materials Review A New Approach to Micromachining: High-Precision and Innovative Additive Manufacturing Solutions Based on Photopolymerization Technology Paweł Fiedor 1 and Joanna Ortyl 1,2,* 1 Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, 31-155 Cracow, Poland; pawel.fi[email protected] 2 Photo HiTech Ltd., Bobrzy´nskiego14, 30-348 Cracow, Poland * Correspondence: [email protected] Received: 9 June 2020; Accepted: 29 June 2020; Published: 1 July 2020 Abstract: The following article introduces technologies that build three dimensional (3D) objects by adding layer-upon-layer of material, also called additive manufacturing technologies. Furthermore, most important features supporting the conscious choice of 3D printing methods for applications in micro and nanomanufacturing are covered. The micromanufacturing method covers photopolymerization-based methods such as stereolithography (SLA), digital light processing (DLP), the liquid crystal display–DLP coupled method, two-photon polymerization (TPP), and inkjet-based methods. Functional photocurable materials, with magnetic, conductive, or specific optical applications in the 3D printing processes are also reviewed. Keywords: 3D printing; high-resolution; additive manufacturing; photopolymerization; industry 4.0; stereolithography; two-photon polymerization 1. Introduction Three dimensional (3D) printing is currently an extremely important branch of Research and Development (R&D) departments. This is because of its rapid prototyping, swift elimination of design errors, and improvements of the product at the prototyping stage. This approach significantly accelerates the implementation of new solutions without incurring significant production costs and eliminating in-production testing of underdeveloped models. Thanks to 3D printing techniques, making a prototype with complex geometry has become possible in a short time with unprecedented precision [1]. Progressive computerization of manufacturing processes introduces us to a new era called Industry 4.0. This level of smart production was enabled by significant breakthroughs in artificial intelligence, robotics, nanotechnology, and 3D printing observed in the twenty-first century. Thanks to the extreme customization and personalization of production technologies, the practice of Industry 4.0 is an observed phenomenon in every section of manufacturing processes. The application of artificial intelligence (AI) algorithms for the preparation and transformation of 3D models significantly quickens and improves the quality of 3D figures. AI has been successfully applied in printability checking, slicing acceleration, nozzle path planning, and, among others, cloud service platforms [2]. Evolution of industry models are presented in Figure1. Materials 2020, 13, 2951; doi:10.3390/ma13132951 www.mdpi.com/journal/materials Materials 2020, 13, 2951 2 of 25 Materials 2020, 13, x FOR PEER REVIEW 2 of 25 FigureFigure 1. 1.Industrial Industrial revolution—from revolution—from Industry Industry 1.0 1.0 to to Industry Industry 4.0. 4.0. TheThe conceptconcept ofof Industry 4.0 is based onon encompassing encompassing a a combination combination of of traditional traditional manufacturing manufacturing andand industrialindustrial platformsplatforms and practices withwith the the latest latest smart smart technologies. technologies. TheThe industry industry has has just just begun begun to to adopt adopt additive additive manufacturing manufacturing methods,methods, which they use mainly toto prototype prototype and and produce produce individual individual items. items. WithWith IndustryIndustry 4.0,4.0, 3D3D printingprinting methods will be widely usedused to to produce produce small small batches batches of of made-to-order made-to-order products products thatthat oofferffer constructionconstruction advantages, such as complex,complex, lightweight lightweight designs designs [3 [3].]. TheThe flexibility flexibility of of 3D 3D printing printing opens opens the the way way for for sharing sharing productionproduction capacitiescapacities betweenbetween diverse companiescompanies for for better better utilization utilization of of assets assets and and supporting supporting growthgrowth andand willwill alsoalso open up opportunities toto share share production production capacities capacities between between di differentfferent companies,companies, therebythereby better utilizing these assets assets [4]. [4]. ResearchResearch projects projects such such as as Horizon Horizon 2020 2020 are are focused focused on on creating creating such such business business modelsmodels inin industry.industry. ManufacturersManufacturers have have implemented implemented 3D3D printingprinting technologiestechnologies throughthrough