Open Access proceedings Journal of Physics: Conference series [311447]

Additive technologies for realization of microfluidic systems

D Besnea1, E Dinu2, M Avram1, A Spanu1, V Constantin1 and I C Panait2

1[anonimizat], Splaiul Independenței nr. 313, postal code 060042, sector 6, Bucharest, ROMANIA

2[anonimizat], Splaiul Independenței nr. 313, postal code 060042, sector 6, Bucharest, ROMANIA

E-mail: [anonimizat]

Abstract. In the paper is presented a 3D DLP (Digital Light Processing) [anonimizat], [anonimizat] ([anonimizat], electronics), [anonimizat] a [anonimizat] (certified for medical use) and transparent resins (prototypes in the packaging industry), characterized by the high printing speed for complex geometry and high productivity with numerous applications in the industrial and medical field.

Keywords. 3D printing, photopolymerisation , digital ligth processing.

1. Introduction

3D printing is an emerging technology with applications across multiple domains. The flexibility of the 3D printing system allows the use of a variety of materials. Most Additive Manufacturing (AM) systems that use liquid raw material use a heat source that scans the 2D [anonimizat] "point-to-point" or "surface-to-surface". The piece is built on a horizontal platform submerged in the liquid polymer. The solidification is achieved by the photopolymerization produced at the impact of a light ray with the upper surface of the liquid. [anonimizat] ([anonimizat], etc.). [anonimizat] a reaction and changes the properties of photopolymers. The photopolymer has special properties in the sense that radiation in the ultraviolet or visible field initiates the polymerization a [anonimizat] a shape corresponding to a cross section of the piece. After solidifying a layer, the piece is submerged with a new section thickness and a new cross section of the piece is solidified. The whole cycle is repeated until the piece is solidified in its entirety. Usually used is the C9H8O2 photopolymer, made by mixing cinamic acid with alcohol in the presence of light[2]. After obtaining the part through this process, a post-processing operation with a light and / or heat source is to be chosen so as not to destroy or distort the piece ([anonimizat], sunlight, etc.). Light from these sources initiates chemical reactions that alter their structure and modify their chemical and mechanical properties. The fact that the photopolymerization occurs only a few tenths of a millimeter thick on the surface of the liquid allows the use of less translucent polymers and a lower intensity of the light source. [anonimizat] (PA), polyisoprene, polyimides, epoxies, acrylics, etc. Typically, monomers, oligomers and additives are used together with these polymers. [3]. Binders are reactive intermediate molecular weight molecules consisting of several monomer units, usually dimers (two units), trimers (three units), and tetramers (four units). They are normally in the state of liquid aggregation at room temperature and are used as ink, adhesives and coatings.

Typical photopolymers consist of (50-80%) of such binders / oligomers and some of them are listed as follows: [2]

– Styrene family: cumene end-styrene-tetramer-alpha group oligomer, A-methyl styrene-dimer, A-methyl styrene-tetramer etc.

– The methacrylate family: acrylic acid oligomers, methyl methacrylate oligomers, methyl methacrylate tetramer, etc.

– Vinyl alcohol family: vinyl alcohol trimer, vinylacetate trimer, vinylacetate oligomer.

– The Olefineolefine family: polyisobutylene.

– The glycerin family: Triglycerol.

– Family of polypropylene glycol: polypropylene glycol (finished dihydroxy) etc.

