1. Field of the invention
The present invention is generally concerned with the manufacture of molds for making optical lenses from thermosetting synthetic materials.
It is more particularly (but not exclusively) concerned with molds for ophthalmic lenses.
2. Description of the prior art
Such molds comprise two molding shells whose facing inside surfaces are a negative image of the surfaces required on the optical lens to be molded.
To set the distance between the molding shells and to seal the molding cavity delimited by them an annular closure member is required between the molding shells. It must allow the molding shells to follow the shrinkage of the molded thermosetting material as it sets.
At present this closure member is usually a relatively thick gasket in which the molding shells are nested.
Although it can be satisfactory, the use of a gasket imposes a number of constraints, mainly because it must be made in advance, making it require an injection press that represents a not inconsiderable investment, a whole range of different gaskets catering for different geometrical characteristics is needed to cater for various possible conditions of use and a large and therefore costly stock of gaskets must be kept.
Furthermore, the gasket does not of itself locate the molding shells relative to each other, in that it only secures them in one direction.
To secure the molding shells in the opposite direction it is usually necessary to provide a clamp which surrounds them from the outside.
This complicates use of the system.
Finally, the gasket is relatively rigid in itself, which makes handling difficult, and gives rise to recycling problems.
An early proposal to overcome these drawbacks was to make the closure member on demand, and in practise at the last minute, from an annular closure sleeve which is fitted around the two molding shells after they are appropriately located relative to each other.
To be more precise, it was proposed that this closure sleeve should be made by winding a strip material around the edge of the molding shells.
French patent No 973 216 describes the use of a strip of paper, for example, while French patent No 1 018 486 describes the use of a strip of rubber.
In either case, this has the advantage that all that needs to be stored is a roll of the strip.
The strip is simply cut to length as required for use.
Nevertheless, there remains the problem of fastening the closure sleeve to the molding shells it surrounds to locate the two molding shells correctly relative to each other.
French patent No 973 216 proposes to achieve this by gluing.
However, subsequent separation of the molding shells from the closure sleeve to remove the finished optical lens from the mold is then difficult and leaves traces of glue on the molding shells and on the finished lens. These traces of glue must be removed from the molding shells at least, entailing the use of a specific cleaning procedure.
In French patent No 1 018 486 the required fastening is achieved by binding the closure sleeve onto the molding shells but this complicates assembly and separation of the molding shells.
A more recent proposal, for example in published Japanese patent application-No 55-105519, is to make the closure sleeve from adhesive tape.
Apart from the fact that if adhesive tape is used traces of glue remain on the molding shells after the latter are separated and have to be removed, if the adhesive tape has a width less than the overall height of the molding shells, this can reduce its grip on them to the detriment of securing them, or if its width is greater than their height, the excess width of the adhesive tape (i.e. the width of the adhesive tape projecting beyond one or both of the molding shells) must be removed for reasons of cleanliness (as otherwise the surplus adhesive tape could retain dust and other soiling substances).
A general object of the present invention is an arrangement avoiding all these drawbacks.