Chapter 3. Molding Materials for Hot Embossing

Molding processes, also hot embossing, cannot be described without the fundamentals of the molding materials. Especially topics like thermal behavior, flow behavior, shrinkage, and warpage and their mathematical descriptions are important issues for an analysis of molding and demolding. The objective of this chapter is therefore to give the reader a basic knowledge of the behavior of molding materials. Most applicable for hot embossing are thermoplastic polymers. Thus, the priority of this chapter is in the characterization of thermoplastic polymers, with the focus on topics relevant for molding. Nevertheless, alternative molding materials like glass, metals, or ceramics will also be discussed. In detail the following aspects will be described:

• A description of suitable materials for replication will be presented. A distinction between unconventional materials and polymer materials shows the bandwidth of molding materials suitable for hot embossing.
• The rheologic behavior of polymer melts determines the flow behavior in the residual layer and during mold filling, and it is therefore an essential issue in molding processes. The stationary flow based on shear and strain, the viscoelastic behavior of the melt, and the modeling are also topics like the molecular orientation and the stress relaxation.
• During cooling the polymer behavior is characterized by solidification. The process parameters in this step decide the orientation and crystallinity of molded parts. But also stress, shrinkage, and warpage of the molded parts are influenced by the process parameters.
• The behavior of solid polymers especially near the softening range is an important factor to understand the behavior of polymers during demolding. Aspects of viscoelasticity will also be discussed, like the stress strain behavior at different temperatures, the creeping of polymers, and the measurement by dynamic mechanical analysis. An important point is also the friction between mold and polymer. High values of friction force are one of the reasons for the damage of structures during demolding.
• The thermal behavior of thermoplastic polymers, based on the aggregate states, already defines molding windows. An analysis of the thermal behavior will give a first estimation about the suitable molding and demolding temperatures for amorphous and semicrystalline polymers and will allow further the specification of the bandwidth of molding temperatures—here, called molding windows.
• Finally, the measurement of the thermal behavior of polymers will be presented, with the aim to measure all necessary data for the further analysis and modeling of the material behavior. This section of the chapter also gives an overview of the most important thermal properties of polymers and discusses the affiliated measurement methods.