Monomers are small chemically bonded molecules that join other monomers, oligomers or polymers in a repeated manner to form new polymers. Mostly, photopolymers consist of monomers based on acrylates or methacrylate, which vary from 10 to 40%. In the polymerization process, two types of monomers can be used: multifunctional monomers and monofunctional monomers. Multifunctional monomers can act as both diluents and crosslinking agents, while monofunctional monomers may be either diluent or crosslinked. Some examples of monofunctional and multifunctional monomers are presented below. [2]

Photoinitiators convert light energy into chemical energy by forming free radicals or cations to exposure to UV radiation. They can break into two or more UV-reactive particles and at least one of the particles will react with monomers or oligomers and bind them together. They exist naturally or can be synthesized chemically and are sensitive to light specific light wavelengths. The initiators form only a small component of photopolymers. [2]

In photopolimerization of free radicals, the radicals or ions remove the initiators when reacting the UV, and the ions will begin to react with monomers to initiate the polymerization. During the cationic reaction strong acid will be released from the initiator and a binding process begins. Diarylodonium and triaryl sulfonium salts are stable crystalline compounds which can be synthetically prepared and commercially available for commercial purposes. [2]

2. DLP –Digital Light Processing process

DLP Printing Technology (Digital Light Processing) is an additive manufacturing process based on the use of UV light for the solidification of liquid polymer resins. DLP technology has as its principal element the DMD (Digital Micromirror Device) chip – a matrix of micro-mirrors used for fast spatial light modulation. Initially, the 3D CAD model is converted by the 3D printer's software into the cross-sections (slices) of the object, then the information is sent to the printer and the DMD chip. For each cross section of the 3D CAD model, the UV light emitted by a projector is modulated and projected by means of the chip on the surface of the polymer resin in the construction bucket. Each individual micro-mirror of the DMD chip projects pixels from the cross-section of the 3D model. Under the action of UV light, the photoreactive liquid (sensitive to ultraviolet light) solidifies in successive layers. Because the entire cross section is projected into a single exposure, the construction speed of a layer is constant regardless of the complexity of the geometry [5]. Fig.1

Figure 1. DLP Digital Ligth Processing printing technology.

3D objects of more complex geometries are printed with support materials that are later removed. The resin remaining in the construction tank can be reused for later printing. Certain printing materials may require subsequent curing processes in UV ovens. Table 3 lists some of the features of Digital Ligth Processing technology.

Advantages of DLP (Digital Ligth Processing):

Fine and precise printed surfaces (use in the jewelry industry, dental technology, electronics), prototypes quite robust for processing, a variety of resins including bio-medical materials (certified for medical use) and transparent resins (prototypes in the packaging industry) , stable printers with few moving parts. The technology allows the prototyping of complex and detailed geometry pieces, the high speed of printing for complex geometries and the simultaneous printing of multiple pieces (high productivity). Printed parts can be used as master molds for injection molding, thermoforming, metal casting.

Disadvanteges of DLP (Digital Ligth Processing):

Higher building materials, higher 3D printer prices (for large volumes), require post-processing operations (UV curing, removal of support material) require handling of resins.

Applications of DLP (Digital Ligth Processing):

Prototypes resistant to functional testing, prototypes and fine, precise models (jewelry, dental models, electronic models), prototypes with complex geometries; manufacturing of small series of models in medicine (hearing aids, dental restorations, medical implants), prototypes and models in the media (animation, cinema, etc.) casting models of jewelery, tools and tools, parts and components in automotive and aerospace.

3.Experimental research

Experimental research aimed at making a microfluidic element with a 3D Duplicator 7 printer that uses the direct imprinting of the photosensitive resin with UV light transmitted through an LCD screen, Fig. 2.

There are two surprising advantages of this new technology:

a) we can use high-resolution LCD screens at low cost;

b) The energy used by printers using the DLP (Digital Ligth Processing) principle is smaller in size compared to other systems because all available photons hit photoinitiators and surrender their energy to the laser where there is much excess energy, which may lead to overload [4].

Figure 2. 3D printer Duplicator 7 general view.

The Creation Workshop software allows you to view and position the workpiece on the work plane as well as mirroring, scaling (Scale%), and rotating (Rotate), Fig. 3 as well as the possibility of simulating the deposition of the layers during the construction process of the piece, Fig. 4.

Figure 3 Creation Workshop soft-ware.

Figure 4. Layers visualisation.

Technical specifications are show in table.

