Wednesday, February 11, 2009
Dispensing at the SMD technique independent of viscosity
Press release, February 11, 2009
In modern manufacturing sequences in which the dispensed quantities continually decrease with the reduction of component dimensions, the smallest variations of the quantity become more apparent than was the case a few years ago. Even with the best high-tech dispensing valves, the characteristics of the medium to be dosed change with environmental conditions. As a result, the differences in the volume have to be avoided or corrected with suitable measures.
By Anton Knupfer, Product Manager Dispenser, Essemtec AG, Switzerland
The reasons for the varying dispensing volumes are set using multiple parameters that have influenced the dispensing process. The pressure at the cartridge and the time of dispensing can be quoted as the most obvious variables. But less obvious values including the environmental temperature, the level of the cartridge and the retention time of the cartridge at the machine also may have considerable influence.
Influencing factors of the time/pressure dispensing procedure
Varying dispensing procedures react with different intensity to various process variables and that is why they produce various outcomes in the volume of dispensing. The least expensive method of dispensing is definitely the “time/pressure” procedure.
The effect of such a dispensing system is simple. By pressurizing cartridge, the medium will be pressed out through the dispensing needle. Because of this, the volume of dispensing depends on the level of pressure on the cartridge. If you always leave this pressure pulse the same, you will notice a decrease in the volume of dispensing of the emptier the cartridge. The reason for this is the space that gets larger with the increase of empty space at the injection (fig.1). Using a constant entry pressure level and constant period of time, at which the space can fill itself with air, the pressure in the cartridge gets lower. Because of this, the dispensed volume reduces as well.
Another factor that causes an irregular result is the heating of the dispensing medium. This is caused by the continuous compressing and removal of the contents of the cartridge. Because of these changes in pressure, the same effect is caused that can be noticed as the blowing up of a bicycle tire by hand — Heat. This heating of a liquid leads to a change of the viscosity and leads to variable results as well.
At the screw valve, the medium for dispensing is moved with a conveyor worm in the form of an Archimedean screw. The medium will be pressed out of the cartridge into the conveyor worm using air pressure. However, the pressure at the cartridge is so low that at the standstill of the screw, no medium will be conveyed. Therefore, the pressure can be constantly upon the cartridge. Basically, the conveyed volume only depends on the number of turns of the screw and the pitch. However, depending on viscosity, variations can still result, as the screw in the feeding pipe cannot tighten 100 percent. Thus, the age of the medium to be conveyed, as well as changes in the environmental temperature and the various viscosities compelled by this, become apparently different at the conveyed volumes.
Influence on the jet dispensing valve
More and more often the “Jetting”, an extremely quick, exact and contactless dispensing method, that is suitable for a wide range of applications, is being used. By a quick pulsating ceramic fibre plunger, the smallest drops are “shut” out of the single valve. Today, this technique is the dispensing method with the best reproducibility of dispensing volume. However, air pressure and viscosity also have an influence on the result.
Fig.1: Varied air volume (3) at full (a) and empty cartouche leads to differences of the volume at the time/pressure dispenser. Legend: 1) compressed-air supply 2) stopper 3) air volume 4) plunger 5) medium to dispense.
Heat or cool
As previously described, such variations of volume are undesirable in manufacturing. Methods and technologies have to be used to prevent or compensate for the variations.
The best method to dismiss the influence of the temperature is the use of a heating/cooling valve. Such valves include Peltier elements that feed or dissipate heat to the valve face as required and so hold the temperature constant. Because of this, the conditions at dispensing are independent from the environmental temperature. Time/pressure valves or screw-dispensing systems are available and can work with an air or liquid-heat exchanger.
The method of the temperature control demonstrated above is still not efficient for long periods of time for a very exact constancy of the volume alone, as it cannot eliminate all influential factors. Better results can only be obtained with a controlled member, which measures and calibrates the real dispensed volume regularly. In this connection, a set reference volume will be dispensed and checked using a suitable procedure to determine whether this corresponds to a quantity settled before. After the result of this check comes in, the machine makes the adjustments of a factor of quantity like that, so the dispensed reference volume is again in the range of tolerance (fig.2). The dispensing result stays stable, despite the changing of parameters.
This procedure is called “automatische Dosiermengen-Kalibrierung” or in English “Automatic Dot Size Adjustment” (ADSA). To determine the dispensed volume, an optical measurement method is used. The diameter of a dispensing point will be recorded with a camera and the space of the point determined with quick picture processing routines. If the diameter of the drop is too small, the process of dispensing will be influenced in a way that a larger volume will be dispensed the next time. Such test points are mostly dispensed out of the object products at a known reproducible surface, as the dimension of the point not only depends on the volume but also the wetting angle of contact.
Fig.2: ADSA at the example of the time/pressure dispending valve: The diameter of the point stays constant by readjustment of a quantity factor (time or pressure).
In the past, various tests have been done to control the dispensing volume by empirical models, which were based on the measurements of several parameters such as temperature, pressure and level. Such models were expensive and were missing something important, namely the check of the actual volume. Only with an active controlling procedure, like the one presented by ADSA, can the demanded quality be guaranteed to be reproducible.
ADSA is a long-standing, tested procedure that compensates active variations in manufacturing because of varied environmental conditions. With adequate adjustment, the system is able to distinguish empty cartridges or clogged dispensing needles. The safety of the operation will be repaid with a slightly higher flow time. ASDA can be used during dispensing with needles as well as at jetting. Because of ADSA, a very constant dispensing result is also available with a simple time/pressure dispensing system.