EP0842030A1

EP0842030A1 - Compression molded composite material fixed angle rotor

Info

Publication number: EP0842030A1
Application number: EP96915412A
Authority: EP
Inventors: Alireza Piramoon; Robert Wedemeyer; Michel Mark Fournier; Guillaume Crete; Don Fritche
Original assignee: Piramoon Technologies Inc
Current assignee: Piramoon Technologies Inc
Priority date: 1995-05-01
Filing date: 1996-04-30
Publication date: 1998-05-20
Also published as: JPH11504873A; AU5719396A; WO1996035156A1; EP0842030A4

Abstract

A method and apparatus for compression molding of fiber fixed angle rotors. Female mold (F) defines cylindrical bore (14) for molding the bottom of the rotor. Bore (14) defining frustum shaped central cavity (C) complementary to and concentric with the spin axis of the rotor. Male mold (M) contains frustum shaped inner cavity (C) with the apex of frustrum (18) disposed to the inner portion of bore (14) and the base of frustum (18) exposed to the cylindrical opening of mold (F). Inner cavity (C) defines the exterior shape of the rotor and defines between the exterior profile and the inner cavity (C) a rotor body wall. At the apex of inner cavity (C) there is a lock system for maintaining cores (K) in alignment with sample tubes of the rotor. Loading with resin pre-impregnated fiber occurs in inner cavity (C) and at the bottom of mold (F).

Description

Compression Molded Composite Material Fixed Angle Rotor

This invention relates to composite material centrifuge rotors of the so-called "fixed angle" variety. More particularly, a method and apparatus for the compression molding of a fixed angle rotor is disclosed. This invention also relates to composite fiber centrifuge rotors. More particularly, a composite fiber centrifuge rotor fabricated from discontinuous fibers by compression molding is provided with sample tube aperture inserts. In this rotor, sample tube aperture inserts are placed over sample tube aperture cores during compression molding of the rotor body. This enables rotor body to be produced with a so-called "net shape" requiring a minimum of machining to finish the centrifuge rotor.

BACKGROUND OF THE INVENTION

Fixed angle centrifuge rotors are known. In such rotors, sample tube apertures of the rotor are disposed at a

"fixed angle" in the normal range of 20° to 34°. Material to be centrifugated is placed in sample tubes within the sample tube apertures in the rotor body and spun at high speed.

Classification of the material within the sample tubes occurs.

At the end of such centrifugation, the classified sample is withdrawn and further processed.

It is known to make fixed angle rotors from composite materials. Further, it has been suggested to make such fixed angle rotors with chopped or discontinuous fibers.

Unfortunately, fiber alignment such chopped or discontinuous has not possible.

It is known that composite materials have anisotropic strength of material properties. Specifically, such materials have great resistance to tension, but are generally poor in resistance to all other modes of loading.

In order to take maximum advantage of the tensile strength of such fibers, fiber alignment to a disposition where stresses of centrifugation can be resisted is required. This usually — but not always — requires that the fibers be aligned either normal to the spin axis or radially about the spin axis.

Compression molding of composite fiber parts is known. To date, such compression molding has not be applied for the manufacture of centrifuge rotors.

In the molding and testing of such centrifuge rotors, we have discovered that there can be a weakness where fibers are compression molded for forming a rotor body. Specifically, the sample tube aperture contains the sample tube with the sample being centrif gated. This sample tube when fully loaded tries to move downward within the sample tube aperture. Such downward movement places increased strain on the bottom of the rotor. Similar to the case of the composite rotor constructed of laminates, this strain can either de-laminate the rotor or actually cause rotor failure. Additionally, we have tried to compression mold rotors to a "net shape." This is a shape where major machining is not required on the rotor surface. Unfortunately, we have also found that withdrawing sample tube aperture cores from such net shape molds requires extraordinary force; the sample tube aperture cores for forming the sample tube apertures are not easily withdrawn.

An attempt has been made to remedy this condition by compression molding the rotors without the sample tube aperture cores being present. This has proved unsatisfactory for several reasons. First, it is most desirable to compression mold rotors utilizing so-called sheet molding compound. Sheet molding compound in the vicinity of the sample tube apertures is usually placed normal to the spin axis of the rotor. Where a rotor is molded with the sample tube aperture cores not utilized to form the sample tube apertures, the sheet molding compound remains largely undisturbed; the sheet molding compound naturally disposes the fibers normal to the spin axis of the rotor. Unfortunately, when the discontinuous fibers utilized in compression molding are largely undisturbed, and remain normal to the spin axis of the rotor, they can easily de-laminate. We have in fact observed delamination under centrifugation when the sample tube apertures of a fixed angle rotor having machined sample tubes are loaded and centrifugated with samples.

This invention is in response to these observed problems.

The reader will understand that invention can be claimed in understanding the problem to be solved. We are unaware of the prior either disclosing or suggesting the problems that we have encountered. Accordingly, we claim invention related to the discovery of the above problems as well as the required solution.

PRIOR ART It is known to have sample tube aperture inserts of materials other than composite materials. In this regard, attention is directed to Keunen et al. U.S. Patent 4,824,429 issued April 25, 1989 where stainless steel inserts are placed within a molded plastic rotor having pre-wound and cured fiber rings placed over the rotor for reinforcement.

Other than show the presences of these inserts, this reference makes no mention of the purpose of the inserts. Further, no mention is made of the necessity to reinforce the bottom of the rotor during centrifugation to avoid failure of the rotor body at the bottom surface. Additionally, and because of the high forces encountered, stainless steel does not have sufficient tensile strength to resist the forces involved in centrifugation of rotors of this kind. Further its weight is not acceptable. Finally, there is no teaching of suggestion of both the problem encountered herein nor the solution to that problem. SUMMARY OF THE INVENTION A method and apparatus for the compression molding of composite fiber fixed angle rotors is disclosed. A female mold member defines a closed cylinder cavity for molding the bottom surface of the rotor, this cavity usually defining a frustum shaped central cavity complimentary to and concentric with the spin axis of the ultimately formed rotor. A male mold member having a complimentary cylindrical profile contains a frustum shaped inner cavity with the apex of the frustum disposed to the inner portion of the cylinder and the base end of the frustum exposed to the cylindrical opening of the female mold. This frustum shaped inner cavity defines the exterior frustum shape of the ultimately produced rotor and defines between the exterior frustum profile and the frustum shaped inner cavity a rotor body wall having sufficient thickness to receive the sample tube apertures. At the apex end of the frustum shaped cavity in the male mold member, there is located a locking system for maintaining sample tube aperture cores. These sample tube aperture cores are locked within the frustum cavity in the precise alignment of the ultimately formed sample tubes of the rotor. Loading of the mold with resin pre-impregnated fiber typically occurs in the frustum shaped cavity of the male mold member and at the bottom of the female mold member. Sheet molding compound — flat strips of resin impregnated discontinuous fibers — are pre-cut and placed within the mold with the plane of the material normal to the spin axis of the ultimately produced rotor. Reinforcement either with composite cloth, tape, or pre-wound and cured fibers can likewise be loaded with fiber alignment anticipating the strength characteristics of the ultimately produced rotor. With pre-heating, ramped heating to curing temperatures accompanied by ramped compression of the male and female mold sections one towards another, a rotor is rapidly formed in about one hour. Upon rotor formation, the sample tube aperture cores are released from the male mold section, the male and female mold sections parted, and the molded rotor withdrawn. Thereafter, the sample tube aperture core members are individually withdrawn, leaving the net shape compression molded rotor.