Figure 5. 3D Duplicator 7 printer: worktank (a) and printed part (b).

While FDM technology produces a mechanical link between layers, DLP (Digital Ligth Processing) technology creates a chemical bond by bonding the photopolymers through the layers, resulting in very dense components (components), the bond is water and air tight, and the resistance does not change depending on the orientation. The experimental results have led to the development of a passive fluid mixing system based on the continuous flow of liquids through microfabricated channels. Liquid flow control is implemented either by external pressure sources, external mechanical pumps or mechanical micro pumps. Precise control and manipulation of fluids that are geometrically constrained thanks to easy-to-use fixed geometry interaction chambers without the possibility of contamination. The microfluidic element achieved has the advantage that the surface is transparent to provide a clear view of the interior as well as a control of correct and stable dosing of liquids in internal channels with practical applications in the design of systems where low fluid volumes are processed; presented in the article were mixed three red and green blue colors [6, 7, 8, 9].

Figure 6. Microfluidic element obtained through DLP (Digital Ligth Processing).

4. Conclusion

Lately, FDM technologies and stereolithography have gained popularity in 3D printing are used to produce high performance products. Because FDM technology is accessible, it is widely used in offices and educational institutions; however, their applications are limited compared to polymer-based polymerization technology where 3D printers are useful for creating better quality products surface. The development of photopolymers with desired properties, such as chemical composition, mechanical composition, bio-compatible materials and materials, will be useful in the creation of complex products and thus increase the application areas. Making resins according to the destination of models made with biocompatible properties when it comes to medical applications or materials with high elasticity when the pieces obtained are used for testing of springs and low melting resins when used in casting with models easily fused, respectively, or making materials for the development of transparent optical devices. Also, using liquid crystal screens similar to those used on portable computers, the CAD software directly controlling the LCD screen interposed between the light source and the polymer surface to be solidified, allows a high speed in the solidification of a layer, the piece will be made being more precise from a dimensional point of view and especially of the precision of shape and position, because the deformations of the piece accompanying the solidification process are no longer present. [1]

The clean and post-treatment of the workpiece is chosen in relation to the characteristics of the material used, regardless of whether it is made with light and / or heat, which must be so chosen that it does not destroy or distort the piece.

5. References

Berce P, Balc N, Caizar C, Pacurar R, Radu A S, Bratean S, Fodorean I 2014 Tehnologii de fabricație prin adăugare de material și aplicațiile lor (Editura Academiei Române, București)

Ramji Pandey 2014 Photopolymers in 3D printing applications (Degree Thesis Plastics Technology)

John T. 2014 Sheridan Photopolymers Materials (Light Sensitive Organic Materials): Characterization and Application to 3D Optical Fabrication and Data Storage

www.wanhao3Dprinter.com

www.3dhubs.com

Yuanyuan Xu, Xiaoyue Wu, Xiao Guo, Bin Kong, Min Zhang, Xiang Qian, Shengli Mi and Wei Sun 2017 The Boom in 3D-Printed Sensor Technology, Sensors 17, 1166; doi:10.3390/s17051166

Bethany C. Gross, Jayda L. Erkal, Sarah Y. Lockwood, Chengpeng Chen and Dana M. Spence Evaluation of 3D Printing and Its Potential Impact on Biotechnology and the Chemical Sciences XXXX American Chemical Society, dx.doi.org/10.1021/ac403397r

3D Printing with Desktop Stereolithography, An Introduction for Professional Users, June 2015, formlabs.com

Rizescu D, Rizescu C I 2017 Experimental Research Relating to the Manufacture of Parts by the Addition of Material Proceedings of the International Conference of Mechatronics and Cyber-Mix Mechatronics (Springer)

Acknowledgement: This work has been funded by University POLITEHNICA of Bucharest, through the “Excellence Research Grants”, Program UPB-GEX 2017. Identify: UPB-GEX2017, Grant no. 48/25.09.2017, ME 14-17-05, ID98.