flow time through the needle tip can change the volumetric consistency. Initial flow of sealant material takes time to reach the surface.
COORDINATION WITH MOTION CONTROL The dispense head starts to accelerate at the first dispensing location to reach dispensing velocity. It requires a certain distance in order to reach pre-defined dispensing velocity. At the first starting point, the system will put down more material at given flow rate because applicator velocity is initially slow. Plus, the applicator decelerates to stop at the end position and thus more fluid will be dispensed at a given flow rate by ramping down applicator velocity. For instance, if a 5 mm line is dispensed from start to finish at given motion profile (figure 8) to reach 35 mm/s dispensing velocity, it takes 0.173 s to dispense one straight line. During this time 0.53 mm distance is used for acceleration (31 ms) and 3.95 mm distance is used for constant velocity.
FLUID BREAK-OFF When the needle is close enough to the substrate surface with a certain dispense gap (the distance between needle tip and substrate) during dispensing, fluid is still in contact with the surface. Fluid can break off when the needle moves up. The amount of fluid in the needle tip and the amount of fluid on the surface are difficult to control when the fluid break-off point is inconsistent.
SOLUTIONS There are two issues to address: knitting point and inconsistent volume. Precise time control of turning fluid pressure or valve motor before the dispense head reaches the first dispensing
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location or turning on delay time before dispensing begins would minimise the ‘dog-bone’ shape line. This timing is critical to control the amount of material volume. Timing control of the pressure off before the end of the sealing line could eliminate the excessive amount of sealant at the knitting point due to slow response time of the flow similar to the start location. In addition, motion parameters should provide needle move-up distance, acceleration and velocity control for consistent fluid break-off. Those three parameters can be optimised based on sealant type. The shortest needle move-up distance with optimised maximum acceleration and velocity for a given application set-up would result in high throughput if needle move-up and -down time is a significant portion of total dispensing time.
Jetting small uniform droplets Another dispensing applicator can eliminate the ‘dog-bone’ issue fundamentally: the jet applicator. Jet dispensing delivers small droplets without contacting the substrate. High energy is delivered by a piston to the tip of the nozzle and fluid is ejected through the small nozzle orifice as figure 9 shows. An array of small dots can be connected to form a uniform line as shown in figure 10. Precise valve control of a millisecond achieves consistent volumetric accuracy and eliminates ‘dog-bone’-shaped lines. Jetting can be optimised by moving the dispense head to compensate for acceleration distance. Thus, at the starting point, dispensing already reaches constant velocity when it begins to dispense dots continuously.
If a sealant fluid contains fillers, then the choice of nozzle size will be limited by particle size distribution. Larger nozzle choices due to filler size can make it challenging to put down a fine line width.
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43 | commercial micro manufacturing international Vol 6 No.5
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