It will be understood that compression molding imparts to the ultimately produced the ability to maintain a high fiber to resin ratio in the ultimately produced rotor. Rotors having high fiber content capable of withstanding the forces of centrifugation are produced.

It is further possible to load the mold with pre- cured fiber parts. In one embodiment, pre-wound fiber rings are added between the frustum shaped mold exterior and the locked sample tube aperture cores to both reinforce the ultimately produced rotor and to assist in supporting the sample tube aperture cores against the considerable forces encountered during compression molding. In the compression molding of the sheet molded composite discontinuous material, the discontinuous fibers are disposed normal to the spin axis of the rotor before the rotor is molded. As the rotor is molded, these fibers conform to the molding forces but maintain there general alignment normal to the spin axis of the rotor. Fibers flow around the sample tube aperture cores radially and from below the sample tube aperture cores. There results a centrifuge rotor having discontinuous fiber where the fibers are aligned in the finally produced rotor for optimum resistance to the forces of centrifugation.

In the compression molding of a fixed angle centrifuge rotor utilizing discontinuous fibers, sample tube apertures cores present in the net-shape mold are covered with pre-cured sample tube aperture inserts. The sample tube aperture inserts have a constant and unchanging inside diameter and are formed from resin impregnated composite fiber. This composite fiber extends from the closed bottom of the inserts, upwardly along the sides of the inserts to the open top of the inserts to provide tensile strength longitudinally of the sample tube aperture inserts. The sample tube aperture inserts have a regular and smooth inside for first accommodating the sample tube aperture cores when the rotor is compression molded and later the sample tubes themselves when the rotor is fully fabricated. These inserts have an irregular outside for forming an interference fit with the subsequently formed compression molded rotor body. Finally, the sample tube aperture inserts have a tapered thickness with the top and open portion of the inserts having a thick wall construction and the bottom and closed portion of the inserts having a thinner wall construction. In use, and when the compression mold is loaded, the sample tube aperture inserts are placed over the sample tube aperture cores. When the mold is fully loaded, and compression molding occurs, the inserts integrally form to the rotor body and the sample tube aperture cores are removed. During centrifugation, the sample tube aperture inserts distribute the load of the sample tube evenly along the length of the inserts through to the body of the rotor.

These sample tube aperture inserts strengthen the compression molded fixed angle rotor in at least six discrete ways. First, the sample tube aperture cores can easily be withdrawn or de-molded from the net shape fixed angle centrifuge rotor; strain on the net shaped molded rotor body from either machining of the sample tube apertures or withdrawal of the sample tube aperture cores under great force is avoided.

Second, the pre-cured sample tube aperture inserts are formed of composite material so as to have considerable strength longitudinally of the sample tube aperture. Thus, the inserts can distribute the loading of the sample tube with contained sample vertically over the length of the sample tube aperture. Third, the sample tube aperture inserts have an irregular exterior surface. During compression molding of the rotor, the inserts form an interference fit with the compression molded discontinuous fiber of the rotor body. Forth, the sample tube aperture inserts have a tapering section with relatively thick walls at the open, top, inside of the sample tube aperture and thinner walls at the closed, bottom, outside of the sample tube aperture. The sample tube aperture inserts in effect wedge themselves into the rotor, distributing their loading over the length of the inserts.

Fifth, the sample tube aperture inserts make possible net shape molding of the sample tube apertures. This net shape molding disturbs the horizontally disposed discontinuous fiber of the rotor during compression molding of the fixed angle centrifuge rotor. The fiber adjoining the sample tube aperture is given a vertical component, further strengthening the rotor. Sixth, and finally, the sample tube aperture inserts for a continuous surface adjacent the sample tubes.

Discontinuities of construction in the rotor body cannot propagate through the sample tube aperture inserts; therefore the sample tubes encounter an uninterrupted and continuous preformed fiber surface which is preformed in advance of the main rotor body.

An important advantage is present over prior art laminated rotors that are diagonally wound with fiber.

Specifically, in such rotors, it has not generally be possible to have the sample tubes at angles exceeding 19°. This has been because the side slope of the rotor body would not allow the diagonally wound fibers to permanently adhere to and reinforce such rotors.

In the present rotor, because of the presence of the sample tube aperture inserts, sample tube apertures can be inclined to the full 23.5°, making conventional rotor construction possible.

In this portion of the application, we relate a discovery which we have made in the process of molding such rotors as well as the product produced by the net molding process.

Fixed angle centrifuge rotors are well known. By convention, such fixed angle centrifuge rotors include plurality of sample tube apertures having inserted inclined sample tubes. Typically, the open top of the sample tube is adjacent the spin axis of the rotor; the closed bottom of the sample tube is remote from the spin axis of the rotor and extends towards the rotor bottom. The sample tube is placed within a plane including the spin axis of the rotor and inclined at 23.5° with respect to the spin axis of the rotor.

During centrifugation, heavy particles within the sample migrate under enhanced gravity fields to the bottom and outside of the sample tubes; light particles remain at the top inside of the sample tubes.

So called fixed angle rotors of composite material have usually been fabricated from layers of composite cloth, the cloth layers being normal to the spin axis of the rotor. In order to prevent delamination of such rotors, it has been necessary to provide the rotors with a spiral wind of exterior composite fiber. Because of the necessity of maintaining such fiber on the exterior of the rotor, sample tube inclination is not standard. Specifically, sample tube inclination in such rotors is at about 19°.

