Cleaning 2,000 different parts ready for coating

Die empfindlichen Gläser der Linsen und Prismen erfordern eine schonende Reinigung und Trocknung. (Bildquelle: Fisba) / The sensitive glass of lenses and prisms necessitates gentle cleaning and drying. (Image source: Fisba)

Die kompakte Anlage zur Feinstreinigung von optischen Linsen und Optiken ist komplett gekapselt und verfügt über acht Wannen. (Bildquelle: UCM) / The compact system for ultra-fine cleaning of optical parts and lenses is fully enclosed and comprises eight tanks. (Image source: UCM)

Der Durchsatz liegt bei zwölf Chargen pro Stunde. Die Eingabe der teilespezifischen Programme erfolgt mit einem Barcodeleser (Tisch). (Bildquelle: UCM) / The equipment has a throughput capacity of 12 loads per hour. Part-specific programs are selected by means of a bar code reader (table). (Image source: UCM)

Die letzten beiden Wannen werden für die HFE-Trocknung genutzt und sind verdeckelt. Das Umsetzen der Teile erfolgt ohne Öffnen der Deckel. (Bildquelle: UCM) / The last two tanks are reserved to HFE drying and are covered with a lid. Parts can be transferred from one tank to the other without removing the lids. (Image source: UCM)

Nach der Reinigung transportiert das gekapselte und mit einer Flow-Box ausgestattete Entladeband die Optiken zu einer Kontrollstation, wo sie unter Reinraumbedingungen geprüft werden. (Bildquelle: UCM) / After the cleaning process, a fully enclosed discharge conveyor equipped with a Flow Box delivers the lenses to a quality control station where they are inspected under cleanroom conditions. (Image source: UCM)

In cleaning optical lenses and prisms the challenge lies not just in the diversity of products. At FISBA Optik, part dimensions and widely differing glass grades likewise call for the use of a sophisticated final cleaning solution. In developing such a system, UCM AG opted for the integration of a HFE drying stage because even de-mineralized water is too aggressive for some of the glass types to be treated.

Based in St. Gallen / Switzerland, FISBA Optik AG has ranked among the world’s leading vendors of optical systems, devices and components for over 50 years. Their products are made to customer-specified dimensions for a host of applications such as, e.g., medical equipment, image processing, metrology, biophotonics, laser diode technology, astronomy and aerospace engineering. Also, the company is among the first in Europe to rely on precision glass moulding for the cost-efficient series production of powerful, high-quality glass aspherics and lenses of complex geometry. “The quality of our optical components is critically dependent on their cleanliness”, explains Peter Boner, Head of Precision Lens Cleaning at FISBA.

Cleanliness – a decisive quality factor

About three years ago, aiming for the most exacting standards in this field as well, the company invested in a new ultrasound cleaning system for its pre-cleaning operation. The contract went to UCM AG of Switzerland, a Dürr Ecoclean Group company. “Needless to say, a key factor prompting our decision was cleaning quality. UCM is very flexible and can boast enormous experience and know-how in the optical industry, enabling us to benefit from this expertise. Their cleaning solution was tailored to suit our needs in terms of both equipment technology and chemistry, so that we are achieving a very good result. This is in addition to our geographic vicinity and an open, partnering cooperation”, Peter Boner summarizes his company’s decision-making criteria at the time.

Accordingly, UCM was high on the list of vendor candidates when, in 2012, the time came to replace the final cleaning line. On the one hand, the existing system was getting on in years. On the other, it did not support cleaning of different glass types so that a laborious manual cleaning process was required.

The challenge: a diversity of products, dimensions and glass types

FISBA produces between 1,500 and 2,000 different optical systems. These include high-precision lenses with a diameter of as little as 0.5 millimeters. The huge product diversity and broadly varying dimensions presented a challenge in the design of the new cleaning equipment. It also had to be considered that the new system had to be capable of cleaning even highly delicate and acid-sensitive glass parts ready for coating. Irrespective of size and material, ultra-exacting cleanliness levels had to be attained in terms of both particulate and film-type contamination. After cleaning, some of the lenses must be coated on one or more sides.

High flexibility ensures part-specific cleanliness

Based on the existing technology of the pre-cleaning system, UCM developed a cleaning process meeting all of the above criteria. “As a matter of fact, most parts are ready for coating as they come out of pre-cleaning, which combines the use of solvent-based and aqueous fluids, so this solution suggested itself. Final cleaning, on the other hand, is carried out with an all-aqueous medium developed through trials”, the Head of Precision Cleaning notes. The reconditioning systems for the cleaning and drying media were both included in UCM’s scope of supply. The cleaning system is fully enclosed and comprises eight immersion tanks, including six equipped with a multi-frequency ultrasound capability (40, 80 and 120 Hz). The power level can be individually adjusted between 2 and 15 watts/liter. Thus, the ultrasound cleaning intensity can be perfectly matched to the given product type. The ideal parameters for the different lenses – e.g., ultrasonic power and frequency as well as the dwell time in each tank – are defined in part-specific programs which are stored in the equipment controller. To ensure an accurate compliance with defined tank dwell times, the flexible control system allows the user to enter “priority time” settings. This also results in an optimization of throughput. “We can store 64 programs. At present we are working with approx. 20 programs in series production. However, there are new developments coming in all the time, so we are grateful for the flexibility provided by the additional spare program slots”, Peter Boner adds. A program is selected by the operator reading a bar code from the associated order documents. From that point the process takes place fully automatically.

Tailor-made cleaning processes

In the first tanks, contamination from handling and shipping operations as well as gloveprints are removed from the lenses by ultra¬sonic treatment. All cleaning tanks are equipped with this ultrasound capability. The subsequent rinsing process is a combination of immersion and spraying techniques. This tank is therefore set lower, so that parts dipped into the fluid can be rinsed with ultrasound assistance and then sprayed off with de-mineralized water as they are lifted out. On the one hand, this method prevents any extended exposure of the lenses to de-mineralized water. On the other, it provides an unsurpassed rinsing effectiveness while minimizing the carryover of water into the downstream dewatering tank.

Gentle drying with hydrofluoroether (HFE)

Reliable dewatering is made necessary by the HFE-based drying step. This process allows even sensitive glass parts to be cleaned ‘ready for coating’ without impairing their optical performance.

For the drying process, the last two tanks – which are also equipped with multi-frequency ultrasound units – are installed in a closed chamber. The system is so designed that product trays can be transferred from the first to the second tank without opening the tank cover. The HFE is cascaded from the last to the first stage before entering a distillation system in which it is reconditioned continuously. A condensing zone placed above the tanks prevents HFE fumes escaping during drying and lift-out operations from getting released into the ambient atmosphere. The fumes condense on cooling fins whose temperature may be as low as -23°C and then enter the distillation system.

The dried lenses exit on the fully enclosed discharge conveyor equipped with a filter/fan unit (Flow Box) which delivers them to an inspection station where they are checked under cleanroom conditions. “We are very satisfied with the cleaning results. Moreover, the cleaning process has become more cost-efficient due to the elimination of manual operations. We are convinced that with UCM, we have made a very good choice”, Peter Boner concludes.