Compression molded composite material fixed angle rotor

A fixed angle centrifuge rotor body is constructed from compression molded composite material. The rotor body has a frustum shaped peripheral contour about a central spin axis between a base end and an apex end. The body has net shaped 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 is 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 exceed 40 weight percent of the cured rotor product and have no visible layering between respective fibers. The resin cured discontinuous composite fibers have minor vertical excursion parallel to the spin axis of the rotor and relatively no kinking along their respective lengths. The resin cured discontinuous fibers are disposed parallel to the surfaces of and about molded sample tube apertures in the rotor body.

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. 
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.degree. to 34.degree.. 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withdrawn and further processed.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 of such chopped or 
discontinuous fibers has heretofore not been 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. 
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.

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 (See FIG. 9A). 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. 
Sample tube aperture core cluster K is some what more complicated. Cluster 
K here includes six sample tube aperture cores R. This number can vary to 
greater or less numbers of sample tube aperture cores R. Referring for 
example to FIG. 7B, sample tube aperture cores R can be understood. Those 
having experience with centrifugation will understand that fixed angle 
rotors are required to have sample tube apertures A (See FIG. 9A). In the 
compression molding process here described, it is the function of sample 
tube aperture cores R to form sample tube apertures A during the 
compression molding process. For this to occur, sample tube aperture cores 
R must be properly held in sample tube aperture core cluster K before and 
during compression molding and conveniently removable after compression 
molding has occurred. 
Continuing with FIG. 7B, each sample tube aperture core R includes male 
cylindrical body portion 40 having relieved bottom surface 42 and circular 
bottom 44. 
The upper portion of each sample tube aperture core R includes frustum 
shaped upper end 46 with rounded apex 48. The structures of each sample 
tube aperture core R is completed with circular segment notch 50. It is 
the purpose of circular segment notch 50 to enable the respective sample 
tube aperture cores R to be held in sample tube aperture core cluster K 
during the compression molding process. 
Viewing FIG. 7A, the function of male mold member M at frustum shaped 
central cavity C to hold sample tube aperture cores R in sample tube 
aperture core cluster K can be easily understood. At the upper portion of 
male mold member M in frustum shaped central cavity C, central male mold 
aperture 52 is configured. Machined at six equal angular intervals around 
central male mold aperture 52, there are frustum shaped core retaining 
apertures 36. These respective frustum shaped core retaining apertures 36 
each receive and hold frustum shaped upper end 46 of each sample tube 
aperture core R. 
It remains to securely hold the respective sample tube aperture cores R in 
sample tube aperture core cluster K during the compression molding 
process. Specifically, keying disc 34 fits interior of circular segment 
notch 50 on each sample tube aperture core R. Keying disc 34 is urged 
upward by gathering disc 32. Such upward urging occurs through attached 
gathering shaft 33 which is typically urged upward by standard threading 
or other conventional apparatus (schematically shown). There results 
sample tube aperture core cluster K held together with relatively great 
force sufficient to withstand the dynamic forces of compression molding. 
It will be understood that once compression molding is finished, release of 
sample tube aperture cores R from the formed sample tube apertures A and 
rotor body B is required. To effect such release, male mold member M and 
female mold member F are first parted. Once this has occurred, keying disc 
34 is rotated. Upon rotation, keying slots 54 in keying disc 34 register 
to circular segment notch 50 in each sample of the tube aperture cores R. 
Thereafter, rotor body B is withdrawn from frustum shaped central cavity C 
of male mold member B. (See FIG. 7E) Keying disc 34 can thus be removed. 
It only remains that the respective sample tube aperture cores R are 
removed from the now formed sample tube apertures A to compete the net 
shaped rotor body B illustrated in FIG. 9A. 
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 required. 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. Some discussion of this commercially 
available compound and its applicability is warranted. Accordingly, the 
reader's attention is directed to FIG. 10A. 