In our Pira oon et al US Patent Application Serial No. 08/431,544 filed May 1, 1995 entitled Compression Molded Centrifuge Rotor and Method Therefore, we have disclosed the production of a compression molded centrifuge rotor body. This compression molded centrifuge rotor body makes possible inclination of the sample tube apertures at the conventional 23.5° from the spin axis of the rotor body. Further, it allows the sample tube apertures to be created as part of the net shape molding process; the sample tube apertures no longer have to be independently machined.

In the molding and testing of such centrifuge rotors, we have discovered that there can be a weakness where fibers are compression molded for forming a rotor body. Specifically, the sample tube aperture contains the sample tube with the sample being centrifugated. This sample tube when fully loaded tries to move downward within the sample tube aperture. Such downward movement places increased strain on the bottom of the rotor. Similar to the case of the composite rotor constructed of laminates, this strain can either de-laminate the rotor and cause rotor failure.

An attempt has been made to remedy this condition by compression molding the rotors without the sample tube aperture cores being present. This has proved unsatisfactory for several reasons.

First, it is most desirable to compression mold rotors utilizing so-called sheet molding compound. Sheet molding compound in the vicinity of the sample tube apertures is usually placed normal to the spin axis of the rotor. Where a rotor is molded with the sample tube aperture cores not utilized to form the sample tube apertures, the sheet molding compound remains largely undisturbed; the sheet molding . compound naturally disposes the fibers normal to the spin axis of the rotor. Unfortunately, when the discontinuous fibers utilized in compression molding are largely undisturbed, and remain normal to the spin axis of the rotor, they can easily de-laminate. We have in fact observed delamination under centrifugation when the sample tube apertures of a fixed angle rotor having machined sample tubes are loaded and centrifugated with samples.

Second, machining places strains on the rotor body; it is desirable to construct and vend a rotor body which has not be subjected to machining — especially in the interior of the rotor body.

Third, machining creates a cost factor. Rotors where the sample tube apertures are individually machined are very much more expensive than rotors that can be molded to net shape — including the sample tube apertures.

Fourth, we have discovered that machining leaves imbedded stress cracks — especially in the areas of the sample tube apertures. At a minimum, these imbedded cracks are unsightly and unsettling to the owner of the rotor. Further, such cracks can propagate to produce rotor destruction.

Because of at least these deficiencies, we have undertaken to "net shape" our rotors. In the process of this net shaping, we have discovered a problem related to the release of the sample tube aperture cores which form the sample tube apertures during the net shape molding.

To understand this problem, the mold must first be described with respect to Fig. 1. Thereafter, the so-called yoke of the mold in holding the sample tube aperture cores will be discussed. Areas of "undercut" will be shown. Finally, it will be seen how undercut prevents convenient withdrawal of the sample tube aperture cores and damages the net shape molded rotor body. Thereafter, the solutions — two in number — will be set forth.

Referring to Fig. 16, closed mold M is illustrated having upper section 214 and lower section 215. Upper section 214 includes upper base 217, ram sleeve 218, and ram 219. Supported on upper base 217 is mold insert I having rotor bottom forming surface 230 and rotor bottom step forming surfaces 232.

Lower section 215 includes ejector bar 220 and lower base 222. Lower base 222 defines interiorly thereof frustum shaped cavity 223 having steps 224 defined on the surface thereof. When a rotor body is formed interiorly of open mold M, these steps 224 form exterior and complimentary steps on the surface of the rotor body which when machined allow windings to reinforce the rotor. Ejector bar 220 supports ejector plate 226 which in turn supports yoke Y. Some special attention can be given to the construction of yoke Y.

Formed rotor body B is shown interior of lower base 222. Presuming that rotor body B has been molded interior of lower base 222, release from frustum shaped cavity 223 interior of the lower base must occur. To this end, sample tube aperture cores A must be held interior of frustum shaped cavity 223. The construction of the sample tube aperture cores A can be best understood with reference to Fig. 18. Referring to Fig. 18, sample tube aperture cores A have frustum shaped portion 234 and cylindrical shaped portion 236 with rounded bottom 238. Cylindrical shaped portion 236 forms sample tube aperture P.

Returning to Fig. 16, frustum shaped portion 234 insures release of sample tube aperture cores A from mold insert I. It will be observed that frustum shaped portion 234 of sample tube aperture cores A fits within female frustum aperture 242. Both frustum shaped portion 234 and female frustum aperture 242 have a slope adjacent mold axis 240 that permits upward withdrawal of rotor body B with sample tube aperture cores A within the rotor body. Once rotor body B is clear of mold M, sample tube aperture cores A may then be withdrawn.

Upon the original construction of mold M, removal of sample tube aperture cores A was intended. Unfortunately, the required shape of yoke Y with respect to sample tube aperture cores A at frustum shaped portion 234 was not understood. It was at this juncture, that the magnitude of the problem relating to "undercut" occurs.

Referring to Figs. 17 and 18, sample tube aperture cores A can be observed where they join to yoke Y. Specifically, upper surface 245 of yoke Y joins to frustum shaped portion 234 of sample tube aperture cores A. It will be observed that undercut 248 exists adjacent frustum shaped portion 234 of the sample tube aperture core A.

The affect of this undercut 248 can be understood with specific reference to Fig. 18. Specifically, and when the rotor body is molded, compressed fibers will form about frustum shaped portion 234 of sample tube aperture cores A. The sample tube aperture cores A will be trapped within molded rotor body B.

Removal of sample tube aperture cores A will only aggravate the problem. Specifically, forcible removal will cause sections of rotor body B to break away. This breaking away can weaken the rotor body, especially at web 250 between sample tube apertures P.

