Disposable culture dish with reinforcement ribs

Abstract: A Petri dish (1) in which the flat bottom (2) is provided with a plurality of preferably radial ribs (6) is disclosed. The ribs do not project more than about 0.05 inches above the upper surface of the bottom (2), and preferably not more than about 0.03 inches. The thickness of the bottom portion (2) other than at the ribs is not greater than about 0.03 inches, and preferably not greater than about 0.025 inches. Various rib arrangements are contemplated. In the preferred embodiment, there are at least three radial ribs (6), and preferably four ribs (6) spaced at 90 degrees to each other. A lid (10) having the same basic structure may also be produced. Generally any suitable rib layout could be selected. A large number or ribs could be provided, if desired, though not a requirement. The ribs (6) need not necessarily be radial. The ribs (6) need not necessarily be continuous, and they need not necessarily run all the way from the center to the side wall (4), or from side wall (4) to side wall (4). The ribs (6) need not necessarily be straight, but could be curved. The ribs (6) could project downwardly instead of or in addition to upwardly.

Description: BACKGROUND OF THE INVENTION This invention relates to Petri dishes. Petri dishes are well known and in common usage, particularly in laboratories for producing cultures in a culture medium such as agar. Such dishes have a flat bottom portion and short cylindrical sides. Flatness of the bottom portion is important, so that a uniform layer of the culture medium can be produced using as little of the medium as possible. The market for Petri dishes is highly competitive and price sensitive. It is therefore encumbent on manufacturers to produce Petri dishes at as low a price as possible. At the same time, however, it is of course essential that the quality of the product be excellent. In Petri dishes, curvature of the bottom is a problem because the bottom tends to develop significant curvature after removal from the mold, especially due to rapid cooling caused by the relatively fast cycle times in production. Curvature is unacceptable because it confuses accurate assessment of the culture in the dish. This problem traditionally has been dealt with by thickening the walls of the dish and increasing the cooling time before it is ejected from the mould. However, these solutions are unsatisfactory because they involve the use of more material than strictly required for the mechanical strength of the finished article, and they lengthen the mould cycle time, thereby making the process expensive while lowering the production rate of the mould. Moulded Petri-dishes are known wherein the bottom of the dish is provided with upstanding walls or ribs, primarily for the purpose of dividing the dish into a number of compartments. Dishes in which divider walls are provided are described in the following patents: US Patent 2,874,091 (Fisks); US Patent 3,005,808 (Henderson); US Patent 3,816,264 (Winter et al); British Patent 1,026,253 (Scheidt). In general, there is a need for an improved Petri dish, such that manufacturing costs can be reduced and quality either enhanced or at the very least not sacrificed. Since the cost of materials, i.e. resins, is the primary cost factor in producing Petri dishes, the best way to reduce cost is to reduce the amount of material required. However, it has not hitherto been possible to reduce the amount of material to the extent possible in the present invention, without sacrificing quality. "Quality" in this context means primarily that the Petri dish must be reasonably strong, and that the bottom must be as close as possible to being absolutely flat. Other factors include transparency and the ability of agar to bond to the dish. SUMMARY OF THE INVENTION It is an object of the invention to provide a reduced cost Petri dish with the quality being enhanced or at the very least not reduced. Thus in accordance with the present invention there is provided a Petri dish as defined in claim 1. The ribs result in a very flat bottom surface despite the rapid cooling brought about by short cycle times, thus avoiding the problems of the prior art tendency for the bottom portion to become curved. The ribs also permit the Petri dishes to be manufactured using much less resin than in the prior art. Various rib arrangements are contemplated. In accordance with one arrangement, there are at least three radial ribs, each not less than 0.9R in length, where R is the radius of the bottom portion, and in which the ribs are evenly distributed around the area of the bottom portion. In accordance with another arrangement, there are four radial ribs each not less than 0.9R in length, in which the ribs are spaced at 90 degrees from each other. In