Abstract
The present paper reports on the development of a biodegradable overmolded orthopedic implant: a metal bone fixing screw, which has been overmolded with a functionalized thin layer of biodegradable polymer to enhance cell adhesion during the healing process. The main challenges were to integrate precise, high-throughput and repeatable solutions to achieve a thin, defect-free structured polymer layer and to ensure a high and consistent implant quality. The work carried out entailed determining proper materials (Purasorb PDLG 5010) for the biodegradable overmolding layer and its economical substitute (NaKu PLA 100HF) to be used during initial tool and process development, designing the surface structure of the overmolded polymer layer, development of injection molding tools, as well as feeding and handling procedures. The injection overmolding process of Purasorb PDLG 5010 polymer was controlled, and the process parameters were optimized. In particular, the dominant process parameters for the overmolding, namely injection pressure, barrel temperature and mold temperature, were experimentally examined using a circumscribed three-factor central composite design and two quality marks; overmolding roughness and mass of polymer. The analysis of the experimental results shows that the mass of the overmolding is not feasible for use as the quality mark. However, the optimal parameters for the overmolding of a metallic implant screw with a thin, micro-structured polymer layer with a predefined roughness of the surface texture have been identified successfully.
Highlights
Mass production of miniaturized components still poses serious challenges to overcome in terms of precision manufacturing [1,2,3], and offers great benefits in terms of production time and Polymers 2018, 10, 707; doi:10.3390/polym10070707 www.mdpi.com/journal/polymersPolymers 2018, 10, 707 costs [4,5]
A design of experiment (DoE) study was carried out to determine the influence of the different process parameters, the interaction effect of the independent parameters on the quality of the polymer overmolding and their optimization
The roughness values and the mass of the polymer will be used as quality marks in the (Figure 17), which can be directly correlated with the height of the pyramidal structuring
Summary
Mass production of miniaturized components still poses serious challenges to overcome in terms of precision manufacturing [1,2,3], and offers great benefits in terms of production time and Polymers 2018, 10, 707; doi:10.3390/polym10070707 www.mdpi.com/journal/polymersPolymers 2018, 10, 707 costs [4,5]. Micro injection molding (μIM) has shown enormous potential as a cost-effective technique to fabricate micro-components and/or meso-parts with intricate 3D features for various industrial applications [6,7] This is due to its ability to mass produce near-net-shape replication, molded from a wide range of engineering materials, with tight tolerance, high repetition quality and minimal material waste [8,9]. Biodegradable polymers have received substantial attention as molding materials for medical applications [13] since, after a certain time, they can be absorbed and, e.g., can be used for drug delivery purposes [13,14,15] Products made of such materials have been used for scaffold engineering [16], the fabrication of micro-needles [17], bone surgery and dentistry [18]. The reason for using different material components to manufacture one part, making its manufacturing process more complex, is to optimize the characteristics and tune the part’s properties to its functional requirements
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