It is also desirable for cosmetic purposes that finished rotor body B have the top of sample tube apertures P at the same elevation. Further, for the purposes of required biocontainment, having the sample tube aperture terminate an a uniform elevation from the bottom of the sample tube allows biocontainment structures to be conveniently attached between the top of the sample.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates exactly one half of the male mold member and the female mold member showing the male mold member overlying the female mold member with the sample tube aperture cores attached within the frustum shaped cavity of the male mold member;

Fig. 2 illustrates the male and female mold members of Fig. 1 filed with resin impregnated composite material and placed under compression to form a net shaped composite rotor body;

Fig. 3A illustrates the male and female mold members of Fig. 2 in exploded relationship one apart from the other showing both mold members loaded with composite materials here illustrating the male mold member assembled and the female mold member having sheet molding compound placed within the bottom portion of the open cavity;

Fig. 3B illustrates the sample tube aperture cores attached to one another in the molding relationship with the frustum shaped periphery being warped with material selected from the group consisting of unidirectional tape, woven composite fabric, or sheet molded compound;

Fig. 4A illustrates the male and female mold members of Fig. 3A loaded with composite materials here illustrating the male mold member assembled and having a fabric winding at the bottom of the sample tube aperture cores;

Fig. 4B illustrates the sample tube aperture cores attached to one another similar to Fig. 3B with the frustum shaped periphery and the lower portion of the sample tube aperture cores being wound with material selected from the group consisting of unidirectional tape, woven composite fabric, or sheet molded compound;

Fig. 5 is a view similar to Fig. 3A illustrating the loading of the female mold member with laminates of materials chosen from the groups consisting of sheet molded compound, resin impregnated tape and resin impregnated woven cloth;

Fig. 6A is a view similar to Fig. 3A of the male and female mold members here illustrating the sample tube aperture cores with tubularly wound braided composite material for molding the braided composite material integrally with the rotor body;

Fig. 6B illustrate a single sample tube aperture core having the tubularly wound braided composite material wrapped about the sample tube aperture core before installation to the core cluster illustrated in Fig. 6A;

Fig. 7 is a section of a compression molded rotor having a ring configuration suitable for use with that centrifuge shown and disclosed in Centrifuge Construction Having Central Stator Attorney Docket No. 16532-5 — Serial

No. 08/288,387 filed August 10, 1994 now U.S. Patent _, , issued ;

Fig. 8 is a side elevation section of a rotor being compression molded in a mold contained within a compression molding press with the sample tube aperture cores being shown covered with a sample tube aperture insert;

Fig. 9 is an expanded side elevation section of the mold illustrated in Fig. 1 so that the individual parts and sequential function of the mold can be understood; Fig. 10 is a side elevation section of a sample tube aperture insert separate and apart from a rotor body;

Fig. 11 is a side elevation section of a sample tube aperture insert integrally molded to a sample tube aperture within a rotor body illustrating the interference fit, the wedging of the insert to the rotor body, the disturbance of fibers adjacent the sample tube aperture from the horizontal to the vertical so that vertical loading placed on the insert can be vertically distributed over the length of the rotor; Figs. 12A and 12B, are respective exploded and assembled and cured perspective sections of sample tube aperture inserts for insertion to compression molded centrifuge rotors; Figs. 13 is a partially assembled view of a sample tube aperture insert;

Figs. 14A and 14B illustrate a sample tube aperture insert being wound on a mandrel; Figs. 15A and 15B are yet another embodiment of sample tube aperture inserts being wound on a mandrel;

Fig. 16 is a side elevation section of the mold of this invention shown in the closed position for the net shape molding of compressed composite fiber material into a fixed angle centrifuge rotor;

Fig. 17 is a perspective view of the yoke of the mold of Fig. 16 with the sample tube aperture cores assembled to the yoke and illustrating the exposed undercut between the cylindrical portion of the sample tube aperture core and the male conical portion of the sample tube aperture core which is mounted to a complimentary female conical portion within the mold yoke;

Fig. 18 is a side elevation section of a rotor with the sample tube aperture core shown in broken lines illustrating how a rotor molded with undercut as illustrated in Fig. 17 has the sample tube aperture cores trapped within the molded rotor body making removal only possible with some damage to the molded rotor body;

Fig. 19 is a side elevation illustrating the desired condition for extraction of the sample tube aperture cores;

Fig. 20 illustrates the first of two solutions in the form of discrete facets placed adjacent the sample tube aperture cores at the juncture between the cylindrical portion and conical portion of the sample tube aperture cores; Fig. 21 illustrates the second and preferred solution in the form of a continuous spherical surface intersecting each of the sample tube aperture cores precisely at the plane defined by the base of the conical portion of the sample tube aperture cores and the cylindrical portion of the sample tube aperture cores; and.

Fig. 22 is a perspective view of the fixed angle rotor illustrated in Fig. 21 showing the generation of the continuous spherical surface about the sample tube apertures cores.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, male mold member M is shown overlying female mold member F. Neither mold member is charged with material to be compression molded. The configuration of the respective mold members will be set forth first; the operation of the respective mold members will be thereafter discussed.

Taking female mold member F, which includes mold member or forging 12 having cylindrical bore 14 for fitting to cylindrical contour 16 of male mold member M. Sufficient clearance is provided between cylindrical bore 14 and cylindrical contour 16 so that resin only and not significant amounts of fiber can escape from the joined, compressed, heated and vibrated male mold member M and female mold member F during compression molding of a rotor body.

Female mold member F must define the lower contour of the rotor body ultimately formed. Consequently, it includes male frustum protrusion 18 having apex circular surface 20 with base 22 integral with the female cavity of the mold. Female mold member F is completed with ring surface 24, cambered surface 26, and step surface 28. As is conventional, gathering surface 30 is provided at the top of cylindrical bore 14 of female mold member F.

It will be understood that during compression molding, heating, application of a vacuum, and vibration are utilized. Accordingly, vibrator V, heater H, and vacuum pump U are all schematically shown. As such members are conventional, they will not be further illustrated or discussed herein.

Having set forth female mold member F, male mold member M will now be discussed. Male mold member M includes frustum shaped central cavity C and sample tube aperture core cluster K.

Frustum shaped central cavity C is relatively easy to understand. It includes a plurality of machined internal female steps S following the frustum profile of frustum shaped central cavity C. These internal female steps S will be shown later to leave corresponding male steps T in the ultimately formed rotor body B. Thus, the process of compression molding here disclosed will be understood to result in the so-called "net shape" or finished state of rotor body B.

One factor related to the difference between compression molding as illustrated herein and injection molding should be emphasized. We have found that it is required that centrifuge rotors have high fiber content to withstand the considerable forces of centrifugation. This being the case, a high fiber content — in the order of 50% of the weight percent of the resin fiber mixture is require. Such a high fiber content material is absolutely unsuitable for injection molding; injectors cannot conveniently handle or inject a resin/fiber mixture with such a high fiber content.

Further, we do not here rely on so-called resin transfer molding. That is to say, we do not charge the mold first with totally unimpregnated fiber and thereafter inject resin without supplying the considerable compression forces here illustrated. Such molding would have the possibility of leaving voids in rotor body product which would ultimately render the final product not suitable for centrifugation. It will be understood that we show female mold member F underneath male mold member M. This can be reversed. Further, a vertical relative disposition between the respective portions of the mold is not required. For example, the mold members could move horizontally towards and away from one another — although this is not preferred. Having set forth the mechanics of the mold, the loading of the compression mold with material for compression molding can now be discussed in detail.

