Method of molding optical recording drums

A method for fabricating in a mold a rotatable recording drum. Initially, the mold is rotated and a predetermined quantity of a flowable substance is poured into the mold cavity and is centrifugally flung against the mold wall to form a mold layer with a smooth inner surface. Next, a surface substance is poured into the mold cavity and is allowed to harden against the mold layer. A core outer layer substance is then poured into the mold cavity and allowed to harden against the surface substance, and rotation of the mold is stopped. The mold layer fluid is removed, and the cavity inside the outer layer is filled with a core inner layer substance which is allowed to harden before the drum core is removed from the mold cavity. Following removal of the core from the mold cavity, the hardened surface substance may be vacuum coated, and a protective overcoat applied to the core. The drum core and the surface substance can also be formed separately.

TECHNICAL FIELD 
The present invention relates to optical recording drums. More 
particularly, the present invention relates to methods of spin molding 
optical recording drums using solid and liquid mold systems. 
BACKGROUND OF THE INVENTION 
Rotating drums have been used for the storage of data in electronic digital 
computing systems since the first electronic digital computer built in the 
1930's. Although magnetic recording superseded these early drums, the use 
of rotatable drum memories has persisted. The general configuration of a 
rotating drum memory is a cylinder which is rotated at a constant speed 
around its axis. A recording medium, such as a magnetic recording 
material, is deposited on the drum surface. Data is recorded, usually in 
bit form, by a recording device located adjacent the rotating drum 
surface. The data is fed from a data source such as a digital computer, 
and is recorded in circumferential lines, called data tracks, on the drum 
surface. To read the data, a reading device is placed over the data track 
to feed the data back to the computing system as the drum rotates. The 
time required to find and read a particular item of data on the rotating 
drum is the access time. 
Recording drums are superior to disks in that the surface velocity of a 
drum is constant over the entire surface whereas with disks the velocity 
varies with the radius. With disks, the maximum data transfer rate is a 
function of the innermost ring, and the disk operates at a very low 
efficiency when at the outermost rings unless the disk speed varies with 
the changing radius. For example, when the outermost rings are twice as 
long as the innermost rings, the disk operates at 50% efficiency at the 
outer rings. Drums operate at a constant velocity and have a higher data 
transfer rate as they operate at 100% efficiency. Additionally, because 
the mechanical precision of drums is typically greater than disks, less 
stringent performance is required of the optical focusing and tracking 
servos used with optical drums. While the coating of magnetic media and 
photographic emulsions onto drums is technically and economically 
feasible, the adaptation of the drum configuration to new recording 
technology such as optical recording is less feasible. 
While the first rotating drum memories used capacitors as the recording 
medium, and most commercial drum memories use magnetic media, optical 
memory rotating drums have been disclosed in U.S. Pat. Nos. 3,383,662; 
3,408,634; 3,440,119; and 3,500,343. In the optical drum memory disclosed 
in this group of patents, the outside of a cylinder is coated with a 
photographic emulsion, and data bits are recorded on the resulting 
photosensitive surface. Data is read from the cylinder by a microscope and 
a photodiode. This device provides greater bit density than prior magnetic 
recording media, and is insensitive to strong magnetic fields. However, 
this device can only record data photographically; the data must be 
developed and can not be rewritten. Thus, this device can not interact 
with associated computers or other digital systems in real time as it is 
read only. 
In optical recording technologies using laser recording, a laser beam is 
focused to a very small spot to record data onto an optically sensitive 
coating on a substrate. The substrate is an inert substance on which the 
optically sensitive layer is coated. The data is immediately readable 
after recording, without any intermediate processing such as chemical 
development of latent images. Such recording systems are called direct 
read after write (DRAW) systems. Such a recording mode is permanent and 
therefore is not erasable and reusable. 
Magneto-optic recording is an erasable, reusable method of laser recording, 
and uses a tightly focused laser spot to heat an area of magnetic material 
above its Curie point while subjecting the area to a magnetic field. The 
size of the recorded data bit is determined by the size of the heated 
area, and is smaller than the area covered by the magnetic field. 
Therefore, bit areas much smaller than those achievable with conventional 
magnetic recording heads can be obtained. The magnetically recorded bits 
can be read by a laser beam. 
Magneto-optic recording can attain bit densities as great as ten times that 
of rigid disk magnetic media as the data bits have an area on the order of 
one square micrometer, and the bit and track pitch are of similarly small 
dimensions. To achieve these levels of resolution, the surface of the 
recording medium must be extremely smooth, a condition not easily 
producible in cylindrical form with existing technologies. Moreover, 
apparatus for locating the data on such a small scale must be extremely 
precise. 
Focusing a beam of light, usually a laser beam, to a sufficiently small 
spot size to achieve resolution on the order of one micrometer requires 
the distance from the focusing lens to the recording surface to be held to 
tolerances of one micrometer. While reading a data track, the recording 
medium surface inevitably moves in a direction normal to its axis of 
rotation, thereby changing the lens-to-surface distance. With rotating 
drums, this surface wandering is expressed as the total distance the 
surface wanders during one revolution of the drum and is commonly referred 
to as runout, or total indicated runout (TIR). To compensate for this 
surface wandering, servo systems maintain focus by adjusting the lens 
location as the surface wanders. In optical disk recording, surface 
wandering can be over 100 micrometers, and servo systems can adjust the 
lens location to a tolerance of less than 1 micrometer at typical 
rotational speeds. As surface wandering in rotating drum memories is 
considerably less, a simpler focusing mechanism can be used, while 
providing greater focus accuracy and higher speeds. 
Typical diameters of commercially available rotating drum memories are in 
the range of 26.7-83.3 cm (10.5-32.8 inches). Maintaining surface 
wandering to within a few micrometers for these drums requires greater 
precision than is achievable by conventional manufacturing processes, 
absent expensive and time-consuming finishing operations. Surface 
wandering of cylindrical drums is caused by the bearings, the eccentricity 
of the drum surface, and various surface waves, out-of-roundness, and 
other defects. Even assuming a stationary drum center, the large drum size 
contributes to dimensional variations which would lead to surface 
wandering. Furthermore, the large mass and any vibration-causing unbalance 
increase surface wandering. 
Finally, manufacture is complicated by the high level of smoothness 
required on the outside of the drum. While techniques for producing smooth 
surfaces on flat optical recording disks are known, these techniques are 
not suitable for applying similar surfaces to drums. The smooth flat 
recording surfaces on optical disk recording media can be achieved by 
coating a curable polymeric liquid onto a horizontal substrate to form a 
free surface and allowing the liquid to harden. The coating can be thinned 
by spinning the horizontal substrate around a vertical axis, before 
hardening, thereby flinging off excess material by centrifugal force. This 
coating process, referred to as spin-coating, provides very high quality 
flat surfaces, but can not coat the outside of a drum, as cylindrical 
surfaces cannot be made horizontal, and gravity causes sagging and other 
non-uniformities. 
