MOLD SPRAY

REVETEMENT A VAPORISER POUR MOULES

English 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.

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.

Description

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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 ~
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Case 5537
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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
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Case 5537
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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
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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.