In my recent post Metal Can Manufacturing – A Study in Process Viscosity Control, I discussed the origin of the metal can, and how it has changed over the years. Of particular interest to us here is the incorporation of advanced sealing technologies into the end attachment process:
“New sealant materials have been added to the rim of the can to assure an air-tight, leak-free, long-term seal when the end is hemmed onto the can body.”
Can manufacturers call this the “lining” process. As with any sealing process, two of the most important criteria are:
- Volume of the sealer applied
- Placement of the sealer (called “compound”)
If there is not enough, or too much compound applied to the end, or if it is not applied in the proper location, the end will not seal properly and the can will leak. Since the primary function of a can is to “contain” a product, leaking constitutes a total failure.
Defects Can Pile Up Quickly…
Modern can making is the consummate high-volume repetitive manufacturing process! The typical production line operates at rates exceeding 5 ends/sec! Should a parameter get “out-of-whack” and go undetected, it does not take very long to create a lot of scrap! For this reason, high-speed vision systems are employed to inspect each end as it leaves the forming and liner application process. This is great for catching physical defects like dents and scratches, and even compound lines that are misplaced, or missing altogether. Unfortunately, a camera cannot detect the volume of compound that has been applied.
After-the-Fact is Way Too Late!
The problem with detecting the volume dispensed is that it is specified as weight (in mg) of dry compound, after it has been cured. As the curing is performed in a baking process, it cannot be evaluated for about 5 minutes after the compound has been applied. That means that more than 1,500 ends have been manufactured in the interim. Moreover, this is a manual process in which an end is weighed prior to lining, then weighed again after lining. The difference is the weight of compound applied. This is shown in the inset photo above, where we can see that 77.3 mg of compound has been applied to this end. Fortunately, this is within specification and all is good!
Less is More…
…likely to succeed that is. Obviously, applying less compound is important from a cost perspective, but too much compound applied to an end is just as likely to prevent the can from sealing properly as applying too little. It’s the “Goldilocks” thing. It has to be “just right” – and if it isn’t, you’ve wasted precious line time creating thousands of bad ends – and that is not good!
Holds are Costly
So now you have to place the product on hold and either sort, scrap, or both. This takes time. It consumes precious resources. It adds cost. And, if you look at the end pictured above, you can probably surmise that this is not exactly a high-margin product. This is literally one of those cases where “we make it up on volume”! But the time factor is equally important. Though the raw materials are quite compact, once formed, the empty cans take up a lot of space. This creates a storage issue. The solution: “just-in-time." Make them and ship them.
But now they’re on hold.
And you’re at risk of missing your delivery schedule.
And the tomatoes that your customer is planning to put in those cans are en route to their factory.
And you can’t sleep…
Inspection is Not the Solution
This is clearly one of those situations where control is the only solution. But how do you accurately control this goop being shot on a rotating end, a few milligrams at a time, more than 5 times per second?
That’s a good question, and one that we'll address in an upcoming blog post.
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