Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Patent
1989-03-31
1990-10-30
Heitbrink, Jill L.
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
264336, 425542, 425555, 425577, B29C 4500
Patent
active
049667440
DESCRIPTION:
BRIEF SUMMARY
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 substantially 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 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-inject 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 present this adverse shaping it is known
REFERENCES:
patent: 3196483 (1965-07-01), Eyles
patent: 4254065 (1981-03-01), Ratkowski
patent: 4422998 (1983-12-01), Sorensen
patent: 4447372 (1984-05-01), Kreuttner
F. J. Lupton, "Injection Moulds-Part 2", British Plastics, vol. 43, No. 10, Oct. 1970, pp. 142-145.
Hoechst, "Artikelgestaltung und Werkzeugkonstruktion bei der Spritzgussverarbeitung von Niederdruck-Polyathylen und Polypropylen", Der Plastverarbeiter, vol. 12, No. 10, Oct. 1961, pp. 453-454.
Callan Edward W.
Heitbrink Jill L.
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