Dual Material Rapid Prototyping Techniques for the Development of Biomedical Devices. Part 2: Secondary Powder Deposition

Polymeric drug delivery devices play an important role in drug administration. However, the current polymeric drug delivery device fabrication methods lack precision. This impairs the quality of the devices, resulting in a decrease in the efficiency and effectiveness of drug delivery. The concept of building parts layer by layer out of powdered raw materials makes selective laser sintering (SLS) a suitable process for fabricating polymeric matrix drug delivery devices. The current SLS process is not capable of processing two or more materials separately. This work explores the possibilities of using SLS to perform a dual material operation by developing two process models. The two processes can then be integrated to form a dual or multimaterial fabrication technique and act as a foundation for future work in multimaterial applications such as polymeric drug delivery device fabrication. Accordingly, two papers are presented. In this paper, Part 2, the emphasis is on a secondary powder deposition method, which is an electrostatic technique based on electrography. Developed toner on the photoconductor is scraped off using mechanical shearing and is deposited using an electrostatic force by electroplating. Results have shown that by reducing the distance between the photoconductor and surface of deposition, the resolution of the printout can be refined. Other important factors include the efficiency of powder removal from the photoconductor, printing speed, and the traversing speed during deposition.     

Characterization of microfeatures in selective laser sintered drug delivery devices

From initial applications in the fields of prosthesis, implants, surgery planning, anthropology, paleontology and forensics, the scope of rapid prototyping (RP) biomedical applications has expanded to include areas in tissue engineering (TE) and controlled drug delivery. In the current investigation, the feasibility of utilizing selective laser sintering (SLS) to fabricate polymeric drug delivery devices (DDDs) that are difficult to make using conventional production methods was studied. Two features, namely porous microstructure and dense wall formation, inherent in SLS fabricated parts were investigated for their potential roles in drug storage and controlling the release of drugs through the diffusion process. A study to determine the influence of key SLS process parameters on dense wall formation and porous microstructure of SLS fabricated parts was carried out. Composite-type DDDs incorporating dense wall and porous matrix features were designed and fabricated using SLS. The characteristics of the fabricated devices were investigated through microstructural examination and in vitro release tests carried out using a drug model or dye in a simulated body environment.     

Building Porous Biopolymeric Microstructures for Controlled Drug Delivery Devices Using Selective Laser Sintering

The capability to build parts with predetermined porous microstructure and dense walls using powdered biomaterials makes selective laser sintering (SLS), one of the more flexible rapid prototyping (RP) process, a strong candidate for building biodegradable controlled release drug delivery devices (DDD). The objective is to design a varying-porosity circular disc with outer region being denser acting as diffusion barrier region and inner more porous region acting as drug encapsulation region. This is to achieve a zero-order of release over a desired duration of time in drug administration.A key study in this paper was to determine the influence of critical SLS process parameters namely, laser power, laser scan speed and part bed temperature on dense wall formation and control of the porous microstructure of SLS-fabricated parts built with biomaterials. The physical characteristics of the fabricated devices were investigated through microstructure examination using the scanning electron microscope (SEM). Two biodegradable polymers, namely Polycaprolactone (PCL) and Poly (-L) Lactic Acid (PLLA), were investigated. For PCL varying-porosity disc, the laser power is set at 3 W (inner region) - 4 W (outer region), the scan speed at 5080 mm/s and the part bed temperature at 40°C. For Poly(-L) Lactic Acid (PLLA), the laser power is set at 12 W, the scan speed at 1270 mm/s and the part bed temperature at 60°C. With the set of SLS parameters tabulated for specific polymers, polymeric matrix with specific porosity can be fabricated as drug delivery devices.     