Nonetheless, spinning a liquid layer around a vertical axis can form 
symmetric curved optical surfaces in a technique known as spin casting. 
Various aspects of spin casting contact lenses are described in U.S. Pat. 
Nos. 4,416,837; 4,534,915; 4,637,791 and 4,659,522. In the first patent, a 
mold is spun and used to spin cast contact lenses. In the second patent, 
UV light is used to minimize stresses by curing the cast product more 
rapidly near the center. In the third patent, vibration is reduced during 
spin casting to prevent surface waves in the lens during curing. The last 
patent describes spin casting of an annular lens. These patents illustrate 
that spin casting can form optical quality concave surfaces, and that a 
spin cast polymeric mold can, in some cases, be used to produce an optical 
quality convex surface. 
However, the lenses produced by spin casting in these patents bear little 
relation to the optical quality surfaces required in a rotating drum used 
in laser optical recording. Contact lenses are not right circular 
cylinders and do not require precise mechanical tolerances of the type 
required of memory drum components. Moreover, when placed in the eye, 
contact lenses are covered by liquid layers which coat surface roughness. 
Optical recording involves no liquid layer and requires a high level of 
smoothness to prevent the optically sensed signal from being lost in noise 
generated by roughness. 
SUMMARY OF THE INVENTION 
The present invention provides an optically smooth, concentric cylindrical 
surface by providing a layer on the surface of a cylindrical drum which 
exhibits levels of smoothness normally achieved only by coatings or by 
complex grinding and polishing procedures. The cylindrical surface 
eliminates larger scale eccentricities, out-of-roundness, and other 
sources of surface wandering found in drums produced by conventional 
volume production methods. 
A method for fabricating a rotatable recording drum in a single cylindrical 
mold rotatable around an axially central mold shaft includes the following 
steps. First, the mold is rotated around its axis of rotation and a 
measured, predetermined quantity of a flowable substance is poured into 
the mold cavity. This substance is centrifugally flung against the mold 
wall to form a cylindrical mold layer having an interior surface 
equidistant from the axis of rotation to create a concentric, smooth, 
circular inner surface. Next, a surface substance having a lower density 
than the mold layer, such as activated epoxy, is poured into the mold 
cavity and is allowed to harden against the mold layer. 
A drum core first layer substance such as a mixture of glass microspheres 
and epoxy is then poured into the mold cavity and allowed to harden 
against the epoxy, and the mold rotation is stopped. The mold layer fluid 
is removed from the mold cavity, and a metallic sleeve is placed over the 
mold shaft to serve as a central hole for the drum core. The cavity 
between the drum core first layer and the metallic sleeve is filled with a 
drum core second layer substance such as a paste of ceramic spheres and 
activated epoxy which is also allowed to harden. Finally, the drum core is 
removed from the mold cavity. 
Where the mold axis of rotation is vertical, the axis is slowly shifted 
while the mold is rotating while containing both the flowable substance 
and the activated epoxy. The mold is shifted to a horizontal position to 
remove the wedge shape of the profile between the flowable substance and 
the epoxy which is caused by the gravitational force. Alternatively, the 
axis of rotation can remain at a constant 30.degree. angle with the 
horizontal. 
Following removal of the drum core from the mold cavity, the activated 
epoxy hardened on the drum core may be vacuum coated with a magneto-optic, 
thin film, metallic layer identical in composition to the layers in 
magneto-optic recording disks. The drum core is replaced into the mold 
cavity, and a protective overcoat is applied to the surface of the 
magneto-optic thin film to protect it and to prevent oxidation of the 
metals in the thin films. This protective overcoat is applied by rotating 
the mold around its axis of rotation; pouring a measured, quantity of the 
flowable substance into the mold cavity to form a mold layer; 
centrifugally flinging the flowable substance so that its interior surface 
is equidistant from the axis of rotation to create a concentric, smooth, 
round inner surface spaced from the outer surface of the drum core; 
inserting the protective overcoat material into the mold cavity between 
the outer surface of the epoxy and the inner surface of the mold layer to 
fill the space therebetween; and allowing the protective overcoat to 
harden onto the outer surface of the drum core. 
Alternatively, the drum core and the surface substance can be formed 
separately. In forming the core, the mold is first rotated around its axis 
of rotation and a measured, quantity of a flowable substance is poured 
into the mold cavity. The flowable substance is centrifugally flung 
against the mold wall to form a cylindrical mold layer having a 
concentric, smooth, round inner surface. A drum core first layer substance 
is poured into the mold cavity and allowed to harden before the mold 
rotation is stopped. The mold layer is removed from the mold cavity, a 
metallic sleeve is placed over the mold shaft to serve as a central hole 
for the drum core, and the cavity between the drum core first layer and 
the metallic sleeve is packed with a drum core second layer substance. The 
drum core second layer substance hardens, and the drum core is removed 
from the mold cavity. 
To form the surface substance on the preformed core, the preformed 
cylindrical drum core is placed in a cylindrical mold which may be the 
same or different from the mold used to form the core. This mold is 
rotated around an axially central mold shaft, and a measured, quantity of 
the flowable substance is poured into the mold cavity to form a mold layer 
as discussed above. A surface substance is inserted into the mold cavity 
between the outer surface of the drum core and the inner surface of the 
mold layer to fill the space therebetween and is allowed to harden onto 
the outer surface of the drum core. Finally, rotation of the mold is 
stopped and the drum core is removed from the mold cavity. 
These methods form a rotatable recording drum for use in erasable optical 
recording having an outer surface substance which is an activated epoxy 
and an inner core layer formed of microspheres within an epoxy. The drum 
may also include an outer core layer disposed between the inner core layer 
and the outer surface. The outer core layer is a mixture of epoxy and 
ceramic microspheres. Alternatively, the inner core layer could be a 
mixture of epoxy and glass or ceramic microspheres. 
The liquid mold system has several advantages over a solid mold system. 
First, the finished part will not stick to the mold surface, and the mold 
need not have any taper to facilitate removal of the finished part. 
Second, the mold is not damaged by scratching or adhesion during casting. 
Third, the mold shell can be used to apply more than one layer to the drum 
section without having to remake the mold, since the inner diameter of the 
layer can be varied by changing the amount of liquid added to the mold 
shell in each casting operation. Finally, in liquid molding systems the 
smoothness and cleanliness of the cast surfaces are determined almost 
entirely by the purity of the mold liquid, rather than by the cleanliness 
of a solid mold surface. Methods of purifying liquids are simpler than 
comparably effective methods of cleaning solid surfaces.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The apparatus and method of the present invention produce cylindrical 
surfaces which exhibit low runout during rotation, and which produce 
sufficiently smooth surfaces suitable as optical recording substrates. The 
surfaces produced are located on the curved peripheral surfaces 10 of drum 
sections 12 shown in FIG. 1A. The drum sections 12 can be mounted 
side-by-side coaxially on a shaft 14 to form a drum 16 usable as a 
rotating drum memory, as shown in FIG. 1B. As shown in FIG. 1C, the drum 
sections 12 include a core 18 and a substrate or surface layer 20. 
