Child-resistant closures, and when they are required
Updated
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The clearest case in this estate of a container specified by law rather than by preference, and the trigger is a number in your formula rather than anything about the bottle.
The formula decides the closure
The Poison Prevention Packaging Act lists substances that require special packaging above a stated concentration in the product as sold.
Cross one and a child-resistant closure is a legal requirement. Stay below and it is a choice.
Which means the closure cannot be chosen until the formula is settled, and it is a decision that can be reversed by a reformulation in either direction. The thresholds and the arithmetic are on the check page, because they are a calculation rather than a discussion.
It is a tested performance standard, not a feel
"Child-resistant" is defined: significantly difficult for children under five to open, or to obtain a harmful amount from, within a reasonable time, and not difficult for normal adults to use properly.
That second half is a requirement too, and it is why the closures are as they are rather than simply stiff.
16 CFR 1700.15 sets the effectiveness requirement and 1700.20 the protocol, which is run with panels of children and of adults. A closure is child-resistant because it passed that, not because it clicks.
The closure and the bottle are one system
The requirement is on the package, so the tested combination is a closure on a particular finish, not a closure alone.
Putting a certified closure on a different finish, or on a bottle with a different thread profile, is outside what was tested.
Which in practice means buying the bottle and the closure as a pair, from a supplier who can tell you they were tested together, and not mixing suppliers for the sake of a price.
What to get from the supplier
Documentation that the package meets the standard, naming the closure and the bottle finish.
A liner suitable for your chemistry, because the liner is what seals and it is the component nobody specifies.
And torque guidance, because a child-resistant closure applied at the wrong torque may not perform as tested.
Keep all of it with the product file. It is the evidence that the package was chosen rather than assumed.
When to re-check
Any reformulation, in either direction.
A supplier change on a raw material whose concentration might differ.
A move from ready-to-use to a concentrate, which raises every concentration in the bottle and is the common way a product acquires this requirement.
And a new size, if the closure finish changes with it.