Materials processing towards development of rapid prototyping technology for manufacturing biomedical implants (Binder development).

摘要

Materials processing towards development of fused deposition of materials (FDM) method for manufacturing biomedical implants has been studied experimentally. Main processing steps consisted of thermoplastic binder development in the ethylene vinyl acetate (EVA)-microcrystalline wax system, feedstock extrusion, characterization and optimization of binder degradation, and sintering of calcium deficient hydroxyapatite. Differential scanning calorimetry (DSC) revealed that the melting index (MI) of the copolymer affects the temperature location of the solidification exotherm, whereas the effect on the temperature location of the melting endotherm was negligible. Nonisothermal measurement of viscosity of different blends as a function of VA content of the EVA component revealed that the microcrystalline wax is compatible with 25–14% VA-containing EVA grades. Further DSC analysis revealed that co-crystallization leads to compatible EVA-microcrystalline wax blends.; A typical binder formulation that was developed in the present work has a viscosity of about 700 cP at 140°C, a compressive yield strength of 6 MPa and an elastic modulus of about 600 MPa, and contained 15–20% EVA and 80–85% microcrystalline wax. Various filaments with a nominal diameter of 1.8 mm were extruded by using such a binder, and calcium pyro-phosphate powder that had a distribution modulus of about 0.37. Measurement of physical dimensions of the filament revealed that fluid state can be achieved in the filaments.; Simultaneous thermal analysis of degradation characteristics of the typical binder formulations revealed that degradation sequence is oxidation of the hydrocarbons, evaporation of the hydrocarbons, degradation of the vinyl acetate, and degradation of the ethylene chain. A rate controlled binder removal system was developed and used in order to optimize the binder removal schedule.; Sintering of gel-cast calcium hydroxyapatite was studied by means of thermal analysis, XRD, mechanical testing, and SEM, both for sub-micron and the agglomerated powder type. Transformation temperatures of tri-calcium phosphate upon cooling were determined as 1475°C for (α′ to α, 1190°C for α to β, and 800–700°C range for β to hydroxy-oxyapatite transformation. Dilatometry revealed that dimensional change reaches to saturation at 1200°C. Mechanical testing of the parts sintered at 1200–1500°C revealed that compressive strength in the 2–10 MPa range was achieved.