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Engineering

Abstract

3D printers are an easy and powerful way to produce 3D structures out of a solid polymer filament or a liquid resin. In this years iGEM project the team from TU Darmstadt created a fully working SLA-DLP 3D printer using a self-written software running on a RaspberryPi. As a light source a commercially available beamer manufactured by Acer Inc. is used. This beamer hardens the resin layer-by-layer on a metal base plate. The hardened structure is lifted out of the resin basin. For testing purposes, two different resins are used. Hardening is achieved using a commercially available resin and the self-manufactured resin of the iGEM team of TU Darmstadt.

Introduction

In the last few years 3D printing became a popular and cheap method to manufacture three dimensional structures in small numbers of units. At first it was only used in Rapid Prototyping in industry but as the process is quite simple and can be realized with atoxic substances, 3D printers later on became also available for personal use [1][2]. Among other procedures of Rapid Prototyping, the process of 3D printing is either established by heating a solid polymer filament and placing it via a movable nozzle in desired shapes or by hardening a liquid resin by energy contributions from a light source. The latter process is also referred to as Stereolithography (SLA) [1][3][4]. In Stereolithography, a base plate is inserted into the resin and the structure is either built by lowering the base plate stepwise and hardening new resin on top of the structure or by lifting the base plate stepwise and hardening resin which flows below the base plate. This layer-by-layer manufacturing technique is repeated until the structure is finished [3][5]. If a laser is used as light source it hardens the resin pointwise in a certain depth until a complete hardened layer is produced before the base plate is moved. This requires laser optics to guide the laser beam towards defined points in space. If a less focused light source is used, the whole layer can be lighted in one step [1][3][4]. This is then referred to as Digital Light Processing (DLP) [4]. The hardening process of the resin depends on the light source and the photoinitiator in the resin used to start the polymerization.

Chemical Background

During the printing process a radical polymerization occurs at the illuminated spots which leads to the hardening of the resin. This way the shape of the desired structure can be created layer by layer. This radical polymerization is achieved by adding a photoinitiator to the photocurable prepolymer.
Photoinitiators are fragile molecules which decay into radicals when hit by light of a specific wavelength. These radicals react with the unsaturated carbon-carbon double bonds in the resin kicking off a radical polymerization. In our case the double bonds of itaconic acid in prepolymer are responsible for the crosslinking. The radical forming reaction initiated by electromagnetic radiation can be seen in Figure 1.

Goal

The goal was to construct and build a fully functional and at the same time easy-to-use and low-cost SLA-DLP 3D printer using a RaspberryPi as a control unit and a self-written software to print structures out of commercial resins. Therefore the software should access both Beamer and stepper motor and provide a user interface where the user can load CAD models, slice them and submit the printing job to the printer. Furthermore, the integration in the iGEM project of TU Darmstadt by means of building structures using the self-manufactured resin was striven for.

Results

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References

1. P. Hehenberger. Computergestützte Fertigung. Eine kompakte Einführung. 1st ed. Springer-Verlag Berlin-Heidelberg, 2011.
2. P. Fastermann. 3D-Drucken. Wie die generative Fertigungstechnik funktioniert. 1st ed. Springer Vieweg, 2014.
3. P. Fastermann. 3D-Druck/Rapid Prototyping. Eine Zukunftstechnologie - kompakt erklärt. 1st ed. Springer-Verlag Berlin Heidelberg, 2012.
4. A. Gebhardt. Generative Fertigungsverfahren. Additive Manufacturing und 3D Drucken für Prototyping - Tooling - Produktion. 4th ed. Carl Hanser Verlag, 2013.
5. J. Eschl. “Die mechanischen Eigenschaften von Stereolithographiematerialien während der Aushärtung”. PhD thesis. Universität Stuttgart, 2001.

Attachments

• Collection of printer parts and models
• Building instructions and Bill of materials