Lego blocks are awesome—they snap together easily and perfectly. These blocks are made using injection molding and wouldn’t fit so flawless together without the precise features of the metal molds used. Currently, most injection molding templates are made from steel because of their strength and durability. But the steel molds are typically limited to features that are about 10 micrometers in size.
So researchers at the University College Dublin in Ireland have turn to an alterative material—bulk metallic glasses—to make molds with smaller feature sizes, in the micro to nanometer range. Nan Zhang and his colleagues present a prototype of their new mold in this month’s Materials Today.
Bulk metallic glasses (BMGs) are a new class of man-made materials and have a disordered, amorphous structure meaning atoms are lined up only for very short distances. This is unlike traditional metallic materials such as iron or nickel which are crystalline, where the iron or nickel atoms are arranged in a regular and periodic manner.
The amorphous nature of BMGs allows them to be patterned and machined to dimensions not easily done with conventional metals. Conventional metals go through volume shrinkage (~5%) due to crystallization, making direct casting to form small surface features difficult. But with BMGs it is possible to make robust, multi-scale tools (with features in the centimeter to nanometer range) at high-volumes and low cost.
The team chose a BMG with the composition Zr47Cu45Al8 to make their injection-molding template. The features on the template were machined using focused ion beam (FIB) milling with a gallium ion beam. The patterns created on the BMG template was then replicated onto a polymer (HDPE) using injection molding. Figure 1 shows a BMG template with features as small as 100 nanometers and the subsequent HDPE sample that was molded with the replicated features.
Now you’ve seen what the final product looks like, I just wanted to quickly mention how FIB milling was used to make the pattern on the BMG templates (because the process is pretty cool!)
The researchers used a FIB microscope to image and pattern the BMG templates. A FIB microscope is both a high-resolution microscope and a milling machine all in one. It is like a scanning electron microscope, except an ion beam (usually gallium ions) rather than electron beam is used. It’s a powerful tool because of its imaging and machining capabilities at the micrometer to nanometer length scale.
The image of the entire sample is displayed on a computer monitor in real time, and you can zoom in to identify which part of the sample you would like to remove. Then, the computer guides the gallium ion beam to precisely mill away the area that is selected—leaving behind the patterned BMG template in this case.
This is unrelated research by the Roma Tre University in Italy, but their video gives an idea of how an FIB works. The video captures how an FIB is used to cut out a thin foil from a bulk piece of metal. In the video you see in the how the computer highlights the area to be milled away. The milling part is done about 3 minutes into the video, and the foil is subsequently attached onto a probe and transferred onto a sample holder (“TEM grid”) for further analysis in another electron microscope.
All right, now back to the original research. The team’s molds were used in over 20,000 cycles. However replication of the micro- and nano-features on the polymer component is influenced by the surface roughness of the mold. The starting BMG template (in Fig. 2) showed surface flaws from the initial casting process, resulting in surface roughness in the final molded polymer. The surface flaws remained on the BMG template despite attempts to remove them using conventional polishing methods.
The team concludes their new metallic glass sub-micron injection molding technique needs to be optimized for the processing conditions (and to eliminate surface flaws), but it should allow for high-volume micro- and nano-fabrication of polymers in the future.
Zhang, N., Byrne, C., Browne, D., & Gilchrist, M. (2012). Towards nano-injection molding Materials Today, 15 (5), 216-221 DOI: 10.1016/S1369-7021(12)70092-5
Featured photo credit: Wikipedia user Arne Hückelheim