Venting, swelling and paneling
Updated
A bottle is a pressure vessel with thin walls, and cleaning products are unusually good at generating pressure.
Pressure goes both ways
Positive, from heat, from altitude, or from a formula generating gas.
Negative, from cooling, or from product being drawn out faster than air comes in.
Both deform a bottle and both can compromise a closure.
Paneling
The bottle collapsing inward, with flat sides where the wall has buckled.
Causes: filling hot and cooling, a solvent permeating out and reducing internal volume, or a non-vented closure on a product being dispensed.
It is usually cosmetic and it always looks like a defect, so it is a returns problem whatever the product does.
Fixes: fill at ambient temperature, choose a resin the formula does not permeate, and specify a bottle with paneling resistance designed in.
Swelling
The more serious one, because it can indicate a reaction.
Heat and altitude account for most of it and are benign.
Gas generation is not: an acid meeting a carbonate, a peroxide decomposing, or microbial growth all produce gas in a sealed bottle, and all three mean something is wrong with the formula rather than with the transport.
A swollen bottle should be investigated rather than vented and shipped.
Design decisions that help
Adequate headspace.
A bottle designed for the job, with ribbing or a vacuum panel, rather than a decorative shape.
A liner suited to the formula, since the liner is what actually seals.
And a warm-storage test on capped, filled, labeled bottles for several months, which catches all of this before a customer does.