Additionally, the core 18 may include two or more layers, as shown in FIG. 
1D in which the core 18 includes a central sleeve 22, an inner core 24, 
and an outer core 26. Fabrication of cores 18 from non-metallic materials 
reduces cost, weight, and the susceptibility of the core 18 to magnetic 
fields. 
Drum sections 12 may have protrusions or indentations (not shown) on the 
flat side faces 28 which mate with corresponding parts on adjacent drum 
sections 12 to link together and facilitate cementing drum sections 12 
together into a drum 16 during use. Alternatively, linking the drum 
sections 12 may be accomplished by adhesive. Using separate drum sections 
12 simplifies fabrication, and permits adding or removing drum sections 12 
on a shaft, to change the capacity of the rotating drum memory. 
Mold Shell Apparatus 
The methods of fabrication of drum sections 12 use the mold shell apparatus 
30 shown in FIG. 2, which can fabricate drum cores 18 and complete drum 
sections 12 using various different materials. The basic apparatus 30 
includes a cylindrical mold having an outer mold shell 32 rigidly and 
coaxially mounted to a rotatable shaft 34. The mold shell 32 may be 
machined from a dimensionally stable material such as aluminum. Because 
the mold shell 32 is spun at rotational velocities above 3000 RPM, the 
density of the mold shell material should be low, to reduce weight and 
minimize the effects of any residual unbalance. The mold shell 32 also 
should be sufficiently strong to resist deformation by the centrifugal 
forces arising from spinning. 
In some cases, such as when photocurable resins are cast, part or all of 
the mold shell 32 may be made from materials transparent to light, such as 
ultraviolet light, which is used for curing. Polymeric materials are 
particularly suitable for their transparency and ease of fabrication. 
Additionally, a variety of plastic tooling compounds, particularly 
epoxies, can be used to cast part or all of the mold shell 32. Polymeric 
materials used for the mold shell 32 must be dimensionally stable, to 
prevent dimensional changes during use because of warping, heat 
relaxation, or other uncontrolled phenomena. 
The portion of the shaft 34 which lies within the cavity 36 of the mold 
shell 32, the critical dimension region, must exhibit a very low level of 
runout when the shaft 34 rotates and should closely approximate a perfect 
cylinder. This critical dimension region of the shaft 34 should locate the 
geometrical center of produced drum sections 12 within 12.7 micrometers 
(0.0005 inches) of the rotational axis of the rotating drum memory shaft 
used with the drum section, and the shaft 34 diameter should correspond to 
the diameter of the rotating drum memory shaft. The shaft 34 must be large 
enough to resist bending during use, while small enough to reduce weight 
and expense, and improve dimensional accuracy. Shaft 34 may be hollow. 
The rotatable shaft 34 is supported on bearings 38 to fix its axis of 
rotation. Gas lubricated bearings, such as air bearings which provide less 
than 0.051 micrometers (2 microinches) of runout, are preferred when the 
apparatus is well balanced. Non-rotational movement could occur in journal 
bearings if bearing clearances are too large, or in ball or roller 
bearings if the bearings' centers of rotation are not coincident with the 
center of the shaft 34. Additionally, ball and roller bearings are less 
desirable because they produce high frequency vibrations. 
The upper edge of the mold shell 32 is covered by a removable annular 
retaining ring 40. The retaining ring 40 provides an inner lip over the 
top of the mold shell 32 to prevent liquid from spinning out of the shell 
32, while leaving open the majority of the top of the mold shell 32. This 
is accomplished because the inner diameter of the retaining ring 40 is 
always smaller than the inner diameter of the liquid in the mold shell 32. 
The liquid is always poured into the mold shell 32 while the mold shell 32 
is spinning so centrifugal force holds the liquid against the outer wall 
of the mold and below the level of the retaining ring 40. The retaining 
ring 40 may be made of aluminum or stainless steel, or a chemically 
stable, non-adhesive polymeric material such as polytetrafluoroethylene 
and can be attached to the mold shell 32 by machine screws. 
An upper gasket 42 is placed between the retaining ring 40 and the mold 
shell 32 to form a liquid-tight seal. The upper gasket 42 may be a soft 
material, such as fluoroelastomers or silicone rubber, which is chemically 
unreactive with and does not adhere to the casting compounds. The upper 
gasket 42 may be flat, as shown in FIG. 2, or it may be an O-ring residing 
in an annular groove cut in the top edge of the mold shell 32. The upper 
gasket 42 may alternatively be a thin coating of sealant applied to the 
upper edge of the mold shell 32, to the bottom surface of the retaining 
ring 40, or to both. 
Where a preformed inner core 18 is used to make the drum section 12, as 
shown in FIG. 5, a lower gasket 44 may be placed in the bottom of the mold 
shell 32. The lower gasket 44 may be formed from an elastomeric material 
such as fluoroelastomers or silicone rubber. Alternatively, the lower 
gasket 44 may be an adhesive sealant applied as a thin layer to the bottom 
of the mold shell 32 capable of releasing any finished parts to be removed 
from the mold. Beeswax, which can be melted onto the mold shell bottom, 
and releases when melted at temperatures from 61.degree. C. to 67.degree. 
C., can be used. A layer of polytetrafluoroethylene also can be used. 
A variable speed electric motor 46 is coupled to the shaft 34 through one 
or more belts 48. The motor 46 is mounted to a support base 50 through 
rubber mounts and O-ring belts (not shown) and air bearings 38 to prevent 
the transmission of vibrations to other components. Alternatively, the 
motor shaft 46A can be connected directly to the end of the air bearing 
shaft 38A of the air bearings 38. 
The orientation of the axis of shaft 34 of the mold shell apparatus 30 can 
be varied, as discussed below, via a gimbal mount 52 shown in FIG. 2. The 
mold shell can be held in the desired position by mechanical stops, 
locking worm gears, or pinion mechanisms (not shown). 
