[0001] The present invention relates to the use of flash dressings in foundry practice.
[0002] It is conventional to coat porous moulds and cores with a dressing which serves as
a barrier between molten metal and eg the sand in the casting process. By its use,
a number of other casting defects are prevented. Where hardened sands are used it
is customary - to apply a dressing in the form of a water-based coating by brush,
swab or spray gun. Such water-based coatings may comprise a base or filler and a liquid
vehicle. The filler may be carbonaceous, graphitic, or based on a mineral flour such
as zircon. The dressing is normally bonded by a clay-based binder.
[0003] In the case of chemically-hardened sands, it is necessary to use dressings in which
the vehicle is a volatile liquid which can be "flashed-off' by ignition. Such dressings
are referred to as "flash dressings". The volatile liquids for instance may be pure
organic solvents or mixtures of organic solvents. Volatile liquids now commercially
in use include isopropyl alcohol, and mixtures of ethoxy ethanol with isomeric butanols
or white spirit 50:50. Other mixtures used for this purpose include mixtures of ethoxyethanol
and liquid hydrocarbons. All these dressings suffer from one or more of the following
disadvantages:
1. Isopropyl alcohol when used alone is highly inflammable (flash point below 90°F
equivalent to 32°C) which imposes restrictions on its use.
2. Some dressings such as those containing a high proportion grains, refractory fillers
may be used. These fillers may be carbonaceous, graphitic or mineral based. The fillers
are similar to those of water-based dressings but in this case the binder may be a
resinous material. "Blackings", carbonaceous, graphitic and mineral based fillers
are preferred.
[0004] In the flash dressings of the present invention suspension agents such as bentonite
may be added to prevent the settling out of heavy fillers during use. Other minor
components such as iron- oxide may be added to mould coatings to assist in preventing
reaction between nitrogen-containing bonding agents and molten ferrous metals. This
latter reaction is usually responsible for defects such as "pinholing" under the casting
surface. Smoke supressants such as for example ferrocene may also be added to the
flash dressing compositions. In some cases addition of a minor proportion, eg up to
20% by volume of the total composition, of a liquid hydrocarbon aids. not only to
burn off completely-the flash dressing applied to the casting but also enhances the
luminosity of the flames. This is desirable in some instances to ensure that the flash
dressing has in fact been ignited. The amount of liquid hydrocarbon is suitably not
more than 15%, preferably not more than 10% by volume of the total composition.
[0005] The flash dressings of the present invention may be brushed, sprayed or swabbed onto
the mould surface. Where small cores are being dressed, the components may be dipped
into the liquid dressing suspension.
[0006] After application, the mould is ignited, leaving behind a film on the surface of
the sand, and when the casting has been made, the sand core can then be broken away
easily leaving a smooth surface on the casting.
[0007] The invention is further illustrated with reference to the following tests the results
of which are tabulated below. The results in Table 1 are Comparative Tests not according
to the invention. The results in Table 2 illustrate the present invention.
1. Smoke Tests
(a) The amount of smoke produced by a dressing was assessed by of the dressings tested
in 1 (a) above.
2. Flash Point Tests
[0008] The flash points of various dressings listed in 1 above was measured using the Abel
closed cup apparatus according to the Institute of Petroleum Standard Test No IP 170/70.
[0009] The results of these tests are also shown in the Tables below:
[0010] The dressing may contain in addition 1-2% by weight of a natural or synthetic resin
such as a phenol-formaldehyde resin.
