Cassette for disposable microorganism culturing media and automated scanning system

An apparatus for counting microorganism colonies on at least one disposable microorganism culturing medium having a substantially planar substrate. The apparatus is adapted to interact with a cassette holding a number of similar substrates at once. The apparatus includes an imaging device capable of detecting colonies on the substantially planar substrate, a cassette positioning device for moving the cassette so that the substrates supported within the cassette are moved sequentially into a predetermined position relative to the imaging device, and an ejecting device for ejecting a substantially planar substrate in turn from the cassette when that substrate is in the predetermined position from the cassette and into an imaging position adjacent the imaging device. Suitable cassettes and a method for counting colonies on substrates supported within a cassette are also disclosed.

TECHNICAL FIELD 
This invention relates to a method and apparatus for queuing and moving 
multiple similar objects for imaging, particularly to allow counting the 
number of distinct elements in the images so obtained. In particular, it 
relates to microbiological testing, and more particularly to improvements 
in the handling and reading of disposable microorganism culturing devices. 
BACKGROUND OF THE INVENTION 
Different methods and devices are known for counting microorganism colonies 
in, for example petri dishes or in thin film culture plate devices such as 
PETRIFILM.TM., manufactured by 3M of St. Paul, Minn. In the latter case, 
such devices have a very thin layer of growth medium, making all colonies 
visible with surface illumination. Manual counting of colonies on such 
media by trained laboratory personnel is well-known; typically a film will 
be inoculated and marked as to the source of the inoculant, stacked 
together with similar samples, and placed into an incubator. A manual 
inspection and counting is performed after a period of 12 to 24 hours. 
This method has known disadvantages, particularly the costs associated 
with the use of skilled technicians to perform the time-consuming task of 
manual counting, as well as the limited accuracy of the counts achieved. 
Also desirable in the matter of microbiological counting is the early 
detection of colonies, particularly when food products are being tested. 
If the samples indicate excessive contamination, the product must often be 
discarded. Reliable early detection of excessive contamination in the 
range of 6 to 12 hours after inoculation would be welcomed by 
manufacturers because it would allow them to identify contaminated 
products early in processing, thereby avoiding additional expenses 
incurred in processing product that will be discarded and possibly 
contaminating additional product by running it through contaminated 
processing equipment. 
International Publication No. WO 94/26870, which is hereby incorporated by 
reference, discusses improvements in colony counting in e.g. disposable 
microorganism culturing media having a substantially planar substrate. 
These include scanning and imaging the inoculated surface more than once 
and processing the images to produce a scaled time lapse image. Processing 
this scaled time lapse image allows the identification of hit pixels which 
can be clustered to identify the appearance of colonies. Thus early 
indication of colony growth can be obtained. 
U.S. Pat. No. 5,403,722, which is also incorporated by reference, discloses 
a method and an apparatus for automated counting. However, a limitation on 
the apparatus disclosed therein is the physical properties of the 
disposable microorganism culturing media. Optimally, the art could wish 
for a more robust handling of the planar substrates, which have to be 
manipulated as a loose pile. 
International Publication No. WO 95/16768, which is also incorporated by 
reference, discloses a specialized tool for handling disposable media 
under automatic control. The apparatus has an imaging means for detecting 
colonies on the disposable media, which media are contained within 
individual holders. Cooperating with the imaging means is a holder 
positioning means for storing and queuing one or more of the holders. The 
holder positioning means is adapted for moving the holders sequentially 
into a predetermined position relative to the imaging means so that images 
can be obtained. While this system provides a very accurate early 
detection, it is more complicated and expensive than is required in all 
industries where microbiological assays are performed. 
OTHER BACKGROUND ART 
For a more complete description of disposable devices for culturing 
microorganism such as PETRIFILM.TM. disposable thin film culture plates, 
the reader is directed to U.S. Pat. No. 4,565,783 to Hansen et al., which 
is hereby incorporated by reference. Additional chemistries which are 
preferred for use with the imaging means contemplated by the present 
invention are more completely described in U.S. Pat. No. 5,364,766. That 
copending and coassigned application is also hereby incorporated by 
reference. 
