Production methods for nano/micro-scale features on molds
We have taken a multi-faceted approach in this research endeavor as the field has yet to settle on a particular material or production method. In fact, current literature is replete with tales of very poor mold performance, often citing lifetimes of less than ten parts.
The key problem is the use of silicon based tooling typically produced using Deep Reactive Ion Etching (DRIE). Given a proper mask, such tools have excellent dimensional fidelity and are rather easy to create. However, the resulting tool is plagued by brittleness, and poor wear resistance. As larger tools are anticipated (to increase through put as well as to make larger parts), this brittleness can be fatal, both from the effect of differential thermal expansion and from demolding stresses, as shown in Figure. 1.
Figure 1. Failed silicon tool from a normal PMMA embossing cycle. Failure occurred during demolding at 50C.
We have begun to investigate a number of alternatives to silicon tools including: Powder metal injection molded tools for embossing, Micromachined glassy metals, Electroformed nickel tools, Polymer tools from low surface energy materials.
In each case the goal is to create tools with long lifetimes and good shape fidelity. The metal tools achieve this, in general, by having superior toughness while the polymer tools tend to minimize the demolding stresses attendant with surface energy and differential shrinkage.
We have begun a comprehensive study of the basic factors involved in demolding stresses, starting with surface energies of the mating materials. We have conducted preliminary tests of very flat parts in order to compare to theoretical adhesions limits. Results so far indicate the actual adhesion forces are far below the theoretical, most likely owing to lack of cleanliness and imperfect surface mating. An initial study of the thermal effects has shown that differential shrinkage may indeed dominate the demolding forces particularly for regions of the part that are far from the centroid of the mold (see Figure. 2).
Figure 2. Evidence of damage to a channel from large differential shrinkage of PMMA against a silicon model. The part was formed at 120C and cooled to 50C before demolding. Not that this feature was on the periphery of the tool, far from the centroid.
Initial trials with polymer tools were reviewed under task one. We explored the use of silicon masters in a conventional (albeit high temperature) embossing operation. So far we have successfully produced tools from PEEK, Polycarbonate and COP polymers. Initial results (see e.g. Figure. 3) are encouraging, and may lead to a significant breakthrough in the concept of embossing tooling.
Figure 3. Profiles of specimens. PEEK working tool, PC working tool. Working tool profiles have been inverted for comparison.
A “first” metal powder tool was recently produced. It involved injection molding against a silicon master with 2-micron nickel powder in a polymer binder. After sintering, the part shows good shape fidelity, but significant shrinkage was evident. Work is now underway to characterize the shrinkage and resulting surface finish.
We have also explored the use of electroformed tools, and recently created and tested such a tool (see Figure 4). This too looks like a promising technique and will be investigated further with respect to tool toughness, shape fidelity and dimensional uniformity.
Figure 4. Electroformed test tool (top) and resulting test part.
We have initiated a study exploring the use of cast or hot embossed amorphous metals to create high strength, low wear tools. Such super-plastic forming of these materials is not well researched, and we have started with a study of the basic processing characteristics. In particular, we are looking at stress strain relationships at elevated temperatures typical of those used in forming of these materials and the formation of some simple test channels by embossing with a silicon master. We believe this material and methods of forming has great promise as a low cost, long life tooling production method.
A second-generation set of tools have been fabricated from a Nickel-Cobalt alloy, using a combination of lithography and electro-forming techniques. (Figure 5.) Electroformed Nickel-Cobalt alloy tool has high hardness, stiffness and smooth surface due to very fine grain sizes and should be suitable for mass manufacturing of microfluidic devices by hot-embossing. The performance of the Ni-Co hot-embossing dies has been evaluated. Although numerous embossing experiments can be conducted with a single die, the feature sizes are currently limited to 50 microns in height, and as such are not suitable in the long run for manufacturing
Figure 5. Tool fabrication by the lithography - electroforming process.