Vat Photopolymerization Processes PDF

Chapter 4 Vat Photopolymerization Processes  Vat Photopolymerization Processes Objectives: – Discuss on: Vat Photopolymerization processes Photopolymerization materials – UV curable photopolymers; Overview of photopolymer chemistry; and R...

Chapter 4 Vat Photopolymerization Processes  Vat Photopolymerization Processes Objectives: – Discuss on: Vat Photopolymerization processes Photopolymerization materials – UV curable photopolymers; Overview of photopolymer chemistry; and Resin formulations and reaction mechanisms (photo initiator system; monomer formulations; and interpenetrating polymer network formation) Reaction rates Laser Scan Vat Photopolymerization Photopolymerization Process Modeling – irradiance and exposure; Laser-resin interaction; photospeed; and time scales Vector scan VP Machines Scan Patterns – Layer-based build phenomena and errors; WEAVE; STAR-WEAVE; and ACES scan pattern; Vector Scan Micro-Vat Photopolymerization Mask projection VP technologies and Processes – Mask projection VP technology; Commercial MPVP systems; and MPSL modeling Vat Photopolymerization Processes Two-Photon Vat Photopolymerization Process benefits and drawbacks Summary Assignment: – Read Chapter 4, Pages 63 - 106 Homework: – Exercises: 1-4 YouTube Videos: Photopolymer Processing -https://www.youtube.com/watch?v=Y8hBGDhC0CE Photopolymer Exposure and Development -https://www.youtube.com/watch?v=wiKYlszhYYo QuikArt Photopolymer Plates and ReVerseArt Film and Developer MADE EASY! -https://www.youtube.com/watch?v=FI_o4VqptI4 How To Make a Liquid Photopolymer Plate Using AVantage Liquid Photoplymer Resin - https://www.youtube.com/watch?v=2r3HN_cfRQM  Photopolymerization Processes Photopolymerization processes: – Make use of liquid, radiation curable resins, or photopolymers as their primary materials – Most photopolymers react to radiation in the ultraviolet (UV) range of wavelengths – Some visible light systems are also used – Photopolymers – developed in 1960; soon widely used in coating & printing industry – Photo-curable resins are used in dentistry, for sealing the top surfaces of teeth to fill in deep grooves and prevent cavities; coatings are cured by radiation that blankets the resin without the need for patterning either the material or the radiation – Mid 1980s: Charles Hull – Experimenting with UV curable materials by exposing them to a scanning laser (similar to the system found in laser printers); discovered that solid polymer patterns could be produced; by curing one layer over the previous layer, he could fabricated a solid 3D part; beginning of SL technology. – Various types of radiation used to cure commercial photopolymers: Gamma rays, x-rays. Electron beams, UV, and in some cases visible light In SL systems, UV and visible light radiation are used most commonly In the microelectronic industry: Photomask materials are often photopolymers and are typically irradiated using far UV and electron beams In dentistry, visible light is predominantly used  Photopolymerization Processes Two primary configurations developed for the Photopolymerization Processes in a vat and one additional configuration that has some research interest: – Vector scan, or point-wise approaches typical of commercial SL machines – Mask projection, or layer-wise approaches that irradiate entire layers at one time – Two-photon approaches that are essentially high resolution point-by-point approaches – See Fig. 4.1 for these configurations – Scanning laser beams are needed for the vector scan and two-photon approaches – Mask projection approach utilize a large radiation beam that is patterned by another device – Digital Micromirror Device – Two-photon case: photopolymerization occurs at the intersection of two scanning laser beams – other configurations use a single laser and different photoinitiator chemistries – There is the need to recoat, or apply a new layer of resin, in the vector scan and mask projection approaches – in two-photon approach, the part is fabricated below the resin surface, making recoating unnecessary – approaches that avoid recoating are faster and less complicated.  4.2 Vat Photopolymerization Materials UV curable photopolymers – Photopolymers are used as photoresists in the microelectronic industry – Various types of radiation may be used to cure commercial photopolymers: gamma rays, X-rays, electron beams, UV, and in some cases visible light – UV and electron beam are most prevalent Photopolymer chemistry – Free radical polymerization – acrylate – Cationic polymerization – epoxy and vinylether Resin formulation and reaction mechanisms – Basic raw materials such as polyols, expoides, (meth) acrylic acids and their esters, diisocyanates etc., are used to produce monomers and oligomers used for radiation curing – Most of the monomers are multifunctional monomers(MFM) Photoinitiator system: The role of photoinitiator is to convert the physical energy of the incident light into chemical energy in the form of reactive intermediates Monomer formulation: Both di-functional and higher functionality monomers are used typically in SL resins Interpenetrating polymer network formation: Acrylates polymerize radically; epoxides cationicaly polymerize to form their respective polymer networks 4.4 Laser Scan Vat Photopolymerization – Laser scan VP creates solid parts by selectively solidifying a liquid photopolymer resin using an UV laser – The physical parts are manufactured by fabricating cross- sectional contours, or slices, one on top of the other – After building the part, the part must be cleaned, post- cured, and finished – During the cleaning and finishing phase, the VP operator may remove support structures – During finishing, the operator may spend considerable time sanding and filing the part to provide the desirable surface finishes 4.6 Vector Scan VP Machines - The machine subsystem hierarchy is shown in Fig. 4.8. The five main subsystems are: Recoating system, platform system, vat system, laser and optics systems, and control system. The process can be described as follows: - After a layer has been cured, the platform dips down by a layer thickness. - The recoater blade sides over the whole build depositing a new layer of resin and smoothing the surface of the vat. 4.7. SL Scan Patterns: 4.7.1. Layer-based built phenomena and errors: The most obvious phenomenon is discretization – a stack of layer causes “stair steps” 4.7.4. ACES Scan Pattern  4.8. Vector Scan Micro-Vat Photopolymerization Several processes were developed exclusively for microfabrication applications based on photopolymerization principles using both lasers and X-rays as the energy source. Specifications of atypical point-wise Microsterolithography process: – 5-µm spot size of the UV beam – Position accuracy is 0.25 µm in the x-y directions and 1 µm in the z-direction – Minimum size of the unit of hardened polymer is 5 µm x 5 µm x 3 µm (in x, y, z) – Maximum size of fabrication structure is 10 mm x 10 mm x 10 mm Mask Projection VP Technologies and Processes – Mask Projection VP technology – Figure 4.15 4.9.2 Commercial MPSL System 4.9.3. MPSL Modeling 4.10. Two-Photon VP 4.11. Process Benefits and Drawbacks Two main advantages of vat photopolymerization technology over other AM technologies: Accuracy and surface finish – Accuracy for SLA-250 = 0.002 in/in – Surface finish of SL parts ranges from submicron Ra for upfacing surfaces to over 100 um Ra for surfaces at slanted angles – Another advantage: flexibility – supporting many different machine configurations and size scales – Mask projection VP technologies: have speed advantage over laser scan SL – Trade-off between resolution and the size of the pattern Drawback – Usage of photopolymers since their chemistries are limited to acrylates and epoxies for commercial materials. – Current SL materials do not have the impact strength and durability of good quality injection molded thermoplastics – They are known to age, resulting in degraded mechanical properties over time. – These limitations prevent SL processes from being used fro many production applications.  Chapter 4 Solution to Problems