Note: Descriptions are shown in the official language in which they were submitted.
CA 02497853 2005-02-21
P-6259
Patent Application
for
CONTAINER ASSEMBLY HAVING COUPLING LUGS
by
Charles D. Muller
Field of the Invenltion
[0001] The present invention is directed to a container assembly for
use in the sampling and growth of microorganisms. More particularly,
the invention is directed to a container assembly having a base and a
lid where the lid is removably coupled to the base by lugs on the base.
Background of the Invention
[0002] Container; for culturing microorganisms are commonly
referred to as petri dishes and are well known in the art. These
devices typically include a bottom container or base for the growth
medium and a peripheral side wall. The bottom container is typically
used in conjunction. with a lid having a top wall and a peripheral side
wall that is able to fit over the side wall of the bottom container.
Generally, the side wall of the lid forms a loose fit with the bottom
container to prevent the lid from binding with the bottom container.
Containers such as these are manufactured by several manufacturers
and are readily available.
(0003] Sample containers and petri dishes for microorganisms are
commonly used in l,~rge numbers each day by a technician. A
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technician can use several hundred containers each day to test for
various microorganisms. To enable the technician to handle the large
number of containers efficiently, the lids must be easily removed and
replaced without sticking or binding together. In some instances, it is
desirable to have tree lid sufficiently loose to enable the technician to
remove the lid with one hand while dispensing the sample into the
petri dish for testing. Other uses of the petri dish require that the lid
be secured to the dish to prevent inadvertent separation of the lid
from the bottom container but still allow the lid to be removed from
the dish with minimal effort.
[0004] The petri dishes which have been inoculated with a sample
are often transported in a stack. The petri dishes often have some
form of restraining structure to prevent sliding of the dishes with
respect to each other when they are stacked. The dishes generally
have a recess in the' bottom portion that can mate with the lid of
another petri dish when stacked.
[0005] One example of a petri dish having a container and a lid is
disclosed in U.S. Patent No. 5,021,351. The petri dish as disclosed in
this patent include: a bottom container with an annular base that is
smaller than the outermost dimension of the base. The lid includes a
recessed portion th;~t can receive the annular ring of the base to
prevent sliding of the petri dishes when stacked. The petri dishes are
stacked by nesting the bottom of the base in the recess of the lid.
[0006] One problem that often occurs with petri dishes during use
is the risk of the technician touching the agar media or other growth
media. This typically results in inaccurate sampling results since
unwanted microorganisms are often transferred to the growth media
by the inadvertent touching. To reduce the risk of inadvertent contact
with the growth media, various methods have been proposed for
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securing the lid to the base container to prevent inadvertent
separation of the lid from the base container. These efforts have often
resulted in high fricaional forces that can make it difficult to separate
the lid from the container. Other methods have been generally
ineffective or difficult to operate. One example of such a device is
disclosed in U.S. Patent No. 5.725,123. This patent discloses a petri
dish having a lid with several elevations that contact the side wall of
the base container to form a secure friction fit. The sides of the lid are
compressed to allow the elevations to be released from the base. This
arrangement has the disadvantage of requiring the technician to grip
the lid in specific locations to separate the lid from the base.
[0007] Examples of other petri dishes are disclosed in U.S. Patent
No. 6,602,704, U.S. Patent No. 5,854,065, U.S. Patent No. 3,769,936
and U.S. Patent No,. 4,945,061. These patents generally disclose a
base container having a lid that closes the open top end of the base
container.
[0008] The prior petri dishes have been generally successful for
their intended purpose. However, there is a continuing need in the
industry for improved containers for culturing microorganisms.
Summary of the Invention
[0009] The present invention is directed to a container assembly for
use in sampling microorganisms. The invention is particularly
directed to a container assembly having a base and a lid where the lid
can be secured to the base while being readily separable from the
base.
[0010] Accordingly, an object of the invention is to provide a
container assembly having a base and a removable lid. The container
base has a peripheral edge with a plurality of lugs that contact the lid
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when the lid is placed on the base. The lugs define a friction fit to
resist inadvertent separation of the lid from the base while allowing
the lid to be separated easily by the technician during use.