Fabrication of porous polymeric matrix drug delivery devices using the selective laser sintering technique

New techniques in solid freeform fabrication (SFF) have prompted research into methods of manufacturing and controlling porosity. The strategy of this research is to integrate computer aided design (CAD) and the SFF technique of selective laser sintering (SLS) to fabricate porous polymeric matrix drug delivery devices (DDDs). This study focuses on the control of the porosity of a matrix by manipulating the SLS process parameters of laser beam power and scan speed. Methylene blue dye is used as a drug model to infiltrate the matrices via a degassing method; visual inspection of dye penetration into the matrices is carried out. Most notably, the laser power matrices show a two-stage penetration process. The matrices are sectioned along the XZ planes and viewed under scanning electron microscope (SEM). The morphologies of the samples reveal a general increase in channel widths as laser power decreases and scan speed increases. The fractional release profiles of the matrices are determined by allowing the dye to diffuse out in vitro within a controlled environment. The results show that laser power and scan speed matrices deliver the dye for 8-9 days and have an evenly distributed profile. Mercury porosimetry is used to analyse the porosity of the matrices. Laser power matrices show a linear relationship between porosity and variation in parameter values. However, the same relationship for scan speed matrices turns out to be rather inconsistent. Relationships between the SLS parameters and the experimental results are developed using the fractional release rate equation for the infinite slab porous matrix DDD as a basis for correlation.     

Process optimization for PA12/MWCNT nanocomposite manufacturing by selective laser sintering

Nanocomposites produced through the addition of carbon nanotubes to a polymeric matrix can improve the material properties. The mobility of the polymer chains is usually affected, and this is also related to the properties. Parts produced with the free-form fabrication process using the selective laser sintering (SLS) technique can be used in different high-performance applications as they do not require expensive tools for their manufacture. A specific field of interest is the aerospace industry which is characterized by a low production volume and the need for materials with a high performance to weight ratio. In this study, the free-form fabrication by SLS of parts made from nanocomposites comprised of polyamide 12 and multiwalled carbon nanotubes (MWCNTs) was investigated. Specimens were manufactured by SLS to identify the appropriate processing parameters to achieve high mechanical properties for aerospace applications. Laser energy density was adjusted to improve the material density, flexural modulus, and stress at 10 % elongation. Design of experiments was used to identify and quantify the effects of various factors on the mechanical properties. The results obtained showed that there was a limit to the amount of MWCNTs which could be mixed with the polyamide powder to improve the mechanical properties since a higher content affected the laser sintering process.     

Development and application of copper–nickel zirconium diboride as EDM electrodes manufactured by selective laser sintering

The production of electrical discharge machining (EDM) electrodes by conventional machining processes can account for over 50 % of the total EDM process costs. The emerging additive manufacturing (AM) technologies provide the possibility of direct fabrication of EDM electrodes. Selective laser sintering (SLS) is an alternative AM technique because it has the possibility to reduce the tool-room lead time and total EDM costs. The main difficulty of manufacturing an EDM electrode using SLS is the selection of an appropriate material. This work investigated the direct production of EDM electrodes by means of the SLS using a newly developed non-conventional metal–matrix composite material composed of a metallic matrix (CuNi) and an advanced ceramic (ZrB₂). The influence of important SLS parameters and material content on the densification behavior and porosity of the electrodes was investigated. EDM experiments were conducted to observe the electrodes behavior and performance. It was found that the ZrB₂-CuNi electrodes could be successfully manufactured by SLS. Interlayer bonding and porosity are directly influenced by the layer thickness. Smaller layer thicknesses improved bonding between layers and decreased the porosity of the parts. The laser scan speed has a significant effect on the densification behavior. The scan line spacing affects the pore structure by means of overlapping. The surface morphology of the samples was not affected by varying the scan line spacing. The ZrB₂-CuNi electrodes presented a much superior performance than SLS copper powder electrodes, but inferior to solid copper electrodes.     

选择性激光烧结新型扫描方式的研究及实现

选择性激光烧结（Ｓｅｌｅｃｔｉｖｅ　ｌａｓｅｒ　ｓｉｎｔｅｒｉｎｇ，ＳＬＳ）快速成形技术是近１０年来发展起来的一种先进制造技术，采用此项技术可以显著地缩短产品投放市场的周期，降低成本，提高产品质量，增强企业的竞争能力。而扫描方式是ＳＬＳ成形过程中的一项关键技术，在分析了现有扫描方式及其对ＳＬＳ制件精度、强度以及成形速度影响的基础上，指出了现有扫描方式的缺陷。为了克服这种缺陷，特提出并实现了一种新的分区变向扫描方式及其分区算法，它通过将扫描线段进行分组，以避开截面内孔和凹槽。这种新的分区变向扫描方式及其算法已经成功地应用到华中科技大学开发的ＨＲＰＳ系列ＳＬＳ系统中。实际应用表明：这种新型扫描方式能大幅度地提高烧结成形效率和减小烧结体的翘曲变形量。     

