Bore control, siliconization & device integration
Five properties that stop being packaging and start being metrology.
A cartridge is not opened by a user. It is loaded into a pen or pump that pushes its plunger a measured distance to deliver a dose. That makes internal diameter, bore straightness and plunger friction into dosing parameters, and it means the container is qualified with the device rather than on its own.
Internal diameter is a dose variable
The device moves the plunger a fixed distance; the volume delivered is that distance multiplied by the bore area. A bore a few hundredths of a millimeter wider therefore delivers proportionally more with no change to the device. Cartridges are held to tighter internal tolerances than most glass containers precisely because this arithmetic is unforgiving.
Siliconization is a specification with two failure directions
The bore is coated so the plunger slides evenly at low, consistent force. Too little silicone and the plunger sticks and releases in jumps, giving inconsistent dosing and a poor feel in the hand. Too much and it migrates into the product, where it can drive protein aggregation or show up as subvisible particles. Both directions are real failures.
The plunger seals against the wall along its whole travel
Unlike a stopper that seals against a finish, a plunger seals against the barrel wherever it happens to sit. That makes wall straightness, out-of-round and internal surface quality part of the closure system for the entire length of the cartridge, not just at one end. It is why cartridge glass is inspected differently from vial glass.
The septum end is a crimped closure like any other
At the front the cartridge is closed by a rubber septum under an aluminum ferrule, pierced by the device needle at each use. Coring, reseal and residual seal force apply exactly as they do on a vial, with the difference that the needle is chosen by the device manufacturer rather than the clinician.
The container is qualified with the device
Dose accuracy, gliding force, break-loose force and needle penetration are properties of the combination, not of the glass. Changing cartridge supplier without repeating that work is not a like-for-like substitution, even when every published dimension matches, because friction and silicone distribution differ in ways dimensions do not capture.
The finished device sets the tolerance, not the glass alone
A cartridge is loaded into a pen whose mechanism advances a fixed distance per click, so any variation in bore or in plunger position becomes a variation in delivered dose. The tolerance the glass has to hold is derived from the device geometry and is tighter than a general-purpose vial would ever need.
