In our last segment, Fixed Displacement Dispensing Doesn’t Guarantee Fixed Results, we examined how viscosity variation impacts your robotic fixed displacement dispensing system and the many defects that can still exist, even in the most highly advanced of these systems. In this installment, we look more closely at the phenomenon of bead profile, it’s importance to the quality of your end product, and why simply controlling path, dispense rate, and total volume does not guarantee that your process will produce consistently high-quality parts.
The Importance of Bead Profile
The importance of bead profile in sealer and adhesive processes cannot be overstated. This is obvious in gluing operations where the right bead height assures proper contact of the adhesive to both mating parts. This is essential to assure that the proper bond strength is established to insure proper performance of the finished part.
Sealer, beads are not that much different. They must be placed in the proper location with the proper width and depth (profile) to assure that they provide the proper protection properties in the finished part.
In both applications, improper placement or bead profile can result in sealer or adhesive in areas where it does not belong (and not where it should be) which can produce performance and/or aesthetic issues that can result in part rejects, or even worse, product recalls.
Imagine a diving mask that doesn’t seal when placed under pressure and fails 20 meters below the surface of the ocean…
Why Bead Profile Varies
One of the most common reasons for a variation in bead profile is a change in fluid viscosity.Often this is the result of a change in fluid temperature. A good example is shown in the photograph at right. These beads were all dispensed with a robot to assure that the nozzle path, speed, distance and angle to the part were consistent. The only change was the temperature of the fluid which, of course, changed its viscosity. We can see that the bead dispensed at 100°F is very nearly twice as wide as the bead dispensed at 60°F.
The Rest of the Story
Another important factor here is that these robotically placed beads were dispensed with a fixed-displacement system. This means that the volume of each bead is consistent.
So how can the width of each bead vary by so much?
The answer is the “slumping” that we discussed in our last post. What the photograph above doesn’t show you is the height of each bead. The graphic at the left shows “the rest of the story”.
As the fluid temperature increases, the viscosity of the fluid decreases, making it easier for it to flow out under the force of gravity. As a result, even though the bead volumes are identical, the bead width increases and the height decreases. If we require the 7mm bead profile produced at 70°F to properly mate our parts without exceeding our glue line limits, and we dispense the bead at 100°F, we are likely to create parts that do not have the appropriate bond strength and have glue outside the bond area that may cause aesthetic rejects.
Substrate Temperature Matters
Just as fluid temperature is important to the performance of the bead, so too is the temperature of the substrate. The primary reason is mass. In most instances, the part you are dispensing onto is several orders of magnitude greater than the mass of the bead. The bead will immediately start trying to assume the same temperature as the substrate. Most of these fluids, however, are not as thermally conductive as the substrate and take time to make this transition. Often, a bead temperature can be selected that will allow it to be at exactly the right temperature/viscosity by the time the operation is completed, assuring that the results are both consistent and predictable.
A couple of good examples (the hot door example and the hot can example) can be found in our February 2014 blog post “So Which Temperature Is Important in Fluid Process Control? (Part 2)”.
Bottom line here is that the physics of the fluid often outweigh the ability of the mechanics (and associated electronics) to compensate for variability. It is generally much easier, reliable, and cost effective to control the physical properties of the fluid as well as the mechanics of the dispense system. Only then can a totally predictable outcome be achieved.
In our next segment we’ll answer the question, “Can Temperature Really Change Viscosity Enough to Disrupt My Robotic Fixed Displacement Dispensing System?”