APPARATUS FOR INJECTION MOULDING A PETRI DISH

Abstract: Apparatus for moulding a thin-walled article, such as a Petri dish, in which the moulding is effected by injecting from the side of a planar portion of the article. The moulding apparatus has a core piece (10) co-operable with a cavity (12) which together define a space (14) in which the article is to be moulded. At least one of the core (10) or the cavity (12) has a moulding surface (16, 18), corresponding to the planar portion of the finished article, which is non-planar. Preferably, the moulding surface (18) of the cavity (12) has a concave curvature, while the opposing surface (16) of the core (10) inserted therein has a convex curvature. The radius of curvature preferably being larger at the core than at the cavity. The particular configuration allows more material to be supplied to the central area corresponding to the planar portion of the finished article. This material cools more slowly than other portions of the article causing contraction so that the planar portion of the finished article is substantially flat.

Description: Apparatus for injection moulding a Petri dish. This invention relates to an improved apparatus for moulding a thin-walled article such as a Petri dish or other product which is injected from the side during moulding; and to a method of moulding such an article. Petri dishes are well known and widely used, especially in biological laboratories for producing cultures of mirco-organisms in a culture medium, such as agar. It is important that the base of the inside of the dish is substan ially flat so as to allow a thin, uniform layer of the culture medium while using as little of the medium as possible. This ensures uniform light transmission through the medium over the whole area of the dish, which is important for accurate assessment of the development of the culture. Furthermore, when the base is substantially flat, considerable savings may be made on the usage of the medium in order to cover the whole base area with fluid. A Petri dish must be injected from the side during moulding because the injection gate to the cavity leaves an impression which would confuse accurate assessment of the culture in the dish. Because the Petri dish must be injected from the side a major problem with existing manufacturing methods is that the base of the dish tends to assume a significant curvature after removal from the mould. The mould traditionally has planar moulding surfaces corresponding to the surfaces of the base of the Petri dish which is to be planar and, in order to prevent unacceptable curvature, the problem 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 required for the mechanical strength of the finished article; and they lengthen the mould residence time, thereby making the process expensive while lowering the production rate of the mould. This invention seeks to overcome or mitigate these drawbacks with an apparatus which allows satisfactory Petri dishes to be produced at a considerably higher rate, and lower cost, than present apparatus. According to this invention, an apparatus for moulding by injecting from the side of a planar portion of a thin-walled article, such as a Petri dish, comprises a core piece co-operable with a cavity and defining a space between in which an article is to be moulded, at least one of the core or the cavity having a moulding surface, corresponding to the planar portion of the finished article, which is non-planar. In a preferred embodiment of the invention, the moulding surface of the cavity has a concave curvature, while the opposing surface of the core inserted therein has a convex curvature. In this embodiment, the radius of curvature of the moulding surface of the cavity is less than that of the opposing core surface. In alternative embodiments, these radii may be the same, or the moulding surface of the core may be flat or may have a concave curvature. In the cases where the core is concave we have observed that the cavity may be flat or even slightly convex. The invention also includes a method of moulding by injecting from the side of a planar portion a thin- walled article, such as a Petri dish which has a flat base, comprising the steps of forming the article in a mould whose shape is such that the planar portion initially has a central region which is thicker than the regions near the side or rim. In the moulding lids in soft plastics material for containers where visual distortion is not a problem it is known to centre-i ject plastics material and mould the surface with a curve ' ; However, this is done for a completely different purpose from the present invention. With the lid, where injection is from the centre, the centre is hotter than the rim when the lid is ejected from the mould. Thus the centre area tends to shrink more than the rim so that, if moulded flat, a lid with a saddle shape would be produced because, with shrinkage, there is effectively too little material area left in the centre. In order to prevent this adverse shaping it is known to add more plastics material area at the middle by curving to compensate for the shrinkage which does occur. In the present invention a different problem is being addressed. Because with the dish, which is made of hard plasticsas opposed to the above-mentioned lid which is made of soft plastic, it is important to inject from the side, with side injection there is a tendency for the hard plastic rim of the dish to pull the centre of the base up between the sides of the rim. Thus, there is the opposite problem that there is effectively too much material area in the centre of the base. In order to compensate or counter for this factor the hard plastic base of the Petri dish is moulded with a curve which is opposite to the way in which the base tends to be drawn between the sides of the rim. Further the present invention proposes to increase the wall thickness at the centre of the base to actually make the centre undergo shrinkage. The invention will now be described by way of example with reference to the accompanying drawings In which:- Figure 1 is a schematic cross-sectional view of a moulding apparatus in accordance with the invention, showing a preferred embodiment; Figure 2 Is a schematic cross-section view of a second embodiment of the invention; Figure 3 Is a schematic cross-sectional view of a third embodiment of the invention. Referring to Figure 1 of the drawings, a preferred embodiment of moulding apparatus for the invention comprises a core piece 10 which fits within a cavity 12, thereby substantially defining the shape of the dish within the space 14 between. The core piece 10 has a convex surface 16 which corresponds to the inside surface of the base of the dish, while the cavity 12 has a concave surface 18 which corresponds to the outer surface of the base. The convex surface 16 has a radius of curvature greater than that of the concave surface 18 thereby producing a base wall which is thicker in the middle than at the edges. Typical values, for the case of a 9 centimetre dish, of the radii of curvature are 2.5 metres and 1.5 metres for the core and cavity respectively, while the base wall is typically 0.6mm thick in the middle and 0.4mm thick at the edges. It is emphasised that the drawings are not to scale and that they exaggerate these differences for clarity. Arrow A shows the central and conventional position of the gate suited for most products. Arrow B shows the side position of the gate which is necessary when injection moulding a Petri dish. In an alternative arrangement (not shown), the radius of curvature of the convex surface 16 equals that of the concave surface 18. Another embodiment of the invetion, shown in Figure 2, shares the concave cavity surface 16 with the preferred embodiment described above, but employs a core with a planar corresponding surface 20. A further embodiment, shown in Figure 3, employs a similarly- shaped cavity but has a core with a concave, base surface 22. If a Petri dish could be moulded from the centre of the base of the dish then when injected, it would be warmer near the centre and therefore contracting In this warm area thereby keeping the base flat. In the method of the Invention plastics material is fed to the space 14 through a side injected runner system diagrammatically indicated by Arrow B. By producing a base wall which is thicker in the middle than at the edges, the central portion of the wall cools less rapidly than the edges having a smaller surface area-to-volume ratio. The contraction associated with this slower cooling draws the base into a substantially flat configuration, shortly after ejection from the mould. 9cm diameter transparent polystyrene Petri dishes, manufactured using prior art apparatus, must typically weigh 15 to 18 grams and have a cycle time of 5 to 6 seconds, in order to achieve an acceptable flat inner base wall. By employing a prefered embodiment of the Invention, this flatness may he maintined within tolerances of 0.3mm convex and 0.4mm concave, while reducing the weight and the cycle time to within 9 grams and 2.5 seconds respectively. This reduction of material usage, and production time, per unit will produce cost savings over large production volumes.

