Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION 2 ~
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This invention relates to Petri dishes. More particularly, the invention
relates to an improved reduced cost Retri dish with enhanced quality.
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Petri dishes ares commonly used, especially in laboratories for producing
cultures in a culture medium such as agar. These Petri dishes generally have a ~`
flat bottom and short cylindrical sides. The flatness of the bottom portion is
15 necessary so that a uniform layer of the culture medium can be produced using as little of the medium as possible.
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2 o Because the market for Petri dishes is highly competitive, it is necessary
for manufacturers to produce Petri dishes at as inexpensive a price as possible,
without reducing the quality of the product.
In Petri dishes, the bottom often tends to develop significant curvature
after it is removed from the molds. This is particularly due to rapid cooling
caused by the relatively fast cycle times in production. This curvature of the
bottom is a problem because it prevents accurate assessment of the culture in the
dish. Traditionally, this problem has been dealt with by thickening the walls ofthe dish and increasing the cooling time before it is ejected from the mold.
0 However, these solutions involve the use of more material than is required for
the mechanical strength of the finished article, and furthermore they lengthen
the mold cyde time. This makes the process expensive while at the same time
lowering the production rate of the mold.
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Thus, an improved Petri dish is needed, in order to reduce manufacturing
costs, and in which quality is either enhanced or at least not sacrificed.
The best way to reduce costs is to reduce the amount of material, i.e. resins,
required, since the cost of these materials is the primary sost factor in producing
Petri dishes. However, this has not been possible up until now to the extent
possible in the present invention without sacrificing quality. By "quality" is
15 meant that the bottom of the Petri dish must be as close to being absolutely flat as
possible, and the Petri dish must be reasonably strong. Other factors include the
ability of agar to bond to the dish and transparency.
SUMMARY OF THE INVENTION
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The present invention provides a Petri dish having a flat bottom and
shallow cylindrical sides. The bottom is provided with a plurality of preferably ; ~
radial ribs, the upwardly projecting portion of the ribs projecting not more than ~ -:
about 0.050 inches above the upper surface of the bottom, and preferably not
more than above 0.020 inches. The thickness of the bottom portion other than at
the ribs is not greater than about 0.030 inches, and preferably not greater than
about 0.025 inches. A mold is also provided for producing the said Petri dish. :;~
, The problems of the prior art relating to curvature of the bottom portion
4 5 are avoided by the ribs which result in a very flat bottom surface despite the rapid
cooling brought about by short cycle times. The ribs also permit the Petri dishes
to be manufactured using much less resin than in the prior art.
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Further features of the invention will be described in the course of the
following detailed description.
BRIEF DESCRIPTION OF l~IE DRAWINGS
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The present invention will now be described by way of example only with
reference to the following drawings, in which like numerals refer to like parts,15 and in which~
Figure 1 is a perspective of one embodiment showing both the Petri dish
and a lid;
Flgure 2 is a top view of the lid;
Figure 3 is a cross-section of the lid;
Figure 4 is a more detailed cross-section of the lid;
Figure 5 is a cross-section showing one of the ribs in the lid;
Figure 6 is a top view of the dish; `
Figure 7 is a cross-section of the dish;
Figure 8 is a cross-section of one of the ribs of the dish;
Figure 9 is a more detailed cross-section of the dish;
Figure 10 is a cross section showing the lid positioned on the dish; and,
Figure 11 is a top view of one alternative rib configuration.
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DETAILED DESCRII'TION OF THE PREFERRED EMBODIMENT
Referring n~w to the drawings, the Petri dish 1 has a substantially Iqat
horizontal bottom portion 2 and a short substantially cylindrical side wall 4
integral with and around the bottom portion. The bottom portion is provided
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with a plurality of ribs 6. These ribs are preferably but not essentially radially
oriented. They do not exceed about 0.050 inches in height above the upper
surface of the bottom, and preferably do not exceed about 0.030 inches in height.
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The thickness of the bottom portion other than at the ribs is not greater than
lo about 0.030 inches and preferably not greater than about 0.025 inches. The ribs
provide greater rigidity for the bottom than would otherwise be possible with
such a thin section. The ribs ensure that the bottom remains substantially flat
when it is removed from the mold and thereafter. ~
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2o In the embodiment of the dish 1 shown in Figures 1 and 6 to 10, there are- ;~
four radial ribs 6 spaced at 90 degrees to each other. It is, of course, possible to
have more than the four ribs, evenly distributed around the area of the bottom -
portion. Three ribs would be a practical minimum, each 120 degrees from the
` other.