Once the mold shell apparatus 30 has been balanced, it can be used to 
produce a cylindrical mold which is coaxial with the axis of rotation of 
the shaft 34 and which has virtually smooth, perfect interior wall 
surfaces. The mold shell 32 is rotated by the motor 46 at a speed which 
produces a centrifugal force at the inner surface of the mold shell 32 of 
about 1800 times the force of gravity, 1800 g. The speed required to 
achieve this force depends upon the inner radius r of the mold shell 32, 
and can be derived from the well known relationship 
EQU Z=r.omega..sup.2 /g (1) 
where Z is the ratio of centrifugal force to gravitational force, .omega. 
is the angular velocity in radians per second, and g is the acceleration 
due to the earth's gravity. Thus, in revolutions per minute (RPM), 
EQU .omega.(RPM)=9.549[gZ/r].sup.1/2 (2) 
When the shaft is rotated at speeds such that Z is above 1000, a small 
amount of liquid placed in the mold shell 32, with the shaft 34 oriented 
vertically will form a substantially vertical thin layer lining the inner 
wall of the mold shell 32. As the centrifugal force is hundreds of times 
greater than the gravitational force, the thin liquid layer forms a smooth 
surface, which is critical to the operation of the mold shell apparatus 
30. However, the gravitational force causes the inner free surface of the 
layer to deviate from vertical and be perpendicular to the resultant of 
the centrifugal and gravitational force vectors. The shape of this free 
surface is a paraboloid of revolution with an axis coincident with the 
axis of rotation of shaft 34 and has the equation: 
EQU y=.omega..sup.2 x.sup.2 /2g (3) 
where y is measured along the axis of rotation, and x is the location of 
the point under consideration. Solving equation 3 for x and taking the 
derivative, and noting that because the layer is quite thin in comparison 
with radius of the mold shell 32, x can be approximated by r, yields: 
EQU dx/dy=g/[.omega..sup.2 r] (4) 
where dx/dy is the rate at which the radius of the inner surface of the 
thin layer changes along the axial direction of the mold shell 32. The 
rate of change in diameter of the inner surface of the thin layer, 
commonly referred to as the taper, is 2dx/dy, and is inversely 
proportional to the square of angular velocity w. The taper, as well as 
the high degree of smoothness and concentricity of the free surface of the 
thin liquid layer, are the features most desired in molds suitable for use 
in making drum sections for optical drum memories. 
The mold shell apparatus 30 described above can be used with various novel 
methods to form optical recording drums. In the preferred embodiment, the 
drums are formed using a single liquid mold as described below. 
Alternatively, the core 18 and the surface layer 20 can be formed 
separately, and a solid mold system also can be used. 
The Drum Core Materials 
The core 18 can be conventionally machined from a flat plate of aluminum or 
other suitably lightweight, dimensionally stable, durable yet workable 
metal. The central hole must be precisely machined to achieve low levels 
of surface wandering when the drum section 12 is placed in a rotating drum 
memory. However, metal cores 18 are very heavy, expensive to fabricate, 
may require special balancing operations, and are electrically conductive 
and subject to the magnetic field effects present in many recording 
processes. Therefore, dimensionally stable polymeric resins or other 
nonmetallic materials or composites are preferred. Non-metallic cores are 
lighter, less prone to thermal expansion, less affected by the magnetic 
fields which are often a part of the recording process, and less expensive 
to fabricate. As nonmetallic resin cores 18 may lack dimensional stability 
if made with known methods, unique methods of fabricating mixtures for 
nonmetallic cores which meet the requirements for rotating drum memories 
are described below. 
Small hollow glass or ceramic spheres can be used with nonmetallic resins 
to enhance dimensional stability, improve strength, and reduce weight. 
Spherical fillers can be added to the resin in larger amounts without 
unduly increasing the viscosity of the composition. Spherical fillers are 
available in various sizes and compositions. Spheres having diameters of 
200 micrometers or less, known as microspheres, are particularly useful 
when surface quality is an important consideration, since their small size 
permits a fine surface texture. Where surface quality is less important, 
larger macrospheres, which can be a few millimeters in diameter, are 
easier to handle and lower in cost. 
In a composite core using both microspherical and macrospherical fillers as 
shown in FIG. 1D, the inner core 24 includes macrospherical fillers, and 
the outer core 26 includes microspherical fillers. This improves the 
quality of the surface to which the surface layer 20 adheres. Because of 
the different densities of the layers 24 and 26, their interface should be 
concentric to the center of rotation of the finished drum section to 
prevent unbalance of the core. 
Suitable resins should have a sufficiently low viscosity to flow into the 
narrow gap between the core 18 and a mold layer 54, and cure at or near 
room temperature without producing gas bubbles or other contaminants. 
Also, the cured resin should adhere to the core 18 but not to the 
metallized mold surface, and should not shrink excessively during cure. It 
is desirable that the casting resin be of microelectronic grade. Finally, 
the casting resin, when cured, should be capable of forming a substrate 
suitable for the optical recording medium to be used for the rotating drum 
memory. 
Liquid Mold System Without Prefabricated Core 
Fabrication of drum sections is preferably accomplished by casting the drum 
section 12 in layers, starting from the surface layer 20 and working 
inwardly, as illustrated in FIG. 3. This is done using a liquid mold 
system in which a liquid mold layer 54 is formed in the mold shell 32 to 
serve as the mold surface. Liquid mold systems are better than molding 
within a machined surface because the molded material cannot adhere to the 
liquid surface, and because the molded surface replicates the smooth inner 
surface of the liquid. 
The empty mold shell 32 and the inside of the pouring lip, retaining ring 
40, can be coated with a light coating of beeswax to seal the retaining 
ring 40 to the mold shell 32 and to serve as a thermally excited coating 
which releases the finished drum when the mold shell 32 is heated. 
The first step in forming a drum section 12 is to place the mold shell 
apparatus 30, with retaining ring 40 in place, in a vibration free 
environment. Next, the empty mold shell 32 is rotated around a vertical 
axis. When the required rotation speed is reached, a measured quantity of 
mold liquid is deposited into the mold shell 32, where it forms mold layer 
54 against the inner surface of the mold shell 32. The axis of the shaft 
34 may be horizontal to eliminate any wedge shape of the free surface of 
the mold layer 54 due to the effects of gravity and to form a more 
optically perfect drum section 12. 
The mold liquid forming the mold layer 54 must be of a density 
significantly higher than the casting resin used to form the surface layer 
20, and should be chemically inert with respect to, and immiscible with, 
the material forming the surface layer 20. A suitable liquid is a 
fluorocarbon of the type commonly used in many electronic manufacturing 
processes, which is available in specific gravities over 1.8. The 
preferred material is a fluorocarbon, Type FC-5311, manufactured by ISC 
Chemicals, Limited, U.K. It has a specific gravity of 2.08. The chemical 
description of the material is phenanthrene, 
tetracosafluorotetradecahydro-(cas 306-91-2). Since many casting materials 
suitable for the surface layer 20 have specific gravities below 1.1, they 
will float on the inner surface of the fluorocarbon fluid under the 
effects of centrifugal force and will float on top under the influence of 
gravity. Also, fluorocarbon liquids are chemically stable and do not 
interfere with curing or react with the casting materials. Additionally, 
due to their chemical stability, fluorocarbon liquids are reusable after 
filtration, using filters having submicron pore sizes, or other 
purification. For example, centrifugation eliminates light impurities 
which migrate to the interface between the layer 54 and the surface layer 
20 during casting. 