SUMMARY OF THE INVENTION 
The present invention addresses the above identified limitations in the art 
by allowing planar substrates to be more accurately monitored under 
automatic control. In one aspect, the invention might be considered as an 
apparatus for counting microorganism colonies on at least one disposable 
microorganism culturing medium having a substantially planar substrate, 
the substrate adapted to fit within and be supported by a cassette. Using 
a cassette this way provides a convenient way to handle a number of 
substrates at once. The apparatus itself includes an imaging means capable 
of detecting colonies on the substantially planar substrate. It also 
includes a cassette positioning means for moving the cassette so that the 
substantially planar substrates supported within the cassette are moved 
sequentially into a predetermined position relative to the imaging means. 
Finally, there will be an ejecting means for ejecting a substantially 
planar substrate in turn from the cassette when that substrate is in the 
predetermined position from the cassette and into an imaging position 
adjacent the imaging means. 
In some embodiments, the imaging means and its associated electronics 
provides an indicating signal as to whether or not a substantially planar 
substrate matches predetermined criteria chosen by the operator of the 
apparatus. This indicating signal may be transmitted to the operator 
directly via panel displays or printouts, or it may be logged 
electronically for later retrieval. In addition, the signal may be used to 
control a means for sorting the substantially planar substrates physically 
into two or more categories based on the indicating signal. 
Other embodiments of the apparatus may be adapted to work with a particular 
type of cassette. This cassette will be adapted for supporting a plurality 
of substantially planar substrates, and can generally be described as an 
enclosure having two ends. Between the two ends, and disposed within but 
not rigidly affixed to the enclosure, will be a plurality of plates. There 
will be some means for retaining the plates within the enclosure. This 
could be, for example, rigid strips tying the edges of the ends together. 
More conveniently, the ends can be tied together by a number of rods 
extending between the ends. Mounted as a stack on these rods will be the 
plates, each having holes therein to permit the mounting, with the rods 
passing though the holes. There will be some means for supporting the 
plates at least some minimum distance apart from each other. This allows 
the ejection means to move each substrate in turn robustly, and allows an 
operator to readily separate the plates when loading the cassette with 
substrates. This may conveniently be accomplished by placing annular 
spacers, such as thin washers, on the rods between the plates. This is 
particularly appropriate when the plates are fabricated by die cutting 
from polymeric sheet stock. Alternatively, projections can be attached or 
molded onto to the plates to stand them off in the required fashion. 
In preferred embodiments, the plates each have a cut-out section in one 
side, and are mounted with these cut-out sections one above another. As 
will be detailed more specifically below, such an arrangement facilitates 
the ejecting means acting on the substrates as the positioning means moves 
the cassette. 
In a second aspect, the invention may be considered to be a method of 
counting microorganism colonies on at least one disposable microorganism 
culturing medium having a substantially planar substrate, said method 
comprising the steps of: 
(a) providing a cassette adapted to support a plurality of substantially 
planar substrates; 
(b) moving said cassette so that the substantially planar substrates 
supported therein are moved sequentially into a predetermined position; 
(c) ejecting sequentially the substantially planar substrates in said 
predetermined position from the cassette into an imaging position; and 
(d) imaging the substantially planar substrates in said imaging position. 
Optionally, the method may further include the step of sorting the 
substantially planar substrate into categories based on the results of the 
imaging step. 
In a third aspect, the invention may be considered to be related to the 
cassette described above per se, with its special utility in supporting 
the substantially planar substrates. 
The invention provides an apparatus for automatically counting colonies 
during microbiological testing. 
A feature of the invention is the use of a cassette holding a number of 
substantially planar substrates at once, which cassette is inserted into 
the reading apparatus. The substrates are then ejected and imaged one at a 
time as their location within the cassette passes by a predetermined 
position. 
An advantage of the invention is that a number of substrates can be handled 
and imaged automatically as a group, with the handling of the thin, flimsy 
substrates being accomplished robustly. 
Another advantage of the invention is that the apparatus can deal with a 
cassette which is not totally filled; rather it can handle, without 
special operator intervention, any arrangement of the loading of the 
cassette.