Referring now to Figures 1 to 5 and 10, a lid 10 for the Petri dish may be
produced according to the same principles. The lid includes a top portion 12, side
wall 14, and ribs 6. `
i 40 Figure 11 shows an alternative embodiment, in which there are eight ribs,
namely four ribs 6 preferably being at least about 0.9 R, where R is the radius of
the bottom portion, and the ribs are spaced at 90 degrees to each other, and four
ribs 14 preferably being at least about 0.25 R running from near the centre of the
bottom portion, and being spaced at 90 degrees from each other, offset by 45 ~;
5 o degrees from the first four ribs. This is the preferred embodiment for the dish,
while the four rib configuration is preferred for the lid, although either could be
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used for either the dish or the lid. Again, it is possible to have more than thebasic eight ribs, evenly distributed around the area of the bottom portion.
Another alternative embodiment exemplary of one of many possible
variations, consists of at least about 16 ribs, the majority of which are not greater
in length than about 0.25 R. The ribs coulcl be evenly distributed around the area
of the bottom portion.
The injection gate may be in the centre of the bottom, at the bottom offset
from the centre, or at the side wall. In each of the embodiments, there may or
may not be a dimple 18 at the centre of the bottom portion, depending on the
i location of the injection gate. In the preferred embodiment illustrated in Figures
25 1 and 6 to 10, the ribs run from the dimple to the side wall, although it is possible
for the ribs to stop short of the dimple and/or side wall.
The preferred embodiment illustrated in Figures 1 and 6 to 10 for a Petri
dish of 3.3 inches nominal diameter contains the following key dimensions: -
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Bottom portion thickness: 0.018 inches
Rib total height: 0.030 inches
Rib height above upper surface of bottom 0.012 inches
These dimensions may be scaled up or down appropriately for larger or
4 5 smaller Petri dishes without departing from the principles of the invention, with
only routine testing and experimentation being required for optimization.
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Table 1 shows some of -the characteristics of prototype Petri dishes
produced according to the invention:
Petri Weight Thick- Flatness Base Base Side Twist
Dish (g3ness (10-3 in) Load Stiff Stiff. Stiffness
l o Resin (in) (k,~ kg/mm kg/mm if tn/lb cu
Ribbed 8.440.019 5.27 41.6 15.1 5.0 29.90
tPyrpoeto- o = 0.06 o = 2.14 o = 2.3 o = 0.05 o = 0.08 o = 10.5
615 APR
2 o Ribbed 8.660.021 1.95 25.2 11.3 6.5 24.90
Proto- o = 0.05 o = 1.8 o = 9.6 o = 3.3 o = 1.2 o = 15.7
type 688
2 5 Note: Weight i~ combined weight of base plus lid; other columns are fo~ bas~ only.
These embodiments are by way of example only, and many variations will
be obvious to those skilled in the field, which variations are in the scope of the
inven~ion as described and claimed, whether or not expressly described.
Specifically, any suitable layout could be selected and it is a matter of
routine testing of prototypes to determine whether or not a given rib layout is `
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acceptable from a strength viewpoint. The ribs do not have to be radial
(although that is preferable), nor do they have to be continuous (i.e. they could
5 0 be "broken"). Also they do not have to run all the way from the centre to the :
side wall, or from side wall to side wall, and they do not have to be straight but
could be curved. A large number of ribs could be provided, if desired, though a
large number is not a requirement.
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Furthermore, the ribs could projec~ downwardly from the underside of the
.~ dish, or some could project upwardly and others downwardly, or individual ribs
5 could project both upwardly and downwardly (for example, the bottom of the
~.~ dish could be in effect centred on the ove:rall height of the rib). However, the
.:' height of the ribs above the upper surface of the bottom should still not exceed
l~ the above-mentioned heights, and in the case of ribs which project only
:~ downwardly, the height may indeed be zero.
~, The dishes are made using conventional manufacturing processes known
in the art, such as the dash-fill method.
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