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with the injected part. The eroded parts were mainly the boss edges (Fig. 7).Wear and consequent surface failure may be expectable from various mechanisms: abrasion, erosion, adhesion and surface fatigue. Abrasion occurs when material is removed or displaced by contact with a harder part or particles. The mechanical properties of the polypropylene are lower than Neukadur; thus, clearly, abrasion is not the major mechanism for degradation of the core. Moreover, at the lateral movements, no wear was observed. The high shear stress of the melt flow can induce erosion wear, particularly at the gates [24]. In this study, the wear was particularly noticeable close to the pin gate. Adhesion occurs when two surfaces in contact bond under Fig. 8 Isotropic shrinkage of the moulding high pressure or by chemical affinity.Fig. 9 Structural simulation a CAD model; b boundary conditions Fig. 10 Stress simulation results Int J Adv Manuf Technol (2010) 50:441–448 445 Upon breaking the bonds, loss of material occurs at the weakest part. In polymers, the chemical affinity can be inferred from the Hildebrand solubility parameters [25]. When using epoxy moulding blocks on hybrid moulds, wear by adhesion is expected, as reported by Gonçalves et al. [23]. Pitting was observed on the core and cavity moulding surfaces as a result from some kind of surface fatigue. According to Williams [26], subsurface cracks may be originated at microstructural defects or inclusions. The crack propagation originates the characteristic pitting of surface fatigue. The heterogeneous nature of the Neukadur resin can lead to the filler particles to act as cracks raisers, especially at high temperatures [27]. Degradation can also be related to mechanical failure if details of the moulding blocks are under high stress during ejection. At this stage of the injection moulding cycle, the core is subjected to a contact force arising from the moulding shrinkage and to the frictional forces resulting from the action of the ejection system. The contact forces resulting from shrinkage of the part are the main component of the force required to eject the moulding [28]. The contribution of this phenomenon to core degradation was assessed with CAE simulations. First, the shrinkage of the moulding was estimated with the software package Moldflow Plastics Insight 6.1 (Autodesk, USA). Then, the resulting contact pressure and frictional force were calculated and input in a simplified model of the core details that failed, using the software package ANSYS Simulation 11 (Ansys, USA). The injection moulding processing conditions and the properties of the materials already referred to where used as initial conditions for a Flow + Warp simulation on the MPI software. The Fusion-type mesh with 42,000 elements was generated from the CAD 3D model of the part to estimate shrinkage of the part at the ejection time. The result of this analysis is shown in Fig. 8.The ejection force, Fe, can be estimated in simple geometries from the contact pressure of the moulding on to the core surface, pc, the contact area, Ac, and the coefficient of friction ( ) at the moment of ejection: This equation can be deployed showing the influence of the main variables [29]: The simplified model of the damaged pins, used in the structural simulation of the stress field at ejection, is presented in Fig. 9. The contact pressure is exerted perpendicularly to the cylindrical surface of the stub. The friction force of 114.7 N, calculated from Eq. 2, is applied tangentially to the same surface. Further, simplifications of no shrinkage occurring in the mould core and no restraint resulting from the other stubs and geometric features were also considered. The equivalent (von Mises) stress field is shown in Fig.10. The stress–strain data at the most stressed point are listed in Table 4.Table 4 Structural simulation results summary Equivalent (von Mises) stress 29.96 MPa Equivalent (von Mises) elastic strain 1.5% Total deformation 50.4 μm Fig. 11 Temperature and pressure variation during the cycle time for SLSm moulding blocks 446 Int J Adv Manuf Technol (2010) 50:441–448 The maximum equivalent stress is close to 12.5 MPa, which is mechanically close to the tensile yield strength of the Neukadur composite. Considering that the Neukadur composite is not an homogeneous material including some particulate fillers, the fatigue of the material during injection moulding, the decrease of the material mechanical properties with the temperature and the imperfections caused by the manufacturing process, it is expected that the predicted maximum equivalent stress is enough to cause the premature failure of the pins.
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