Referring to FIG. 10A, it can be seen that such sheet molded compound 65 
consists of chopped fiber layers 67 alternating with resin layers 68. This 
alternating construction can be found in SMC produced by Quantum Composite 
of Midland, Mich. 
Referring briefly to FIG. 9A, it will be understood that when rotor body B 
spins about rotor spin axis 70 at high rotational velocity, major stress 
will be exerted normal to the spin axis. It will be understood that if the 
discontinuous fibers 69 illustrated in FIG. 10A could be oriented overall 
substantially normal to rotor spin axis 70, rotor body B would have 
maximum resistance to the forces of centrifugation. 
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 seen 
in FIG. 10A, 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. With mold separation 
as previously described, rotor body B as illustrated in FIG. 9A results. 
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 (See 
FIG. 9A). 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. 
Referring to FIGS. 7A and 7B, integral molding of a rotor body is 
illustrated where wound and pre-cured fiber tension rings 85 are utilized. 
Specifically, and referring to FIG. 7A, wound and pre-cured fiber tension 
rings 85 are wound with a diameter to fit into internal female steps S. 
These respective rings 85 fit to the internal portion of finished rotor 
body B and leave exteriorly thereof the respective male steps T seen in 
FIG. 9A. Thus it is possible to internally reinforce rotor body B. 
Referring to FIG. 7B, an additional advantage of wound and pre-cured fiber 
tension rings 85 can be seen. Examining male mold member M at frustum 
shaped central cavity C in the interface between internal female steps S 
and sample tube aperture cores R, it will be seen that wound and pre-cured 
fiber tension rings 85 occupy the interface precisely. That is to say, 
wound and pre-cured fiber tension rings 85 contact internal female steps S 
on one side and sample tube aperture cores R on the other side. With this 
configuration, the proper angularity of sample tube aperture cores R in 
sample tube aperture core cluster K is assured, even in the presence of 
the considerable forces encountered during compression curing of the resin 
and fiber. Thus wound and pre-cured fiber tension rings 85 have a 
secondary function in bracing sample tube aperture cores R relative to 
male mold member M at frustum shaped central cavity C. 
It is also possible to use ring segment 90 for this same effect. Referring 
to FIGS. 7C and 7D, ring segment 90 is shown backing sample tube aperture 
core R relative to internal female steps S of male mold member M. 
Referring to FIGS. 8A-8C, it is possible to reinforce the compression 
molded rotor body with wound and pre-cured resin fiber rings 105. This can 
be done when frustum shaped central cavity C is configured without 
internal female steps S or with smooth frustum shaped internal female 
surface 106. Referring specifically to FIG. 8C, it will be seen that wound 
and pre-cured resin fiber rings 105 occupy the interstitial volume 107 
between sample tube aperture cores R and frustum shaped central cavity C 
interior of male mold member M. At the same time, and referring to FIG. 8A 
and 8B, it will be seen that pre-impregnated fiber mass 100 fits between 
respective sample tube aperture cores R. In this location, it is possible 
to use pre-impregnated fiber mass 100 with ring supporting notches 102 to 
support respective wound and pre-cured resin fiber rings 105. Thus, with 
frustum shaped central cavity C of male mold member M charged with sample 
tube aperture core cluster K, pre-impregnated fiber mass 100 and wound and 
pre-cured resin fiber rings 105, a rotor body without male steps T can be 
fabricated which has ring reinforcement. Such a rotor body requires no 
further finishing. 
Referring to FIG. 9B, finishing of rotor body B is illustrated. 
Specifically, resin fiber windings W are tension wound and cured over male 
steps T. Such winding and curing secures under tension resin fiber 
windings W to the exterior surface of rotor body B to provide additional 
resistance against the forces of centrifugation. 
It will be apparent that the illustrated molding process provides superior 
flexibility. Specifically, and using the compression molding cavities here 
illustrated, all shapes of fibers can be compression molded. For example, 
and referring to respective FIGS. 10B and 10C successive angularly 
alternated impregnated tape 110 or successive angularly alternated woven 
fiber 120 can be pre-impregnated and compression molded in female mold 
member F here illustrated. Likewise, this invention will admit of other 
variations to the compression molding herein set forth. 