DETAILED DESCRIPTION 
Referring now to FIG. 1, a side view of a cassette according to the present 
invention, suitable for supporting a disposable microorganism culturing 
medium having a substantially planar substrate, is illustrated. The 
cassette 20 includes an enclosure 22 having a top end 24 and a bottom end 
26. The ends 24 and 26 are held rigidly together by several rods 28, and 
optionally for more structural strength, one or more side walls 30. A 
number of plates 32, each having a plurality of holes 34 therein, are 
mounted within the enclosure 22 with the rods 28 passing through the holes 
34. The plates 32 are kept a minimum distance apart from each other by 
means of annular spacers 36, also mounted on the rods 28. In the 
illustration, a disposable microorganism culturing medium having a 
substantially planar substrate 40 is shown being loaded into the cassette 
20. The cassette 20 is lying on its back, and in this position, the plates 
32 can be manually opened to the correct place for substrate insertion. 
This action is facilitated by the fact that the plates 32 are held apart 
by the spacers. 36, which allows a fingertip to separate them, and by the 
fact that the size of the holes 34 in the plates 32 is sufficient to allow 
the plates to flop over somewhat and provide a gap 42 as illustrated in 
this figure. Insertion of a substrate 40 is further enhanced if the plates 
32 are made of a thin, but bendable, polymeric material. In preferred 
embodiments, there will be a stack weight 44 mounted on the rods 28 at the 
top of the stack of plates 32 to close gaps and provide a predictable 
downward force on the stack of plates when the cassette is held upright. 
Such a downward force facilitates the action of the ejecting means as will 
be described below. For the convenience of the operator, a handle 46 may 
be attached to the top end 24. 
Referring now to FIG. 2, a top section view taken along section lines 2--2 
in FIG. 1 is illustrated. For clarity, in this view a substrate 40 is 
illustrated in phantom lines showing a typical placement when loaded 
within the cassette 20. In this view it is more readily seen that besides 
rods 28 which pass through holes 34 in plates 32, there may also 
conveniently be guide rods 48 and 49 positioned at the rear of the 
cassette 20 to limit how far the substrate 40 can go when inserted from 
the front into the cassette. It is also convenient to provide a projecting 
tab 50 on the front of each plate to make separating the plates for the 
insertion of the substrates as easy as possible for the operator. 
It will also be seen that the plate 32 has a cut-out section 52 in the back 
side. As will be noted with more specificity later, the ejecting means 
resides partially within the space defined by the cut-out sections when 
the cassette 20 is engaged with the positioning means in the counting 
apparatus. 
Preferred substrates suitable for use with this apparatus will have a base 
sheet 54, and a cover sheet 56 which is slightly longer. This arrangement 
not only makes the substrate easier to handle during inoculation, but in 
the context of the present invention helps the action of the ejecting 
means be more certain. 
Referring now to FIG. 2a, a disposable microorganism culturing device 40, 
such as the PETRIFILM.TM. product described above, is illustrated. The 
device includes a base sheet 54 having microorganism colonies 57 located 
on its surface. Once again, it is to be noted that the cover sheet 56 
extends further than the base sheet 54. In a preferred embodiment, the 
substrate 40 also preferably includes a barcode 58 at its upper edge for 
identifying individual samples. PETRIFILM.TM. products also typically 
include a series of grid lines 59 which are useful for manual counting of 
colonies. Although the preferred apparatus is designed for use with 
PETRIFILM.TM. products, it will be understood that any similar product 
having a substantially planar substrate with nutrients adhered to its 
surface for the culturing of microorganisms could be used in place of 
PETRIFILM.TM. products. 
Now referring to FIG. 3, a top perspective view of an embodiment of the 
counting apparatus 60 of the present invention, adapted for use with the 
cassette of FIG. 1, is illustrated. The bulk of the apparatus is protected 
by an enclosure 62, and mounted on the enclosure is a control panel 64 and 
a display screen 66 to allow operating commands to be entered by the user, 
and for data to be displayed. A recess 68 is provided in the top of the 
enclosure 62, adapted to receive a cassette 20 of the type described 
above. The imaging means 70 (not seen) and its associated electronics are 
located under access cover 72. Substrates which are ejected from the 
cassette are delivered to the imaging means 70 through slot 74, and 
discharged from the imaging means through slot 76. Once discharged from 
the slot 76, the substrate falls into a sorting cradle 78, and onto a 
rotatable platform 80. A signal from the imaging means cues a mechanism 
which can tilt the rotatable platform 80 one way or the other, dropping 
each substrate into one side or the other of a generally W-shaped bin 82, 
thus sorting the substrates into categories based on the results of the 
imaging step. For example, the electronics may be set to sort the 
substrates into the categories of minimally acceptable and non-acceptable 
colony counts according to a criterion chosen by the operator. 
