Injection Moulded Syringe Force Test Validation

We have inherited a mature product that involves injection moulding syringe barrels, plungers and pistons then assembling together with a coating of silicone grease in the barrel to lubricate the piston.

We are ok with the components and are a good way to having validated moulding processe for them, the problem is in validating the test that measures the force required to push the plunger in the assembly. We have a computer controlled force tester that can measure and record the necessary forces (the initial break-out force and the running force) but have issues in validating and achieving consistant results. Everything is within spec but R&R values are much higher than we would normally accept, there is also a significant difference between our data and historical data.

I believe the problem with the test is the grease quantity/dispersal in the barrel which is likely to be different on every device and will alter on a repeat test of the same device.

Does anyone have any experience of validating the force tests for syringes with grease?

Many thanks.