In our previous post in this series, Can Liner Weight Variations Impact Quality, Delivery, and Cost Structure, we detailed how variations in can end liner weight can negatively impact delivery and cost due to quality holds and rejects.
In this post, we will look at how those variations enter into the system.
A Fixed-Orifice System
The can end liner process is a perfect example of a simple fixed orifice dispense system. Compound is stored in a bulk tank and, as shown in Figure 1, is pumped to the process location(s) through a header system. At each dispense station, a drop from the header carries compound to a regulator to stabilize the pressure.
The outlet of the regulator is tied to the gun via a hose. To dispense, the gun opens a valve which allows compound to flow through the dispense nozzle and onto the spinning can end.
As with all simple fixed orifice dispense systems, the volume dispensed is determined by:
- The size of the nozzle orifice in the gun
- The length of time that the gun is ON
- The outlet pressure of the regulator
- The viscosity of the compound
Controlling a Fixed Orifice Dispense System
On first blush, it seems like this would be a very simple system to control. After all, the hole in the nozzle orifice is “fixed” by the drill that made it – thus the name.
Simple – no variation there.
The length of time that the gun is ON is controlled by an electronic timer. The resulting pulse to the gun should be very accurate and repeatable on every cycle. Because the opening and closing mechanism of the gun is mechanical, it should respond to the pulse the same way every time.
That’s two out of four variables – this is getting easier all the time!
Next is pressure. A regulator, by definition, is designed to maintain stable pressure at its outlet. Once set, this shouldn’t change. But the typical passive regulator used in these systems is a “reactive” device, therefore, its accuracy will vary as a function of the pressure variation presented to its inlet. For this reason, the same regulator will perform differently in different systems, and as upstream conditions vary within the same system.
So, pressure has the potential to be a problem…
That brings us to the viscosity of the compound. We always use the same compound from the same supplier. The formulation is controlled by our supplier and should be the same all the time, right?
Well, not exactly…
Variations in Compound Viscosity
The compound we receive from our supplier is going to be pretty stable, but there will likely be variations from batch-to-batch. If they allow a total tolerance of just 5% (±2.5%), we have to assume that our dispense volume – all other factors being equal – will also vary within that same 5% window.
But it doesn't stop there.
All liquids change viscosity as a function of temperature – and compound is no exception. In fact, the manufacturers publish the curves on their materials for their customers. A typical curve from a supplier’s lab is shown in Figure 2.
Figure 2: Viscosity vs. Temperature Curve for a Typical Compound
So what does this mean?
On a typical summer day, temperatures can fall to 60°F overnight and reach 90°F during the day. Looking at Figure 2, the viscosity of our compound will range from 2800 cps overnight to 2400 cps during the day. That’s a change of roughly 15% over the course of the day!
When Fixed Isn’t Fixed
It probably isn’t fair (or accurate) to oversimplify the mechanical components in the system.
Over time, the solids in the compound wear out the nozzle. Likewise the needle wears as it opens and closes over millions of cycles. This can change both the response time and flow rate through the gun. The regulator is also subject to similar wear on its internal components.
Fortunately, these can be addressed through timely preventative maintenance.
So, the parameters critical to controlling the liner process are temperature and pressure.
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