Method Article

Stereolithographic 3D Printing with Renewable Acrylates

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DOI:

10.3791/58177

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September 12th, 2018

 ,  ,  ,  ,  , 
1Professorship Sustainable Polymers, NHL Stenden University of Applied Sciences, 2Macromolecular Chemistry and New Polymeric Materials, Zernike Institute for Advanced Materials, University of Groningen

In This Article

Summary

A protocol for additive manufacturing with renewable photopolymer resins on a stereolithography apparatus is presented.

Abstract

The accessibility of cost-competitive renewable materials and their application in additive manufacturing is essential for an efficient biobased economy. We demonstrate the rapid prototyping of sustainable resins using a stereolithographic 3D printer. Resin formulation takes place by straightforward mixing of biobased acrylate monomers and oligomers with a photoinitiatior and optical absorber. Resin viscosity is controlled by the monomer to oligomer ratio and is determined as a function of shear rate by a rheometer with parallel plate geometry. A stereolithographic apparatus charged with the biobased resins is employed to produce complex shaped prototypes with high accuracy. The products require a post-treatment, including alcohol rinsing and UV irradiation, to ensure complete curing. The high feature resolution and excellent surface finishing of the prototypes is revealed by scanning electron microscopy.

Introduction

Rapid prototyping enables on-demand production and design freedom and allows the efficient manufacturing of 3D constructs in a layer-by-layer manner1. As a result, 3D printing as a fabrication technique has developed rapidly in recent years2. Various technologies are available, all relying on the translation of virtual models into physical objects, and applying processes such as extrusion, direct energy deposition, powder solidification, sheet lamination and photopolymerization. The latter involves stepwise UV curing of liquid photopolymer resins. In 1986, Hull and co-workers developed the stereolithography apparatus (SLA), a UV laser-based 3D printer. More recently, a similar process called digital light processing (DLP) has become available, in which photopolymerization is initiated by a light projector. Together, DLP and SLA are referred to as stereolithography 3D printing3.

SLA is applied in high-resolution prototyping and fabrication of biomedical devices4,5. This technology is superior to the widely used fused deposition modeling (FDM) in terms of accuracy, surface finishing and resolution6. Depending on the architecture of the product, a support structure is integrated in the 3D model to stabilize the construct during fabrication. Furthermore, a post-printing treatment of manufactured parts is required7,8. Typically, printed objects are washed in an alcohol bath to dissolve unreacted resin, and subsequent curing in an UV oven is performed to guarantee full conversion of the polymerization9.

In general, resins for lithography-based additive manufacturing rely on photocurable systems containing multifunctional acrylates or epoxides10. Current photopolymer resins on the commercial market are fossil-based and expensive, while the availability of low-cost renewable resins is needed to facilitate waste-free and local manufacturing of sustainable 3D products for a biobased economy1,6. Recently, photopolymer resins based on renewable acrylates were developed and successfully applied in stereolithography 3D printing11,12. In this detailed protocol, we demonstrate the rapid prototyping with biobased resins on a commercial stereolithography apparatus. Special attention is paid to critical steps in the procedure, i.e., resin formulation and post-printing treatments, to help new practitioners in the field of additive manufacturing.

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Protocol

CAUTION: Please consult all relevant material safety data sheets (MSDS) before use.

1. Preparation of Photocurable Resin

NOTE: Please use personal protective equipment (safety glasses, gloves, lab coat) during the following procedure. See our previous work12 for more details on this section.

  1. Pour 50 g of 1,10-decanediol diacrylate (SA5201) in a 500 mL Erlenmeyer flask.
  2. Add 1.0 g of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) and 0.40 g of 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBOT) to the flask.
  3. Equip the flask with a mechanical stirrer and stir the mixture at 200 rpm for 5 min at room temperature in order to dissolve TPO and BBOT in the acrylate monomer.
  4. Add 100 g of pentaerythritol tetraacrylate and 100 g of multifunctional epoxy acrylate (see Table of Materials) to the mixture.
  5. Stir the mixture at 200 rpm for 30 min at 50 °C to ensure a homogeneous resin.
  6. Remove the mechanical stirrer and fit the flask with a stopper. The flask is wrapped in aluminium foil to protect the biobased acrylate photopolymer resin (BAPR) from light.
    NOTE: The protocol can be paused here.
  7. Cover the bottom plate of a rheometer with parallel-plate geometry with the photoresin.
  8. Set the gap between the plates at 1 mm and cover the rheometer with a UV resistant hood.
  9. Measure the resin viscosity at room temperature at shear rates from 0.1 to 100 s-1; e.g.,0.100, 0.126, 0.158, 0.200, 0.251, 0.316, 0.398, 0.501, 0.631, 0.794, 1.00, 1.26, 1.58, 2.00, 2.51, 3.16, 3.98, 5.01, 6.31, 7.94, 10.0, 12.6, 15.8, 20.0, 25.1, 31.6, 39.8, 50.1, 63.1, 79.4, and 100 s-1.