Alternatively, the sorting criterion might be successfully scanned and not 
successfully scanned substrates. Other mechanisms which can sort the 
substrates into more than two categories are also contemplated within the 
definition of the invention. 
Within recess 68 is a means to position the cassette as required, including 
a movable stand 84 which has a pair of threaded trucks 86 and 88. The 
trucks are adapted to engage a pair of threaded shafts 90 and 92, so that 
synchronously rotating the shafts causes the trucks, and hence the stand 
84 to move up or down. Housed in a central column 94 within the recess 68 
is the means for ejecting a substrate out of the cassette and towards slot 
74. This column 94 fits within a cut-out portion 96 in stand 84, so that 
the stand can move downwards within the recess 68 from the starting 
position here illustrated. As it does so, column 94 begins to enter the 
space defined by the cut-out sections 52 in plates 32. 
During the operation of the counting apparatus 60 to count colonies on 
substrates contained within a cassette 20, the stand 84 is moved to its 
uppermost position by sending an appropriate signal to the motor 
controlling the rotation of threaded shafts 90 and 92. Then the cassette 
beating the substrates of interest is placed on stand 84, and the read 
operation started. The threaded shafts 90 and 92 begin to rotate so that 
the stand 84 is lowered relative to column 94. As column 94 begins to 
enter the space defined by the cut-out sections 52 in plates 32, 
eventually a sensor lever 98 will contact the lowest substrate within the 
stack of plates 32. This lightly spring loaded sensor lever 98 will 
depress, signalling the motor controller to temporarily halt the rotation 
of the threaded shafts. Attached to the sensor lever 98 is an extended arm 
100 which will signal the motor controller to stop the rotation of the 
threaded shafts when the stand 84 is being raised and arrives at its 
uppermost position. 
At the point when the actuation of sensor lever 98 halts the lowering of 
the movable stand 84 due to its contacting the lowermost substrate 40 in 
the stack, pusher arm 102 will be under the extended edge of its cover 
sheet 56 but behind the edge of its base sheet 54. Thus the cover sheet 56 
of the lowermost substrate 40 shields the pusher arm 102 from accidently 
snagging the next substrate upwards and causing a misfeed, when as next 
occurs, the pusher arm 102 moves in a circular fashion within curved slot 
104 to eject the lowest substrate. The pusher arm 102 moves the substrate 
through slot 74 and into engagement with pinch rollers associated with the 
imaging means. Then the pusher arm 102 retracts backwards, and the 
threaded shafts 90 and 92 are activated again so that the next substrate 
in the stack within the cassette 20 can be located and the cycle started 
again. Eventually, all the substrates are read, and the stand 84 bottoms 
out near the bottom of recess 68 and contacts a limit lever 106 (seen in 
FIG. 4), notifying the electronics that the entire cassette has been read. 
FIG. 3a is a horizontal cross-section detail view of the area of the top of 
column 94, taken along the plane defined by the top of the column, when 
the cassette 20 is mounted on the movable stand 84. As in FIG. 2, the 
substrate 40 is shown in phantom in order to visualize the structures on 
the top of the column 94 and their relationship to the lowest substrate in 
the stack of substrates within the cassette, and more readily illustrates 
how spring loaded sensor lever 98 will depress as the cassette 20 moves 
downwards and the substrate 40 contacts it. In this view it is also more 
readily appreciated how the leading edge of pusher arm 102 will be under 
the extended edge of cover sheet 56 but behind the edge of its base sheet 
54. When commanded by the electronic controls of the apparatus, the pusher 
arm 102 moves in a arced fashion within curved slot 104, turning its flat 
leading edge 105 towards the substrate 40 as it moves so that an even 
pressure is applied when the substrate is pushed into engagement with the 
pinch roller pair within slot 74. 
Once again with reference to FIG. 3, the recess 68 is closed on one side 
with a side panel 108 for the convenience of the maintenance technician. 
Referring now to FIG. 4, a side perspective view of the apparatus of FIG. 
3, with side panel 108 removed to more readily display the interaction 
between a cassette and the cassette positioning means, is illustrated. 