sevenseven didifferentfferent additive manufacturingmanufacturing processes: processes: powderpowder bedbed fusion,fusion, vatvat photopolymerization,photopolymerization, binder jetting, material extrusion,extrusion, directed directed energy energy deposition, deposition, material material jetting, jetting, and sheet and lamination.sheet lamination. Each method Each ismethod achieved is throughachieved a di throughfferent variation a different of variation 3D printing of technology,3D printing whichtechnology, varies which based onvaries its material based on state, its material sources state, sources of light or heat, print axes, feed systems, and post-production processing [5]. of light or heat, print axes, feed systems, and post-production processing [5]. • Powder bed fusion: It works by melting powder to fuse particles together. It is ideal for most Powder bed fusion: It works by melting powder to fuse particles together. It is ideal for most • types of manufacturing [6]. • typesVat photopolymerization: of manufacturing [6]. It uses a liquid instead of a powder or filament in its build platform Vat photopolymerization: It uses a liquid instead of a powder or filament in its build platform • and is a light-activated process. It is ideal for low-run injection molds [7,8]. • andDirected is a light-activated energy deposition: process. It uses It is highly ideal for focused low-run thermal injection energy molds delivered [7,8]. via a laser, electron Directedbeam, or energy plasma deposition: art to melt It uses and highly fuse focusedmaterial. thermal It is used energy exclusively delivered viain ametal laser, additive electron • beam,manufacturing or plasma [9,10]. art to melt and fuse material. It is used exclusively in metal additive • manufacturingMaterial jetting: [9, 10Tiny]. nozzles dispense droplets of a waxy photopolymer, layer by layer, which Materialis hardened jetting: via Tiny UV light. nozzles It is dispense ideal for droplets items requiring of a waxy high photopolymer, detail and high layer accuracy by layer, [11,12]. which is • • hardenedBinder jetting: via UV It light.is similar It is to ideal material for items jetting requiring but instead high uses detail a powdered and high material accuracy and [11 ,a12 binding]. Binderagent. jetting: It is primarily It is similar used to in material furniture jetting design but models instead [13,14]. uses a powdered material and a binding • • agent.Material It is extrusion: primarily A used thermoplastic in furniture filament design modelsis extruded [13, 14through]. a heated nozzle onto the build Materialplatform, extrusion: which solidifies A thermoplastic as it cools. filament It is the most is extruded commonly through referred a heated to additive nozzle manufacturing onto the build • platform,method whichwhen someone solidifies is as discussing it cools. It 3D is the printing most commonly[15,16]. referred to additive manufacturing • Sheet lamination: Ultra-thin layers of solid material are bonded by alternating layers of adhesive. method when someone is discussing 3D printing [15,16]. It is best used for non-functional models [17]. Sheet lamination: Ultra-thin layers of solid material are bonded by alternating layers of adhesive. • ItThe is best history used forof 3D non-functional printing in modelsits extremely [17]. turbulent beginnings revealed many ideas and solutions that have been verified by the market over time [18]. Among the ideas that have been developedThe history for commercialized of 3D printing in technologies its extremely based turbulent on the beginnings polymerization revealed of many liquid ideas resin and are solutions thermal thatprocessing have been of verifiedthermoplastic by the materi marketals, over melting/sintering time [18]. Among of metals, the ideas and thatmethods have creating been developed prints from for commercializedcomposite materials technologies or rubber. based The on list the of polymerization methods that should of liquid be resinhighlighted are thermal in the processing topic of 3D of thermoplasticprinting is presented materials, below melting [19]:/ sintering of metals, and methods creating prints from composite • Photopolymerization of liquid resin: Materials 2020, 13, 2951 3 of 25 materials or rubber. The list of methods that should be highlighted in the topic of 3D printing is presented below [19]: Photopolymerization of liquid resin: • o SLA—stereolithography, o DLP—digital light processing, o MJP—multijet printing, o BJ—binder jetting, o CAL—computed axial lithography, o SGC—solid ground curing, o CLIP—continuous liquid interface production. Thermal processing of thermoplastic polymers: • o FDM—fused deposition modeling, o BPM—ballistic particle manufacturing, o SLS—selective