The mold liquid should not contain contaminants which are of lower density 
than the liquid itself, since the centrifugal force of the spinning mold 
shell 32 would cause these particles to move to the inner surface of the 
mold layer 54 and produce defects in the surface of the surface layer 20. 
Furthermore, the mold liquid should not contain dissolved impurities which 
would leave an undesirable film on the cast surface. As fluorocarbon 
liquids are poor solvents, it is unlikely that they will dissolve any 
materials which might later be deposited as films when the fluorocarbon 
evaporates. 
Once the mold layer 54 is formed in the mold shell 32, a layer of liquid 
casting resin, such as an activated epoxy, is deposited onto the free 
surface of the layer 54 to form the surface layer 20 having a thickness of 
about 2.5 mm (0.1 inch). The rotational speed of the mold shell 32 is 
maintained until the casting resin cures and hardens. This surface layer 
20 forms the outside surface of the drum section 12. Preferably, the resin 
is placed in a vacuum prior to pouring to extract any air that was 
entrapped during mixing as large bubbles could affect the surface profile 
of the resin. 
Materials suitable for use as casting resins include acrylics, polyesters, 
and epoxies. The primary requirements for the casting resins are that they 
cure at or near room temperature, that they do not shrink excessively or 
undergo dimensional changes during curing, and that they form optically 
smooth, durable surfaces suitable for optical or magneto-optic recording 
materials. Moreover, it is highly desirable that the cured casting resin 
adhere well to the core 18. Cure times could be reduced by using a resin 
system curable by ultraviolet light. This would require that the mold 
shell 32 and the layer 54 be transparent to ultraviolet light. 
Because the surface layer 20 is thin, delicate, and easily deformed, a 
second support layer or outer core 26, shown in FIG. 3B, is applied onto 
the inner surface of the hardened surface layer 20, without stopping 
rotation. The material used for layer 26 is chosen primarily for strength 
and dimensional stability, rather than surface quality or optical 
properties. A suitable material is a mixture of glass or ceramic 
microspheres and epoxy having sufficiently low viscosity. This material is 
poured into the spinning mold and is spread out by centrifugal force to 
form a uniform layer 26 covering the surface layer 20. A sufficient amount 
of epoxy/microsphere mixture is added to give layer 26 a thickness of 
about 12 mm (0.5 inches). 
Once the outer core layer 26 has cured, rotation of the mold shell 32 is 
stopped, and the liquid layer 54 and the retaining ring 40 are removed. A 
solid, preferably metal, center sleeve 22 is then placed over the shaft 
34, as shown in FIG. 3C. The sleeve 22 provides a precise and durable 
inner diameter for the drum section 12. Therefore, the inner diameter of 
the sleeve 22 must precisely fit the shaft 34, with substantially zero 
clearance, but with no interference. The sleeve 22 should be sufficiently 
thick to provide adequate strength and dimensional stability both during 
fabrication and during use of the finished drum 16. The length of the 
sleeve 22 should be equal to the width of the drum section 12. Since the 
core 24 should adhere well to the sleeve 22, the outer surface of the 
sleeve 22 may be roughened, primed, or otherwise treated to promote 
adhesion. In an alternative embodiment, no sleeve 22 is used and the 
finished drum section 12 is contacted at its axial ends for rotation 
rather than being placed on a shaft 14. 
The final step in casting the drum section 12 is to form the inner core 24. 
Suitable compositions for forming the core layers 24 and 26 include a 
binder resin, such as an epoxy and hardener blend, and a hollow spherical 
filler particulate. The core 24 preferably includes a paste formed of a 
mixture of activated epoxy and hollow glass or ceramic spheres. Ceramic 
spheres are preferred to glass spheres as the ceramic spheres are 
comparatively inexpensive, lightweight, and strong. A suitable epoxy blend 
can be prepared by combining 35 parts of a modified cycloaliphatic amine 
hardener with 100 parts by weight of a low viscosity, non-crystallizing 
bisphenol A epoxy. A suitable macrosphere-filled inner core layer 24 can 
be prepared by adding 600 milliliters of the epoxy blend to 4000 
milliliters of ceramic spheres having diameters in the range of about 
0.6-1.4 mm and stirring until a uniform dispersion is obtained. A coarser 
grade of ceramic spheres having particle diameters in the range of 1.4-2.8 
mm results in a coarser surface texture. Despite the high proportion of 
filler in relation to the amount of epoxy resin used, the resin readily 
wets the filler particles and the resulting mixture exhibits flow 
properties which are very satisfactory for casting purposes. 
Although the same binder resin can be used for both layers 24 and 26, the 
outer core layer 26 requires a smoother, finer textured surface over which 
the final surface is placed. Thus, microspherical fillers, rather than 
macrospherical fillers are used for layer 26. A suitable composition can 
be prepared by preparing a batch of the epoxy blend discussed above, and 
adding hollow glass microspheres until the viscosity approaches the 
maximum suitable for casting. It is important that the filled epoxy blend 
spreads evenly when deposited in the spinning mold. 
The layer 24 is formed by filling the mold cavity between the sleeve 22 and 
layer 26 with the paste, leveling the top surface, and removing the excess 
material. As the paste cures, it bonds to the sleeve 22 and forms the body 
of the drum section 12. The finished drum section 12 can then be removed 
from the mold by overturning the mold shell 32, and allowing it to slide 
down the shaft 34. Where beeswax is used, the mold shell 32 is heated to 
melt the beeswax to remove the drum section 12. Any excess casting 
material can be removed from the drum section 12 by conventional 
machining, while preventing damage to or contamination of the curved 
surface 10 of the drum section 12. Additionally, excess epoxy connects the 
drum section 12 to the retaining ring 40 to help remove the section from 
the mold shell 32. To further assist removal, a tool with compressible 
O-rings (not shown) can be used to create an interference fit with the 
drum section 12 and pull it out of the mold shell 32. 
The resulting drum section 12 can be cleaned by standard cleaning methods. 
As the fluorocarbons evaporate without leaving significant residue, 
special cleaning procedures are not necessary. Next, an optical recording 
material, such as a magneto-optic medium, can be applied by known vapor 
deposition methods such as vacuum coating. Typical materials include those 
suitable for magneto-optic recording, such as combinations of cobalt, 
chromium, rare earth metals, or other magneto-optic thin film metallic 
layers used in magneto-optic recording disks. Alternative recording 
materials include those capable of phase change during recording. 
A protective layer can be provided over the recording medium layer to 
protect it from mechanical damage and corrosion, and reduce the disruptive 
effects of dust or other contaminants. Thicker overcoat layers, called 
dust defocusing layers, reduce optical reading noise due to dust since the 
light beam used for reading has a very small depth of focus. Therefore, 
any dust on the overcoat surface will be outside the focal plane of the 
beam, and will contribute less to the noise level. Materials useful as 
overcoats are optically transparent and mechanically durable. However, 
overcoating materials which are cured by UV light are usable if the mold 
shell 32 and the layer 54 are transparent to the required wavelengths. 