[0011] In one embodiment, the present invention is directed to the
container assembly having a base and a lid that is removably coupled
to the base where the lid grips the base with sufficient frictional force
to enable the container assembly filled with a growth medium to be
lifted by the lid without separation of the base.
[0012] Another object of the invention is to provide a container
assembly with a continuous side wall where the side wall includes a
plurality of lugs protruding outwardly from the outer surface of the
side wall. The lugs are positioned to engage the side wall of the lid
with sufficient friction to couple the lid to the base.
[0013] The inveni:ion primarily relates to a device for use in
connection with sampling of a surface for the presence of
microorganisms. The container assembly includes a base and a lid
where the base has a support surface for receiving a growth media
such as an agar media. A side wall surrounds the support surface
and depends from the peripheral edge of the support surface. The
side wall of the base' includes a flange that extends radially outward to
form a lid support surface. A lip depends from the outer peripheral
edge of the lid support surface and extends in a generally downward
direction with respect to the lid support surface and the media
support surface.
[0014] The side wall of the base includes a plurality of lugs that
protrude outwardly from the surface of the side wall. The side wall
preferably includes an odd number of lugs which are uniformly spaced
apart with no two lugs being diametrically opposed. The lugs have a
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shape and dimension to engage the inner surface of the lid and
frictionally grip the inner surface of the lid.
[0015] The various objects of the invention are obtained by
providing a container assembly comprising a base member and a lid,
where the base member has a support surface and a continuous side
wall surrounding the support surface and has a longitudinal
dimension extending in a direction away from the support surface.
The side wall of the base member has a plurality of lugs protruding
outwardly from an outer surface of the continuous side wall. The lid
has a substantially planar surface and a continuous side wall
depending from the planar surface. The side wall of the lid has an
inner surface and a.n inner dimension complementing an outer
dimension of the continuous side wall of the base member. The lugs
on the continuous ;side wall of the base member are spaced apart in
such a manner that the side wall of the lid deflects outwardly in a
contact area of the lugs and the side wall deflects inwardly in an area
between adjacent lugs on the side wall of the base member.
[0016] The objects of the invention are also obtained by providing a
container assembly for use in sampling the surface for the presence of
microorganisms comprising a base and a lid. The base has a support
surface for supporting a sample media and a continuous side wall
surrounding the support surface and having a longitudinal dimension
extending away from the support surface. The outer surface of the
side wall of the base has an odd number of lugs protruding outwardly
from the side wall. A peripheral lid support surface extends outwardly
from the continuous side wall of the base. The lid has a substantially
planar surface and a continuous side wall extending from the planar
surface and having an inner surface and an inner dimension. The lid
is dimension to be placed on the base and overlie the side wall of the
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base. The lugs on 'the side wall of the base have a dimension to
contact the inner surface of the side wall of the lid and are spaced
apart a distance to deflect the side wall of the lid outwardly at a point
of contact with the lugs and to deflect the side wall of the lid inwardly
at a point between adjacent lugs on the base.
[0017] The various features, objects and advantages of the invention
will become apparent to those skilled in the art in view of the following
detailed description and the annexed drawings which disclose
preferred embodiments of the invention.
Brief Description of the Drawings
[0018] The following is a brief description of the drawings in which:
(0019] Figure 1 is a side elevational exploded view of the container
assembly according to one embodiment of the invention showing the
base and lid;
(0020] Figure 2 is a top view of the base of Figure 1;
[0021] Figure 3 is a bottom view of the base member of the
embodiment of FigL~re 1;
[0022] Figure 4 is a bottom view of the lid of the embodiment of
Figure 1;
[0023] Figure 5 is. a partial enlarged top view of the base showing a
lug in one embodiment of the invention;
[0024) Figure 6 is a top view of the container assembly depicting the
flexing of the lid when placed on the base;
[0025] Figure 7 is. a partial cross-sectional side view of the base and
lid showing the side wall and the lug on the base;
(0026] Figure 8 is. a perspective view of the base showing the lug on
the side wall of the base;
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[0027] Figure 9 is a partial cross-sectional view taken along line 9-9
of Figure 6 showing the lid fitted on the base and showing the lug
contacting the lid and deflecting outwardly from the base;
[0028] Figure 10 is a partial cross-sectional view taken along line
10-10 of Figure 6 and showing the lid fitted on the base and deflecting
inwardly in an area. between adjacent lugs from the coupling
engagement of the lid to the base;
[0029] Figure 11 is a top view of a base in a second embodiment of
the invention; and
[0030] Figure 12 is a side view in cross-section showing the
dimensions of the lugs in one embodiment of the invention.
