Metal Fiber Technology
IntraMicron, Inc. bundle draws corrosion resistant alloys (i.e. nickel alloys, stainless steel alloys, Nichrome, titanium, copper, brass, bronze, etc.) to extremely small diameters. IntraMicron's fibers are typically chopped to specified length then shipped clean and dry (without PVA or other sizing). All metal fibers are custom made from a customer specified alloy then drawn to a customer specified diameter and length. Bundle drawing is more economical than single filament wire drawing for smaller diameters, but not as economical as other processes for diameters larger than 50 micron (0.0020"). So the upper bound of the process's size range (35 micron) does not indicate limited ability, but rather superiority to competing processes. The lower size bound is somewhat flexible depending on alloy choice and varies according to the corrosion resistance of the selected alloy. Extremely small diameters are formed from alloys with the highest resistance to corrosion (i.e. Nichrome, stainless steel, nickel).
The majority of the 16,000 sq. ft. IntraMicron facility in Auburn is dedicated to the production of fibers. The process utilizes seven wire drawing machines, two strand annealers, a rolling mill and wire choppers to produce the composite wire bundle that contains the micron-diameter fibers. After drawing the bundle to size, each filament inside the bundle will have the same effective diameter (but not the identical shape). As a result of this unique bundle geometry, IntraMicron's fibers have greater surface area and more flexibility than round or hexagonal fibers of the same effective diameter. For most applications, this additional surface area and increased width to thickness ration (aspect ratio) aids filtration, fiber bonding, and resin contacting.
BUNDLE DRAWING TECHNOLOGY
WHAT IS BUNDLE DRAWING?
The bundle drawing process starts by the cladding a piece of solid wire (core-wire) with a sacrificial alloy (typically low-carbon steel or copper). This process is done mechanically using a rolling mill to wrap the cladding (thin metal strip) around the solid wire. This process produces spools of round "composite wire", which are referred to as a "single-sheath" or "one-sheath" product.
Since the fibers are chopped while in the composite form, the ends of these fibers are protected from "hooking" which happens when fibers are chopped in tow form. After chopping, the bundling is removed, releasing the fibers, which are subsequently rinsed and dried. Since IntraMicron's fibers are chemically processed in the chopped state, the process is suitable to make nickel alloys economically. Other processes that removed the bundling alloy in the un-chopped state only function cost effectively for a smaller selection of alloys and suffer from high chopping costs. Removal of the bundling alloy or sheath (usually iron or copper) is made possible by the relative ease of dissolving the sheath in acid compared to the fibers. For this reason IntraMicron's process cannot produce fibers made from steel alloys (only stainless steel or other corrosion resistant alloys).
Once the fibers have been produced, they may be dispersed in fluid to form a nonwoven (paper-like structure) or conductive resin. Other products may also be produced from metal fibers without dispersing it in a liquid. These nonwovens may utilize catalysts and sorbents for enhanced performance at the molecular level. Although IntraMicron's Fibers are used for mechanical filtration, most of the company's research is focused on using microfibrous products to enhance molecular level processes.
Other Fiber Technologies
Single filament wire drawing:
The preferred method for sizes larger than about 0.004" (100 micron) in diameter. Of all the processes that produce small fibers, this is the only one that is truly capable of producing truly round fibers. As the wire is drawn increasingly smaller, it is drawn faster, bu the speed increase does not keep pace with the reduction in cross sectional area (proportional to mass flow in lb/hr). As a result, smaller diameter fibers require greater efforts in terms of machine time and labor. At some point competing technologies become more cost effective. Micron-diameter single filament wire is drawn for medical applications but in general is too expensive for most applications, exceeding $1,000,000 / lb as small diameters are approached.
Shaved wire or fiber (steel wool):
This is commonly produced in sizes ranging from 50 micron to 120+ micron (some alloys are available in smaller sizes). Because the fibers are shaved, they are bent and not at all close to round. Also, chopping the fibers to a specifically controlled length would be extremely difficult at best. However, this process produces some of the cheapest fibers in this size range. Because of the nature of the process, the fibers will likely contain lubricants and have random lengths.
Melt Meniscus:
Fibers and wool are produced by rapid solidifcation of molten alloys. This is best for 100 microns (0.004") and larger diameter fibers, but smaller sizes may be possible.
Grown Metal Crystals:
IntraMicron knows little about this process, and it appears to have been abandoned. In the past, National Standard Co. grew nickel crystals in a drop tower on carbon fibers, while others may have produced these using electroplating.