In our recent blog, Can Liner Weight Variations Impact Quality, Delivery, and Cost Structure, we discussed how modern sealing technology has changed the way that cans are manufactured and judged. In particular we noted:
“As with any sealing process, two of the most important criteria are:
- Volume of the sealer applied
- Placement of the sealer (called 'compound')"
The balance of that post focused on issues with variation in liner weight and its impact on the function of the can – which are many. So now it is time to look closely at the second of these criteria – placement.
Right Amount – Right Place
It makes sense that to seal properly, there must be the right amount of sealer in the seam. The key phrase here being “in the seam." The right amount of sealer, running down the side of the cean, will do nothing to seal the seam. It must be carefully placed in, and remain in, the area where the seal ultimately takes place.
Seems simple. (pun intended!)
And yet, there are many factors that affect the placement of the compound when it is applied. This starts with the location and direction of the gun nozzle. Fortunately, this is mechanically fixed in position so, once adjusted, it will point the compound in the same direction on every dispense.
Of course, there’s more to it than that…
A Moving Target
Remember that, on average, the entire lining operation takes just 0.2 seconds. That includes positioning the end, dispensing the compound perfectly, and sending the end on to the inspection and curing processes. Because the nozzle is fixed in position, the end must rotate so that the compound can be evenly distributed in the groove. This means that the end must complete 4 rotations in less than 0.1 seconds – a speed of at least 40 RPM!
Viscosity Affects Placement
Just as with dispense volume, viscosity plays a role in the placement of the bead. Though the compound is rubber-like when cured, when dispensed, it is in a liquid form and, like all liquids, viscosity determines how it flows. The flow behavior is important in the end result because the “rubber” seal formed in the groove must be smooth and even if it is going to do its job. To form a smooth seal, the compound is dispensed slowly – if you can call less than 0.1 seconds slow – during the course of just over 3 rotations of the end under the theory that the 3 lines of dispense will flow together to form a smooth, even seal in the groove. This is where we get that 40 RPM minimum.
If the compound is too viscous, the three lines will not flow together, which can create a rough surface that will not seal. This is evident in the scalloping shown in the attached photo. But an equal problem comes when the viscosity is too low, which is especially true when dispensing on a moving target – like a rotating end.
Movement After Placement?
When the viscosity is too low, the compound can spread out and become too thin to perform the sealing function properly. Even worse, as it flows it is subject to the centrifugal force from the rotating end. This has a tendency to pull the compound toward the outside of the groove. (Strangely enough, this is also shown in the attached photo – but that’s a long story.) This can cause it to be misplaced and/or misshapen so it will not seal properly. In the worst cases, it can cause the compound to leave the groove entirely, ending up on the outside of the end in areas that will ultimately interfere with the hemming process and even foul the equipment, making it hard for your customer to maintain production.
If not caught and removed by the vision inspection station, this can easily result in a customer reject and the host of issues that go with it. If you’re nodding your head in agreement right now, you may want to refer to our post, Why Customer Returns are the Worst for Can Makers for my “rant” on customer rejects!
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