Industry Insight: Why Bottom End Quality Manages the Shelf Life of Paint Cans

Shelf life. An issue in the paint world that every technician knows is critical, yet we avoid talking about it too directly. From the time a can of paint leaves the factory till the time it is opened by the end user, months and years can go by. Will the paint retain its original characteristics during that time? The answer lies chiefly in seal integrity, which in turn lies in bottom end precision.

We are a Chinese factory based in Foshan, Guangdong, we produce Tin Can Components for 30 years, especially bottom ends for paint and coating. This time I want to share how bottom end precision affects shelf life, from a more technical angle.

Where Should Start with Precision?

Many people think that bottom end precision means only outer diameter accuracy. But through a job as a bottom end encompasses far more than that. A single bottom end has at least a dozen critical dimensions to consider: outer diameter, height, curl diameter, curl thickness, curl radius, flange width, compound groove depth, compound width, compound height, rivet height if there is a pull ring, and more. Each has a target value and range of allowable tolerances, and altogether, the scopes define the level of preciseness of the bottom end.

Let’s consider an example. If outer diameter tolerance is too loose (say ±0.3 mm) what are two things that could happen when seaming?Either the fit isn’t secure enough to completely close it off from air and moisture, or it is too tight to easily slide into position, or it get excessively deformed when seamed together. Either way it kills shelf life.

What do we clean to? Outer diameter ±0.08 mm, height ±0.05 mm, curl diameter ±0.1 mm. We are not boasting. It is because we know that over the life of a one-year shelf life those few fractions of millimetres will make the difference between a paint that lasts and one that spoils.

Curl Constancy: The Initial Guard

A proper bottom end curl is critical as it is the first interface that locks onto the can body flange to form the seal. The radius/ thickness and its overall constancy all speak to the tightness of the seam. If the curl profile is not complete or the thickness is not constant so that it has localised thin areas, then areas that do no seal properly are left after seaming. Visually, these have a microscopic size. Gas and water molecules are smaller still, and will penetrate slowly.

Every day that the can sits, solvents at the paint surface begin to evaporate. The paint begins to thicken, to skin over, and finally may actually cure in the can!

More horrific, should there be burrs or fractures in the curl, these become the very nucleation sites for corrosion take-up. Moist will seep in, directly contacting the metal substrate and then sets the rust chain in motion. Rust will drive the burr or crack further open, thus compounding the problem. That by the time the consumer opens the can there are rust rings down, and spoiled paint.Our approach to curl quality: we sample once every half hour, measuring curl characteristics under a projection microscope in order to catch problems on the fly. If anything deviates, we then adjust at once. In addition, we conduct cross-section analyses, observing the curl’s layer count and its tightness under magnified examination to ensure that the material consists of all 7 layers with no voids.

Compound Precision: The Invisible Guardian

The sealing compound is typically the least observed part and most critical part of the bottom end. Applied inside of the curl, it’s compressed during the seaming operation to fill the gap between the curl and the can body, and thus, accomplish the actual seal.

There are 3 aspects of compound precision that we have to deal with. First is quantity; too little compound and we have gaps–the potential for leakage paths. Too heavy a compound and we have a surplus that will squeeze between the curl and can body after drying. Besides disfiguring the appearance of the bottom, this excess could be sticky to the pull ring as its tendency is to linger in the curl itself. We have weighted the compound to within ± .05 gr. on each part, thus insuring that each bottom end receives exactly the right quantity.

Next is position; if the compound is not applied at the right zone inside the curl, say, too far left or right, it will be misalignment, and at the risk of failing to cover a potential leakage path when seamed. Within the canmaking industry we use vision inspection systems that monitor the bead width and the position offset continuously while it is being marked. Parts outside predetermined limits are automatically rejected.

Finally comes the performance of the compound itself. Different paint mixes require different compound types. Oil-based paints require solvent resistant compounds. Water-based paints require hydrolysis resistant compounds. High solid coatings require highly elastic compounds.We know how to match the right compound type to each customer formulation and conduct compatibility tests prior to shipment to make sure the compound does not cause adverse reactions with the paint.

Dimensional Consistency: The Elixir of Mass Production

Making one model bottom end, as said, is within reach. But making every one of them so? That is the real test. If only 1% is bad it is that 1% the customer will complain about!

To ensure batch to batch consistency, there are multiple checkpoints on our production lines. The first piece after every die change is subjected to full inspection. During production operators take rolling samples, QC surveyors take hourly patrol checks and quality supervisor reviews daily. Any hint of excursion will have been detected and corrected.

And we keep track of these variables on file for every batch designator: date of production, machine number, operator, inspection data and so on. Open any customer complaint and trace back to where it started and what went wrong.

Real Case Study: gap precision dictates shelf life

To drive that point home, we devised a driving comparison test last year. The same batch of paint was filled into cans using precision bottom ends, and cans using our mate precision bottom ends. These cans all went into a 45C constant temperature chamber for accelerated aging. Results out of three months (equivalent to about a year at room temp):

cans employing our mate precision bottom ends: paint normal, no obvious solvent loss, no skinned over surface, no rust on interior bottom end.

cans using Other brand mate bottom ends: about 15% lost some degree of solvent with thickening paint, about 8% developed visible rust spots internally at bottom end, odd cans had a noticably off odor when opened.

Final remarks

Paint makers spend tons of cash developing the perfect formulation — and quite right too. But if the packaging doesn’t match the precision, what’s the point? The bottom end may be small but when it comes to shelf life, it is one of the most powerful parts in a complete tin can aside from millage itself.

A fellow manufacturer in China with 30 years in foundry can appreciate what we mean by this. Every micron of tolerance, every gram of compound control counts in our good graces. Should you care to preserve your own paint shelf life, start sweating the bottom end too.