Direct metal laser fabrication: machine development and experimental work

A novel modification of the freeform technique selective laser sintering machine via system integration to develop a direct metal laser fabrication machine suitable for both nonmetal and metal materials is put forward in this paper, the aim of which is to establish an experiment platform for studying the direct metal laser fabrication (DMLF) as a variant of selective laser sintering (SLS). The system integration of two machines is realized by use of the low-power SLS Machine (CO₂, 50 W) and high-power laser processing machine (CO₂, 2,000 W) in our institute after the improvement or rebuilding of light route system, functional units, machinery and electrical system, and control software in the primary SLS machine, which means that direct metal laser fabrication machine with the laser power up to 1,000 W has been developed successfully. Functional tests of two machines and DMLF experiments of a large number of metal powders including Cu-based mixture and 316L-based mixture of powder have been done in detail. The results show that the material suitable for direct metal laser fabrication machine ranges from nonmetal powders to metal powders, including polystyrene, polyamide, polycarbonate, sands, and a large number of metal powders. At the same time, the primary functions or performances of both low-power SLS Machine and high-power laser processing machine are preserved entirely. Metal samples based on copper and 316L powders can be fabricated with the relative density of about 80% and 100%, respectively, by use of developed DMLF machine. The macro appearance and microstructure and processing mechanisms of DMLF are analyzed minutely. The academic application of DMLF machine for metal powders with high melting point has been making its mark.     

航空发动机整体叶轮快速制造的试验研究

选择性激光烧结与铸造技术相结合 ,是快速制造金属零件的有效途径。本文介绍了利用这种技术制作整体叶轮铸造型腔 ,并通过铸造快速获得整体叶轮零件的工艺试验     

Direct Selective Laser Sintering of Hard Metal Powders: Experimental Study and Simulation

Direct selective laser sintering (SLS) technology can be used to produce 3D hard metal functional parts from commercial available powders. Unlike conventional sintering, it does not require dedicated tools, such as dies. Hence, total production time and cost can be reduced. The large shape freedom offered by such a process makes the use of, for example, sintered carbides components viable in domains where they were not applied before. Successful results have been obtained in the production of sintered carbide or hard metal parts through SLS. The investigation focuses on tungsten carbide–cobalt (WC-Co) powder mixture. This material is characterised by its high mechanical properties and its high wear resistance and is widely used in the field of cutting tools. This paper is devoted to the experimental study and the simulation of direct selective laser sintering of WC-Co hard metal powders.     

Two-dimensional modeling of sintering of a powder layer on top of nonporous substrate

Selective laser sintering (SLS) of a two-component metal powder layer on the top of multiple sintered layers by a moving Gaussian laser beam is modeled. The loose metal powder layer is composed of a powder mixture with significantly different melting points. The physical model that accounts the shrinkage induced by melting is described by using a temperature-transforming model. The effects of the porosity and the thickness of the atop loose powder layer with different numbers of the existing sintered metal powder layers below on the sintering process are numerically investigated. The present work will provide a better understanding to simulate much more complicated three-dimensional SLS process.     

Two-dimensional modeling of sintering of a powder layer on top of nonporous substrate

Selective laser sintering (SLS) of a two-component metal powder layer on the top of multiple sintered layers by a moving Gaussian laser beam is modeled. The loose metal powder layer is composed of a powder mixture with significantly different melting points. The physical model that accounts the shrinkage induced by melting is described by using a temperature-transforming model. The effects of the porosity and the thickness of the atop loose powder layer with different numbers of the existing sintered metal powder layers below on the sintering process are numerically investigated. The present work will provide a better understanding to simulate much more complicated three-dimensional SLS process.     

