Eye opener

Making contact lenses is one of the ultimate high-precision tasks. Tolerances are necessarily tight — any deviation, and the lens may not provide the required correction and could irritate the eye, making it worse than useless. But it’s also a high-volume operation, and manufacturers want to make the largest number of lenses, from the smallest amount of material, as fast as possible.

The UK-based Clearlab facility in Plymouth is now operating a production process which, it claims, is the world’s first fully-automated contact lens manufacturing system. Developed over three years along with system integrator Barr & Paatz, the entire system occupies just eight square metres, yet produces over 24 million daily disposable contact lenses, every year, with considerable savings in materials consumption.

Contact lenses are not easy to make, explained Clearlab’s technical support manager Jim Curtis. ‘You can’t mould lenses in a press,’ he said, ‘because the polymers they’re made of act like glue — they stick to the tooling and gum everything up.’

Injection moulding is the only option, and the mould has to be made from something the lenses won’t stick to; generally, polypropylene. ‘Most manufacturers make a two-part mould, inject the lens material, cure by heat or UV, then split the mould, discard it, hydrate the lens and package it in a blister pack,’ said Curtis. Generally, this process comprises several automated steps, but the lenses still have to be transferred from one part of the process to the next.

The new process, developed by Clearlab’s UK general manager, Wade Tipton, is simplified considerably. The main change is that, rather than making a two-part mould that is then discarded, the lens monomer is injected into a single dish-shaped polypropylene mould, which is spun to distribute the monomer evenly and then retained throughout the process. The mould eventually becomes the rigid component of the final packaging of the lens.

Pneumatic grippers, mounted on robot arms, transfer the pallets of lens moulds from one part of the process to another.

Even so, it’s a seven-stage process. Barr & Paatz’s machine produces a batch of eight lenses. The first stage sees the injection moulding of the polypropylene moulds, which are moved by a robot arm on to a transportation pallet. This moves to the dosing station, where the liquid monomer is injected into the mould, and then to the spinning stage. Two UV curing steps follows, then the lens is hydrated with a saline solution. Finally, the mould is sealed with a foil cap, and the whole assembly is sterilised in an autoclave.

The single-piece mould was the major innovation in the process, said Curtis. ‘Another manufacturer keeps the mould as part of the packaging, but it uses a two-piece mould and discards the top half.’ The new process, which has been dubbed Freeforming, means Clearlab has cut its polypropylene requirement by 2g per lens. ‘Our recycling contractor complains that he gets half as much material from us than he used to,’ said Curtis.

‘Our focus in designing the machine was to implement the best and smallest technical solution,’ said Barr & Paatz managing director Stirling Paatz. The process uses nine flexible robotic handling arms to transfer the lenses between the production stages, and over 400 pneumatic actuators, organised into cells for each process stage.

These all came from Festo, which allowed Barr & Paatz to take advantage of a common interface system, and a valve terminal backplane system which also provides digital I/O expansion for the robot controllers. ‘This has saved a considerable amount of space, which in turn allows our client to increase its manufacturing capacity within a very small footprint,’ said Paatz.

Clearlab now plans to build similar machines at its Singapore headquarters. ‘Technology transfer is now underway,’ said Curtis.