Optovision - Creating an oil-free vacuum revolutionises spectacle lens coating

Optovision produces high-quality plastic varifocal lenses for the customers of opticians throughout Europe. On its state-of-the-art production line, Optovision applies a coating to approximately one-and-a-half million lenses each year. Once the machining stage is complete, the PVD coating process is carried out in a vacuum, and Optovision relies on Busch's dry-running vacuum systems to generate the initial vacuum of 10-2 mbar. A cryogenic vacuum pump is then used to evacuate the process chamber to 10-5 mbar. This enables evacuation with no operating fluids whatsoever, which makes the process both safe and efficient and ensures that high-quality spectacle lenses are produced.

Optovision is a German spectacle lens manufacturer that specialises in free-form applications and sees itself as a service provider for opticians. Its product range includes spectacle lenses from a variety of different materials, in many different colours and finishes, from single-vision lenses to custom-made free-form varifocal lenses. The main production focus is indisputably the production of the plastic varifocal lenses that opticians order directly from Optovision on behalf of their customers. These are produced individually, each lens in its own right, in accordance with specifications provided by the optician. Three to five days later the optician receives the ordered lenses and can then fit them into the spectacle frame. The starting material used by Optovision is a semi-finished product, one side of which has already been cast in the correct form and does not require any further machining. Optovision keeps tens of thousands of these so-called blanks in stock. They differ in terms of the shape of the cast side and their material. The other side of the lens is machined in accordance with the prescription from the ophthalmologist or optician. The first step in this process is to turn and subsequently polish the lens surface. The lenses are then thoroughly cleaned before being provided with a special anti-scratch protective coating by immersion in a coating vat. They can also be tinted if desired. This process is followed by the physical vapour deposition (PVD) process, which takes place in a vacuum chamber in one of the 18 coating units. Here, the lenses are treated to minimise reflection.

In the coating machines the spectacle lenses are inserted individually in a small round cup, which is then introduced into the vacuum chamber. The coating material is introduced and then vaporised using an electron gun. This vapour is then deposited on one side of the lens. The coating materials used include oxides of silicon, titanium and aluminium. A succession of different layers are applied in the vacuum chamber. The first layer is an adhesive layer. This is followed by separate anti-reflective and anti-static layers, and lastly by a superhydrophobic layer, which has a dirt-repellent effect. The entire process takes around an hour, during which time the individual layers on the lenses are monitored continuously and controlled fully automatically. Next, the chamber is flooded, the lenses turned round, and the same process set in motion to coat the other side of the lens.

The process takes place in a vacuum of 10-5 mbar. The initial vacuum is created using a completely dry-running vacuum system from Busch. A Roots vacuum pump and an upstream COBRA screw vacuum pump evacuate the chamber to 10-2 mbar. Once this initial vacuum is achieved, the mechanical vacuum system is separated from the chamber by means of a valve, and a cryogenic vacuum pump continues to evacuate the chamber until the process vacuum is reached. On each coating machine, Optovision has installed a mechanical vacuum system for generating the initial vacuum plus a cryogenic vacuum pump for the final evacuation. Originally, all the vacuum systems were equipped with oil-lubricated rotary-vane vacuum pumps as preliminary pumps, plus one roots vacuum pump on each system. To make sure that no oil or oil vapour can get from the rotary vane vacuum pump into the process chamber, it proved necessary to install noble metal catalysts between the process chambers and vacuum systems. These costly catalysts were intended to trap any oil that may start to flow in from the pump in order to prevent it from penetrating into the process chamber.

In 2007, Optovision installed and tested its first dry-running vacuum system from Busch. The company's technical director, Hans Joachim Loch, thereby succeeded in generating the vacuum completely free from operating fluids; this in turn removed any risk of oil from a mechanical vacuum pump getting into the process. The resulting improvement in process reliability cannot possibly be overestimated, since even the tiniest particles of oil would render an entire high-quality batch of spectacle lenses practically unusable. The fact that the noble metal catalyst is no longer needed results in huge savings. Hans Joachim Loch also achieved something else: it has been possible to drastically reduce the operating costs for generating the vacuum, because there is no longer any need for an oil change (this had to be done regularly with the rotary-vane vacuum pumps). At the same time, the costs associated with oils, filters, their disposal and downtime of the machines have also been dramatically reduced. As all vacuum systems operate continuously around the clock, this results in a significant number of extra operating hours — the fact that no maintenance work needs to be carried out has an extremely positive effect. A further benefit has been gained in terms of noise emissions: the use of water-cooled motors has had the effect of significantly reducing the noise level compared with the rotary-vane technology that was used originally. Following good experience with the first vacuum system from Busch, the oil-lubricated rotary-vane technology has now been replaced by the COBRA screw system on many coating machines. The aim in future is to use this innovative, dry-running vacuum technology exclusively.

Busch has been using screw technology successfully for many years now in many different applications. One of the most important areas relates to the coating processes in many industrial sectors and in the production of solar modules; in the latter case, COBRA screw vacuum pumps are installed for the most demanding processes. COBRA screw vacuum systems are also used in chemical engineering, the pharmaceuticals industry and in metallurgical processes.


Captions:
A round cup filled with varifocal lenses in the open vacuum chamber

Dry-running COBRA vacuum system in a PVD process


Busch vacuum pumps and systems
Marketing/Marketing Services
Uli Merkle • 26 June 2010