accordance with yet another arrangement there are at least sixteen ribs, the majority of which are not greater in length than 0.25R, which ribs are evenly distributed about the area of the bottom portion. In accordance with a further arrangement, there are at least eight radial ribs each having a length of at least 0.5R, in which the ribs are evenly distributed around the area of the bottom portion. In accordance with a still further arrangement, there are eight ribs, namely a first four ribs having a length of at least 0.90R spaced 90 degrees from each other, and a second four ribs each having a length of at least 0.25R running from near the centre of the bottom portion, spaced at 90 degrees from each other and offset by 45 degrees from the first four ribs. Further features of the invention will be described or will become apparent in the course of the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more clearly understood, the preferred and alternative embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which: FIG. 1 - is a perspective of one embodiment, showing both the Petri dish and a lid; FIG. 2 - is top view of the lid; FIG. 3 - is a cross-section of the lid; FIG. 4 - is a more detailed cross-section of the lid; FIG. 5 - is a cross-section showing one of the ribs in the lid; FIG. 6 - is a top view of the dish; FIG. 7 - is a cross-section of the dish; FIG. 8 - is a cross-section of one of the ribs of the dish; FIG. 9 - is a more detailed cross-section of the dish; FIG. 10 - is a cross-section showing the lid positioned on the dish, and FIG. 11 - is a top view of one alternative rib configuration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, the Petri dish 1 has a substantially flat horizontal bottom portion 2 and a short substantially cylindrical side wall 4 integral with and around the bottom portion. The bottom portion is provided with a plurality of ribs 6, preferably but not essentially radially oriented, and not exceeding about 1.27 mm (0.05 inches) in height and preferably not exceeding about 0.76 mm (0.03 inches) in height, above the upper surface of the bottom. The thickness of the bottom portion other than at the ribs is not greater than about 0.76 mm (0.03 inches), and preferably not greater than about 0.64 mm (0.025) inches. The ribs ensure that the bottom portion remains substantially flat on removal from the mould and thereafter. The ribs provide greater rigidity for the bottom portion than would otherwise be the case with such a thin section. In one embodiment of the dish 1, shown in Figs. 1 and 6 - 10, there are four radial ribs 6, spaced at 90 degrees to each other. There of course could be more than the basic four ribs, evenly distributed around the area of the bottom portion. A practical minimum would be three ribs, each 120 degrees from the other. A lid 10 for the Petri dish may be produced according to the same principles, as illustrated in Figs 1 - 5 and 10. The lid is of very similar construction, including a top portion 12, a side wall 14, and ribs 6. In an alternative embodiment, shown in Fig. 11, 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, the ribs being 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 degrees from the first four ribs. At present, this embodiment is preferred for the dish, with the four rib configuration being preferred for the lid, although either could be used for either the dish or the lid. Again, there of course could be more than the basic eight ribs of the embodiment, evenly distributed around the area of the bottom portion. In yet another alternative embodiment, not specifically illustrated but merely exemplary of one of many possible variations, there could be at least sixteen ribs, the majority of which are not greater in length than about 0.25 R. The ribs are evenly distributed around the area of the bottom portion. In each of the embodiments, there may or may not be a dimple 18 at the centre of the bottom portion, depending on the location of the injection gate. The gate may be in the centre of the bottom, at the bottom offset from the centre, or at the side wall. In the preferred embodiment, Figs. 1 and 6 - 10, the ribs run from the dimple to the side wall, although there is no reason why the ribs could not stop short of the dimple and/or side wall. The various key dimensions in the preferred embodiment of Figs. 1 and 6 - 10, for a Petri dish of 8.4 cm (3.3 inch) nominal diameter, are as follows: Bottom portion thickness: 0.46 mm (0.018 inch) Rib total height: .76 mm (0.03 inch) Rib height above upper surface of bottom .30 mm (0.012 inch) Obviously, these dimensions could be varied somewhat and could be scaled up or down appropriately for larger or smaller Petri dishes with only routine testing and experimentation being required for optimization. Table 1 below