3.1.2.1. Hot Embossing of Metals
Compared to the molding temperatures of glass, the required molding temperatures of metals often increase significantly. Therefore, the metals that can be molded at moderate temperatures are limited. Cao et al. systematically analyzed the hot embossing of lead [2] and aluminum [1].
Hot Embossing of Lead
A major topic of molding high-temperature materials is the chemical and mechanical interaction of the molding material with the mold insert, in this case an electroplated nickel mold insert. The interaction can result in high adhesion or bonding and can damage the mold insert and the molded part during demolding. Cao et al. demonstrated that between lead and nickel mold inserts no chemical interactions occurs. Therefore, a surface modification of a nickel mold insert is not necessary.
Independent of the melting temperature of lead of 327°C and the soft material, behavior results in a wide range of molding temperatures between 100°C and 300°C. The replication of structure details increases with the increasing molding temperature. In experiments [2] nickel mold inserts with 585 nickel micro pots with 150 μm diameter, a depth of 400 μm, and a spacing of 650 μm were replicated in lead plates (99.9%) in the form of circular discs with a 3.5 mm diameter and a thickness of around 0.6 mm. Already at 118°C and an acting force of around 780 N on the molding area, an increase of temperature in the range of 120°C up to 270°C improves the geometric details of the replicated features. The systematic experiments and the analyzed molding results demonstrate that the contact stress during molding and demolding is well below the yield stress of electroplated nickel-mold inserts. The experiments demonstrate that sharp microscale features can be replicated without degradation of the nickel-mold insert after repeated molding runs.
Hot Embossing of Aluminum
In contrast to the molding of lead, the molding of aluminum is more challenging regarding the molding temperature and especially the interaction of nickel-mold inserts with aluminum. Electroplated pure nickel molds are not suitable for molding aluminum. Because of the strong driving force of nickel and aluminum to form intermetallic compounds, a barrier between both components is required. This barrier can be implemented by a temperature-resistant coating. Cao and Meng [1] used a ceramic coating to modify the mold insert surface. By vapor deposition a Ti-containing hydrocarbon (Ti-C:H) layer was deposited.
Already below the melting temperature of aluminum of 660°C, microstructures can be replicated by hot embossing. Similar to the molding of lead, Cao and Meng [1] replicated a squared array of cylindrical micro pots with a height of 400 μm and diameter of 200 μm on an area of 1.8 × 1.8 cm2 on an aluminum (99.9%) substrate in the form of circular discs with a 3.5 mm diameter and a thickness of around 0.6 mm. To compare the influence of coated and uncoated mold inserts, two mold inserts with and without coating were replicated, with the result that the structures from the uncoated mold insert could not be demolded successfully. The molding temperature was set in a range between 433°C and 460°C, with the molding force up to 1,700 N. It was found that the maximum compressive stress of 18 M Pa at 450°C was sufficient to mold and demold the microstructures without damaging the nickel structures on the mold insert and the corresponding molded aluminum structures.
An important aspect is the longtime stability of the coating used. The Ti-C:H coatings are deposited at 250°C and may suffer hydrogen desorption at higher molding temperatures. The recommended molding temperature for aluminum will, therefore, degrade the coating with the number of molding steps. This degradation increases if the molding temperature increases in the range of 500°C or higher. The loss of hydrogen and the graphitization of the coating used reduce the durability and the lifetime of the coating. The molding of aluminum is, therefore, a function of the coatings and their longtime stability.
Metallic Glass
Independent of the molding of metals, the molding of metallic glass has been investigated. Pan et al. [13] investigated a thermoplastic forming process of bulk metallic glass (Mg58Cu31Y11), in theory by a simulation with commercial simulation software, and verified the study by hot embossing experiments. Due to the moderate molding temperature of 140–150°C, the molding is quite different from the molding of conventional metals or glass. The hot embossing experiments referred to the replication of an Ni–Co electroplated mold insert with a micro lens array. The height of the structures was 14 μm and the diameter 330 μm. Systematic measurements and experiments demonstrated the hot embossing capability of this material class. It could be found that under molding conditions, the shrinkage of the molded parts (0.61%) was about 10 times higher than the measured shrinkage of PMMA (0.06%).
3.1.2.2. Micro Powder Molding
Micro powder (injection) molding (PIM) of metals or ceramics is an alternative to fabricate structures of these materials in large series, especially in the micron range. This method is well established in replication by injection molding and metal injection molding (MIM) or ceramic injection molding (CIM).
Fine metal or ceramic powder is mixed with a binder system into a feedstock and injected into a mold containing a microstructured mold insert. After demolding, the so-called green compact is processed further in the furnace, mostly under an oxidizing atmosphere, to remove the binder system, thus changing into a brown compact. In this step, the binder is removed and the first contact spots between the powder particles are produced. Subsequently, in a defined atmosphere, the brown compact is sintered into a solid microstructure of close-to-theoretical density [16].
Typically, materials for metal powder are, for example, carbonyl iron powder, stainless steel (316L), or alloyed hardenable steel (17-4PH). Typically, ceramic powders are aluminum oxide powder, ZrO2-strengthened Al2O3, or zirconium oxide powder. The mean particle sizes are typically in a range of 0.3 μm up to 5 μm (e.g., iron powder with a mean particle size of 4–5 μm, aluminum oxide powder with a mean particle diameter of 0.6 μm, zirconium oxide powder with a mean particle diameter of 0.3–0.4 μm). The binders are commercially available and consist of a polyolefin/wax compound or a polyacetal-based system [14,17].
Depending on the particle shape and the particle size distribution, the feedstock typically contains 50 vol% up to 60 vol% of metal powder. Because of the high porosity, the linear shrinkage of a molded part after sintering is in a range of 15% up to 22%. The precision that can achieved by these materials is about ±0.3% compared to the original shape [6]. The particle size also determines the quality of the molded part, especially the surface roughness. But also the filling of filigree cavities depends on the particle size. For micro features, the average particle size should be one or two magnitudes smaller than the structural details of a design. This can limit the minimal achievable structure size. The replication of microstructures therefore requires very fine powders in the submicron range. Further, the temperature of the binder determines the molding and demolding behavior of the feedstock. To achieve a successful molding of microstructures, a variotherm molding cycle is required [6]. Especially the demolding behavior and the possible damage to the molded part correspond to the strength of the feedstock after injection and cooling. Fu et al. [5] analyzed the demolding behavior in the case of micro metal injection molding. This analytic approach can be transferred also to the demolding of polymers.
Nevertheless, these materials are successfully implemented for injection molding but are also suitable for hot embossing. By injection molding, a pillar array with pillars of 100 μm [10] or 60 μm [4] have been replicated.