Study on the fracture reason and fatigue life for guide pillar of injection machine

Some injector guide pillars (IGP) used in injection machine were fractured only after 1.5-year service. In order to determine the cause of the fractures, the numerical simulation technology was applied to analyze the mechanical properties of the IGP. To model the contact between the mating surfaces of the clamping mechanism, nonlinear multi-region contacts of surface–surface were applied to establish the contact model of FEA. The constraint of tie was used for modeling thread joint. The simulated results indicated that the smaller area of contact surface yielded higher value of stress in the neck of IGP. The value of maximum stress concentration factor (SCF) was obtained by FEA. The stresses at outer wall of the IGP were measured by strain gages, in order to check the results of FEA and examine the load balance between each IGP. The experimental data agree with the simulated results very well because the relative error of them is no more than 3.7%. The experimental results also proved that the load of the IGP was equal with each other, which means there was no partial load between each IGP. Based on the numerical analysis and experimental study, the structure of the IGP was improved by adopting a smoother double-round neck. The fatigue life of the improved structure is longer than that of the original machine.