If you’ve ever actually noted how long one can of shaving cream lasts, you’ll know that a lot of foam is somehow packed into a small 11 ounce can.
The fact is, the soapy solution expands as it comes out of the can because it contains hydrocarbon propellant, which vaporizes as it hits the air.
The hydrocarbon has a pressure above that of the atmosphere, so at normal temperatures it turns into a gas. Emulsified in the liquid soap, it is kept in the can under pressure, until your finger on the valve presses down on a spring which releases the material, and foam spurts out.
Hydrocarbon propellant constitutes about 3 to 5 percent of the total contents of a can. The soapy solution, which includes lubricants and perfumes, is first mixed and then put into the can as it travels along a filling line.
The propellant is then added by one of two methods. The top of the can (valve and valve cap) may be installed and crimped first.
Then a gasing machine pushes propellant through the valve under pressure. Or the propellant may be added before the valve has been attached. A machine descends over the top of the can and presses a metered amount of propellant into it.
Immediately the valve and cap descend to close the can, sealing the soap and propellant inside.