2. Stereolithographic 3D Printing with Biobased Acrylates

NOTE: See our previous work12 for more details on this section.

  1. Turn on the SLA 3D printer and select open mode.
  2. Start the model preparation software on a computer. Choose the desired print settings: material (Clear), version (V4) and layer thickness (50 µm).
  3. Open the digital model of the complex-shaped prototype, a standard tessellation language (.stl) file (see Supplemental Coding File) and choose the location and orientation on the build platform.  Upload the print job from the model preparation software to the SLA 3D printer.
    NOTE: Depending on the architecture of the product, a support structure can be integrated in the 3D model to stabilize the construct during fabrication. In case of the complex-shaped prototype demonstrated here, a support structure is not required if printed normal to the build direction.
  4. Pour 200 mL of the biobased photoresin into a resin tank. Open the 3D printer and mount the resin tank properly.
  5. Mount the build platform and close the 3D printer.
  6. Start the print job.
  7. Allow the 3D printer to fabricate complex shaped prototypes.  Do not open the printer until the print job is finished.
    NOTE: Before printing, make sure the 3D printer is leveled. For the demonstrated protocol, the wavelength of the UV laser is 405 nm. The print time of the object is 2.5 h.

3. Post-treatment of 3D Printed Objects

NOTE: Please use personal protective equipment (safety glasses, gloves) during the following procedure.

  1. When the print job is finished, open the printer. Remove the build platform, with the produced parts attached, and close the printer.
  2. Open the washing station, filled with isopropyl alcohol, and insert the build platform. Start the procedure and rinse for 20 min to remove any unreacted resin.
  3. When the rinsing procedure is finished, remove the build platform from the washing station and detach the prototypes from the build platform.
  4. Allow the prototypes to air dry for 30 min. In the meantime, preheat the UV oven at 60 °C.
    NOTE: Preheating will take at least 15 min. The UV wavelength of the oven is 405 nm, identical to the wavelength of the SLA laser.
  5. Open the UV oven and quickly place the prototypes on the rotating platform. Close the UV oven and cure for 60 min at 60 °C to ensure complete conversion.
  6. When the post-curing procedure is finished, open the UV oven and take out the prototypes.

4. Characterization of Surface Morphology of Complex-shaped Prototypes

NOTE: See our previous work12 for more details on this section.

  1. Cut ca. 1 cm of internal helix from the complex shaped prototype using a razor blade.
  2. Attach the sample to the sample holder with double sided carbon conductive tape.
  3. Prior to imaging, coat the sample with 30 nm Pt/Pd (80:20) on a sputtering system.
  4. Insert the sample into a scanning electron microscope operating at an accelerating voltage of 5 kV. Acquire several images of the sample at 30X and 120X magnification.

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Results

Four representative resin compositions are displayed in Table 1, along with their average biobased carbon content (BC) derived from the individual BC of the components. The resin viscosity (Figure 1) is influenced by the ratio of acrylate monomers and oligomers and typically demonstrates Newtonian behavior. The mechanical properties of parts manufactured from various resins were determined by stress-strain analysis. Figur...

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Discussion

Additive manufacturing is applied in fabrication of tailor-made prototypes and small series, when the higher production costs per part can compete with conventional processes since there is no need for production of molds and tools. In the last decade, the revenues from services and products related to additive manufacturing have grown exponentially13. The largest fraction of material sales is from photopolymers. The growth attracted attention and initiated the investments of major industries,

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Disclosures

The authors have nothing to disclose.

Acknowledgements

This study was supported by GreenPAC Polymer Application Centre as part of Project 140413: "3D Printing in Production". We would like to acknowledge Albert Hartman, Corinne van Noordenne, Rens van Leeuwen, Anniek Bruins, Femke Tamminga, Jur van Dijken and Albert Woortman for facilitating the video shooting.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Isobornyl acrylate SartomerSA5102Acrylate monomer
1,10-decanediol diacrylateSartomerSA5201Acrylate monomer
Pentaerythritol tetraacrylateSartomerSA5400Acrylate monomer
Multifunctional epoxy acrylateSartomerSA7101Acrylate oligomer
Diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO), 97%Sigma Aldrich415952Photoinitiator
2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBOT), 99%Sigma Aldrich223999Optical absorber
Isopropyl alcohol (IPA), 99%Bleko1010500For alcohol bath (applied in Form Wash)
Paar Physica MCR300 Anton Paar-Rheometer with parallel plate geometry
Form 2 PrinterFormlabs-Desktop SLA 3D printer
Form Wash Formlabs-Washing station
Form CureFormlabs-UV oven
Instron 4301 1KN Series IXInstron-Universal testing machine
Philips XL30 ESEM-FEG Philips-Scanning electron microscope

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Tags

Resin FormulationBiobased ResinsSLA 3D PrinterViscosity MeasurementPost Printing TreatmentScanning Electron MicroscopyMonomer Oligomer RatioUV Curing

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