Thermal model for curing implantable silicone in the moulding process applied to tracheal stents

Abstract

Tracheal stents are a kind of endoprosthesis used to prevent tracheal obstruction due to different illnesses or disorders. Stent design has to fulfil several requirements such as the prevention of migration, easy removal if needed and radiopacity allowance. Although some commercial solutions can be found on the market, there is a constant demand for new designs in order to obtain more anatomical geometries and customized solutions. This study shows a numerical model developed to predict appropriate manufacturing of tracheal stents with implantable silicone by using a moulding process. In this study, tracheal stents have been manufactured using two manufacturing processes, namely heating plate and furnace systems. Both manufacturing processes yield equivalent mechanical properties but with a longer process time for the heating plate manufacturing system. Thermal images captured by a thermo-graphic camera during the heating process using an open plate system were compared with the numerical simulations for the equivalent process revealing inconsistencies in the thermal stabilization time. This inconsistent result could be related with FE modelling as non-acceptable element geometries (distortions and sizing), non-realistic input experiment parameters and/or standard parameters used by the FE software. Elimination by simulation trials showed the reason for this disparity as being a result of inappropriate thermal conductance between all the solid surfaces in contact with the model due to the use of the FE software standard set for conductance, which proved to be critical in this case. Corrections in thermal conductance were performed and the new FE numerical model applied in furnace system conditions was tested showing an experimentally coherent curing time of the silicone stents. The final results lead to a heating system that can produce flexible and customized products with a good prediction of the curing process