Detailed Descriuti~on of the Preferred Embodiments
[0031] An exemplary embodiment of the present invention is a
container assembly 10 having a base member 12 and a lid 14.
Container assembly 10 has a dimension and shape suitable for
supporting growth media for growing microorganisms, such as
bacteria. In preferred embodiments of the invention, container
assembly 10 has a width and a height as commonly used in the
industry for devices used in sampling microorganisms.
[0032] Referring t:o the drawings, container assembly 10 typically
has a substantially circular shape as commonly used in the industry.
The base 14 has a bottom wall 16 that forms a support surface for
growth media for the microorganisms. In the embodiment illustrated,
bottom wall 16 has a slightly convex upper surface 18 and a slightly
concave bottom surface 20. Preferably, bottom wall 16 is a support
surface suitable for supporting a conventional agar growth medium
which can be used for sampling surfaces to determine the presence or
absence of microorganisms. In one embodiment, bottom wall 16
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includes a plurality of perpendicular intersecting ridges 22 to define a
grid system 24. Grid system 24 is provided to facilitate counting of
microorganisms as known in the art.
[0033] Base member 12 includes a side wall 26 that extends away
from the plane of bottom wall 26 in a generally upward axial direction
with respect to an ~~xial dimension of base member 12 and to define a
height of side wall 26. Side wall 26 is a continuous structure that
encircles bottom well 16 and defines an upper cavity 28 in base
member 12. Preferably, side wall 26 has an annular or circular shape
that surrounds the peripheral edge of bottom wall 16 to contain
growth medium and receive lid 14. Side wall 26 has a cross-section
as shown in Figure 7 with a substantially inverted U-shape formed by
an inner wall 30 that extends upward from the peripheral edge of
bottom wall 16, a top portion 32 and an outer wall 34 that extends in
a generally downward direction from top portion 32. In the
embodiment illustrated, inner wall 30 and outer wall 34 are oriented
in substantially the same direction and have a slight taper that
converge toward top portion 32 to assist in manufacturing and
facilitating removal of the base member 12 from the mold used to form
base member 12. Base member 12 is typically made as a unitary,
single piece from a ,suitable transparent plastic material such as
polystyrene. Typically, base member 12 is made by injection molding
so that the taper of inner wall 30 and outer wall 34 facilitate removal
of the injection molded part from the mold cavity.
[0034] Referring t:o Figures 7-10, outer wall 34 has a distal bottom
end and a flange 36 extending therefrom. Flange 36 in the
embodiment illustrated extends in an outward radial direction with
respect to base member 12. Flange 36 in preferred embodiments is
continuous and has a substantially annular shape and defines a lid
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supporting surface with a shape and dimension for receiving and
supporting lid 14. Preferably, flange 36 has a uniform width
surrounding outer wall 34 and lies in a horizontal plane that is
substantially perpendicular to a vertical axis of base 12.
[0035] As shown in Figure 7, a continuous wall 38 depends from a
peripheral edge of flange 36 in a downward direction. As shown, wall
38 extends away from bottom wall 16 and flange 36 in a generally
downward direction and defines a circular supporting wall for base
member 12. Wall 38 includes an upper edge 40 that is contiguous
with flange 36 and extends to a free bottom edge 42. Preferably, wall
38 has a substantially annular shape to stabilize base member 12
when placed on a table or other support surface. In a preferred
embodiment, the outer diameter of bottom edge 42 of wall 38 is
greater than the diameter of upper edge 40 to assist in stabilizing base
member 12 and to .assist in stacking and nesting of multiple container
assemblies.
