Attributes Affecting Rigid Needle Shield Pull-Off Force and Cap Removal Force in Autoinjector Systems

Autoinjector cap removal force (CRF) must be controlled to support reliable cap removal and ease of use. To understand this force, syringe-level rigid needle shield (RNS) pull-off force (POF) must be considered together with the manufacturing and test attributes that can change the RNS interface before or during cap removal. This study evaluated how pull-off test speed, assembly-induced mechanical disturbance, applied axial preload, and elastomer siliconization affect POF and CRF. POF increased linearly with test speed at low-to-intermediate rates before reaching a plateau, showing that measured POF depends on the test rate. Autoinjector assembly reduced device-level CRF relative to syringe-level POF, while applied axial preload similarly reduced subsequent syringe-level POF, indicating that interface conditioning during assembly can alter the relationship between syringe-level POF and device-level CRF. Additional silicone oil modestly decreased POF, whereas one-month room-temperature aging after preload and 60°C/18h heat exposure after recapping each produced only a small additional increase under the evaluated conditions. Together, these results show that device-level CRF cannot be interpreted from syringe-level POF alone without accounting for test speed, assembly-induced interface conditioning, and material or aging effects.