Chapter 8. Hot Embossing Tools

Apart from the molding press (Chapter 7) and the microstructured mold insert (Chapter 9), hot embossing tools are essential components of any hot embossing system. The hot embossing tool may be defined as an interface between the molding press that is responsible for applying the molding force and molding velocity and the microstructured mold insert to be replicated in polymers. Compared to macroscopic molding tools, such as tools for injection molding, where the structures are part of the tool, tools for micro replication are characterized by a reversible integration of a microstructured mold insert. This concept results from the different and incompatible fabrication processes of the macroscopic tool and microscopic structures. The tasks of a hot embossing tool are similar to tasks known from macroscopic molding tools, on the one hand. On the other hand, the embossing tool has to fulfill tasks that are specific to the molding of microstructures, like the generation of a vacuum. In detail, a hot embossing tool has to fulfill the following tasks.

• heating and cooling of the polymer film by heat conduction of the mold insert and substrate plate
• fixation of different kinds and sizes of mold inserts
• hermetic sealing of the mold insert, polymer, and substrate plate against ambient pressure
• generation of vacuum to fill the microcavities completely
• a demolding unit, which allows the vertical demolding of the embossed parts at a controlled demolding velocity
• optional alignment of both mold halves, if double-sided molding or positioned molding is desired

From these tasks, the requirements to be met by a molding tool can be deduced, including minimum requirements for molding and optional requirements for specific tasks.

8.1. Requirements on Hot Embossing Tools

• To achieve reproducible molding results over the molding area, such as a homogeneous viscosity for the complete filling of microcavities and a uniform thickness of the residual layer, a constant temperature distribution over the molding area is required. Heating of the polymer film up to the melting state is characterized by heat transfer from the mold insert and substrate plate and may be achieved, for instance, by heat flow from electrical heating elements. The heat flow that finally arrives at the surface of the mold insert or the substrate plate depends on the heat conductivity of the materials and especially on the heat transfer between the different components of the tool. Here, a good contact of the components is an important factor in determining the quality of the temperature distribution in the mold insert and also the heating time. Good heat transfer can be achieved by a high degree of evenness of the contact surfaces and avoiding any gaps in between.
• This homogeneous temperature distribution is also crucial to demolding, where inhomogeneous temperature distributions will cause anisotropy of shrinkage. Typically, convective systems are applied for cooling. For homogeneous temperature distribution, an arrangement of cooling channels is needed, which ideally guarantees a uniform heat flow over the molding area. Here, the increase in temperature of the coolant during cooling has to be taken into account (Section 8.2.2).
• Integration of mold inserts should be characterized by high flexibility and the use of uncomplicated fixation systems that allow for a quick exchange of the mold inserts. As different kinds of mold inserts are used (Chapter 9), a system is required that allows one to fix, for example, LIGA molds with a thickness of several millimeters and, with some modifications, also thin nickel shims with a thickness of 300 μm. In this respect, the creativity of construction engineers is needed. Furthermore, integration has to guarantee good mechanical contact with the heating and cooling unit to provide for an excellent heat transfer, which is fundamental to reaching adequate cycle times.
• To fill microcavities completely, vacuum is needed. Therefore, a sealing of the mold insert, polymer, and substrate plate against ambient pressure is required. To enable molding and demolding, the sealing has to ensure a relative motion between mold insert and substrate plate of several millimeters after vacuum has been established.
• A challenging task is the demolding of microstructured parts. The tool requires demolding units that, irrespective of the kind of mold insert, guarantee a demolding velocity in the range of 1 mm/min or below in a vertical direction over a distance of several millimeters under vacuum. This vertical motion is required to prevent any perpendicular forces from acting on the filigree structures. Otherwise, the risk of deformation or damage would increase significantly.
• If double-sided molding is required, the tool has to provide for a relative positioning between mold insert and substrate plate or, alternatively, a second mold. Depending on the structure size, an overlay accuracy in the range lower than 10 μm is desired. To achieve this overlay accuracy, an alignment with three degrees of freedom has to be taken into account, motion in a lateral direction (x, y), and one rotation around the center axis of the mold insert.
• An optional requirement is the compensation of uneven mold inserts. To compensate wedge-shaped mold inserts, a mechanism should be integrated to equalize these differences in evenness. With such a mechanism, a nearly homogeneous thickness of the residual layer will be achieved, which may be fundamental to further processing steps of molded parts.