"Hard coat" plastics, such as the hard materials coated over eyeglasses, 
are desirable and would be hardened by exposure to UV light through the 
mold shell 32 and layer 54. 
Where recording is magneto-optic, the overcoat preferably is not 
birefringent to the plane polarized light used to read the data. 
Overcoating resins which do not develop large strains upon curing, as 
might occur in casting resins which exhibit high shrink rates, are less 
likely to produce birefringence. 
The overcoat can be applied with the molding apparatus used to apply the 
recording substrate to the drum section. After the optical recording 
material has been applied, the drum section 12 is placed in the mold shell 
32, and the retaining ring 40 is attached. Rotation is started as before, 
and when a suitable, reduced speed (e g., 450 rpm) is reached, a measured 
quantity of mold liquid which forms layer 54 is deposited onto the top of 
the rotating core 18. The mold liquid is flung outwardly, caught by the 
ring 40, and directed downwardly along the inner wall of the mold shell 
32. The quantity of mold liquid used is less than that used for casting 
the surface layer 20, as the inner surface of the layer 54 is farther from 
the core 18, due to the thickness of the surface layer 20 already applied. 
Once the mold liquid is in place, the overcoat layer is applied. Again, the 
rotational speed is increased, and the mold is slowly rotated into the 
horizontal position as curing occurs. Finally, the drum section 12 is 
removed from the mold, and any excess edge material is removed. This is a 
particularly convenient point at which to perform final machining or other 
finishing operations, since the dust defocusing layer will protect the 
recording surface from any contaminants resulting from such operations. 
Once the final finishing is done, the drum section 12 is ready to be 
mounted on a drum shaft, perhaps with other drum sections, to form a 
rotating drum memory. 
Fabrication Of Cores 
Alternatively, the drum core 18 can be preformed, and the surface layer 20 
formed around the core 18 in a mold apparatus 30. This allows the 
core-forming and surface-forming operations to be performed in parallel. 
Depending upon the material used for the core 18 and the intermediate 
layers, several drum section configurations can be produced. 
In forming the core 18, after the retaining ring 40 is in place, the mold 
shell 32 is rotated at the required speed, and a measured quantity of the 
mold liquid is deposited into the spinning mold shell 32 to form a mold 
layer 54. Next, the outer core 26 is formed by depositing a quantity of 
the casting epoxy resin containing hollow microspheres into the rotating 
mold. As the density of the microspheres are less than the epoxy resin, 
the outer surface of this outer core layer is higher in epoxy content, and 
is smoother than the inner portion of the layer. Once the 
epoxy/microsphere layer has cured, rotation is stopped, the retaining ring 
40 is removed, the center sleeve 22 is positioned on shaft 34, and the 
inner core 24 can be cast. 
The inner core 24 is formed by filling the remaining volume of the mold 
with the epoxy and hollow ceramic macrosphere mixture described above. The 
top surface is leveled and smoothed, and the inner core 24 is allowed to 
cure. The core 18 is removed from the mold, and finishing operations are 
performed. 
In order to keep the weight, inertia, and cost of prefabricated cores 18 
low, the core material must combine light weight with strength and it must 
be low cost. Ceramic spheres, such as those manufactured in 1990 by 3M 
under the trade name MACROLITE.TM., meet these requirements. When wetted 
with epoxy resin and allowed to cure in a solid state, the ceramic spheres 
produce a lightweight solid that is hard and has high tensile strength to 
resist high centrifugal forces encountered in rapidly spinning drum 
applications. This material is low in cost. 
Alternatively, prefabricated drum cores 18 can be formed as a single layer 
by pressing the paste-like substance, produced by wetting the ceramic 
spheres with epoxy, into a cylindrical cavity without using centrifugal 
force. The cavity can be a mold shell 32, having dimensions identical to 
the desired size of the finished core. A metallic sleeve 22 is slipped 
over the shaft 34 prior to inserting the ceramic spheres, and the sleeve 
22 becomes the central structural member of the drum section 12 used for 
mounting the core 18 and for locating the core 18 during subsequent drum 
section 12 fabrication processes and in final mounting in optical 
recording systems. The epoxy which wets the ceramic spheres also cements 
the metal sleeve 22 to the ceramic sphere core. This simple process, 
however, produces drum sections 12 which are very porous at the perimeter, 
and which therefore provide poor surfaces on which to coat optical layers 
by the spinning processes described above. Also, voids can be created in 
the interior mass of the cores 18 which can cause the drum sections 12 to 
be unbalanced when rotated in subsequent fabrication processes and in use 
as optical drums. 
Both of these problems are eliminated by using a core mold which is spun at 
several hundred rpm for a short time after the wetted ceramic sphere 
material is placed inside the mold, and which is then rotated at 
approximately 30 rpm until the epoxy hardens. This mold differs from those 
described above in that rather than using a retaining ring 40, a rigid 
flat end wall 58 is attached to the mold shell 32 to close the mold 
cavity. The core material is inserted into the mold cavity of the mold 
shell 32 before the flat end wall 58 is attached across the upper opening 
of the mold shell 32. The initial fast spin of the mold causes the epoxy 
under centrifugal force to migrate through the ceramic spheres to the 
perimeter of the mold, producing an epoxy-rich, solid layer at the 
perimeter of the core 18. This effect eliminates the problem of porosity 
of the core 18 since the rapid spin also causes the ceramic spheres to be 
flung outwardly from the center of the mold, increasing the density of the 
ceramic spheres and closing any voids that may exist. Once these effects 
have been accomplished by the fast spin, the rotation speed of the mold is 
reduced to prevent further migration of epoxy to the perimeter, but slow 
speed rotation is maintained to prevent gravity from causing epoxy to run 
to the bottom as would happen in a stationary mold. 
However, centrifuging the epoxy and the ceramic spheres to the perimeter of 
the mold creates a void between the ceramic spheres and the sleeve 22 at 
the center of the mold. A mechanism which takes up this void includes 
silicone rubber bladders 56 mounted on the flat end walls 58, 60 of the 
mold shell 32, as shown in FIG. 4. Because the flat end plates 58, 60 
close the mold shell 32 and prevent more core material from being added, 
the bladders 56 are expanded to take up the volume of material lost by the 
compacting of the drum core material and give the cured core 18 an 
indented shape on both edges. After the mold shell 32 is closed, the 
bladders 56 are expanded inwardly by compressed air or another gas 
transported to the mold shell 32 by a hollow shaft 62 and a rotary air 
coupling 64, as shown in FIG. 4. Air passageways 66 are formed in the flat 
end walls 58, 60 and communicate between the bladders 56 and the hollow 
shaft 62. The expanded bladders 56 compress the ceramic spheres into a 
solid mass, and force the mass back into contact with the center sleeve 22 
of the mold. The bladders 56 leave concave depressions on both sides of 
the finished core 18, but these depressions only reduce the mass of the 
cores 18, and insure that drum sections 12 fit together without 
interference when multiple drum sections 12 are mounted side-by-side on a 
common 10 shaft. This is because the depressions insure that the core 
material does not project beyond an ideal plane surface and cause 
interference between adjacent drum sections 12. The two sides of the cores 
18 are maintained symmetrical by using stiff rubber bladders 56 that 
require considerable air pressure to inflate. This stiffness ensures that 
the bladders 56 inflate equally, and since the wetted ceramic spheres 
offer little resistance to the bladder 56, the sides of the cores 18 
become symmetrical. 