[0036] Base member 12 has a bottom cavity 44 defined by bottom
wall 16 and wall 38 as shown in Figures 7, 9 and 10. A plurality of
ribs 46 as shown in Figure 10 extend in a generally inward direction
from an inner surface of wall 38. Each rib 46 has a bottom surface 48
that is contoured to received an upper portion of a lid 14 when
multiple container ;assemblies are stacked. Preferably, bottom surface
48 of ribs 36 have a shape and dimension to contact an upper surface
of lid 14. In one embodiment, four ribs 46 are provided and uniformly
spaced apart about 90° within bottom cavity 44. The fit between a lid
14 and ribs 46 on a.n adjacent base member 12 preferably provide a
reliable stacking arrangement to resist the stacked container
assemblies from sliding laterally with respect to each other.
Preferably, ribs 46 <~nd wall 38 have a dimension to receive an upper
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portion of a lid 14 without binding or sticking so that a container
assembly 10 can be lifted from a stack without inadvertent separation
of a lid 14 from a respective base member 12.
[0037] Lid 14 has a generally planer top surface 50 as shown in
Figure 1. Lid 14 is preferably made of a transparent plastic material
such as polystyrene that is rigid but is able to provide some limited
flex to lid 12 without cracking or breaking. In the embodiment shown,
top surface 50 has a slightly convex shape that is referred to herein as
being generally planar. In alternative embodiments, top surface 50
can have a slightly concave shape as desired. Lid 14 has a side wall
52 depending from a peripheral outer edge 54 of top wall 50 of lid 14.
Side wall 52 extends in a generally downward direction with respect to
top wall 50 and ha;~ a slightly outward taper. Referring to Figure 7,
side wall 52 has a distal end 56 having an outer dimension that is
slightly greater than the outer dimension of top wall 50 so that side
wall 52 has a slightly frustoconical shape. In one embodiment of the
invention, side wall 52 has a taper of about 7° with respect to a
vertical axis of lid 14.
[0038] Distal end 56 of side wall 52 of lid 14 has a flange 58 that
extends radially ou~ward from side wall 52 as shown in Figures 1 and
7. Flange 58 has a substantially annular shape with a uniform width.
Flange 58 has an outer diameter that is greater than the outer
diameter of distal end 56 of side wall 52. Flange 58 provides a visual
and tactile indication to the user of the position of lid 14 to reduce the
risk of the user ina~~vertently contacting bottom wall 16 of base
member 12 and contaminating the growth media on bottom wall 16.
[0039] Flange 58 has a bottom surface 60 that faces in a generally
downward direction. with respect to the normal orientation of lid 14
when placed on base 12. As shown in Figure 4 and Figure 7, a
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plurality of protrusions 62 shown as bumps that extend outwardly
from bottom surface 60. In a preferred embodiment, protrusions 62
are uniformly spaced apart around the periphery of bottom surface 60
of flange 58. Protrusions 62 have a shape and dimension to space
bottom surface 60 of flange 58 from flange 36 of base member 12.
[0040] Flange 36 of base member 12 supports side wall 52 of lid 14
when lid 14 is plat<~d on base member 12 as shown in Figure 10.
Protrusions 62 contact flange 36 to form a small gap between flange
36 of base member 12 and flange 58 of lid 14. The small gap allows
for air to flow into and out of cavity 28 as may be desired or necessary
for many applications in promoting proper growth of microorganisms.
The gap formed between flange 36 and flange 58 also prevent a
moisture lock or seal from being inadvertently created between lid 14
and base 12 so that lid 14 can be readily separated from base 12.
[0041] Referring to Figures 5 and 6, the outer surface of outer wall
34 includes a plurality of lugs 70. In one preferred embodiment of the
invention, lugs 70 ure integral with the outer surface of outer wall 34
and flange 36 as shown. Lugs 70 have a shape and dimension to
contact an inner surface of side wall 52 of lid 14 to provide a secure
friction fit engagement between lid 14 and base 12 while allowing lid
14 to be easily separated from base 12 by the technician. In preferred
embodiments of the invention, lugs 70 are uniformly spaced apart
around outer wall ~~4. Lugs 70 are positioned to engage the inner
surface of side wall 52 of lid 14 to form a point of contact with side
wall 52 and apply a. substantially uniform outward gripping force.
