Method and apparatus for removing a fugitive pattern from a mold

Abstract

A fugitive pattern, such as wax or other meltable pattern material, residing inside of a refractory mold, which can be unsupported or supported in a particulates bed, is removed by discharging steam or other condensable vapor that may include a surfactant inside the mold to contact and melt the pattern while an exterior of the mold is subjected to a non-condensing gas atmosphere such as air outside of the mold. Regardless of whether the condensable vapor includes surfactant or not, the mold can be tilted relative to gravity and rotated while it is tilted to improve the pattern removal. Condensable vapor is condensed inside the mold where the vapor has contacted the pattern while the exterior of the mold remains free of condenate. The condensed vapor and melted pattern material are drained out of the mold with the surfactant, if present, improving drainage.

Description

RELATED APPLICATION [0001] This application is a continuation-in-part of copending Ser. No. 10 / 899,381 filed Jul. 26, 2004.FIELD OF THE INVENTION [0002] This invention relates to method and apparatus for removing a fugitive pattern from a metal casting mold. BACKGROUND OF THE INVENTION [0003] The well-known “lost wax” investment casting process typically uses a refractory mold that is constructed by the buildup of successive layers of ceramic particles bonded with an inorganic binder on a fugitive (expendable) pattern material such as typically a wax, plastic and the like. The finished refractory mold is usually formed as a shell mold around a fugitive pattern. [0004] The refractory shell mold residing on the fugitive pattern typically is subjected to a pattern removal operation, wherein the pattern is melted out of the shell mold. This operation leaves an empty “green” (unfired) refractory shell mold. The fugitive pattern materials typically have a thermal expansion rate many times...

AI Technical Summary

Benefits of technology

[0009] An aspect of the present invention provides method and apparatus for removing a fugitive pattern, such as wax or other meltable pattern material, residing in a refractory mold by introducing a condensable vapor, such as steam, that in a particular embodiment includes a surfactant inside the mold to contact and melt the pattern, while the exterior of the mold is subjected to a non-condensing gas atmosphere, such as ambient air, outside of the mold. The condensed vapor and the melted pattern material are drained out of the mold. The surfactant lowers the surface tension of the condensed vapor in contact with the fugitive pattern inside the mold and increases the ease at which the melted pattern material flows over the freshly exposed mold interior surface to improve draining of the melted pattern material out of the mold, leaving less residual pattern material on the interior mold surface.

[0010] A pressure differential between the condensable vapor inside of the mold and the non-condensing gas atmosphere outside of the mold is small enough as to prevent the condensable gas from exiting outside the mold exterior and the non-condensing gas from entering the mold cavity. The condensable vapor inside of the mold and the gas atmosphere outside of the mold preferably are at substantially the same pressure to this end. In this way, when steam is used as the preferred condensable vapor, the steam is condensed inside the mold where the steam has contacted the pattern while the exterior of the mold remains dry. The condensable vapor including the surfactant can be introduced inside the mold at atmospheric, subatmospheric, or superatmospheric pressure depending upon the melting point of the pattern material.

[0012] Another aspect of the present invention provides method and apparatus for removing a fugitive pattern, such as wax or other meltable pattern material, residing in a refractory mold by subjecting the mold to a combination of rotation and inclination (tilting) during the pattern removal process in a manner to improve draining of melted pattern material from the mold. The mold can be tilted at any desired angle using a mold tilt drive motor, and the mold can be rotated about an axis using a mold rotation drive motor. The angle of mold tilting and the mold rotational speed can be adjusted as required to drain the melting wax from the mold cavities. The mold can be rotated while the mold is tilted at a fixed angle of inclination relative to gravity. Alternately, the mold can be tilted incrementally to selected angles of inclination while the mold is rotated at each of the angle of inclination or continuously. Further, the mold can be continuously tilted while being rotated continuously or intermittently. Steam or other condensable vapor can be introduced to heat and melt the fugitive pattern inside the mold while the mold is subjected to rotation and tilting, although this aspect of the invention can be practiced using any pattern removal technique where the pattern is melted or dissolved.

[0014] The invention is advantageous to remove one or more fugitive patterns residing in a metal casting refractory mold, which may have any mold wall thickness and which may be unsupported or supported by exterior particulate media therearound. The invention is further advantageous to remove one or more fugitive patterns while avoiding saturating the mold wall with steam or other condensate, which may have adverse effects on the binder used to fabricate the mold. The invention may be practiced to reduce mold cracking during pattern removal and to remove pattern material from molds where steam cannot readily access the exterior of the mold wall such as when the mold is supported with particulate support media.

Problems solved by technology

This operation leaves an empty “green” (unfired) refractory shell mold.

This will place the refractory shell mold under tension and will ultimately crack the shell mold.

The connecting together of the refractory shell mold between adjacent patterns is one of the major causes of non-uniform heating of the pattern.

That is, thicker regions of the refractory shell mold will hinder the application of heat to the pattern material and locally delay the melting of the surface of the pattern and disrupting of the continuum.

Such prevention of the passage of surface liquid pattern material causes a buildup of pattern pressure in the remote thinner mold region due to the thermal expansion of the pattern material and can lead to mold cracking.

These problems require the use of a mold strong enough (e.g. thick enough) to resist the expansion pressure of the pattern material and often require the use of supplemental holes or vents through the mold to relieve pressure from unconnected expanding patterns.

Stronger or thicker molds as well as the venting method are undesirable as they increase processing costs.

Thin shell molds however, are more prone to cracking during the pattern removal operation, such as the high pressure steam autoclave or flash fire pattern removal operation mentioned above, wherein the pattern is melted out of the shell mold.

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