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Fixed Displacement Dispensing Doesn’t Guarantee Fixed Results

Posted by Mike Bonner

Nov 13, 2019 9:39:00 AM

It has become commonplace to employ fixed displacement systems when dispensing mid-to-high-viscosity fluids like sealers and adhesives.  It makes sense.  Proper performance in these applications hinges on making sure that the right amount of sealer or adhesive is dispensed in the right place on every part.  When coupled with a robot to assure that path, speed, distance and angle to part are consistent and repeatable, it is easy to believe that the dispense job will be perfect every time.

Unfortunately, there are other factors at play…

Common Defects

This picture shows the common defects that plague modern sealer and adhesive dispensing applications. 

common defects

It doesn’t really matter if it is being applied manually or robotically, the same defects often show up.

It makes sense that combining fixed-displacement control with robotics can virtually eliminate thin beads and heavy beads because the same fluid volume is dispensed over the same path at the same speed.

But what about the others? 

Not All Defects Are Created Equal

While thin beads and heavy beads are a function of dispense rate and tip speed, defects like starting and ending blobs, slumping, and tail-off are related to the viscosity of the fluid being applied.

Probably the easiest of these to understand is slumping.  Here, the sealer simply flows (due to gravity) after it is applied.  This is especially a problem when the bead is dispensed on a vertical surface.  The lower the viscosity of the fluid, the faster the flow.

Tail-off results when the system completes the dispense and then returns to “home position” and the sealer or adhesive which, by their very nature are sticky, extrude from the nozzle, stretching until the string breaks – falling where it may.  This may be addressed by pausing the nozzle at the end of the path, then moving it in a small circle over the end of the bead as it is pulled up and away to break the extruded string.  This can, however, result in an end-blob, depending on how much fluid is pulled from the nozzle.

This may also be addressed with a “snuff-back” valve, which applied a negative pressure to the fluid to draw it back up into the nozzle at the end of the dispense.  The amount “snuffed-back” must be accounted for on the next dispense to assure that the fluid starts dispensing at precisely the right moment in order to assure the desired consistency between parts.  Unfortunately, the amount of “snuff-back” required varies with the viscosity of the fluid, size of the nozzle, fluid flow rate, and other factors, so it is not a one-size-fits-all solution.

Starting blobs can happen when the nozzle is paused over the starting point at the beginning of the dispense.  Or, it can occur when the fluid in the nozzle is cooled between cycles and the resulting higher viscosity fluid doesn’t move as quickly as the rest of the volume in the system when placed under the same pressure.  Once it does move, it can cling and cause “the blob”.  How much the flow changes is determined by the change in viscosity and the volume affected before it is purged during the dispense. 

Again, a lot of variables to consider…

Viscosity is Not a Constant

Viscosity varies as a function of temperature.  This is true of ALL fluids and it is even more pronounced in sealers and adhesives.  Since temperature changes from a combination of environmental and system influences, it often changes from morning-to-evening, from day-to-day, and from season-to-season.  Furthermore, most mid-to-high-viscosity fluids are also shear sensitive, changing their viscosity as a function of the stress that is placed on them by the dispensing system.  To make matters worse, stress and temperature work together resulting in different shear influences at different temperatures and different temperature influences at different shear rates.

That can be a lot to keep track of…

Mechanics Can’t Compensate for Varying Viscosity

In short, like all dispensing systems, a robotic fixed displacement system must still take the physical properties of the fluid into consideration.  The single most important of these is viscosity and, as we’ve shown, it is also the most variable.  This adds complexity to the design and operation of the dispense system just trying to compensate for the changes in this single physical property of the fluid.

In the end, a system that stabilizes the viscosity at a given shear rate will be less complex and therefore more robust and reliable than one that constantly fights this fundamental fluid parameter.

In our next installment, we will look at how this manifests itself in our bead profile and how that affects the success of our application process.

Topics: Manufacturing, Point of Application, fluid process control, automotive, temperature control systems, automotive finishing

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