Typically, lid 14 is made of a transparent plastic material, such as
polystyrene. Polystyrene is slightly flexible but brittle and can easily
crack or break. Lup;s 70 have a radial dimension and a radius of
curvature to be able to flex side wall 52 of lid 14 at a point of contact
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between lid 14 and lug 70 in a substantially radially outward direction
with respect to lid .l4 as indicated by arrows 76 in Figures 6 and 9
without cracking or breaking. The resilience and flexibility of side wall
52 of lid 14 form a ,gripping force against lugs 70 and define a friction
fit. Lugs 70 are spaced apart a sufficient distance so that side wall 52
of lid 14 is deflected inwardly in an area between adjacent points of
contact of adjacent lugs indicated by arrows 78 in Figures 6 and 10.
[0042] Lugs 70 a:re oriented and spaced apart a distance to provide
a limited and controlled distortion of side wall 52 of lid 14 when lid 14
is placed on base member 12. Preferably, base 12 and lugs 70 are
sufficiently rigid so that side wall 26 of base 12 is dimensionally stable
and does not flex o~rr distort when lid 14 is placed on base 12. In
preferred embodiments of the invention, side wall 52 of lid 14 has a
substantially annular shape which can be distorted outwardly at the
point of contact with lugs 70 as indicated by arrows 76 and distorted
or deflected inwardly in the areas between the point of contact of
adjacent lugs as indicated by arrows 78 as shown in Figures 6, 9 and
10. When lid 14 is placed on base member 12, side wall 52 of lid 14
assumes a slightly distorted, non-circular shape as a result of the
relative flexibility of lid 14 and the relatively inflexible base member 12
which is typically manufactured with walls slightly thicker than those
of the lid 14. To achieve the proper distortion of side wall 52 of lid 14
and the desired gripping force, the number, dimensions and spacing
of lugs 70 is important. In the embodiment of Figures 5 and 6, the
surface of outer wall 34 of base member 12 includes three lugs 70
that are uniformly ;paced apart. In this manner, no two lugs are
diametrically opposed to each other on outer wall 34. In addition, the
uniform spacing of lugs 70 form an area for inward deflection of side
wall 52 of lid 14 that is diametrically opposed to a point of contact
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between a respective lug 70 and side wall 52. The space between
adjacent lugs 70 on base 12 define the area for inward deflection for
side wall 52 of lid 14 while lugs 70 define an area for outward
deflection of side w.al1 52 of lid 14. Preferably, side wall 26 and lugs
70 are sufficiently rigid to prevent bending or flexing of base 12 and
cause lid 14 to flex or distort into a non-circular shape when lid 14 is
placed on base 12.
(0043] In another embodiment of the invention, five lugs 70 are
provided on outer vvall 34 and are uniformly spaced apart. It has been
found that five lugs, 70 can provide the proper spacing between the
points of contact between lugs 70 and side wall 52 to provide the
desired outward deflection at the point of contact and the desired
inward deflection or deformation between adjacent points of contact.
It has also been found that more than five lugs on a standard size
container assembly provide a radial surface area that does not enable
proper bending or deflection of side wall 52 of lid 14 and can cause
cracking or breaking of side wall 52 of lid 14. When the surface area
of the lugs becomes too great, the lugs cause the entire side wall of the
lid to expand radial.ly outward rather than flexing and distorting which
can cause the lid to crack or result in excessive frictional engagement
and prevent easy separation of the lid from the base. It has also been
found that fewer than three lugs on the outer wall of base 12 provide
insufficient gripping and frictional forces between lid 14 and base
member 12 to adequately secure lid 14 to base 12.
[0044] Lugs 70 have a dimension and shape to form a gripping
effect through frictional forces between the contact surface of a
respective lug 70 amd the inner surface of side wall 52 of lid 14.
