Method for applying dry powder refractory coating to sand cores

Details Method for applying dry powder refractory coating to sand cores Method for applying dry powder refractory coating to sand cores

1995-06-09

2001-11-13

Bareford, Katherine A. (Department: 1762)

Coating processes

Mold coating

Sand mold

C427S185000, C164S014000, C164S138000

Reexamination Certificate

active

06316047

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to technology for treating and smoothing sand molding surfaces thereby produce higher quality metal castings using such molding surfaces.
2. Discussion of the Prior Art
In the making of automotive metal castings, complex internal cavities are formed by sand cores typically made of coarse sand particles to assure higher strength. If the surface pores of such cores are not coated or filled, metal will penetrate into the interstices between sand grains, causing at best an undesirable rough cast surface, and at worse, difficulty in removing the sand cores from the cast surfaces. To remedy this, a fine refractory slurry is usually applied by dipping, spraying or brushing the surfaces of the sand cores. Although the slurry smoothes the sand core surfaces, defects may occur in the metal castings as a result of cracks or blisters arising in the refractory coating when dried; moreover, the need for drying slows down the manufacturing process considerably.
Coating sand molding surfaces with a dry powder, by use of a fluidized dry powder bed, has been attempted as early as 1985 to eliminate the need for drying. In one attempt, the dry powder had a uniform particle size smaller than the pores of the sand surface. This resulted in an inadequate coating and inadequate penetration of sand pores; the dry powder lacked efficient momentum to penetrate deeply into the pores. In another attempt, the powder was formed as a mixture having a dry powder pushing material to achieve greater penetration of the treating powder. The pushing material had a slightly greater particle size, density and volume, relative to the treating powder; however, the differential was small. This necessitated reciprocal movement of the sand surface to be coated, within the fluidized bed, to reach all of the pores. The need for a period of movement slowed down the coating process and increased the risk of sand surface breakage during such movements, particularly on a repeated basis with a large number of strokes. The randomness of the controlled pushing material in the fluidized bed was not sufficient to achieve a continuous coating on the sand surfaces.
SUMMARY OF THE INVENTION
It is an object of this invention to coat sand cores in a fluidized bed by a method that eliminates the need to move or reciprocate the core within the fluidized bed, and yet produces a sound continuous coating within a few seconds after insertion of the sand core within the bed.
In particular, the invention is a method of applying a refractory coating to densified foundry sand cores having micropores between the sand grains. The method comprises (a) mixing a dry powder refractory material with a dry powder impacting media, the refractory material having a particle size in the range of 5-40 microns and the weight ratio between the refractory material and the impacting media being 1:13 to 1:22; (b) fluidizing the mixture in a contained zone by introducing positive air pressure at one end of the zone and applying a negative pressure at the other end of the zone; and (c) quickly and entirely immersing the sand cores in the zone containing the fluidized mixture and substantially immediately thereafter withdrawing such sand cores, without the need for added motion, to be coated with essentially only the refractory material as impacted by the impacting media. The cores will be coated continuously with the refractory material, impacted within the micropores of the sand core. Advantageously, the refractory material is anhydrous MgOSiO
2
talc, the impacting media is zircon sand, and the foundry sand core body is formed of resin bonded zircon sand.


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