*3.1.2 Laser machining*

The fabrication of a metallic mold insert is generally time consuming and expensive. Laser machining seems to be more competitive than other mechanical machining techniques. It allows the rapid fabrication of micro structure with a feature size of several micrometers at an aspect ratio of ~10 [46]. However, due to the limitation of spot size, making high precision micro mold insert with tight tolerance by direct laser micromachining is challenging. The minimum spot size is usually half wavelength of the light utilized [47]. In this context, the laser LIGA technology combining laser ablation and electroforming process is developed to produce precision mold insert with 3D structures, where laser ablation is applied to form polymer stencil for subsequent metallic replication by electroforming to fabricate nickel micro mold insert. **Figure 8** shows the microstructure metal mold insert and polymer mold insert fabricated by laser machining and laser ablation, respectively.

**41**

**Figure 9.**

*Prototyping and Production of Polymeric Microfluidic Chip*

Of course, when micro mold insert with 3D structures is used for microinjection molding of the microfluidic chip, the use of conventional methods often brings

*(a-d) Micropatterns on metal mold insert fabricated by laser machining [48]; (e) Polymer mold insert fabricated by laser ablation [49]. Copyright (2004) with permission from Royal Society of Chemistry.*

The μEDM technology is a non-conventional electrochemical processing method.

The mold insert is fabricated based on the material removal mechanism with the erosive effect of the electrical discharges that happen between electrode and workpiece. The electrode and workpiece are placed with a small enough gap so that the voltage–current system can efficiently ionize the dielectric. The shape and dimension of the processed workpiece are determined by the electrode pattern. Micro mold insert with the feature size of ~20 μm can be machined by μEDM technology but the forming accuracy and surface roughness are not able to be guaranteed. **Figure 9** demonstrates

*Stainless steel mold insert fabricated by μEDM technology: graphite electrode with better geometry definition on micro-patterns (a); copper electrode with non-uniform edge finish (c); the crater size of both electrodes ((c)* 

*and (d)) [33]. Copyright (2015) with permission from IOP publishing, LTD.*

*DOI: http://dx.doi.org/10.5772/intechopen.96355*

challenges to microstructure demoulding.

**Figure 8.**

*3.1.3 Micro electrical discharge machining (μEDM)*

**Figure 8.**

*Advances in Microfluidics and Nanofluids*

feature size of several micrometers to hundreds of micrometers, although the reachable aspect ratio of this technique is limited in the range of ~20 [34]. Considering the specific microfluidic applications, UV-LIGA is sufficient to fabricate the desired micro structures. Additionally, some LIGA-like processes, such as EUV-LIGA, EBL-LIGA, IB-LIGA, are also developed towards nanoscale micro structuring [44]. However, these LIGA-like techniques still have some shortcomings for mold insert fabrication. First, the mold materials usually are nickel and nickel alloy, the tool steel is not included due to the restriction from the electroforming process. Besides, the integration of draft angle onto mold insert is not easy, but it is critical for demoulding of the polymeric part from mold insert to reduce the potential demoulding deformation and damages of micro structures [45]. The research shows that the draft angle can be achieved by properly adjusting the UV/X-rays

*Basic schemes of manufacturing of mold insert.: (a) metal substrate, lithography, and electroforming; (b) a silicon substrate, lithography, and electroforming; (c) deep reactive ion etching of silicon (d) micro-machining* 

*of non-silicon substrates [9]. Copyright (2020) with permission from IOP publishing, LTD.*

The fabrication of a metallic mold insert is generally time consuming and expensive. Laser machining seems to be more competitive than other mechanical machining techniques. It allows the rapid fabrication of micro structure with a feature size of several micrometers at an aspect ratio of ~10 [46]. However, due to the limitation of spot size, making high precision micro mold insert with tight tolerance by direct laser micromachining is challenging. The minimum spot size is usually half wavelength of the light utilized [47]. In this context, the laser LIGA technology combining laser ablation and electroforming process is developed to produce precision mold insert with 3D structures, where laser ablation is applied to form polymer stencil for subsequent metallic replication by electroforming to fabricate nickel micro mold insert. **Figure 8** shows the microstructure metal mold insert and polymer mold insert fabricated by laser machining and laser ablation, respectively.

**40**

exposure dose.

**Figure 7.**

*3.1.2 Laser machining*

*(a-d) Micropatterns on metal mold insert fabricated by laser machining [48]; (e) Polymer mold insert fabricated by laser ablation [49]. Copyright (2004) with permission from Royal Society of Chemistry.*

Of course, when micro mold insert with 3D structures is used for microinjection molding of the microfluidic chip, the use of conventional methods often brings challenges to microstructure demoulding.