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. 
It is to be understood that the present process of compression molding is 
analogous when it comes to alignment of fibers with respect to the 
resultant compression molded rotor body B as seen in FIG. 9A and 9B. 
Specifically, and when sheet molded compound 65 is utilized, fibers normal 
to the spin axis result. (See FIG. 10D) Cutting such a rotor body in 
section results in a view of cured fibers E "flowing" along planes normal 
to the spin axis or parallel to horizontal vector 130. Such a plane normal 
to rotor spin axis 70 is illustrated by spin axis normal vector 131. It 
will be observed that no measurable kinking is present in such fibers. 
Moreover, only in minor detail do the fibers depart from the original 
alignment of sheet molded compound 65. Taking the case of vertical vector 
131 disposed parallel to rotor spin axis 70, it will be observed that 
individual fiber excursion from the plane of spin axis normal vector 130 
is small, yet present in sufficient amount to enable sufficient vertical 
strength to be present in the rotor body to resist those forces generated 
parallel to rotor spin axis 70. This appearance is distinctive and is 
illustrated herein at FIGS. 10D. 
Referring to FIG. 10E, when fibers E encounter a mold boundary, again the 
appearance is distinctive. First, 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. 
Referring to FIG. 10F, it is preferred to place fabric around the frustum 
shaped periphery of the mold as illustrated in FIG. 3B. Taking the case 
where resin impregnated fiber layer 64 is made from resin impregnated 
fabric, the fabric clearly appears only at the boundary. Such resin cured 
woven composite fabric 164 is illustrated in FIG. 10F. It will be noted 
that the interior of sample tube apertures A when formed with composite 
fiber cloth sock 80 around sample tube aperture cores R has a similar 
appearance. 
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. 
Referring to FIG. 11A, a cross section of sample tube aperture A having a 
triangular cross section 140 relative to rotor spin axis 70 is 
illustrated. 
Referring to FIG. 11B, a cross section of sample tube aperture A having an 
elliptical cross section 142 relative to rotor spin axis 70 is 
illustrated. It will be noted that the major axis of the ellipse is 
radially aligned relative to rotor spin axis 70. 
Referring to FIG. 11C, a cross section of sample tube aperture A having a 
pie shaped cross section 144 relative to rotor spin axis 70 is 
illustrated. It will be noted that the apex of the pie shape is disposed 
towards rotor spin axis 70. 
Finally, and referring to FIG. 11D, a cross section of sample tube aperture 
core R having pyramid shaped three dimensional section 146 is illustrated. 
It goes with out saying that such a sample tube aperture core R will form 
sample tube aperture A having a complimentary female cross section. 
Observing FIGS. 11A-11D, some observations can be made about the varied 
sample tube apertures A. First, they are molded and all other than 
cylindrical. Second, it is required that sample tube aperture cores R all 
be capable of releasing from the mold. Such release here is shown in its 
preferred embodiment from male mold member M. It will be understood that 
release from female mold member F could likewise occur. Further, mold 
members with straight sample tubes--parallel to the direction of mold 
release--will not require release of sample tube aperture cores R. 
In co-pending Centrifuge Construction Having Central Stator, Ser. No. 
08/288,387 filed Aug. 10, 1994, now U.S. Pat. No. 5,505,684 issued Apr. 9, 
1996, 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.sub.1 
has to be ring shaped. Such a ring shaped rotor body B.sub.1 is 
illustrated in FIG. 12. 
Referring to FIG. 12, rotor body B.sub.1 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.sub.1 having central stator aperture 150. 
Secondly, we now understand that rotor body B.sub.1 first disclosed in 
Centrifuge Construction Having Central Stator Ser. No. 08/288,387 filed 
Aug. 10, 1994 now U.S. Pat. No. 5,505,684 issued Apr. 9, 1996, 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. 12. 
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.sub.1 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.