Liquid Mold System With Prefabricated Core 
After the core 18 is fabricated, the surface layer 20 is formed on the core 
18 using the liquid mold system in a separate mold apparatus 30 as shown 
in FIG. 5. A prefabricated solid core 18 is inserted into the mold shell 
32, before the mold layer 54 is formed, by removing the retaining ring 40 
and placing the core 18 over the shaft 34 and into the mold shell 32 until 
it rests on the lower gasket 44. It is preferred that the mold shell 32 
have an inside diameter approximately 6 mm (0.24 inches) larger than the 
desired diameter of the finished drum section 12, and the outer diameter 
of the core 18 be a few millimeters smaller than the diameter of the inner 
surface of the layer 54. The surface of the core 18 can be roughened by 
machining or grinding to enhance adhesion of the epoxy. Adhesion promotion 
is especially important for drums that will operate at high rotational 
speeds. 
Next, the retaining ring 40 and upper gasket 42 are replaced and secured to 
the mold shell 32, so that the gasket 42 seals against the drum section 
12, and the gasket 44 is compressed by the core 18. The bottom of the drum 
section is sealed by the gasket 44 at the bottom of the mold shell 32 all 
of the way around the drum section 12 perimeter. After the core 18 and 
retaining ring 40 are in place, rotation of the mold shell 32 begins. The 
rotational speed is initially relatively low, suitable for the initial 
steps of casting. This prevents deposition of the casting resin from 
unbalancing the apparatus 30, disrupting the interface between the mold 
liquid and the casting resin, and reducing the quality of drum section 
surface 10. A starting speed of 450 RPM has been found suitable. 
Referring now to FIG. 6 as well as FIG. 5, when the desired speed is 
reached, the mold liquid which forms layer 54 is poured into the mold 
shell 32. This is accomplished by pouring the liquid into an annular 
groove 68 at the inside diameter of the retaining ring 40. The liquid 
passes through several small holes 70 spaced around the inside surface of 
the ring 40 into the space between the core 18 and the mold shell 32. The 
quantity of mold liquid that is used establishes a desired average radius 
of the inner free surface from the center of rotation of the mold shell 
32. This radius also becomes the outer radius of the surface layer 20 when 
the drum section 12 is completed. 
An activated resin is then poured into a second annular groove 72 in the 
retaining ring 40. The resin then flows through a separate set of small 
holes 74 spaced around the inside surface of the ring 40 and located on a 
shorter radius than the first set of holes 70. The resin fills the space 
between the free surface of the layer 54 and the core 18, and it contacts 
and adheres to the surface of the core 18. An excess of both the mold 
liquid and resin are poured into the retaining ring 40 grooves 68, 72 so 
that the materials rise to levels having smaller radii than the 
corresponding layers, as shown in FIG. 6. The radii at which the levels 
stabilize depends upon the ratio of the densities of the two liquids. The 
excess liquids within the retaining ring grooves 68, 72 serve as 
reservoirs which replenish liquids which are lost due to evaporation and 
to shrinkage of the resin as the resin cures during mold shell 32 
rotation. The grooves 68, 72 and holes 70, 74 of the retaining ring 40 are 
necessary for the insertion of liquids into the mold shell 32 when a 
preformed core 18 is used as the core 18 blocks access to the outer 
diameter portion of the mold shell cavity. 
The mold layer 54 and the surface substance 20 are poured into the mold 
shell 32 with the shaft 34 axis preferably oriented approximately 
30.degree. from horizontal, a 60.degree. tilt. The mold shell 32 is 
preferably fixed in this orientation to simplify the apparatus and obviate 
the need to pivot or tilt the mold shell 32. This mold shell 32 
orientation, shown in FIG. 6B, substantially eliminates any wedge shape, 
shown in FIG. 6A, which is otherwise created due to the force of gravity 
acting on and enlarging the lower portion of the mold layer 54. 
Additionally, as the amount of wedging is influenced by the rotational 
speed of the mold shell 32, higher rotational speeds further reduce 
wedging. While tilting the mold shell 32 to place the axis of the shaft 34 
horizontal completely eliminates the wedge shape, the resulting wedge 
using a 30.degree. tilt is less than 0.025 mm (0.001 in), well within the 
limits which can be handled by a focus servo of the recording system. 
A horizontal axis for the shaft 34 is not used as it presents air 
entrapment problems. Insertion of the liquids into the space between the 
mold shell 32 and the core 18 must displace air so that air bubbles are 
not trapped between the surface layer 20 and the core 18. If the mold 
shell 32 axis were horizontal, there would be no escape path for the air 
bubbles as the surface layer 20 material would move vertically with a flat 
surface to entrap air. Bubbles are not entrapped when the axis of rotation 
of the mold shell 32 is not horizontal. The 30.degree. tilt, combined 
with the initial 450 rpm rotational speed, creates the wedge shape in the 
liquid mold layer 54 and provides a relative angle between the core 18 and 
the free surface of the mold layer 54. Resin inserted into the space 
between the mold layer 54 and the core 18 rises with uniform thickness to 
meet the core 18. It meets the core 18 first at the closed end of the mold 
shell 32, causing air between the resin and the core 18 to move toward and 
through the small space between the core 18 and the retaining ring 40. 
Once the volume of space between the mold shell 32 and the core 18 has been 
filled, the mold shaft axis is returned to a vertical position, shown in 
FIG. 6A, if the mold shell 32 is pivotable, and any remaining wedge shape 
of layers 20 and 54 gives way to layers of uniform thickness across the 
width of the core. The rotational speed of the mold shell 32 is now 
increased to increase the centrifugal force on both layers so that the 
resin cures under high pressure in contact with the mold layer 54 and the 
core 18. Shrinkage of the resin is compensated for by the addition of 
resin from the reservoir contained in the retaining ring 40. 
After the surface layer 20 has cured, rotation can be stopped. Due to the 
high viscosity of the epoxy and because the entire volume between the core 
18 and the mold shell 32 is filled, wave motion caused by vibration is 
negligible. Any mold layer 54 liquid that spills out of the mold shell 32 
is caught in a shallow tray (not shown) located beneath the mold shell 32. 
