Development of an automatic ultrasonic cleaning machine for contact lenses manufacturing laboratories

During their manufacture in laboratories, most RGP contact lenses are cleaned manually. The use of small ultrasonic cleaning tanks is widely used. This repetitive and boring work cannot guarantee the respect of the duration of the process and an optimal quality of cleaning.

The judicious use of ultrasonic cleaning and the automation of the procedure should lead to a gain in quality and productivity.

Ultrasonic propagation principle (cavitation)

Transducers transform the electric impulses into mechanical energy by forming high-frequency longitudinal wawes (ultrasonic wawes). These wawes are transmitted to the fluid in the vat.

When the pressure drops, a large amount of air bubbles is generated

The bubbles grow, their size which goes from 1 to 15 microns, depends on the frequency.

When the pressure increases, the bubbles are compressed

The temperature in the bubbles increases until they implode, producing micro brushing

During this implosion, the temperature can reach 4700 ° C in the center of the bubble and the pressure 1000 bar. The jet of gas and steam can reach

400 km / h

This explains the gradual increase in temperature in an ultrasonic tank.

The sonic frequency

The sonic frequency play an important role: at low frequencies (20-30 KHz) large bubbles (~ 12 micron) are produced  with high power as they implode. This frequency range is suitable for removing heavy soiling. Areas of application: mechanical components, injection molds for example.

At higher frequency (40 - 60 KHz) the diameter of the bubbles is smaller (~ 8 to 4 micron) with a weaker force. A frequency range suitable for delicate cleaning. For medical, surgical, optical, electronic components

The importance of cleaning liquid

As water is only of limited suitability for ultrasonic cleaning, it is recommended to use cleaning agents. This will equalize the surface tensions between the water and the object to be cleaned, allowing better action of cavitation. Depending on the impurities to be removed, a specific cleaner is added. 

Temperature

40ºC to 60ºC is normal for aqueous alcaline, neutral or acid solutions. With increasing temperature the viscosity and density decrease and the vapour pressure increase. 

Cavitation alone increases the temperature. Depending on the object to be cleaned, a bath temperature control is necessary.

Rinsing

Rinsing is necessary to remove all traces of detergent.

Mixing the water or using ultrasound allows better elimination of the product. In general, deionized water water is used.

Sinner's circle.

Henkel chemist Dr. Herbert Sinner (1900-1988) developed in 1959 a theory of cleaning based on the actions of four elements: temperature, chemistry, mechanical action and duration of cleaning. According to Sinner, for an effective result the decrease in one of the four actions must be compensated by the increase in one or more of the others. Thus, the reduction in chemistry must be offset by an increase in the share of mechanical action or that of time, or that of heat, or a combination of these three other parameters.

BR-6 AMS from A&J Tecno

Automated ultrasonic cleaning is widely used in the field of ophthalmic lenses, especially before the application of anti-reflective layers in vacuum treatment. The application of this technique for contact lenses allows perfectly clean surfaces to be obtained before conditioning or treatment with oxygen plasma.

This technique can be used in the following stages of RGP contact lens manufacturing: 

• Deblocking of the lenses fixed on plastic arbors
• Wax residue cleaning
• Cleaning of polishing residue
• Cleaning after final inspection, before packaging
• Cleaning before plasma treatment

Operations

3 basic operations are possible:

• Ultrasonic cleaning
• Rinsing 
• Air drying

Method

The cleaning is done using 2 cleaning tanks (steps 1 and 3), 2 rinsing tanks (steps 2 and 4) and a drying tank (step 5).

The first ultrasonic cleaning tank and the first rinsing tank as well as the drying tank are used for unblocking and cleaning operations before the final inspection.

The second cleaning tank and the two rinsing tanks are used for final cleaning before conditioning.

Programs

Different programs can be selected.

For each program the cleaning, rinsing times as well as the minimum and maximum temperatures of the ultrasonic baths are set in the software. Access to these parameters is protected by a password.

This ensures that cleaning takes place in an optimal temperature and time space, without operator intervention.

Tools

To maintain the lenses, special tools have been created for optimal ultrasound action. 8 to 16 lenses of diameter 8mm up to 22mm can be cleaned at time.

Cleaning fluid

A cleaning liquid developed for cleaning chirurgical equipment with CE certification (class IIb) improves cleaning. It contains preservatives allowing disinfection of the liquid. The certification of the process in the technical file is simplified.

Conclusions

Faced with ever-increasing demands for quality in the manufacturing process, parameter control and automation of RGP lens cleaning provide added security. Manufacturing staff are relieved of a repetitive task, focusing on more profitable operations.

The Brio BR-6 AMS was created for this purpose.