Referring to Figs. 3A and 3B, a first loading of male mold member M and female mold member F can be understood. In this embodiment, resin impregnated composite fiber precut discs 60 cover the bottom of cylindrical bore 14 in female mold member F. As can be seen, these respective resin impregnated composite fiber precut discs.60 extend over male frustum protrusion 18 to step surface 28 at the bottom of cylindrical bore 14 of female mold member F. It is preferred that these respective resin impregnated composite fiber precut discs 60 consist of preferably of sheet molding compound. They can be chosen from the group including sheet molded compound, pre-impregnated composite fiber tape, or pre- impregnated composite fiber fabric-

Overlying resin impregnated composite fiber precut discs 60 there are placed central fiber layers 62. Central fiber layers 62 are preferably formed from sheet molded compound 65. This alternating construction can be found in SMC produced by Quantum Composite of Midland, Michigan.

Referring back to Fig. 3A, and central fiber layers 62, it will be understood that these respective layers are preferably made of sheet molded compound 65. It has been found that during molding, the respective discontinuous fibers 69 of central fiber layers 62 maintain there respective major horizontal disposition normal to rotor spin axis 70 of the ultimately formed rotor body B. Further, upon curing in the compression molding process here disclosed, the respective layering is no longer visible. Instead, the respective discontinuous fibers 69 have major alignment normal to rotor spin axis 70 but form in the net shape rotor body B without any apparent layering being present. It should be further understood that when sheet molded compound 65 is molded, some vertical orientation of discontinuous fibers 69 occurs. This vertical orientation imparts to rotor body B resistance to vertical forces placed on the rotor during centrifugation. For example, sample tubes within sample tube apertures A can exert a considerable force on the respective bottoms of the sample tube apertures A. Where the rotor is made of composite fiber layers normal to rotor spin axis 70, such composite fiber layers have been known to delaminate under such centrifugation generated forces. It has been found that sheet molded compound 65 and the minor vertical orientation of discontinuous fibers 69 advantageously resists such forces. Finally, it should be understood that during compression molding, central fiber layers 62 when made of sheet molded compound 65 have the advantage of readily deforming and conforming intimately about the shape of female mold member F and particularly the more intricate three dimensional configuration of frustum shaped central cavity C with central, sample tube aperture core cluster K. It is for this reason that in the embodiment illustrated in Figs. 3A and 3B, it is preferred to have central fiber layers 62 made from sheet molded compound 65.

It will be understood that dependent upon the overall strength of the finally manufactured rotor body B, other materials may be added interiorly of the mold. For example micro-balloons (glass, phosphor, or carbon) can be added. Additionally, and dependent upon the stress location in rotor body B, materials such as ordinary fiber glass may be used.

Referring to Fig. 3B, wrapping of sample tube aperture core cluster K with resin impregnated fiber layer 64 is illustrated. Such wrapping here consists of resin impregnated woven fabric. It will be understood that other materials could be used including woven composite fabric not impregnated with resin, composite tape (optionally resin impregnated) , or sheet molded compound. Once the particular female mold member F and male mold member M are respectively loaded, compression molding can occur.

The remaining portions of the description herein will assume that compression molding occurs. Those have skill with composite fibers and resins will realize that the temperatures, pressures and duration required in curing will vary with the resin system mixture involved. While this requires considerable testing when new formulations are utilized with particular molds, persons having skill in the curing of composite fibers impregnated with resins can readily determine such parameters.

Referring to Fig. 4A and 4B, sample tube aperture core cluster K is wrapped at each sample tube aperture core R with portions of depending composite material wrap 72. Depending composite material wrap 72 is slit at intervals between sample tube aperture cores R and has the slit portion extending below sample tube aperture core cluster K wrapped about the lower portion of each sample tube aperture core R. It will be appreciated that this configuration when molded about sample tube aperture cores R produces sample tube apertures A having composite fiber reinforcing the bottom of the apertures A. It will be understood that such sample tube apertures A have high resistance to the force of sample tubes bearing vertically downward at the bottom of the respective sample tube apertures.

Fig. 5 illustrates a loading of female mold member F with sheet molding compound rings 75 and sheet molding compound discs 77. Unlike the example previously given in

Fig. 3A, reliance is placed upon sheet molding compound discs 77 to conform around sample tube aperture cores R when in the fluid state under compression molding. This phenomena can be readily understood. Specifically, as sheet molding compound rings 75 and sheet molding compound discs 77 are heated, compressed and vibrated, the laminate structure of the cut material is lost. The respective fibers within rings 75 and discs 77 conforms around sample tube aperture cores R as held in sample tube aperture core cluster K. Unfortunately, this will interfere with some of the normal alignment of the fibers with respect to rotor spin axis 70. It does have the advantage of causing many fibers to conform to the surface of sample tube aperture cores R and thus form sample tube apertures A having fibers disposed in the plane of the surface of the sample tube apertures.

Individual reinforcement of sample tube apertures A is possible alone or in combination with the other techniques mentioned herein. Referring to Figs. 6A and 6B, the respective sample tube aperture cores R are shown wrapped in composite fiber cloth sock 80. Composite fiber cloth sock 80 can be either pre-impregnated or alternate "dry", in which case reliance on acquiring resin from adjacent pre-impregnated fiber is required. It will additionally be appreciated that respective composite fiber cloth socks 80 can be either fully or partially cured before placement on their respective sample tube aperture cores R. Compression molding is known.

Observation of the compression molded part is helpful. It is common in metallic centrifuge rotors to forge metal blanks or "forgings" for such rotors. When such forging occurs, and the metal resulting from such forging is microscopically examined, especially as to the granular structure, the metallic grains can be treated so as to be optimally aligned to resist the forces of centrifugation. When compression molding occurs, there is little evidence of the fibers being sheared. Second, the fibers present a checkered almost "marbleized" appearance when seen with the eye. Finally, the fibers align themselves parallel to the surface which they encounter at the boundary of a mold.

It is to be emphasized that for the first time, a compression molded rotor body B is produced. No longer is it required that sample tube apertures A be machined.

Specifically, they can now be molded. And more importantly, they can be molded to shapes that are other than cylindrical.

In co-pending Centrifuge Construction Having Central Stator, Serial No. 08/288,387 filed August 10, 1994, now U.S. Patent _, , issued , inventor Piramoon has disclosed the construction of a new centrifuge. Specifically, this centrifuge contains a central stator which produces a rotating magnetic field. The peripheral rotor couples to this rotating magnetic field. It will be understood that to accommodate the central stator, some section of the ultimately produced rotor body B_χ has to be ring shaped. Such a ring shaped rotor body B is illustrated in Fig. 7.