Once rotation has stopped, the retaining ring 40 is removed, and the 
finished drum section 12 is removed from the mold. Since the mold surface 
is liquid, the drum section 12 will not stick to or damage the mold 
surface. Where beeswax is used, the drum section 12 can be removed from 
the mold 32 end surface by heating it sufficiently to melt the beeswax at 
the bottom and then turning the mold upside down and allowing the drum 
section 12 to slide down the shaft 34. 
As slight overfilling of the mold during casting assures complete coverage 
of the core 18 by the surface layer 20, some excess material may be 
present at the edges of the finished part. This material can be removed by 
machining, as long as damage to the curved peripheral surface 10 is 
prevented. Once final edge finishing of the drum section 12 is complete, 
the recording material can be applied to the surface 10 of the surface 
layer 20. An overcoat is applied, and edge finishing is performed as 
described above. 
Solid Mold System With Prefabricated Core 
Alternatively, when forming a surface layer 20 for a drum section 12, the 
mold layer 54 can be formed using a hardenable casting epoxy or prepolymer 
composition, called the mold resin. Although this method is not as simple 
as using a liquid mold system and requires more steps, it represents an 
improvement over known prior art systems. In this system, the liquid mold 
resin is deposited into a spinning mold shell 32 and is allowed to cure 
while maintaining the rotational speed, thereby forming a solid tapered 
cylindrical mold layer 54 having a shape described by equation 4. 
Commercially available casting epoxies which have a very low shrink level 
during curing and do not contain volatile solvents or produce reaction 
products which bubble or form other surface defects are suitable mold 
resin materials for layer 54. Epoxies adhere well to clean metals and 
other durable materials, even without adhesion promoting primers, and are 
curable at temperatures near room temperature. Additionally, some epoxies 
provide a very brittle, optically polishable surface. Because of the need 
for high surface smoothness, the liquid materials used to form the mold 
and the drum surface layer 20 should not contain particulate impurities of 
any significant size. This can be achieved by filtering using filters 
having pore sizes in the submicron size range. Liquid materials having 
this level of purity are microelectronic grade materials. A particularly 
suitable mold resin is a two-part liquid casting epoxy prepared by 
blending 100 parts by weight of a low viscosity bisphenol a compound with 
55 parts by weight of a modified cycloaliphatic amine hardener. The mixed 
composition is aspirated in a vacuum chamber to remove any air bubbles 
entrained during mixing, as is common when casting polymeric materials. 
The aspirated resin blend is deposited onto the rotating inner wall of the 
mold shell 32. The speed of the mold shell 32 is held constant at 
approximately 2500 rpm until the mold resin forming the layer 54 hardens. 
While the free surface of mold layer 54 will be smooth after curing, it is 
not yet suitable for use as a mold. After stopping rotation, any 
protrusions caused by particulate impurities which traveled to the inner 
surface of the mold layer 54 must be removed by polishing. Also, as many 
casting resins adhere to, chemically react with, or diffuse into the mold 
layer 54, the mold layer 54 must be provided with a protective coating. 
The coating isolates the mold layer 54 from the materials used in casting 
but does not reduce its smoothness or adversely affect its concentricity 
or other dimensions. The coating, such as vacuum deposited chromium, also 
effects a release of the outer surface 20 from the solid mold layer 54 by 
preventing adhesion of the outer surface 20 to the mold layer 54. Also, 
wiping a very thin layer of release agent on the mold layer 54 before 
inserting the surface layer material further prevents adhesion. 
The mold resin forms a coating having a thickness causing its surface to be 
equidistant from the axis of rotation of the mold shell 32 to insure 
concentricity, flatness, and roundness of the surface. Due to gravity, the 
lower portion of the surface of mold layer 54 is slightly thicker so that 
the surface forms a wedge or conical shape. This shape assists the release 
of the finished drum section 12 from the mold shell 32. A wedge of any 
taper can be obtained by controlling the rotation speed of the mold shell 
32. A difference in the upper and lower radii of approximately 0.1 mm 
(0.004 in) is acceptable. 
When the drum section 12 is to be formed without a prefabricated core 18, 
fabrication proceeds as described above. 
Where a preformed core 18 is used, the retaining ring 40 is removed and the 
core 18 is placed over the shaft 34 and into the mold, until it rests on 
the lower gasket 44. The retaining ring 40 and upper gasket 42 are then 
replaced and secured to the mold shell 32, and the bottom of the drum 
section 12 is sealed at the bottom of the mold shell 32 all the way around 
its perimeter as done when using a liquid mold system. Next, the surface 
layer 20 is formed by filling the gap surrounding the core 18 with casting 
resin 76. 
The casting resin 76 can be injected into the sealed gap between the mold 
layer 54 and the core 18, using top and bottom openings provided in the 
gap, as shown in FIG. 7. The top opening is connected to a vacuum source 
78 by tube 80 to remove air from the gap. Alternatively, the entire mold 
may be placed in a vacuum chamber, provided that casting resin 76 remains 
exposed to the atmosphere, or is pressurized. The bottom opening is 
connected to a valved tube 82, which is immersed in casting resin 76 in a 
container 84. The casting resin 76 is allowed to flow into the gap until 
it begins to exit from the top opening, whereupon the openings are closed, 
and the casting resin 76 in the gap is allowed to cure. 
Once the casting resin 76 has cured, the retaining ring 40 is removed, and 
the core 18 with a cast surface layer 20 around its outer periphery is 
removed from the mold. The brittle resin in the inlet and outlet tubes 80, 
82 is easily broken off during removal of the finished drum section 12. 
Removal is simplified if the drum section 12 is provided with grasping 
means to pull it out, or if a set of ejector pins (not shown) pushes out 
the drum section 12 from the mold shell 32. The lower gasket 44 can be 
compressed during the sealing of the retaining ring 40, and the release of 
this compression may be sufficient to release the drum section 12 from the 
mold. Additionally, the taper or wedging in the mold layer 54, which can 
be controlled by controlling the centrifugal force according to the 
equations discussed above, increases the physical clearance between the 
mold layer 54 and the drum section 12 as the parts separate to insure that 
neither surface is damaged. Thus, in this situation, wedging is 
beneficial. The drum section 12 is then slid from the shaft 34 by turning 
the mold upside down. These drum sections 12 may require slight edge 
finishing, especially at the entrance and exit holes. Finishing can be 
accomplished by conventional machining, as described above. Finally, a 
magneto-optic recording layer can be deposited onto the surface of the 
drum section 12. A protective overcoat layer may also be applied using the 
same molding techniques. 
Numerous characteristics, advantages, and embodiments of the invention have 
been described in detail in the foregoing description with reference to 
the accompanying drawings. However, the disclosure is illustrative only 
and the invention is not intended to be limited to the precise embodiments 
illustrated. Various changes and modifications may be effected therein by 
one skilled in the art without departing from the scope or spirit of the 
invention. For example, differently-sized molds, operating at different 
speeds may be used to form smaller or larger drum sections. 1:1.3