Preferably, the frictional forces are sufficient to secure lid 14 on base
member 12 without cracking or breaking lid 12 and while allowing the
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removal of lid 12 by the technician without excessive sticking or
binding. In a preferred embodiment of the invention, the frictional
forces between lugs 70 and side wall 52 of lid 14 enable the technician
to remove lid 14 from a respective base member 12 with one hand
during the handling of container assembly 10 and processing of the
microorganisms. In one embodiment, lid 14 is coupled to base 12 by
sufficient frictional forces to enable assembly 10 to be lifted by lid 14
without inadvertent separation of base member 12 from lid 14.
Preferably, the gripping force is in terms of a gram-weight-fit so that
assembly 10 can bf~ lifted by lid 14. In one embodiment, the friction
gripping force is about twice the weight of base member 14. In other
embodiments, the ~;ripping force by the lugs engaging the inner
surface of the side wall of the lid is about 2 to about 20 times the
weight of the base member.
[0045] In one embodiment of the invention, base member 12 and lid
14 are made of transparent polystyrene, which is typically brittle and
can easily crack. Lugs 70 have a radius and a dimension to provide a
rounded outer face that forms a gradual or rounded bending surface
of outer wall 52 of lid 14 to resist cracking or breaking of lid 14. In
addition, the rounded surfaces and dimension of lugs 70 also inhibit
cracking or chipping of lugs 70 during continuous use. It has been
found that a sharp angle on the outer surface of lugs 70 can cause lid
14 to crack and can result in lugs 70 chipping or cracking.
[0046] Referring to Figure 5, lugs 70 have a height extending in an
axial, longitudinal direction with respect to outer wall 34 of base
member 12 that provide a surface area sufficient to contact and grip
side wall 52 of lid 14. In one embodiment as shown in Figure 5, lugs
70 have a height that is about one-half the height of outer wall 34.
The height of lugs i'0 can vary to provide a surface area with the
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necessary friction gripping force for lid 14. Preferably, each of the lugs
70 have substantially the same dimensions and shape so that each
lug contacts the inner surface of side wall 52 of lid 14 with a uniform
gripping force.
[0047] Referring to Figure 6, lugs 70 in a preferred embodiment of
the invention have a substantially arcuate and cylindrical outer side
72 forming a substantially convex surface. In a preferred
embodiment, outer side 72 has a substantially continuous and
uniform shape. Lugs 70 have an upper portion 74 having a
substantially semi-spherical shape that converges from outer side 72
and is contiguous with outer wall 34 of base member 12. Upper
portion 74 has a shape and dimension to guide side wall 52 of lid 14
over lugs 70. Preferably, outer side 72 extends to and is contiguous
with flange 36. In one embodiment of the invention, outer side 72 of
lugs 70 have an arc:uate shape defined by a radius of about 0.125 to
about 0.14 inch. In one preferred embodiment, outer side 72 of lugs
70 have an arcuate shape defined by a radius of about 0.125 inch to
about 0.130 inch.
[0048] It has been found that lugs having a substantially arcuate
shape side provide a sufficient surface area to contact the inner
surface of side wall 52 of lid 14 to provide the desired outward
deflection of side w;~ll 54 at the point of contact without cracking or
breaking side wall ;i4. Lugs that are formed with a sharp point or
ridge can cause side wall 52 of lid 14 to bend around the sharp corner
which can cause side wall 52 to crack and provide insufficient surface
area at the point of contact to secure the lid to the base member with
the desired friction<~1 forces.
