The shear rate of an adhesive is a measure of the stick/slip performance of the fluid. It is measured in rate and force. Simply put, imagine that you place a fluid between two plates of glass and then slide the two plates in opposite directions. The speed of the glass sliding is known as the shear rate.
The amount of force required to move the glass is known as the shear force. The shear rate and shear force are determined by the viscosity of the fluid. For example, when using oil, the glass would move quickly with minimal force required. On the other hand, when using solder paste the glass would move slowly with more force required.
Dispensing Fluids from Syringes
When considering dispensing fluids from syringes equipped with dispense tips, consider this. When dispensing low viscosity oil, you can assume that the shear rate will be high/fast and the shear force will be low/minimal to move the fluid. When dispensing a high viscosity solder paste you can assume that the shear rate will be low/slow and the shear force will be high/significant to move the fluid. The shear stress corresponds with the shear rate.
The higher the shear rate, the higher the shear stress. This performance could also vary depending on the material of the syringe and tip used to dispense the fluid. For example, some fluids may stick more to stainless tips than they would to plastic Free Flow tips. The temperature should also be taken into consideration.
AirFree Fluid Dispensing is Better
AirFree fluid dispensing equipment is better than pneumatic fluid dispensing equipment because it can apply force much greater than the fluid shear force while maintaining force consistency at a controlled speed. Therefore, shear force does not impact the programming of the AirFree dispenser.
An air system requires the air pressure setting (force) to match the shear force of the fluid. If there is too much force the fluid will rapidly squirt out of the reservoir like a SuperSoaker squirt gun. If there is not enough force the fluid does not move forward. The operator is forced, via trial and error, to find what air pressure setting matches the fluid shear force. This process can take hours while the AirFree fluid dispensing system sets up in minutes, lowering your deployment costs.
An AirFree fluid dispensing system has the added benefit that once the dispense program is set, the force and distance traveled never changes. The force and distance traveled constantly changes with an air pressure system due to the unstable air pressure supply, adding insult to injury.
For more information on how AirFree fluid dispensing offers a better solution, contact Fishman Corporation.
*photo credit plastics.tamu.edu