Referring now to FIG. 5, a side view of a cassette interacting with the 
sensor elements usually housed within column 94, with the column removed 
for clarity, is illustrated. The cassette 20 is supported by stand 84, 
which has been lowered to the point that the lowest substrate 40 in the 
stack of substrates in the cassette 20 (designated as 40d) is just 
contacting sensor lever 98. As lowering of the stand continues, the sensor 
lever 98 will depress slightly against the urging of extension spring 110 
and pivot about its pivotal mounting 112, which is fixed to column 94. The 
lower leg 114 of the sensor lever will move away from optical switch 116. 
When the light shield 118 is withdrawn from optical switch 116, the 
controlling electronics are signaled to halt the lowering of the stand 84 
and to activate the pusher arm 102. In this way, all of the substrates 40 
are read in turn until the stand 84 contacts limit lever 106, (this 
position is shown in phantom) which then pivots about its pivotal mounting 
120 and against the urging of extension spring 122 to withdraw light 
shield 124 on upper leg 126 from optical switch 128. This also causes 
extended portion 129 to move lower leg 114 away from optical switch 116, 
which halts the decent of the stand 84. The activation of optical switch 
128 further signals the electronics to return stand 84 to its uppermost 
starting position. 
Referring now to FIG. 6, a cross-section side view of the apparatus 60 is 
illustrated, particularly high-lighting a preferred arrangement for the 
imaging means 70. The counting apparatus 60 includes a motor 130 which is 
preferably a DC stepping motor. Motor 130 is operably connected to-roller 
pairs 132a,b and 134a,b to move a substrate 40 which has entered the 
enclosure 62 through slot 74 through the imaging means 70 for scanning. 
The stepping motor 130 provides accurate incremental movement of the 
substrate 40 though the imaging means 70. The preferred imaging means 70 
includes a linear LED array 136 aimed at the substrate 40. The preferred 
LED array is Model No. LT4665E manufactured by Sharp Corporation. This LED 
array is particularly useful because it produces light in wavelengths that 
neutralize a grid array placed on one version of the PETRIFILM.TM. product 
to facilitate manual counting. A similar effect could be duplicated with 
light sources producing other wavelengths by using a reddish-yellow filter 
(Wratten filter #9 or Corion LL-550-S-4385) between the light source and a 
counting sensor 138. 
The preferred LED array 136 is also useful for the present application 
because the intensity of the light produced across the array is 
substantially uniform, which aids in the accuracy of the apparatus. 
Located above substrate 40 in the preferred counter is a linear array of 
SELFOC lenses 140, Model No. SLA-12-TC43A6, manufactured by Nippon Sheet 
Glass Co. This rod lens array 140 is widely used in facsimile machines as 
well as some photocopying equipment. Above the SELFOC lens array is a 
linear CCD sensor array 138, preferably Model No. TCD 127AC, manufactured 
by Toshiba Corporation. 
FIG. 7 is an enlarged view of the scanning area of the preferred apparatus. 
As shown there, LED array 136 produces light 141 which reflects off of 
substrate 40 and is at least partially reflected through the SELFOC lens 
140 which focuses the light on the CCD image sensor 138. Rollers pairs 132 
and 134 are used to move the substrate 40 through the scanning area. As 
discussed above, the rollers are preferably connected to stepping motor 
130 for accurate incremental movement of the substrate 40 through the 
scanning area. 
FIG. 8 is a block diagram illustrating the preferred control system of the 
present invention. As illustrated, the CCD image sensor 138 is connected 
to the control circuit 142 based around a microprocessor 144. A preferred 
control circuit 142 will incorporate a CMOS microcontroller, Model No. 
80C451, manufactured by Signetics. 
Data from the CCD image sensor 138 is stored in the image memory 146 also 
connected to the microprocessor 144. The LCD display 66 is used to provide 
messages to the operator and prompt actions by the same. The cassette 
switches 116 and 128 are used to indicate the condition and position of 
the cassette as discussed above in connection with FIG. 5. The start key 
148, part of control panel 64, is used to begin the process of moving the 
cassette 20, bringing each substrate 40 in turn into the predetermined 
position, and ejecting the substrate into the imaging position and 
performing the imaging step. An input light switch 150 may optionally be 
present, used to indicate whether a substrate 40 has actually been moved 
into the imaging position, thus identifying misfeeds. 
The stepping motor 130 is used to incrementally advance substrates 40 
through the imaging position and is operably connected to the rollers 
132a,b and 134a,b as shown in FIGS. 6 & 7. The rollers are preferably 
gear-driven, although any suitably accurate connection method can be used. 
