University of Twente Student Theses


XZEED DLP. A multi-material 3D printer using DLP technology

Holtrup, R. (2015) XZEED DLP. A multi-material 3D printer using DLP technology.

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Abstract:The 3D DLP printing process was studied. In DLP printing a projection coming from a DLP beamer is projected on a resin containing photo initiators, monomer and blockers. A thin layer in the shape of the projection is polymerised and a moveable build platform moves this layer one-layer thickness so that a new layer can be polymerised. Since photo initiators are sensitive to specific wavelengths of light mostly in the UV-A (λ<400nm) and near UV band (400-420nm), the optical system has to be designed for curing within these bands. There are many aspects that influence the polymerization process: The formula of the resin, inhibition by oxygen, the wavelength band of the projected light, the intensity, the exposure time, and temperature. They result in the amount of shrink, the density, toughness, the amount of uncured resin, the thickness of the curing gradient and the optimal layer thickness. The resolution of the beamer and the projected size account for the XY accuracy of a printed object, although shrink and bleed cause inaccuracy. The minimal step resolution of the motor driving the Z-axis is called the mechanical resolution. This is the linear movement per step. The resolution in the Z-axis is also influenced by how accurate the focus is. Since the refraction index of a lens system is wavelength-dependent, the focus will differ for lower and higher wavelengths, thus the more monochromatic the light, the lower the range of focus points, thus the higher the Z-accuracy. Projecting monochromatic light at the photo initiator’s peak-absorbance wavelength is also better, since less light energy will be transferred to heat because of absorbance by pigments and blockers. The higher the temperature, the higher to polymerization rate and the higher the amount of shrink. Secondly a range of existing 3D DLP printers was analysed. One of the most important aspects of the construction is the anti-stick solution. Because of the projection of light from the bottom of the vat, the molecules closest to the bottom of the vat will be cured. This will cause adhesive forces with the bottom. Another reason why it sticks is the air pressure: Lifting the platform will cause a vacuum that ‘pulls’ on the printed object. The following solutions are found: Putting a layer of oxygen permeable PDMS on a solid vat floor: Since oxygen inhibits the polymerization process, a thin layer of uncured resin will be generated at the bottom of the vat. Since PDMS is a bit flexible too, it will also help to break the vacuum easier. Another method is to place a flexible film of oxygen permeable FEP at the bottom of the vat. This solution is more reliable since it is inert and not degraded by high temperatures (PDMS does this) but is less easy to make compared to PDMS. The flexibility also makes it easier to break the vacuum. Other solutions to easily break the vacuums include mechanical steps: Shoving, or tilting. These mechanical steps lower the amount pulling force significantly. Having control of the inhibition rate caused by oxygen makes these steps unnecessary. (CLIP technology) Abstract 9 Inspired by the existing printers and by taking care of the theory, a multi material 3D DLP printer prototype was designed (Fig.1) and build. This printer features a moving array of petri dishes covered with a layer of PDMS and a passive tilting system (1) to make the layer peeling easy. A modified DLP beamer (2) was used as the source of the projection and projects via a mirror (3) to the bottom of the petri dishes. A build platform (4) pulls the object out of the vat. The control of the stepper motors (5) was done by a motherboard (6) loaded with firmware that communicates with host software. This host software controls the beamer concurrently to the motors. The printer was mainly build using 3D printing and laser cutting, although some post processing of the parts was needed to bring up the result.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:20 art studies
Programme:Industrial Design BSc (56955)
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