Claims: CLA IMS : 1. An apparatus for moulding by Injecting from the side of a planar portion a thin-walled article, such as a Petri dish, comprising a core piece co-operable with a cavity and defining a space between in which an article is to be moulded, at least one of the core or the cavity having a moulding surface, corresponding to the planar portion of the finished article, which is non-planar. 2. An apparatus according to Claim 1 wherein the moulding surface is on the cavity, and has a concave curvature• 3. An apparatus according to Claim 2 wherein the opposed surface on the core has a convex curvature. 4. An apparatus according to Claim 2 wherein the surface of the core opposed to the moulding surface is planar. 5. An apparatus according to Claim 2 wherein the surface of the core opposed to the moulding surface has a concave curvature. 6. An apparatus according to Claim 3 wherein the radius of curvature of the moulding surface of the cavity is not greater than that of the opposing surface of the core. 7. A method of moulding by injecting from the side of a planar portion a thin-walled article, such as a Petri dish which has a flat base, comprising the steps of forming the article in a mould whose shape is such that the planar portion initially has a central region which is thicker than the regions near the sides or rim. 8. An article having a planar portion made by the method of Claim 7.

Inventors: SORENSEN JENS OLE, SORENSEN, JENS, OLE
Applicants: SORENSEN JENS OLE, SORENSEN, JENS, OLE
Application year: 1987
Main classification: B29C45
IPC codes: B29C45, B29L31, C12M1