shows some of the characteristics of prototype Petri dishes produced according to the invention: Petri Dish & Resin Weight (g) Thickness (mm) Flatness (mm) Base Load (kg) Base Stiff kg/mm Side Stiff. kg/mm Twist Stiffness cm/kg/cc Ribbed Prototype 615APR 8.44 0.48 .13 41.6 15.1 5.0 48.4 σ=0.06 σ=2.14 σ=2.3 σ=0.05 σ=0.08 σ=10.5 Ribbed Prototype 688 8.66 0.53 .05 25.2 11.3 6.5 40.3 σ=0.05 σ=1.8 σ=9.6 σ=3.3 σ=1.2 σ=15.7 Note: Weight is combined weight of base plus lid; other columns are for base only. σ = standard deviation It will be appreciated that the embodiments described in the text above are by way of example only. It should be emphasized that generally any suitable rib layout could be selected, it being a matter of routine testing of prototypes to determine whether or not a given rib layout is acceptable a strength viewpoint. A larger number of ribs could be provided, if desired, though a large number is not a requirement. The ribs need not necessarily be radial, although that is preferable. The ribs need not necessarily be continuous, i.e. they could be "broken", and they need not necessarily run all the way from the centre to the side wall, or from side wall to side wall. The ribs need not necessarily be straight, but could be curved. Furthermore, although the above embodiments describe ribs which project upwardly from the upper surface of the bottom of the dish, it should be readily apparent that the ribs could project downwardly from the underside of the dish, or some could project upwardly and others downwardly, or individual ribs could project both upwardly and downwardly (E.G. the bottom of the dish could be in effect centred on the overall height of the rib). The height of the ribs above the upper surface of the bottom should still not exceed the above-mentioned heights, and indeed may be zero in the case of ribs which project only downwardly.

Claims: A Petri dish (1) having a flat horizontal bottom portion (2) and a short cylindrical side wall (4) integral with and around said bottom portion (2), characterized wherein said bottom portion (2) is provided with a plurality of upwardly projecting ribs (6), the upwardly projecting portions of said ribs (6), projecting not more than 1.27 mm (0.05 inches) above the upper surface of said bottom portion (2), and in which the thickness of said bottom portion (2) other than at the ribs (6) is not greater than 0.76 mm (0.03 inches). A Petri dish as recited in Claim 1, in which there are at least three radial ribs (6), each not less than 0.9 R in length, where R is the radius of the bottom portion (2), and in which said ribs (6) are evenly distributed around the area of the bottom portion (2). A Petri dish as recited in Claim 1, in which there are four radial ribs (6), each not less than 0.9 R in length, where R is the radius of the bottom portion (2), and in which said ribs (6) are spaced 90 degrees from each other. A Petri dish as recited in Claim 1, in which there are at least sixteen ribs, the majority of which are not greater in length than 0.25 R, where R is the radius of the bottom portion (2), and in which said ribs (6) are evenly distributed around the area of the bottom portion (2). A Petri dish as recited in Claim 1, in which there are at least eight radial ribs (6), each having a length of at least 0.5 R, where R is the radius of the bottom portion (2), and in which said ribs (6) are evenly distributed around the area of the bottom portion (2). A Petri dish as recited in Claim 5, in which there are eight ribs (6), namely a first four ribs (6) each having a length of at least 0.9 R, where R is the radius of the bottom portion (2), said first four ribs (6) being spaced at 90 degrees to each other, and a second four ribs (6) each having a length of at least 0.25 R running from near the centre of the bottom portion (2), and being spaced at 90 degrees from each other, offset by 45 degrees from said first four ribs (6). A Petri dish as recited in any one of Claims 1 to 6, in which said ribs (6) project not more than 0.76 mm (0.03 inches) above the upper surface of said bottom portion (2). A Petri dish as recited in any one of Claims 1 to 6, in which the thickness of said bottom portion (2) other than at the ribs (6) is not greater than 0.64 mm (0.025 inches). A Petri dish as recited in Claim 1, in which said bottom portion (2) includes a plurality of downwardly projecting ribs (6). A Petri dish as recited in any one of Claims 1 to 9, in which said Petri dish (1) includes a lid (10) therefor. A Petri dish as recited in any one of Claims 1 to 10, in which said bottom portion (2) is provided with a dimple (18) centrally located thereon.

Inventors: ISKANDER EZZAT, ISKANDER, EZZAT
Applicants: ISKANDER EZZAT, ISKANDER, EZZAT
Application year: 1990
Main classification: C12M1
IPC codes: C12M1