In our post, Temperature Variations Cause Inconsistent Liner Placement, we “proved” that compound temperature and placement are related – by using the transitive property from our old high-school algebra days.
Sounds “geeky," right?
Perhaps, but it proved to be a fun way to look at a couple of otherwise mundane subjects. Through that process we came to the conclusion that:
“…if we can control the temperature of the compound as it exits the nozzle, it will be at a predictable viscosity and we can go a long way toward controlling the liner process.”
And this provides the “secret” to stabilizing liner placement.
A Graphic Example
Sometimes, a picture truly is worth a thousand words – and the effect of temperature on dispense, as shown in Figure 1, is a perfect example. Using a robot to maintain speed, distance to the surface, and angle of the nozzle, we dispensed a simple spray pattern. Moving from left to right, we can see that, at the start of the dispense the cool material resident in the hose produces a narrow dispense pattern. As heated material enters the gun, the pattern widens – all without any changes in the dispense system.
Different Process – Same Problem!
This is an excellent visual representation of the effect of temperature (read: viscosity) on the dispense process – any dispense process!
Now imagine that this is your liner process.
If it’s like most of the liner processes I’ve seen, it starts and stops a lot. That means that sometimes the compound is flowing through the hose while other times it is just sitting there. Regardless, if its temperature is not being actively managed all along the path to the nozzle, its temperature is changing. And as it changes, so does the placement of your compound.
In our recent post, Variation in Liner Placement Impacts Delivery, Quality, and Cost, too!, we showed a photo of an end with scalloping in one area, and another area where the compound is extending up the wall and over the lip of the groove. We said it was a long story, but with our discussion here it makes perfect sense. As cooler compound in the nozzle was dispensed it held to the inside of the groove and did not migrate outward. As warm compound entered the gun chamber and was dispensed it migrated outward toward the lip of the groove as a function of centrifugal force. In this case it was too warm, and therefore too thin, and it migrated over the lip. Hot and cold spots like this are a problem often associated with electrically heated hoses – a “solution” selected for many liner applications.
The “Secret” Solution
So the “secret” solution was set forth at the outset: “…control the temperature of the compound as it exits the nozzle…”
But that’s where things get complicated. To achieve this, you have to bring the compound to temperature at just the right moment – preferably right at the nozzle – but that’s impractical. A more realizable solution involves an in-line system that can bring the compound to temperature at a convenient point in its delivery path relative to the location of pressure regulation, and then carefully hold it at that temperature all the way to the nozzle. That’s how you balance viscosity, flow rate, and pressure drop to assure a consistent dispense.
This requires some pretty specialized skills and devices.
First, you have to accurately evaluate the specific conditions under which your liner will operate. Thermal loads are different for 202 ends than they are for 401 ends – and for liners in Houston versus Minneapolis. Then, you have to determine which of the myriad temperature control devices out there will enable you to achieve your goals and objectives based on your unique set of circumstances.
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