Bending and compression tests for PA 2200 parts obtained using Selective Laser Sintering method

Craniofacial bone defects that produce morphological and For this reason, the collaboration between the field of medicine with engineering, lead to the manufacture of custom implants from biocompatible materials. In the last years it was shown that Additive Manufacturing methods are very helpful to achieve customized medical implants. All the customized implants made of biocompatible materials have to be tested for mechanical properties, in order to have the optimal characteristics for the area were will be used. This paper presents the results for bending and compression tests for a biocompatible material, PA 2200. The samples were achieved using Selective Laser Sintering method and were obtained with different laser power, starting with 4 W. Maximum load, maximum stress, specific deformation and Young's modulus were analysed. The study showed that different mechanical properties can be obtained, depending on the laser power used. At bending tests, increasing values were obtained for the investigated parameters along with laser power increasing, but at compression tests a different trend was observed.


Introduction
A successful result for repairing of complex craniofacial bone defects depends on the defect size, the quality of the soft tissue covering the defect, and the choice of reconstructive method. The complex craniofacial skeleton is involved with various specific functions, such as protection of the brain and optic tracts, breathing, mastication, speech, and hearing [1,2].For this reason is very important to know the forces and which are the required mechanical properties for the future implant that will take over all the forces from the area.
Recent advances in Computer-Aided Design and Computer-Aided Manufacturing have begun to revolutionize craniofacial surgery, which is frequently confronted with the reconstruction of challenging 3D anatomic structures. Additive Manufacturing technology, with widespread availability of high-resolution medical imaging, has allowed for the generation of contoured 3D prostheses for craniofacial reconstruction [2].Computer technology has moved forward with the advent of Additive Manufacturing techniques which allow both the production of models of the hard tissues and custom-made prostheses from computerised scanning data [3].
Selective Laser Sintering (SLS) is a manufacturing technique which enables the final product to be made directly and rapidly, without tooling or additional machining. For biomedical applications, SLS permits the fabrication of implants and scaffolds with complex geometry accurately and economically [4]. Selective Laser Sintering technique has been identified as a promising technique capable of building complex objects with predefined macro and microstructures [5].
In the last decade, the bio-medical industry was focused more and more on developing and exploiting of materials that are easy accepted by the human body [6].Polyamide is a biocompatible material that has a polar molecular structure (CO± NH±) similar with the chemical structure of bone collagen and exhibits excellent mechanical properties [7].
Customized properties of parts manufactured using the Selective Laser Sintering process are achievable by variation of build parameters [8].
Static tensile test is a basic mechanical test originally designed to become the widespread and known testing method for evaluating the mechanical properties of materials [9]. But for a future cranial implant are very important the bending and compression tests, that can show the stresses in normal and accidental situations. These stresses are much closer to the real situation from the human body.
In The manufacturing process was Selective Laser Sintering, using a DTM Sinterstation 2000 System with a CO2 laser. The PA 2200 powder was brought layer by layer from the two powder suppliers on the working place using the roller and was heated to near the melting point with the help of the laser beam, which drawn the shape of the virtual model. Thus, the component particles were bonded one to others, solidifying into the shaped form drawn by beam. When a solid layer was obtained, the platform was moved down with an equal distance to the thickness of the following sintered layer. The powder was again For bending and compression tests it was used a 10 kN testing machine (INSTRON 3366). Figure 1 shows a picture from the bending tests of specimens.

Results and discussions
In tables 1, 2 and 3 are presented the mechanical properties obtained at the bending tests for PA 2200 samples obtained at 4W, 4.5W and 6W.For samples manufactured at 4W, analysing the results obtained for each characteristics, it was observed that the values were close to each other and were determined standard deviations 4.906 for maximum load, 3.098 for maximum stress and 77.464 for Young's modulus. However, the lowest values were recorded at the specimen No. 2 for all the analysed characteristics.   Fig. 2. Mean values of the maximum load and maximum stress for PA 2200 samples at bending tests.