Compositions: coating or plastic – Coating or plastic compositions – Molds and mold coating compositions
Reexamination Certificate
2000-08-21
2002-11-26
Brunsman, David (Department: 1725)
Compositions: coating or plastic
Coating or plastic compositions
Molds and mold coating compositions
C106S272000, C164S035000, C164S521000
Reexamination Certificate
active
06485553
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wax compositions for use in investment casting. More specifically, the invention is directed to the use of polyethylene terephthalate as a filler material for the wax compositions used in investment casting.
BACKGROUND OF THE INVENTION
The lost wax process, also known as the cire perdue process, has been used as a process for investment casting for centuries. In the lost wax process, a refractory mold is built up around a pattern of wax and then heated so as to melt and drain off the wax. The resulting mold is then used to cast the desired object.
Generally, the lost wax process involves injecting a wax into a die having an area defining an object to be manufactured, such as a golf club head or a piece of jewelry. A wax duplicate of the object is then removed from the die. The next steps are directed to preparing a refractory shell of the wax object. These steps typically include dipping the wax object into a ceramic slurry so as to build up a durable ceramic shell about the wax object. Once the shell reaches a desired thickness, the wax is removed from the shell. Wax can be removed from the mold by any one of several means of heating. Of these, only the steam autoclave uses a defined pressure whereas flash fire dewaxing is done in high temperature furnaces with no pressure. Hot wax dewaxing, which is rarely used, is also without pressure. After dewaxing, the molds are fired at high temperatures to cure the ceramic and burn out any wax residues before pouring the metal. All traces of the wax composition are preferably removed from the ceramic mold so as to prevent defects in the final product. Next, a desired alloy is melted and brought to the required pouring temperature. In the interim, the ceramic mold is generally heated so as to prevent thermal shock as the molten metal is poured into the mold. Once poured, the mold is allowed to cool and the metal alloy set. The ceramic mold is then removed from the cast object. The raw casting is then further finished, i.e., patination, deburring, removal of gates and sprues used during the investing steps, etc.
One problem with investment casting occurs when the wax composition is injected into the die. Generally, the wax composition is injected at an elevated temperature so that the wax will flow and fill all the interstices of the object to be manufactured. Waxes injected under pressure are subject to shear causing them to become somewhat more fluid. As such, it is not necessary to heat waxes until they are completely liquid in order to completely fill the interstices of a ceramic mold. However, when the wax composition is injected into a complicated mold pattern having different section thicknesses, the different section thicknesses cool at different rates. As would be expected, the thinner areas cool relatively quickly, whereas the thicker sections cool considerably slower. As a result, it has been found that the thinner areas reproduce that particular section with a high degree of dimensional accuracy. In contrast, the wax in the thicker sections tends to shrink during cooling causing dimensional inaccuracies since the object to be manufactured is an exact duplicate of the wax pattern. Moreover, because of the different rates of cooling and shrinkage between the thin and thick sections, considerable stresses can be imposed on the wax pattern, and when removed from the die, the pattern may distort in order to relieve the strain.
One solution to increase dimensional accuracy and stability of the wax composition has been attempted by the addition of various filler materials in the waxes such as organic acids and inert polymeric materials. The term “filler material” is hereinafter defined as discrete solid particles that do not melt during the investment casting process. The use of filler materials has improved some features of the process but has also introduced new problems as well as leaving other problems unsolved.
Among the various materials that have been suggested as useful fillers for investment casting wax compositions but which have not met all of the desirable physical properties for pattern making or have resulted in new problems include fillers such as polystyrene as disclosed in U.S. Pat. No. 3,465,808. Wax compositions containing polystyrene fillers have a propensity for the wax to melt out first during dewaxing leaving a polystyrene residue in the mold that tends to tear the ceramic mold.
Organic acids, such as fumaric acid, adipic acid, terephthalic acid and isophthalic acid, have also been used as a filler material. In general, the organic acid filler materials have a high thermal conductivity. That is, this type of filler material allows the wax compositions to desirably cool quickly even in the thicker sections. However, the major drawbacks with the use of acid fillers in wax compositions is the possibility that these acidic fillers will react with the refiractory mold material and consequently, affect the surface finish and dimensional accuracy of the cast part or expand too rapidly during dewaxing and cause shell cracking in the mold.
Inert polymeric filler materials do not chemically react with the refractory mold materials. However, the thermal conductivity of these materials is generally poor compared with the organic acid type fillers. That is, cooling is non-uniform for the varying section thicknesses and as such, shrinkage still remains a serious problem with respect to dimensional accuracy. In addition to poor thermal conductivity, these polymeric materials are generally more difficult to remove from the mold during the wax removal steps. As a result, significant amounts of ash residue may form in the mold during the firing steps and these residues will undesirably become imprinted onto the casted object. The most commonly used inert polymeric filler materials are polystyrene and acrylic type polymers.
Prior art fillers suffer additional drawbacks. For example, urea tends to decompose when the wax is melted and organic acids have high specific gravities and thus tend to settle quickly when not sufficiently agitated. Moreover, many fillers have a relatively high thermal conductivity which can lead to premature expansion of the investment casting wax composition upon autoclaving and thereby cause shell cracking. Additionally, many fillers can produce environmentally hazardous or carcinogenic materials upon combustion.
SUMMARY OF THE INVENTION
The present invention is directed to a new filler material and its use in wax compositions for investment casting. In particular, it has been found that polyethylene terephthalate is preferred for use as a filler material in wax compositions. The material is safe to handle and is capable of convenient melting or burning out of the mold. Also known as PET, polyethylene terephthalate advantageously does not chemically react with the refractory mold materials used in investment casting. Unexpectedly, the use of polyethylene terephthalate as a filler material results in wax compositions having very desirable thermal properties which allow high pattern production rates without the shell cracking problems typically associated with organic acid fillers. Moreover, unlike other inert polymeric materials, it has been found that the polyethylene terephthalate wax compositions readily flow from the mold during the wax removal steps which consequently results in little or no appreciable ash residue in the mold upon firing. As such, the use of wax compositions using polyethylene terephthalate as a filler material is especially useful in investment casting of parts that require a high degree of dimensional accuracy. Moreover, the higher specific gravity of polyethylene terephthalate allows the filler to be easily separated from the base wax to allow the base wax to be reclaimed and reused.
In general, the wax composition comprises:
a. polyethylene terephthalate in an amount ranging from about 5 to about 50 parts by weight based upon the total weight of the composition; and
b. wax in an amount ranging from about 50 to about 95 pa
Brunsman David
The Kindt-Collins Company
Watts, Hoffmann, Fisher & Heinke Co. LPA
LandOfFree
Filler material and wax composition for use in investment... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Filler material and wax composition for use in investment..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Filler material and wax composition for use in investment... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2989610