One of my first customers was Trent Haling. He developed a product called Tolicap. The product solves the problem of aerosol can spray nozzles breaking when dropped without the can cap on. To date, no product (as far as I am aware) solves this issue while also allowing you to use the spray nozzle. The concept is pretty simple. As you can see in Figure 1, the cap has a large enough of an opening to allow your finger to push the spray nozzle down. But the cap also serves as a sort of bridge, protecting the spray nozzle. Trent has videos of the Tolicap in action. It works great no matter what angle it makes impact. You can see this video here: http://www.tolicap.com/ (see video on left side)
One of the original challenges with this product is the varying sizes of aerosol spray cans. In general, the area around the spray nozzle that the Tolicap snaps onto is about the same. In other words, it is intended to be the same. All manufacturing processes have a tolerance that can be held. The process of making the top of this type of can involves using a machine to bend the metal in place. This type of process has a fairly wide range of tolerance. In other words, even if we try to make a can with a diameter of exactly 18mm, the actual diameter might vary somewhere between 17 and 19mm. This is the range that can be controlled. If the actual can comes out to be greater than 19 or less than 17mm, then it would be discarded. Generally, this does not happen too often in manufacturing (at least that is the goal of the manufacturer).
Anyways, back to my point. Let’s say this variation is 1mm above or below the target. The Tolicap needs to fit on tightly enough that it will not come off when it impacts the floor. But it also needs to be able to be assembled without too much force for the user. Since the range of sizes that the cap needs to fit to is large, a “one sized fits all” Tolicap will not work. If we tried to use this approach, the 19mm can would be too tight to be assembled and the 17mm can would be too loose to hold in place when dropped.
The solution to this problem was to add in an additional part, a ring, that fits inside the Tolicap. This ring is used when the Tolicap is too loose on the can. Therefore, when the user has a 19mm can, he uses just the Tolicap without the ring. When the can is closer to 17mm, the ring is added to give the tighter fit.
I helped Trent get this fitting part (the ring) designed. Since this is a plastic injection molded part, it was important during this design that the rules for injection molding be followed. We worked together to define the parting line and draft angles so that part would mold properly.