Schell
Flush pressure
cartridge

Classic leak tests are, for example, water tests or differential pressure measurements. Water testing is a procedure that does not require a great deal of technology. A component is held under water and pressurized. If air bubbles rise, the part is not tight. The differential pressure test is a somewhat more cost-intensive but faster test using air pressure.

Such leaks can be caused, for example, by air pockets in the plastic, so-called vacuoles. And this is exactly what happened to the piston of the pressure flushing cartridge. Because we couldn’t change the geometry of the part, we came up with something else. We changed two injection parameters – injection speed and holding pressure – which enabled us to reduce the size of the vacuole. We also adjusted the ventilation in our mold to further reduce the size of the vacuole. Challenge mastered.

But we didn’t stop there. Because we wanted to further improve our process and not just rely on the water test. That’s why we used an additional test method: a roundness test. By measuring the roundness of the inside diameter of our parts to within a tenth of a millimeter, we can track down every vacuole in this area. And can eliminate potential leaks.

For example, we have managed to ensure that the locking head and the piston are each so round that the sealing lip of the piston reliably seals the locking head during the entire stroke movement.

The nozzle bore was particularly important to ensure that the timing of the pressure rinsing cartridge was correct. Here it was not so easy to maintain the tolerances of a few hundredths of a millimeter. This is because plastic parts always have a separation. And therefore also a burr.

What is not a problem with many parts would have changed everything with our piston. This is because a burr would reduce the diameter of our nozzle bore and thus affect the amount of water. And because burrs are never really uniform, the burr would have made the amount of water and therefore the function of the pressure flushing cartridge unpredictable. So the burr on the nozzle bore had to be removed.

It’s a good thing we have our own in-house tool shop! So we were able to put our heads together and find a solution: We redesigned our tool for the piston so that the burr runs upright to the nozzle bore. This means that it does not affect the diameter of the bore. And exactly the right amount of water flows through the pressure flushing cartridge.

The burr in the O-ring groove had to be removed so that the less tightly pressed O-ring would fit tightly on the closure head. But we had no way of getting the burr small enough during the injection molding process. So we turned off the burr in the 2.5 mm wide groove after injection molding, i.e. removed it mechanically.

And we thought of something else. Namely to weigh our bolt heads. We determined a narrow tolerance of +- 0.1 g for the part weight. And we established a correlation between the weight of the cap head and the diameter of the O-ring groove. This enabled us to use our weight test to determine quickly, easily and during series production whether a closure head is OK with regard to the O-ring groove. And have thus further improved our process.