International Journal of Modern Trends in Engineering and Research www.ijmter.com e-issn No.:2349-9745, Date: 28-30 April, 2016 Improving Stereolithography resolution Yogesh D. Patil 1, Richa A. Patil 2 1 Department of Mechanical Engineering, K. J. Somaiya College of Engineering, Mumbai, yogesh.dp@somaiya.edu 2 Department of Mechanical Engineering, K. J. Somaiya College of Engineering, Mumbai, richa.patil@somaiya.edu Abstract-Stereolithography is the first Additive Manufacturing (AM) technique developed & considered as a base for many techniques employing light to solidify the photopolymer resin. This paper provides brief information on stereolithography (SLA) process which uses a UV laser beam to solidify photopolymer resin. It reviews various strategies used to improve anisotropic resolution in both vertical & horizontal direction, by reducing polymerized layer thickness, avoiding local degradation of vertical resolution and improving lateral resolution of SLA process. It also reviews the evolution of polymerized layer thickness with irradiation time for fixed value of irradiation flux (210 mw/cm) for five different monomers. Keywords- Additive Manufacturing; Micro-stereolithography; SLA resolution; Photopolymer resin; Photopolymerization;Polymerized layer thickness; Photoinitiator. I. INTRODUCTION The first Additive Manufacturing (AM) technique developed is the Stereolithography (SLA) & it s one of the several methods used to create solid objects from liquid photopolymer resin. The term Stereolithography was coined by Charles (Chuck) W. Hull when he patented the process in 1986. SLA process is one of the several AM techniques, used to produce objects by curing photopolymer resin layer-by-layer using an ultraviolet (UV) laser beam. The photopolymer resin is a material in liquid form used in SLA process. Figure 1: Stereolithography apparatus [1] Figure 1 show the apparatus used in stereolithography process. It has four main parts: a tank filled with liquid photopolymer resin, a table or a platform to hold the built object, an Ultra Violet laser beam system & computer to control the motion of platform & laser beam. In the initial act of SLA @IJMTER-2016, All rights Reserved 1045
process, a thin layer of photopolymer resin is exposed to a laser beam which hardens or cures the resin by tracing cross-section of the first layer, results in printing first layer of the object. As soon as, the first layer of resin gets cured, the platform moves down, exposing a new layer of fresh liquid resin to UV laser beam. This beam again traces the cross section of the second layer & once the second layer gets cured, it immediately sticks to the cured layer underneath of it, this process repeats until the final object forms inside the liquid resin.sla process related more precisely to microstereolithography (micro-sla) because both, SLA & micro-sla resembles the samemanufacturing principle, but micro-sla carries out process improvements which result in a better resolution. The first ever development in micro-sla happened in 1993 & efforts have taken to modify the SLA process to get better resolution. SLA process which based on the superimposition of layers, resolution, precision & surface roughness are anisotropic. The average resolution is in the range of 150 µm in the three directions. Specifically, the vertical resolution (along the vertical build axis) depends on the thickness of the superimposed layers, whereas the lateral resolution relies on the dimension of the light beam (UV laser beam) used to cure shape of a given layer, at the resin surface. II. IMPROVING STEREOLITHOGRAPHY RESOLUTION There are different factors which affect the inherently anisotropic resolution of SLA in both vertical & horizontal directions [2]. 2.1.Reducing thickness of the layers Even if the objects made by superimposition of layers, the vertical resolution of SLA does not relate to the layer thickness of fresh resin admitted on the surface of the previously cured layers. Also making thinner layers of fresh liquid resin on the surface of the last cured layer won't improve the vertical resolution, but it depends on the light penetration depth inside the liquid photopolymer resin. Thus, the ideal case will be the light should be confined to the surface and not penetrate deep into the medium. Still, if light penetrates deep into the resin, the polymerization process will start & will solidify resin, resulting in thicker cured layers which will badly affect SLA resolution. The photopolymerization phenomenon (free radical or cationic polymerization) where polymer grows by the chain reaction & this chain reaction starts when the photopolymer resin absorbs a fixed amount of photons with sufficient energy results into polymerized (cured) layer. e = ln( ), with t = (1) α α The equation 1 shows the evolution of the thickness of the cured layer with irradiation time. Where, e is polymerized layer thickness (µm), α is the Napierian coefficient of molar extinction for the photoinitiator (L/mol m), c isthe photoinitiator concentration in resin (mol/l), t 0 is the threshold irradiation time required to begin the photopolymerization reaction (s), t is the irradiation time (s), T is the irradiation threshold value (photons/m 3 ), and F 0 is the light flux reaching the surface of the resin (photons/m 3 s). According to Zissi et al. who measured this evolution of thickness with irradiation time at fixed value of irradiation flux (210 mw/cm), for five different monomers with a 0.4mol/L concentration of same photoinitiator DMPA (a,a-dimethoxy-a-phenylacetonephenone). @IJMTER-2016, All rights Reserved 1046