基于零件切片的选择性激光烧结预热温度控制方法

选择性激光烧结(Selective laser sintering,SLS)系统是通过加支撑或采用人工调节预热温度的方法来减少成形件的翘曲变形,它劳动强度大、SLS材料浪费多、成形效率低,且预热温度难以精确控制。基于以上情况,提出一种新的控制方法,使预热温度能随零件断面几何形状的变化而自动调节。以华中科技大学开发的SLS设备为研究对象,验证这种新型控制方法的可行性。试验结果和实际应用表明:与原有控制方式比较,根据文中所提出的方法成形的零件表面质量、尺寸精度和形状精度等都有较大幅度改善,并且真正实现了SLS成形过程的自动化。     

基于SLS原理的快速成形机研制

介绍了选择性激光烧结（SLS）快速成形技术的工作原理,简述了基于SIS原理的快速成形机的样机研制与调试、烧结工艺实验、存在的问题及解决思路。     

Empirical investigation of environmental characteristic of 3-D additive manufacturing process based on slice thickness and part orientation

The significant amount of research has been done in improving the mechanical properties (compressive strength), dimensional accuracy (length, height and width), and build time of the components manufactured from the additive manufacturing process. In contrast to this, the research in the optimization of environmental characteristic i.e. energy consumption for the additive manufacturing processes such as selective laser sintering (SLS), and selective laser melting (SLM) needs significant attention. These processes intakes the significant portion of input laser energy for driving the laser system, heating system and other machine components. With world moving towards globalization of additive manufacturing processes, the optimization of laser energy consumption thus become a necessity from productivity and as well as an environmental perspective. Therefore, the present work performs the empirical investigation by proposing the optimization framework in modelling of laser energy consumption of the SLS process. The experimental procedure involves the computation of energy consumption by measuring the total area of sintering. The optimization framework when applied on the experimental data generates the functional expression for laser energy consumption which suggests that the slice thickness is a vital parameter in optimizing it. The implications arising from the study is discussed.     

Effect of delay time on part strength in selective laser sintering

Selective laser sintering (SLS) is one of the most popular layered manufacturing processes used for making functional prototypes of polymers and metals. It is a powder-based process in which layers of powder are spread and laser is used to sinter selected areas of preheated powder. In the present work, experimental investigations have been made to understand effect of delay time on SLS prototypes. Delay time is the time difference for laser exposure between any two adjacent points on successive scanning lines on a layer. Tensile specimens of polyamide material as per the ASTM standard are fabricated on SLS machine keeping delay time range constant for the entire specimen. Specimens are fabricated for different ranges of delay time and tested on universal testing machine for tensile strength. An optimum value of delay time range is obtained experimentally. As delay time depends on part build orientation, an algorithm has been developed and implemented to find out optimum part build orientation for improving tensile strength. The obtained results from developed code are validated experimentally for tensile specimen. Case study for a typical 3D part is also presented to demonstrate the capabilities of developed algorithm.     

STL数据处理对激光烧结制件表面条纹影响的研究#br#

对光固化立体造型（STL）模型数据的尼龙选择性激光烧结（SLS）制件表面条纹进行研究,利用SLS成形技术原理、STL模型数据存储规则、激光扫描路径形成原理,设计STL模型数据的尼龙SLS实验,分析STL模型数据烧结制件表面条纹形成机理,并提出避免条纹形成的方法。实验分析结果表明：制件表面条纹的产生是由于系统软件读取数据时,数据平面有三角形缺失或冗余;制件表面条纹与STL模型数据在成形坐标系中的Z坐标值有关,且与实际烧结层厚的关系存在一定规律性;通过对STL模型数据三角形均匀化处理或使其Z坐标不在特定坐标,能够避免制件形成表面条纹。     

选择性激光烧结间接成型金属件及其在机械工业中的应用

快速成型的重要发展方向是快速制造。在各种利用快速成型制造金属功能件的方法中，选择性激光烧结间接法制造金属件是已商业化并在模具制造中广泛应用的方法。介绍了间接法的原理、所使用的覆膜金属粉末材料、成型工艺和后处理工艺，总结了该方法的应用现状。     

选择性激光烧结方法加工仿生支架的工艺研究

在利用选择性激光烧结技术加工仿生支架过程中,加工参数(包括激光功率、扫描速度、扫描间距)是影响成形质量的重要因素,通过调整加工参数可以使成形零件内部保持一定的孔隙,从而产生支架内部微孔结构。正交试验方法可以科学地安排和分析多种因素的影响,从而优化加工参数,最终获得具有较高孔隙率的仿生支架。     

不锈钢激光间接烧结粉末材料及其成型工艺研究

开发低成本,成型性好的材料对激光选择性烧结的应用具有重要的意义。本文研究了一种不绣钢间接烧结粉末材料,并对其工艺参数进行了优化,用此种材料在优化的工艺参数下制作出了复杂的三维零件。