Chapter 9. Microstructured Mold Inserts for Hot Embossing

For every replication process a mold or master is necessary to copy the structures of the mold into a molding material. Because of the different structuring methods of classical mechanical mold fabrication in the macroscopic range and the structuring methods of microstructures, the mold has to be split into the tool (Chapter 8) and the mold insert with a microstructured surface. In theory every microstructured surface can be used as a mold insert. The precondition is that the mold material and the microstructures will withstand the temperature and mechanical load during molding. Nevertheless, for successful molding and especially demolding, the mold insert has to fulfill the following requirements.

• The yield stress of the mold material at maximum molding temperature has to be significantly higher than the load effected by the molding force.
• To avoid any bending and to guarantee the best possible evenness of the mold, the residual stress inside the mold, caused by the fabrication process, should be reduced to a minimum.
• The mold material should show chemical resistance to the polymer.
• A high heat conductivity of the mold material will reduce heating and cooling times.
• For cost effectiveness, the lifetime of the mold should be extended over many cycles.
• To support successful demolding, the surface roughness, especially of vertical sidewalls, should be reduced to an unavoidable minimum.
• Demolding angles are advantageous because they facilitate demolding. In contrast, undercuts prevent successful demolding of microstructures. Even small undercuts in the submicron range can increase demolding forces significantly (Section 6.4.5).

Regarding the requirements, especially the requirement of high yield stress, it is obvious that mold inserts fabricated in metals are well suited. The technique of microstructuring of metals is therefore essential for mold fabrication. But also glass or polymers like UV-transparent PDMS or high-temperature resistant PEEK can be used for selected replication tasks. Nevertheless, regarding the lifetime of a mold insert and the high stiffness, molds fabricated of metals are mostly used for replication. Therefore, this section of mold insert fabrication is focused on the fabrication technology and the properties of metal mold inserts.

An overview of the different mold fabrication processes is shown in Fig 9.1.

Figure 9.1. Overview of the mold fabrication processes. The processes are split into direct structuring methods, lithographic methods, and alternative methods like etching processes or PDMS casting.

The structuring processes can be split off into two groups: (1) direct structuring methods, like mechanical machining, electric discharge machining (EDM) or laser structuring [7]; and (2) the field of lithographic methods, like E-beam lithography, UV-lithography, and for structures with high aspect ratio, X-ray lithography. All lithographic processes require the step of electroforming to obtain a metal mold insert. Each structuring method has different characteristics and is therefore suited for different kinds of applications.