

RMF:av 10-11-76 - ' Case 5537 MOLD SPRAY -~

No drawing accompanies this specification, which re~
lates to the art of casting products, such as railway car wheels, in graphite molds.
To protect the casting surfaces of the graphite mold from excessive erosion and to eliminate surface defects on the surfaces o~ products cast in the mold, the casting surfaces are customarily provided with a refractory coating such as quartz, zircon, or the like. Such coatings have always been of the crystalline form and, although generally satisfactory, are subject to several disadvantages. It has been noted, ~or example, that crystalline silica, the most common coating material, has a tendency to occasionally break loose from the mold surface in a random pattern. When tllis occurs, laps or other sur~ace defects ~ppeax on the cast product in areas corresponding to the portions of the mold wi~h the loose coating. Furthermore, some difficulty has been experlenced in removing the coating from the mold a~ter a product has been cast therein. Since the mold coating must be removed and a new coating applied to the mold a~ter each pouring operation, the cost of mold clean-ing can be a significant factor.
It has been theorized that the problem o~ poor adhe-sion of crystalline silica is caused by a peculiarity in its thermal expansion curve. Crystalline silica, when heated, appears to expand up to a temperature of approxi--1 ~

- , . -RMF:av 10-11-76 Case 5537 t~3~

mately 575C- The material then contracts and, with increase in tempera~ure, once again expands. It is pos-sible that this inversion causes some of the coating to break loose from the mold and eventually be the cause of surface defects in products cast in the mold.
According to the present invention, a mold coating has been provided which overcomes the above disadvantages.
The mold coating also has better insulating qualities than prior art coatings using crystalline silica and therefore offers greater protection against mold deterioration.
Furthermore, because the coating constituents do not appear to be subject to a thermal expansion inversion temperature, its adherance to the mold casting surfaces is superior to that of prior art coatings. Therefore, graphite molds coated in accordance with the present in~ention produce castings relatively free of surface defects.
The proposed coating preferably includes fused silica, having a particle size in the range of about 5 to 30 microns, suspended in water as a carrier by means of cel-lulose gum and/or V-gum as is known in the art. A typical spray coating includes about 45.3 kg of finely ground fused silica, about 75.7 liters o~ water, about 0~453 kg o V-gum, and about 56.7 grams of cellulos gum. The pH
of the coating solution should pre~erably be between 7 and 8 to obtain optimum adherance to the graphite mold and the RMF~av 10-11-76 Case 5537 ~ 06 viscosity should be in the range of about 35 ~o 40 Baumé.
The slurry is sprayed onto the casting surfaces of a graphite mold which has been preheated to a temperature preferably in the range of 177C to 232C. The preferred coating thickness is about 0.05 cm but satisfactory results have been obtained with coating thicknesses in the range o~ 0.014 cm to ~.~76 cm.
The particle size of about 5 to 30 microns, with a preferable average size of 15 microns, is critical.
Because the slurry is sprayed on the casting surfaces of the graphite mold with the mold at a temperature of about 205C, the water carrier is transformed into steam. If the average particle size of the fused silica is less than 5 microns, the back pressure of the steam prevents such small particles from adhering to the mold, resulting in a low quality coating which cannot prevent the formation of surface deects on a casting poured in the mold. If, on the other hand, the average particle size is greater than 30 microns 7 the coating loses some of its insulating effect and, when molten metal is poured into the mold cavity, heat is conducted ~rom the metal too rapidly.
This o~ten results in wrinkles or other surface defects in the cast product.
Utilization of a coating for a graphite mold, wherein the coatlng comprises fused silica which is applied in the .. . . . .

RMF:av 10 ~~ Case 5537 . :~0~3~6 manner described, provides castings which are substantially free of surface defects. Furthermore, mold life is increased and the process of stripping and recoating the casting sur~aces of the mold is accomplished more quickly and at less expense.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of producing castings in a graphite mold having casting surfaces defining a mold cavity, comprising the steps of: coating the casting surfaces of the graphite mold with a slurry of water and fused silica, the fused silica having an average particle size in the range of 5 to 30 microns, while holding the mold at a temperature in the range of about 177°C
to 232°C, and then filling the mold cavity with molten steel.

2. A method of producing castingsaccording to Claim 1, wherein the casting surfaces are coated with a slurry of fused silica held in suspension in water, the fused silica and water being in a ratio of about 6:10 by weight.

3. A coating for the casting surfaces of graphite molds, said coating comprising fused silica having an average particle size in the range of 5 to 30 microns.

4. A coating according to Claim 3, wherein the fused silica is suspended in water as a carrier in a ratio of about 45.3 kg of fused silica to 75.7 liters of water.

5. A coating according to Claim 4, wherein the fused silica-water suspension is of a viscosity in the range of about 35° to 40° Baumé and has a pH in the range of 7 to 8.