Referring to Fig. 7, rotor body B_λ includes a central stator aperture 150, and maximum capacity shaped sample tube apertures 152. Some comment is in order.

First, the molding apparatus here illustrated can be modified to make any shape of rotor body B. We prefer the fixed angle embodiment of rotor body B as of this time. It will be understood that with the introduction of additional centrifuges, other rotor bodies may be required such as rotor body B having central stator aperture 150.

Secondly, we now understand that rotor body B_χ first disclosed in Centrifuge Construction Having Central Stator Serial No. 08/288,387 filed August 10, 1994 now U.S. Patent

_, , issued , has several advantages over conventional spindle mounted rotors. First, it requires a larger diameter. This results in a lower speed of rotation. Further, a greater number of sample tube apertures A can be accommodated. For example the reader will observe eight sample tube apertures A in Fig. 7.

Secondly, it is especially advantageous to change of the shape of sample tube apertures A to maximize capacity of the sample tube apertures and any tubes which are subsequently placed within them. As such rotor body B is conventionally reinforced by resin fiber windings W, the maximum capacity shaped sample tube apertures 152 do not appreciable detract from the overall rotor resistance to the forces of centrifugation.

It will therefore be understood that the enclosed described compression molded rotor has wide applicability. Referring to Fig. 8, open mold M is illustrated having upper section 114 and lower section 115. Upper section 114 includes upper base 117, ram sleeve 118, and ram 119. Supported on upper base 117 is mold insert I having rotor bottom forming surface 130 and rotor bottom step forming surfaces 132.

Lower section 115 includes ejector bar 120 and lower base 122. Lower base 122 defines interiorly thereof frustum shaped cavity 123 having steps 124 defined on the surface thereof. When a rotor body is formed interiorly of open mold M, these steps 124 form exterior and complimentary steps on the surface of the rotor body which when machined allow windings to reinforce the rotor.

Ejector bar 120 supports ejector plate 126 which in turn supports yoke Y. Some special attention can be given to the construction of yoke Y. Formed rotor^*bodyB is shown interior of lower base 122. Presuming that rotor body B has been molded interior of lower base 122, release from frustum shaped cavity 123 interior of the lower base must occur. To this end, sample tube aperture cores A must be held interior of frustum shaped cavity 123. The construction of the sample tube aperture cores A can be best understood with reference to Fig. 10.

Referring to Fig. 10, sample tube aperture cores A have frustum shaped portion 134 and cylindrical shaped portion 136 with rounded bottom 138. Cylindrical shaped portion 136 forms sample tube aperture P. Frustum shaped portion 134 insures release of sample tube aperture cores A with the formed rotor body B. It will be observed that frustum shaped portion 134 has a slope adjacent mold axis 140 that permits upward withdrawal of rotor body B with mold insert I within the rotor body. Once rotor body B is clear of mold M, mold inserts I may then be withdrawn.

Referring to Fig. 9, a single sample tube aperture core A is illustrated covered with sample tube aperture insert S. Sample tube aperture insert S covers sample tube aperture cores A at cylindrical shaped portion 136 as illustrated in Fig. 10.

Before discussing the possible constructions of sample tube aperture cores A, operation of mold M in cooperation with sample tube aperture insert S should be briefly set forth. First, mold M is charged with discontinuous fiber material to be molded. Second, sample tube aperture insert S are placed over sample tube aperture cores A. Thereafter, molding occurs. Once molding occurs, mold M is opened and rotor body

B is withdrawn. Thereafter, sample tube aperture cores A are withdrawn from rotor body B. A molded rotor body results.

It should be understood that the presence of sample tube aperture insert S improves rotor body B in at least six ways.

First, the sample tube aperture cores can easily be withdrawn or de-molded from the net shape fixed angle centrifuge rotor; strain on the net shaped molded rotor body from either machining of the-sample tube apertures or withdrawal of the sample tube aperture cores under great force is avoided.

Second, the pre-cured sample tube aperture inserts are formed of composite material so as to have considerable strength longitudinally of the sample tube aperture. Thus, the inserts can distribute the loading of the sample tube with contained sample vertically over the length of the sample tube aperture. This loading is primarily on the bottom of sample tube aperture insert S. Fiber alignment interior of sample tube aperture insert S causes loading from the bottom to be redistributed to sidewalls 142 of sample tube aperture insert S.

Third, the sample tube aperture inserts have an irregular exterior surface. The irregular surface is shown in the form of circular annular protrusions 144. During compression molding of the rotor, the sidewalls 142 at form an interference fit with the compression molded discontinuous fiber of the rotor body. It should be understood that circular annular protrusions 144 are not required to be in the format illustrated in Fig. 10. In practice, it will be found that the fibrous exterior of sample tube aperture insert S is often sufficient by itself to appropriately key the inserts to rotor body B.

Forth, the sample tube aperture inserts have a tapering section with relatively thick walls at the open, top, inside of the sample tube aperture and thinner walls at the closed, bottom, outside of the sample tube aperture. This tapering section is indicated by angle 146 towards the top of sample tube aperture insert S. The sample tube aperture inserts in effect wedge themselves into the rotor, distributing their loading over the length of the inserts. Fifth, it will be understood that rotor body B is formed with flatly disposed fibers in what is commonly referred to as sheet molding compound. The presence of the sample tube aperture inserts insures net shape molding of the sample tube apertures. This net shape molding disturbs the horizontally -disposed discontinuous fiber of the sheet molding compound from which rotor body B is formed during compression molding. The fiber adjoining the sample tube aperture is given a vertical component, further strengthening the rotor. This further strengthening results from some of the discontinuous fibers being vertically disposed adjacent the sample tube aperture. Thus, the vertical fiber disposition of sample tube aperture insert S has fibers in rotor body B adjacent sample tube aperture insert S also vertically disposed.

Sixth, and finally, the sample tube aperture inserts for a continuous surface adjacent the sample tubes. Discontinuities of construction in the rotor body cannot propagate through the sample tube aperture inserts; therefore the sample tubes encounter an uninterrupted and continuous preformed fiber surface which is preformed in advance of the main rotor body.

