Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – To produce composite – plural part or multilayered article
Reexamination Certificate
1999-04-27
2001-10-02
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
To produce composite, plural part or multilayered article
C264S258000, C264S277000, C264S324000, C494S016000, C494S081000
Reexamination Certificate
active
06296798
ABSTRACT:
This invention relates to centrifuge rotors. More specifically, this invention relates to so-called fixed angle compression molded carbon fiber centrifuge rotors with scalloped bottoms. This construction enables lighter construction, easier balancing, accumulation of lesser kinetic energy during centrifuging, and an improved exterior appearance.
BACKGROUND OF THE INVENTION
Carbon fiber centrifuge rotors are known. Accordingly to my earlier U.S. Pat. No. 5,643,168 entitled Compression Molded Composite Material Fixed Angle Rotor issued Jul. 1, 1997 to Piramoon et al, mixed carbon fiber (about 60%) and resin (about 40%) are compression molded to form a net shaped central rotor body with integrally formed sample tube apertures. Typically, this body is frustum shaped between an apex end and a base end. The sample tube apertures are symmetrically arranged about the spin axis of the rotor body and open to the apex end of the rotor. The sample tube apertures extend angularly outward to their closed bottom ends, these ends being near the base end of the rotor and more distant from the spin axis of the rotor than the openings of the sample tube apertures at the apex end. The generally frustum shaped rotor body is provided with circular peripheral steps, these steps being configured to receiving windings. The rotor is wound with continuous carbon fiber at the steps and finished by machining and painting.
It is highly desirable to reduce the weight of a centrifuge rotor to the maximum extent. This is a primary reason why carbon fiber rotors are utilized; they are 40% lighter than their metal equivalents.
Scalloped rotors are known. Until this disclosure, these rotors have all been constructed from carefully machined metal billets. In the case of metal centrifuge rotors, producing differing exterior contours only requires machining. Scalloped bottom metal rotors are known to have problems. Specifically, their lightened metal construction must be constantly inspected for failure. Further, such rotors are now manufactured with carefully machined and dimensioned thin sections which “fuse”, the rotor. When breakage is observed at the fused section, the rotor is taken out of service.
Machining of carbon fiber interferes with its structural integrity. For this reason, scalloped rotor constructions have not been utilized with carbon fiber. Instead, carbon fiber rotors have thus far been compression molded without scallops.
DISCOVERY OF A PROBLEM
I have extensively produced compression molded carbon fiber rotors. An example of the produced carbon fiber rotor is set forth in now U.S. Pat. No. 5,643,168 entitled Compression Molded Composite Material Fixed Angle Rotor issued Jul. 1, 1997 to Piramoon et al. I have discovered that compression molding of the rotors there described can cause cracking—especially as the rotors are molded in larger sizes.
This cracking appears at the bottom of the rotor after it is utilized. The cracking does not cause catastrophic failure of the rotor—as it would in a metal rotor—in that the circumferential windings of the rotor maintain the structural integrity of the rotor.
Unfortunately, as of this date, most rotors used are still made from metal. Users are trained to instantly reject rotors with any kind of surface cracking—even carbon fiber rotors. Because of this training on the part of extant users, it is important to eliminate any appearance of cracking on rotors.
Having discovered this occasional problem, I have tried to identify the causes of this occasional cracking. I therefore list what may be some of the reasons for these occasional occurrences.
First, in compression molding of regular shaped carbon fiber rotors, some areas in the bottom half of the rotor have relative massive amounts of compression molded carbon fiber material. Other areas—especially those areas adjacent the sample tube apertures—have much lesser thicknesses of molded carbon fiber material. I believe that the migration of the fibers under the considerable forces of compression molding is non-uniform. For example, it is not uncommon to use hydraulic presses generating as much as 125tons of pressure over a mold to produce the compressed carbon fiber rotor bodies. As the materials compression molded constitute about 60% fiber and 40% resin, this non-uniformity leads to both non-uniform curing and non-uniform density of the finished product.
The reader will understand that the discovery of a problem, as well as the solution to the problem, can constitute invention. This being the case, I claim invention both in the discovery of the problem—as well as its solution.
SUMMARY OF THE INVENTION
A compression molded fixed angle carbon fiber centrifuge rotor and method for forming the centrifuge rotor are disclosed. My prior art technique of compression molding is utilized in which a frustum fixed angle rotor body is formed between mold parts. As before, sample tube aperture cores are clustered from the apex end of the mold in that array which duplicates the intended number, size, and angularity of the sample tubes desired in the finished rotor. The bottom boundary of the rotor mold has no longer has a substantially planar boundary; instead the bottom of the rotor mold is supplied with a series of scallops, these scallops being equal in number to the sample tube aperture tubes. The shape at the bottom boundary of the rotor is maintained to give a substantially uniform thickness between the sample tube apertures and the exterior bottom surface of the rotor. Sufficient thickness of material is maintained to resist natural load of sample within the sample tube apertures. This has been found to produce a more uniformly molded part, prevent the referred to stress cracking, further reduce the weight of the rotors compared with their metal counterparts, and reduce the totality of material which is molded. Necessary balancing is easily accomplished in the vicinity of the scallops. The scalloped rotor bottom is provided with a compression molded carbon fiber cover to prevent windage—this cover not requiring painting. As a result, a superior carbon fiber fixed angle rotor body is compression molded.
REFERENCES:
patent: 4820257 (1989-04-01), Ishimaru
patent: 4824429 (1989-04-01), Keunen et al.
patent: 5362301 (1994-11-01), Malekmadani et al.
patent: 5411465 (1995-05-01), Glen et al.
patent: 5643168 (1997-07-01), Piramoon et al.
patent: 5776400 (1998-07-01), Piramoon et al.
patent: 5833908 (1998-11-01), Piramoon et al.
patent: 5942068 (1999-08-01), Adams et al.
patent: 6033612 (2000-03-01), Adams et al.
patent: 6056910 (2000-05-01), Fritsch et al.
Piramoon Technologies, Inc.
Silbaugh Jan H.
Staicovici Stefan
Townsend and Townsend / and Crew LLP
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