Bundle Drawing Technology
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.
After application of the initial cladding, the single-sheath composite wire is annealed in an inert or reducing atmosphere to soften the composite wire and prepare it for drawing. Wire drawing is the process of pulling the wire through drawing dies (typically made of diamond or carbide). As the wire passes through each die, it is made ~10 to 20% smaller in cross sectional area while it becomes longer by the same proportion. The initial dies can be more aggressive in their reduction than subsequent dies because the wire gets progressively harder as it passes through each die. IntraMicron uses a cold fiber drawing process, which prevents the manufacture of hot-draw alloys like tungsten.
IntraMicron typically draws each composite wire through 20 to 40 dies per cladding. Each die must be threaded by hand, but once threaded, the drawing machines will pull the wire down to size. Eventually the wire must be further annealed to continue size reduction. The annealing frequency is alloy dependent, and is inversely proportional to ductility. More ductile alloys like copper, nickel, and Nichrome require fewer anneals to achieve a given size than less ductile alloys like stainless steel and aluminum.
After the single-sheath composite wire is drawn to size on many spools, the wire is bundled together inside a second cladding. This second cladding typically contains 40 to 470 wires, and the wire count depends on the final fiber diameter. For smaller sizes larger wire counts are utilized. This "two-sheath" composite bundle is drawn down to smaller sizes just as the single-sheath was.
If the desired fiber size is less than four microns, then multiple spools of the two-sheath wire will be bundled a second time to form "three-sheath" material. This material will be drawn to the final size in a similar fashion to the other composite wire. Typical three-sheath wire counts would be in the range of 2,000 to 15,000.
The cost advantage of bundle drawing is that one machine and one person can reduce hundreds or thousands of filaments to the desired size all at once and at a reasonable rate (lbs / hour). Although there is considerable cost in the cladding application and removal, it is considerably more cost effective for sizes below 50 micron.