In our recent blog, Can Liner Weight Variations Impact Quality, Delivery, and Cost Structure, we noted that the two most important criteria in evaluating liner performance are:
1. Volume of the sealer applied
2. Placement of the sealer (called “compound”)
There are No Independent Variables
When we were all studying our STEM curriculum, we were told to “isolate the independent variable so you can study the dependent variable." Then, they sent us out into the real world and the first lesson we learned was, where processes are concerned, it is almost impossible to truly isolate any variable – and all of the variables interact!
This is exactly the situation we find ourselves in when studying the liner process.
We have these two important criteria to judge our control success. But there are so many variables that affect them…
Temperature is a “Constant Variable”
Temperature is one of the most reliable variables that we have in any dispensing process. And by that, I mean that we can rely on it to change – continuously. It is as certain as the sun rising in the morning and setting in the evening. The problem is, the compound changes viscosity as a function of temperature, and its behavior in our dispensing process is dependent on its viscosity.
How can we ever control our liner process if the compound is continuously changing?
That’s a good question.
We Live by Pressure Regulation – We Die by Pressure Drop!
In this series, we’ve already demonstrated that pressure has an impact on both liner weight and placement – so we go to great lengths to regulate it. The problem is that we regulate it at a point in our process – and everything that happens to our fluid after that point endeavors to change all our hard work.
The pressure drop in the hose from our regulator to our nozzle is a complex function of hose diameter and length, flow rate, friction, and viscosity. The hose diameter, length, and materials of construction (which determine the coefficient of friction) are constant. And armed with that knowledge, we fool ourselves into believing that the pressure drop will be constant.
But it’s not. And here’s why:
First, this process starts and stops – a lot. And that means the flow rate changes – a lot.
Next, we’ve already determined that temperature is constantly changing – often by 30°F or more from morning to evening – and that’s a lot. And we already know that compound viscosity is dependent on temperature which means our compound viscosity changes – a lot.
So, the pressure drop in our hose is constantly changing and as a result, the pressure at our nozzle is constantly changing. This means that our liner weight, and placement are also constantly changing. This is pretty evident from the data table shown here from one of our liner process control experiments.
So what are we to do?
Control What is Controllable
Though it may not be apparent at first glance, the image below points us in the right direction. First, we see that the temperature changes by 25%, while our regulator holds pressure variation to less than 2%. These combine to produce a nearly 31% change in compound weight. As all of these variables increase, the placement also creeps toward the edge. This is a combination of excess compound in the groove, lower viscosity (due to elevated temperature), and centrifugal force pulling the compound outward.
So what is controllable?
And when we do so, we stabilize most of the rest of the variables.
When the temperature is stable, the viscosity of the compound is stable. This holds changes in pressure drop in the hose to those flow related changes created by the starting and stopping of the liner (a factor over which we have little to no control). And after dispense, it controls migration from centrifugal force.
So, stable compound temperature equals stable dispense pressure, which equals stable compound weight and repeatable placement.
It’s just that simple.
Get your free copy of our Process Temperature Control Case Study to see how one manufacturer was able to improve production and reduce issues via temperature control solutions.