Figure 2: Evolution of thickness of polymerized layer with the irradiation time in SLA for Five monomers [3] Tri(ethyleneglycol)diacrylate (TIEGDA), 1,6-hexanediol diacrylate (HDDA), trimethylopropane triacrylate (TMPTA), pentaerythritol triacrylate (PETIA), and 2, 29-bis[4-(methacryloxyethoxy)phenyl] propane (Diacryl 101) these are the five different monomers used. Figure 2 shows the evolution of the polymerized layer thickness with the irradiation time showing the good agreement of the equation 1 with the experimental results, provided that the resins should not undergo the photobleaching, else the relation will be no more logarithmic. Depending on the contribution of terms present in the equation 1 to the polymerized layer thickness shows two ways to improve the vertical resolution: 2.1.1. Irradiating resin for a short duration, close to the threshold irradiation time. Reduction in the polymerized layer thickness achieved by irradiating photopolymer resin for the time slightly more than the threshold irradiation time required for starting the polymerization reaction. In that case, equation 1 reduces to following: e (2) α Using this technique to reduce the layer thickness appears to be simple & an efficient way to produce thinner layers, but it has more drawbacks than benefits. First, a small change in irradiation time results in the significant change in the layer thickness makes accurate control of layer thickness as an extremely tough job. Second, layers obtained by this method have poor mechanical properties. 2.1.2. Using reactive media having a small optical thickness. When the irradiation wavelength falls onto the photosensitive resin having strong the absorption property, the light restricts to the surface of the resin and resulting into the minimum layer thickness, irrespective of the irradiation duration. Then the equation 1 simplifies to the following: e α, withr = ln(α τ ) (3) Where τ is a small variation in irradiation time induces small changes of R when τ t 0. In that case, the layer thickness described by term µ=1/αc, calledthe optical thickness strongly depends on the absorption property of the resin. There exist two ways to produce a resin with a minimum optical thickness. First, an addition of photoinitiator or a photosensitizer into the resin increases reactivity and reduces optical thickness, as photoinitiator becomes more absorbent for irradiation wavelength shown in figure 3(a). Second, the addition of neutral absorbers, nonreactive chemicals, once added to the resin, strongly absorbs the light at the irradiation wavelength and dissipates the corresponding energy that does not interfere with the polymerization reaction. Thus, the addition of neutral absorbers reduces the energy available to start the photopolymerization reaction, results in a reduction of the optical thickness of the resins shown in figure 3(b). But, it has the serious drawback that it reduces the reactivity of resin. @IJMTER-2016, All rights Reserved 1047
Figure 3: Evolution of thickness of polymerized layer (e) with (a) Photoinitiator concentration; (b) Neutral absorber concentration [2] Zissi et al. [3] demonstrated how cure depth of resin drastically reduced by addition of small amount of inert UV dye (2-(2-hydroxy-5-methylphenyl)benzotriazole, also called Tinuvin P). Since then, many preferred to use neutral absorbers, derived from Tinuvin family, for cure depth adjustment. Bertsch and Renaud analytically described the relation between polymerized layer thickness and irradiation time, when neutral absorbers added to the resin and presented equation 4: e = + ln( ), with t α α = (α α ) (4) α Where, α N is the Napierian coefficient of the molar extinction of the neutral absorber (L/mol m), c N is the neutral absorber concentration (mol/l), and t 0 is the threshold irradiation time required to start photopolymerization phenomenon after adding neutral absorbers (s).zabti [1] experimentally demonstrated the effect of the addition of the neutral absorber on the mechanical properties such as surface roughness, density, and accuracy of acrylate-based resin components produced by micro- SLA and J. W. Choi, et al., showed that the addition of the light absorber Tinuvin 327 TM into an acrylate-based resin reduces penetration depths and thus cure depths for dynamic mask projection microstereolithography system [4]. 