The LED bar array 136 is used to illuminate the surface of the substrate 40 
and any objects on it for sensing by the CCD image sensor 138. The CCD 
image sensor 138 is used to detect objects from the reflected light. The 
image sensitivity circuit 152 is used to adjust sensitivity of the CCD 
image sensor 138 to a level below which a blank substrate 40 will not 
trigger the image sensor. An advantage of image sensitivity circuit 152 is 
that it automatically adapts each element of CCD image sensor 138 to 
differences in the background image caused by variations due to different 
substrate materials, variations in light density across the surface of 
each substrate, and variations in the manufacturing tolerances of the CCD 
sensor. 
The image memory 146 is used to store data from the CCD image sensor 138 
and is used to detect the number of distinct objects on the surface of the 
substrate 40 using an appropriate algorithm. The RS-232 serial interface 
154 is used to provide communication between the present invention and a 
computer or other electronic device for purposes such as long-term data 
storage and analysis. 
To begin the process, the counting apparatus 60 is powered up and the LCD 
display 66 indicates to the operator that a cassette 20 containing a 
number of substrates should be loaded into the onto the movable stand 84. 
In some embodiments, the operator may be prompted to make sure the 
lowermost position in the cassette is loaded with a blank (empty) 
substrate. The blank substrate is used to adjust the sensitivity of the 
CCD image sensors via the image sensitivity circuit 152 in order to avoid 
errors in the detection process. Once the cassette is loaded, the operator 
begins the counting operation, for example by pressing a start key 148, 
and the read cycle begins as discussed above. 
The microcontroller 144 may also interact with the positioning means by 
having control over a motor 156 used to control the drive of threaded 
shafts 90 and 92. The exact programming required depends on the devices 
chosen, but the methods required are well known to those conversant with 
the art of programming embedded controllers. Any suitable machine vision 
algorithm may be used to analyze the scanned image, but the algorithm 
discussed in U.S. Pat. No. 5,403,722, incorporated by reference above, is 
considered particularly preferred. 
In preferred embodiments, the roller pair 132a and 132b eject the just 
scanned substrate 40 through slot 76 and into a sorting mechanism. 
Referring now to FIG. 9, a cross-section side view sectioned a vertical 
plane of symmetry through an exemplary sorting mechanism is illustrated. 
Conveniently, the sorting cradle 78 is attached to a bracket 158 which is 
adapted to removably engage enclosure 62 near slot 76. The bracket 158 
supports rotatable platform 80 on a pair of bearings 160 and 162. 
Rotatable platform 80 conveniently has an extension rod 164 which engages 
tilt mechanism 166 through a hole 168 in the enclosure 62 when the bracket 
158 is mounted on the enclosure so that mechanism 166 can tilt the 
rotatable platform 80 one way or the other, dropping each substrate into 
one side or the other of a generally W-shaped bin 82. These elements are 
sized so that the substrate cannot fall directly down beyond rotatable 
platform 80 while platform 80 is in its horizontal, neutral position. 
Depending on the results of the scan performed by the imaging means 70 the 
control circuit 142 will cue a tilt mechanism 166 which can tilt the 
rotatable platform 80 one way or the other, thus sorting the substrates 
40. 
Conveniently, the bracket 158 will have a stud 170, which fits into an 
aperture 172 in W-shaped bin 82, and a pair of extended rods 174, 
positioned to support the underside of the bin when the stud is in the 
aperture. This arrangement supports the bin 82 in a stable, but removable, 
fashion. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Cassettes suitable for use with the present invention can be fabricated 
from numerous materials, but the rigid structural components of the 
enclosure are conveniently formed from a metal such as aluminum by 
machining, or from thermoplastic polymers by injection molding. A certain 
amount of toughness in order to endure repeated handling by the operator 
and by the positioning means is desirable, so plastics such as 
polycarbonate are considered particularly preferred. Proper material 
selection for the plates helps to make a durable, yet easy to load 
cassette; sheet stock of polyetherimide, commercially available as ULTEM 
DL1648 from Cadillac Plastic, is considered preferred. 
While certain embodiments of the present invention have been described in 
detail herein and as shown in the accompanying Drawings, it will be 
evident that various further modifications are possible without departing 
from the scope of the invention.