Referring to Fig. 11, a cut away section of rotor body B is illustrated. Sample tube aperture insert S is shown imbedded in rotor body B. It will be seen that top 150 of sample tube aperture insert S is short of rotor body top 152. This is the preferred construction and imparts to rotor body top 152 a continuous construction which gives no exterior appearance of the presence of sample tube aperture insert S. Having set forth the general purpose and operation of sample tube aperture inserts S, differing possible constructions can be set forth.

Referring to Fig. 12A, an "iron cross" construction is illustrated. Central bottom portion 160 has flare portions 162 with generally narrower base 164 and wider distal section 166 remote from central bottom portion 160. As suggested by broken line insert portion 170, more than one of these respective iron cross fiber sections 159 is placed one on top of another. The sections are then molded about a cylindrical mold which for all practical purposes has the same dimension as cylindrical shaped portion 136 of sample tube aperture cores A. Conventional curing of sample tube aperture insert S occurs by placing plastic about iron cross fiber sections 159, drawing a vacuum on iron cross fiber sections 159 over the molding cores, and subjecting the impregnated fiber with sufficient heat to effect setting of its resin system. Sample tube aperture insert S_λ is formed. Referring to Fig. 13, an alternate construction of sample tube aperture insert S₂ is illustrated. Discrete tape layers 172 are laid over bottom 174.of sample tube aperture insert S₂. Circular wrapping layers 176, 178, and 180 are placed wrapping peripherally about sample tube aperture insert S₂. When these respective layers are conventionally compressed under a vacuum wrap and appropriately heated, sample tube aperture insert S₂ results having the preferred tapered wall construction and irregular surface.

Referring to Figs. 14A and 14B, mandrel 182 is shown diagonally wound with continuous fibers 184. Diagonal winding occurs so that circular bottom 185 of mandrel 182 is covered at successively decreasing diameters 186, 188, and 190. When these fibers are impregnated by resin and cured, sample tube aperture insert S₃ as shown in Fig. 14B results. It will be realized that continuous fibers 184 when wound at an angle have a vertical component. This vertical component distributes the vertical loading of a contained sample tube within the sample tube aperture vertically of rotor body B. Further, continuous fibers 184 lend a discontinuous surface to the exterior of sample tube aperture insert S₃ which enables keying of the insert to rotor body B.

Referring to Fig. 15A, two part mandrel 190 is illustrated. As in Figs. 14A and 14B, continuous fibers 184 are wound diagonally with the wind extending over two part mandrel 190 at the respective ends. The windings — impregnated with resin — are thereafter conventionally cured. Thereafter, cutting of the cured body at cut line 194 separating the two rotor sections occurs. Two sample tube aperture inserts S₄ and S₄' results. Referring to Fig. 19, the solution to the release from the mold problem is set forth. Sample tube aperture core A is shown immediately before extraction from rotor body B. It is required that rotor body B terminate precisely at the interface between cylindrical shaped portion 236 and frustum shaped portion 234. This enables free withdrawal of sample tube aperture cores A without entraining portions of rotor body B. There are two solutions to this problem. The less preferred solution is shown in Fig. 20. Sample tube aperture P is shown in Fig. 20 having central axis 244. At the top of sample tube aperture P there is placed facet F. Facet F is designed so that at all points about sample tube aperture P it defines a single plane. Further, this plane terminates at the base of frustum shaped portion 234 of sample tube aperture cores A.

This solution has two draw backs. First, facets F are difficult to machine in any mold. Second, between the discrete facets F, a discontinuity 246 exist where the facets F come together with adjacent sample tube apertures P. Further, and in some applications, web 248 between adjacent sample tube apertures P can be too thin.

Referring to Figs. 21 and 22, the preferred solution is indicated. Specifically, intersection 252 between central axis 244 of sample tube aperture P and spin axis 250 of rotor body B is located. Thereafter, concave spherical surface S is defined at radius R₂ and placed within rotor body B by machining a corresponding male spherical surface S_M. There results from this continuous and uninterrupted surface a preferred solution.

Over the solution of Fig. 20, the solution shown in Figs. 21 and 22 is easy to machine, and does not have a discontinuity on concave spherical surface S. Further, radius R₁ can be adjusted so that web 248 is given a optimum thickness.

It is to be noted that this problem is an unusual problem which we have noted only after actual construction of molds. Accordingly, we claim invention in the discovery of the problem to be solved — as well as the solution to that problem.

Claims

WHAT IS CLAIMED IS:

1. A method of compression molding a fixed angle centrifuge rotor body having a frustum shaped peripheral contour about a central spin axis between a base end and an apex end, the rotor body having angled sample tube apertures extending from openings in the apex end adjacent the spin ax-is of the rotor body to bottom portions of the sample tube apertures more remote from the spin axis of the rotor body, the compression molding comprising the steps of: providing a mold member; forming a frustum shaped cavity interior of the mold member with a periphery having the frustum shaped peripheral contour of the rotor body; providing sample tube aperture cores, each sample tube aperture core for forming a sample tube aperture; clustering the sample tube aperture cores to form a cluster of sample tube aperture cores to form openings for sample tube apertures in the apex end of the rotor body adjacent the spin axis to bottom portions of the sample tube apertures more remote from the spin axis of the rotor body; mounting the cluster interior of the frustum shaped cavity of the mold member; filling the mold member with resin impregnated composite material to be pressure molded; heating and compressing the resin impregnated composite material in the mold member with the resin impregnated composite material to be pressure molded for a sufficient interval to cure the resin impregnated composite material; detaching the sample tube aperture cores from the mold member; separating the rotor body from the mold member; and, separating the detached sample tube aperture cores from the rotor body.

2. A method of compression molding a fixed angle centrifuge rotor body according to claim 1 and wherein the filling step includes: placing resin impregnated composite material in the mold member with fibers aligned normal to the spin axis of the rotor.

3. A method of compression molding a fixed angle centrifuge rotor body according to claim 1 and wherein the filling step includes: placing resin impregnated composite material in the mold member chosen from the group consisting of sheet molding compound, resin impregnated fiber tape, and resin impregnated fiber cloth.

