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One helpful activity when making recommendations to customers is a theoretical calculation referred to as the “stack-up assessment.” This calculation allows West to determine the amount of excess skirt length available for completing the crimp under the vial crown by using the nominal or mean values for the glass vial crown height, stopper flange thickness and proposed aluminum skirt lengths. We make an assumption of approximately 20 to 40 percent rubber stopper flange compression (Check with your equipment vendor for specific recommendations for your specific system.) To determine the excess skirt length achieved, the below calculation is performed:
excess skirt length = [mean skirt length – aluminum thickness] –[vial crown height + ((flange thickness) x [(100 - % compression)/100])]
An aluminum thickness of 0.008 inches is assumed. An excess seal skirt length of approximately 0.030 inches or slightly greater is required for an acceptable crimp on a vial.
This theoretical calculation is a helpful guideline, but it is not a substitute for component testing. Each dimension mentioned has a tolerance, so there is some variation in the parts. Therefore, it’s important to ensure that the components in their final state are compatible via appropriate container closure integrity testing.