We use indium wire for creating cryogenic seals in the HYPER lab. We buy most of our wire from Indium Wire Extrusion (IWE). Indium is expensive however, quoted by IWE at $180/oz. Rather than selling the wire back for half its original price, we like to recycle our indium by melting it down and re-extruding it into usable indium wire. Indium’s low melting point of 156.6 °C (313.9 °F) and softness (Mohs hardness 1.2) make this process cheap and easy. We have the capability of extruding 0.0625 inch (1.5875 mm) and 0.1 inch (2.54 mm) diameter indium wire with the equipment we have in the lab.
The following article provides an easy-to-follow guide to the HYPER lab’s indium extrusion process. The hope is that this guide will serve as training for those working in the HYPER lab, as well as a good starting point for others who would wish to extrude indium for cryogenic applications.
- Hydraulic extruder
- Hot plate capable of at least 200 °C (the higher the better)
- Flat copper plate
- Puck cast mold sized for extrusion chamber
- Indium scrap
- Empty spool
- Metal bin with 2-3 inches water for quenching
- 3/8″ Hex key
- Metal shaft with diameter smaller than puck diameter
- Slip-joint adjustable pliers
- Retractable blade knife
- Large pipe wrench
- 2 wood or metal blocks of the same size
- Gather indium scrap. Make sure scrap pieces are small enough to fit into puck mold.
- Place copper plate onto hot plate. Place puck mold onto copper plate.
- Heat the hot plate to around 250 °C (or higher for faster melt).
- Put indium scrap into mold and wait for indium to melt. For faster melting, push indium down into the mold.
- While waiting for indium to melt, extruder can be assembled. The assembly consists of three parts: die, die holder, and extrusion chamber. Begin by placing the die into the die holder.
- Screw the extrusion chamber into the die holder.
- Bolt the assembly onto the end block of the hydraulic extruder.
- Once indium has begun to melt, it will fill the mold cavity. At this point, add more indium scrap until the mold is full.
- Using the adjustable pliers, grab the hot copper plate (with mold on top) and dip plate, mold, and indium into the water to quickly cool the assembly. You may now turn off the hot plate.
- Once water has stopped boiling and making sure all parts of the assembly have been cooled, take the assembly back out of the water.
- Placing the mold with indium stuck inside it onto two wood or metal blocks, use a hammer and shaft to knock the indium out of the mold.
- Cut off impurities using the retractable blade knife.
- Place indium puck into extruding chamber of extruder. Place the metal spacer behind it.
- Turn on the hydraulic extruder by opening the compressed air valve and set it to ‘EXTEND.’ Then get ready on the output end of the extruder with the empty spool. It is best to have two people for this task: one controlling the extruder, one spooling.
- Utilize the pressure regulator to modulate the speed of the extruder piston. This can be increased during free float piston motion and modulated during extrusion to achieve desired spooling speed.
- Wait until the plunger reaches the indium and indium wire begins to emanate from the output end of the extruder. Be prepared, since wire will come out at high speed depending on the pressure regulator settings.
- As wire emanates from the output end, wrap it around the spool while keeping a small amount of tension on the extruding indium.
- When you hear the hydraulics begin working harder than they were initially, set the extruder to ‘RETRACT.’ When you hear this sound, the plunger is trying to extrude the metal spacer.
- Break the end of the indium wire away from the extrusion outlet. You should now have a spool of indium wire.
- Disassemble the extrusion assembly in order to remove the metal plunger. At this point a large pipe wrench will likely be necessary to unscrew the extrusion chamber from the end block. The remaining indium can be left in the extrusion die until the next extrusion.