4. A method of compression molding a fixed angle centrifuge rotor body having a frustum shaped peripheral contour about a central spin axis between a base end and an apex end, the rotor body having angled sample tube apertures extending from openings in the apex end adjacent the spin axis of the rotor body to bottom portions of the sample tube apertures more remote from the spin axis of the rotor body, the compression molding comprising the steps of: providing a female mold member having a cylindrical shaped cavity of sufficient diameter to accommodate the base end of the frustum shaped peripheral contour of the rotor body; providing a male mold member having a cylindrical periphery for fitting to the cylindrical shaped cavity of the female mold member; forming a frustum shaped cavity interior of the male mold member with a periphery having the frustum shaped peripheral contour of the rotor body; providing sample tube aperture cores, each sample tube aperture core for forming a sample tube aperture; clustering the sample tube aperture cores to form a cluster of sample tube aperture cores to form openings for sample tube apertures in the apex end of the rotor body adjacent the spin axis to bottom portions of the sample tube apertures more remote from the spin axis of the rotor body; mounting the cluster interior of the frustum shaped cavity of the male mold member; filling the male and female mold members with resin impregnated composite material to be pressure molded; heating and compressing the resin impregnated composite material between the male mold member and the female mold member with the resin impregnated composite material to be pressure molded for a sufficient interval to cure the resin impregnated composite material; detaching the sample tube aperture cores from the male mold member; separating the male and female mold members to expose the rotor body; and, separating the detached sample tube aperture cores from the rotor body.

5. An apparatus for compression molding a fixed angle centrifuge rotor body having a frustum shaped peripheral contour about a central spin axis between a base end and an apex end, the rotor body having angled sample tube apertures extending from openings in the apex end adjacent the spin axis of the rotor body to bottom portions of the sample tube apertures more remote from the spin axis of the rotor body, the mold apparatus comprising: a mold member; a frustum shaped cavity interior of the mold member with a periphery having the frustum shaped peripheral contour of the rotor body; sample tube aperture cores, each sample tube aperture core for forming a sample tube aperture; means for clustering the sample tube aperture cores to form a cluster of sample tube aperture cores to form openings for sample tube apertures in the apex end of the rotor body adjacent the spin axis to bottom portions of the sample tube apertures more remote from the spin axis of the rotor body; and, means for mounting the cluster interior of the frustum shaped cavity of the mold member.

6. A fixed angle centrifuge rotor body comprising: a frustum shaped peripheral contour about a central spin axis between a base end and an apex end; angled sample tube apertures defined-in the rotor body extending from openings in the apex end adjacent the spin axis of the rotor body to bottom portions of the sample tube apertures more remote from the spin axis of the rotor body; the rotor body formed from resin cured about discontinuous composite fiber with the major axis of the discontinuous fiber disposed normal to the spin axis of the rotor body interior of the rotor body; the resin cured discontinuous composite fibers exceeding 40 weight percent of the cured rotor product and having no visible layering between respective layers of fiber; the resin cured discontinuous composite fibers having minor vertical excursion parallel to the spin axis of the rotor and relatively no kinking along their respective length; and, the resin cured discontinuous fibers disposed parallel to the surfaces of and about molded sample tube apertures in the rotor body.

7. In a fixed angle centrifuge rotor having, a resin bound discontinuous fiber rotor body symmetrical about a spin axis having top and a bottom, the fixed angle centrifuge rotor further defining sample tube apertures having tops opening to a top of the rotor body, the sample tube apertures having closed bottoms extending near the bottom of the rotor body, and the sample tube apertures inclined with respect to a spin axis with the opening of the rotor body toward the spin axis and the closed bottom of the sample tube apertures more remote from the spin axis, an apparatus for reinforcing the sample tube aperture comprising: a plurality of sample tube aperture inserts having sides with an inside diameter defining the inside diameter for each receiving a sample tube and the closed bottoms for holding a bottom of the received sample tube, the sample tube aperture inserts formed from composite fiber material; the sample tube aperture inserts having the composite fiber material aligned with individual fibers having tensile components for transferring load on the bottom of the sample tube aperture inserts to the sides of the sample tube aperture inserts; and, the sample tube aperture inserts having an exterior surface molded to the resin bound discontinuous fiber rotor body to distribute vertical loading from the sample tube on the bottom of the sample tube aperture inserts to the exterior surface of the sample tube aperture inserts and thence to the resin bound discontinuous fiber rotor body surrounding the sample tube aperture inserts.

8. In a process for molding a fixed angle centrifuge rotor having, a resin bound discontinuous fiber rotor body symmetrical about a spin axis having a top and a bottom, the fixed angle centrifuge rotor further having defined sample tube apertures having tops opening to the top of the rotor body, the defined sample tube apertures having closed bottoms extending near the bottom of the rotor body, and the sample tubes inclined with respect to a spin axis with the tops opening of the rotor body toward the spin axis and the closed bottoms of the rotor body more remote from the spin axis, the processes of molding comprising the steps of: providing a mold for compression molding at an exterior surface of the resin bound discontinuous fiber rotor body; providing sample tube aperture cores for attachment to the mold for occupying the sample tube apertures of the resin bound discontinuous fiber rotor body; providing sample tube aperture inserts having sides defining an inside diameter defining the inside diameter for receiving a sample tube and a closed bottom for holding a bottom of the sample tube, the sample tube aperture inserts formed from composite fiber material; the sample tube aperture inserts further having composite fiber material aligned with individual fibers having tensile components for transferring load on the bottom of the sample tube aperture inserts to the sides of the sample tube aperture inserts; placing the sample tube aperture inserts over the sample tube aperture cores before molding the resin bound discontinuous fiber rotor body; molding the resin bound discontinuous fiber rotor body interior of the mold to cause the exterior surface of the sample tube aperture inserts to be molded to the resin bound discontinuous fiber rotor body; and, removing the resin bound discontinuous fiber rotor body from the mold; and, removing the sample tube aperture cores from the sample tube apertures inserts to leave the sample tube aperture inserts attached to the resin bound discontinuous fiber rotor body of the centrifuge rotor.

9. In a fixed angle rotor having: a rotor body symmetrical formed about a spin axis extending through a top and a bottom of the rotor body; a plurality of sample tube apertures configured within the rotor body and each inclined at an identical angle with respect to the spin axis of the rotor body and each sample tube aperture symmetrically defined about a sample tube aperture axis which intersects the spin axis of the rotor body at a common point, each sample tube aperture further having a central axis with the sample tube apertures opening to the top of the rotor body nearer the spin axis and extending into the rotor body to a closed sample tube aperture bottom adjacent the bottom of the rotor body further from the spin axis of the rotor body; the rotor body defining at least one top surface adjacent the opening of the sample tube apertures, the top surface intersecting the edges of the sample tubes along a plane which is normal to the axis of each of the sample tube apertures.

10. In a fixed angle rotor according to claim 9 and wherein: the at least one surface includes a plurality of facet surfaces defined at the edge of each sample tube aperture.

11. In a fixed angle rotor according to claim 9 and wherein: the at least one surface includes a single spherical surface generated from the common point on the spin axis of the rotor body.