[0049] The dimensions and the contact surface area of lugs 70 are
selected to provide the desired and substantially uniform gripping
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force for lid 14. The actual dimensions of lugs 70 will vary depending
on the dimensions of lid 14 and the flexibility of lid 14 and particularly
side wall 52 of lid 14. The height and thickness of side wall 52 of lid
14 determine the flexibility of the lid and the gripping force applied to
the lugs 70. The dimensions and thickness of flange 58 also
influences the flexibility of side wall 52. The dimensions of lugs 70 are
preferably selected according to the flexibility of lid 14, and
particularly side wall 54 of lid 14. For example, the dimensions and
particularly the radlial dimension of lugs 70 are increased to
complement a lid 14 having a thin and flexible side wall 54. A lid with
a thin and very flexible side wall will easily flex and distort to a non-
circular shape and may not provide the desired gripping force. In this
situation, the radial dimension of the lugs 70 are increased to increase
the gripping force. In a similar manner, a more rigid side wall 54 of a
lid 14 will not flex as easily and can cause the gripping force to be too
high such that the lid is not easily removed from the base member. In
the embodiments where the side wall 54 of lid 14 is less flexible, the
radial dimension of lugs 70 are decreased to prevent an overtight fit
and reduce the risl~; of cracking or breaking the lid while ensuring the
desired friction fit.
[0050] In one embodiment of the invention, side wall 34 of base
member 12 and side wall 52 of lid 14 are formed such that the
respective side walls are substantially parallel when lid 14 is placed
on base member 12. In one embodiment, side wall 34 and side wall
52 are oriented to provide a taper of about 7° with respect to a
vertical
axis. Outer sides 7 2 of lugs 70 are preferably oriented at an angle
that is sufficient to grip the inner surface of side wall 52 of lid 14
without binding or sticking. An angle of outer surface 72 that is too
shallow can prevent side wall 52 of lid 14 from properly engaging the
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respective lug such that the flexibility of side wall 52 can cause side
wall and lid 14 to slide upwardly along outer side 72 of lug 70. In one
preferred embodiment of the invention, lugs 70 are provided with side
wall 72 having an angle that is slightly less than the angle of side
walls 52 of lid 14. :In the embodiment illustrated, outer side 72 of lug
70 is oriented at a 1=aper of about 4° with respect to a vertical axis
of
base 12 and has been found to effectively grip lid 14 without binding
or sticking when side wall 52 of lid 14 has about a 7° taper. In this
embodiment, lugs 70 have a bottom section defined generally by outer
side 72 and a top section at adjacent upper portion 74. The bottom
section of outer side 72 has a radial dimension with respect to base 12
that is greater than the radial dimension of the top section of outer
side 72.
[0051] Preferably, the mating surfaces of lugs 70 and side wall 52 of
lid 14 have a slightly different taper to prevent binding. Mating
annular surfaces that have little or no taper often bind and stick
together. Outer wall 34 of base member 12 and side wall 52 of lid 14
have a 7° taper in one embodiment. However, the inner surface of
side wall 52 of lid 14 is spaced from outer wall 32 by lugs 70 so that
side wall 52 of lid 14 typically does not directly contact outer wall 34
of base member 12. In one preferred embodiment, when lid 14 is
placed on base member 14, side wall 52 of lid 14 contacts the outer
gripping surfaces of lugs 70 without directly contacting outer wall 34
of base member 12. The deflection and distortion of side wall 52 of lid
14 provide the desired gripping force.
(0052] Preferably, container assembly 10 has a dimension to be
compatible with other commercially available petri dishes and
handling apparatus>. In one preferred embodiment, base member 12
has a radius of about 2.75 inches and a height of about 0.375 inch.
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Outer wall 34 of side wall 26 preferably has a radius of about 2.46
inches and a height 82 of about 0.15 inch as shown in Figure 12. In
this embodiment, lugs 70 have a height 84 of about 0.075 inch and a
taper of about 4°. Outer side 72 of lugs 70 preferably have a height 86
of about 0.040 inch and upper portion 74 has a height 88 of about
0.35 inch. Upper portion 74 defines a curved portion extending from
the outer surface o:f outer wall 34 to outer side 72 of lugs 70 having a
radius 90 of about 0.070 inch. Outer side 72 of lugs 70 preferably
have a radius of about 0.125 inch.
[0053] The foregoing description is intended to be exemplary of a
preferred embodiment of the invention. It will be understood by those
skilled in the art that various changes and modifications to the
disclosed embodiment can be made without departing from the
purview and spirit of the invention as defined in the appended claims.
