In this series we’ve been discussing the impact of changes in fluid properties, specifically viscosity, on the performance of robotic fixed displacement dispensing systems. In the first installment, “Fixed Displacement Dispensing Doesn’t Guarantee Fixed Results”, we examined common dispensing defects, contrasting those that can be readily addressed through the implementation of a robotic fixed displacement dispensing system, and those that are the result of changes in physical fluid properties, which prove to be more resilient.
In the second installment, “Why Robotic Fixed Displacement Dispensing Systems Can’t Guarantee Consistent Bead Profiles”, we examined the impact of variations in bead profile that are beyond the control of robotic fixed displacement dispensing systems on the successful outcome of our process and the ultimate quality of our product.
But one burning question remains…
Can Temperature Really Change Viscosity Enough to Disrupt My Robotic Fixed Displacement Dispensing System?
Worth a Thousand Words
In “Why Robotic Fixed Displacement Dispensing Systems Can’t Guarantee Consistent Bead Profiles”, we showed a photograph of bead profiles, dispensed under controlled conditions, that doubled in width and halved in height as the dispense temperature was increased from 60°F to 100°F. This is a fairly normal factory ambient temperature range for most of the manufacturing facilities we have been in. But is that kind of change in viscosity characteristic of common sealers and adhesives, or did we just “pull a ringer” to support our claim?
The answer is “Yes, it is”, and “No, we didn’t”!
A Common Example
The graph at right, which shows the viscosity vs. temperature behavior for a common adhesive, really tells the story. Here, as we moved down in temperature, the formulation has a very sharp “knee” at 80°F and another, more gentle knee at 65°F. Over this small 15°F range, the viscosity of this adhesive more than doubles in viscosity from less than 40,000 cP at 80°F to 80,000 cP at 65°F! This is a huge change in viscosity over a change in temperature that is may be observed from 6:00A to 3:00P on most any given day!
This kind of performance is characteristic of the mid-to-high viscosity materials commonly used in sealer and adhesive applications.
A Task Too Large for Mechanics
Under a change in viscosity of this magnitude, it is easy to see how the bead profile can vary despite the advanced control provided by modern robotic fixed displacement dispensing systems. Though they are quite capable of placing the right volume of material in the right place time after time after time, the behavior of that bead after the dispense is beyond the capability of the dispensing system.
Exploit the Material
In short, to achieve success in dispensing, some kind of viscosity control is still necessary. And the best way to address the problem is to exploit the physical properties of the fluid itself. The change in viscosity as a function of temperature, shown on the graph above, is a physical property not a defect. It is both predictable and repeatable for each formulation. As a result, it provides the opportunity to use this knowledge to stabilize not just the dispense, but the behavior of the final beads as well – all by managing the temperature of the fluid as it is being dispensed to assure that the viscosity is stable and consistent from dispense to dispense.
The key phrase here is “as it is being dispensed”. Controlling the temperature in the drum, or in a pumping room, or even in the header doesn’t guarantee the temperature, and therefore the viscosity, of the fluid as it exits the nozzle – and that is the time that this viscosity matters most. Temperature control at the point of application is the only solution that makes sense.