2.2. Avoiding local degradations of vertical resolution SLA techniques depend on an adequate tuning between the irradiation & the absorption wavelength of the resin to effectively control the thickness of each polymerized layer. If the absorption peak of the resin does not match with the irradiation wavelength, the light penetrates deep into the resin and results in the thick polymerized layers. There exist four cases, depending on the reactivity of the resin and light penetration in the medium [2]. The desirable conditions for photopolymerization reaction, when the resin having a strong reactivity &strong absorption at the irradiation wavelength used, the light will not penetrate deep inside the resin, but restricts at the surface of resin and results in the minimum polymerized layer thickness. When the resin has a low reactivity and the strong absorption then the light confines to the resin surface, there will be no polymerization at all. If still polymerization occurs, it will result in the thinner polymerized layers at the resin surface & this happens only when the resin contains a huge concentration of the neutral absorber. When the resin has a strong reactivity and low absorption at the irradiation wavelength, the light penetrates deep inside the resin, polymerization begins quickly and results in the thick @IJMTER-2016, All rights Reserved 1048
polymerized layers. For SLA, the thickness of polymerized layer strongly depends on irradiation time thus these conditions stands undesirable. When the resin has a low reactivity & low absorption at the irradiation wavelength, the light penetrates deep inside the resin, takes a longer time to start the polymerization reaction, but whenthe reaction starts, the threshold reaches everywhere at the same time and results in unwanted polymerization conditions. These conditions arise when there is the mismatch between irradiation wavelength & the absorption spectrum of resin. The degradation of vertical resolution in SLA occurs due to z-overcure error, also called Printthrough phenomenon and occurs when irradiating the resin close to its threshold value or using the resin with low light absorption for the producing objects with overhanging structures. In this case, after building the first layer of the overhanging structure, a small amount of light penetrates the resin underneath of it, but not enough to reach the threshold value; result in the local sensitization of the resin. Building additional layers on the top of the overhanging structure causes more energy to deposit below of it. Once the polymerization threshold reaches in the sensitized area, unpredictable structure grows below the first polymerized layer of that overhanging structure, results in the local degradation of the vertical resolution and worsens features of the object to be produced. By using the resin with high absorption & a strong reactivity, z-overcure error can be solved. 2.3.Improving the lateral resolution The strategies used in the SLA process to improve the vertical resolution have not much impact on the lateral resolution. As shown in figure 4(a), the addition of the photoinitiator makes the resin more reactive, reduces the threshold energy needed to begin the photopolymerization process and increases the polymerized width (l) leading to the slight decrease in lateral resolution. Exactly Opposite happens after the addition of the neutral absorber because the reactivity of resin decreases shown in figure 4(b). Figure 4: Evolution of width (l) of polymerized layer with (a) Photoinitiator concentration; (b) Neutral absorber concentration [2] In the SLA process, the laser beam scans the surface of the resin to produce polymerized layers. The laser beam diameter & accuracy of the scanning system determines the smallest feature of the object and lateral resolution of the SLA process as well. CONCLUSION Improving the SLA resolution in both lateral & vertical direction using above strategies provides good results. To improve the vertical resolution by the exposing resin for the duration slightly longer @IJMTER-2016, All rights Reserved 1049
than its threshold value, an addition of photoinitiator & neutral absorber reduces the thickness of polymerized layer to a much extent. Choosing the resin having a strong reactivity & strong absorption avoids the local degradation of vertical resolution. But the addition of photoinitiator increases polymerized layer width consequently decreases the lateral resolution and addition of neutral absorber decreases resin reactivity thereby increasing the curing time. In the case of scanning micro-sla machines, lateral resolution depends on laser beam diameter & accuracy of laser beam system. REFERENCES [1] M.M. Zabti, Effects of Light Absorber on Micro Stereolithography Parts, University of Birmingham, Edgbaston, Birmingham, 2012 [2] A. Bertsch, P. Renaud, Microstereolithography: Stereolithography, Springer, New York, 20 29, 2011 [3] S. Zissi, et al., Stereolithography and microtechniques, Microsystem Technologies, Springer-Verlag, 2 (1), 97 102, March1995 [4] J. W. Choi, et al., Cure depth control for complex 3D microstructure fabrication in dynamic mask projection microstereolithography, Rapid prototyping journal, 15 (1), 59-70, 2009 @IJMTER-2016, All rights Reserved 1050