There are difficulties associated with conventional injection moulding equipment when it is required to mould a thermoplastic article which is very thin. Such thin articles may comprise contact lenses or particularly very thin lenses utilised in equipment such as pap smear testing apparatus.

Where the article being moulded is of a thickness of less than 0.25 millimetres there is difficulty in flowing thermoplastic material into the mould. This difficulty occurs due to the fact that injected thermoplastic material, such as polycarbonate, will not flow properly through such narrow cavities but rather starts to roll causing air bubbles and possibly cessational flow altogether.

Compressive injection moulding may alleviate such problems to some extent. Compressive injection moulding is a process whereby thermoplastic material is not flowed into a cavity of fixed dimensions but rather the mould cavity is charged with thermoplastic material and then the mould cavity is compressed and reduced in dimension during the charging and/or immediately thereafter. In this manner the cavity into which the thermoplastic material is flowed is originally of sufficiently large a dimension to permit adequate flow but before the flowed-in material is set the mould cavity is reduced in dimension to the required dimension for the finished article. This process requires mechanical rams or other devices in order to close the mould from its expanded position whereat the thermoplastic material is flowed-in to its compressed position. More importantly this compression apparatus must be controlled with respect to the timing of closure and the rate of closure in order that a satisfactory finished article may be produced. To date there have been difficulties synchronising the timing of the closing of the mould (and the rate of closure) with the injection of thermoplastic material through the sprue such that satisfactory moulding of articles such as polycarbonate lenses of less than 0.25 millimetres has either not been possible or has only been possible with extremely expensive and elaborate equipment.

Accordingly it is an object of the present invention to ameliorate one or more of the disadvantages with existing compressive injection moulding equipment or at least to provide the market with an alternative.

According to the present invention there is disclosed compressive injection moulding equipment comprising a main sprue for channelling molten thermoplastic material into a compression type mould adapted to reduce the dimensions of the mould cavity during the injection process; a secondary sprue and runner system in fluid communication with the main sprue adapted to divert thermoplastic material to a position whereat it may directly or indirectly urge a component of the mould towards a complementary mould component thereby reducing the cavity dimensions; means to control the flow of thermoplastic material in the main and secondary sprue so as to control the timing and rate of mould closure with respect the timing and rate of injection of thermoplastic material into the mould cavity.

One embodiment of the present invention will now be described with reference to the accompanying drawing in which: Figure 1 is a sectional schematic view of part of a mould and sprue runner system in accordance with the present invention.

According to the embodiment of Figure I there is depicted a mould cavity 1 of a shape appropriate for forming a lens and defined by upper and lower mould components (not shown). It will be observed that the mould cavity 1 is in communication with a main sprue runner 2 fed by main sprue inlet 3 through which molten polycarbonate material (not shown) is injected.

The main sprue runner system 2 is linked at position 4 via a secondary sprue 5 to a secondary runner system 6 adapted to flow thermoplastic material.

A flow restrictor/controller is provided at 7 in order to control the timing for and volume of injection of thermoplastic material into cavity 1. A further flow restrictor 8 is provided in the secondary sprue runner 6 in order to control the flow of thermoplastic material from first end 9 of secondary runner system 6 to the second end 10 of secondary runner system 6.

The purpose of admitting molten thermoplastic material to the second end 10 of secondary runner system 6 is to urge compression piston 11 upwardly thereby urging the lower mould component (not shown) against the upper mould component (not shown) with the upper mould component being controlled by cavity piston 12. This urging of the lower mould component against the upper mould component reduces the thickness of cavity 1.

In use the main sprue is pressurised hence causing thermoplastic material to be injected into cavity 1 via flow restrictor 7 on the main runner system. At this moment the upper and lower mould components are positioned to define the mould cavity 1 at a volume corresponding to the maximum which is permitted by the stroke of the compression piston and which will admit thermoplastic material to the cavity in such a manner as to promote good flow through the cavity. At the same time this pressurising of the main sprue will pressurise the secondary sprue and cause thermoplastic material at the second end 10 of the secondary runner system 6 to be urged against the bottom of compression piston 11 hence moving it and the attached lower mould component upwardly against the upper mould component (not shown) thereby causing cavity 1 to be progressively reduced in dimension to the minimum volume position defined by the maximum extent of the upward stroke of the compression piston. During the reduction in size of the cavity 1 the thermoplastic material which has been or is being flowed into the cavity via flow restrictor 7 is urged throughout the cavity and compelled to adopt the dimensions desired for the finished product.

Adjustable pressure relief mechanisms 14 are provided in the secondary runner system so as to assist in regulating the speed and timing of the compression stroke of the compression piston.

It will be appreciated that in order that the compression piston be capable of urging the lower mould component upwardly against the pressure of thermoplastic material in cavity 1 the area of that piston against which the molten thermoplastic material must act should be greater than the area of molten material against which the lower mould component is acting and in this regard test units have used a ratio of 16.4 : 1.

A component ejector 15 is also provided as is the case with conventional compressive moulding equipment and it will be observed that this component ejector operates through the hollow core 16 of compression piston 11.

It has been found that compressive injection moulding equipment in accordance with the present invention and utilised in accordance with the methods as above described facilitate the moulding of lenses such as lenses used in the course of pap smear tests having a thickness of 0.22 millimetres or less. Testing has shown that the moulding of lenses down to 0.13 millimetres can be reliably performed and indeed if uses can be found for articles down to approximately 0.05 millimetres in thickness it is expected that apparatus and methods in accordance with the present invention will be capable of reliably producing such articles.

It should be appreciated that alternative embodiments of the present invention may be devised apart from those above described without departing from scope and intendment of the present invention and it is the intention of the present document to cover such embodiments.