Machining

POCOFoam and POCO HTC can be cut and shaped using a variety of conventional machining techniques. The following information describes the general parameters needed to create parts from POCOFoam or a porous graphite (POCO HTC).

Although graphite foam does not have sufficient strength to withstand the machining stresses, porous graphite does have sufficient strength to withstand normal machining stresses. The same care must be taken during the fabrication process for both materials. Taking a light final skim cut on thin sections with limited support does not work well. Where practical, relatively large cuts should be made to finish size in one pass.

Tool loading by milling cutters has not posed a problem.

Cup grinding can take a large depth of cut easily and leaves a relatively clean surface. Loading of the wheel does not occur.

Turning

• Suggest 500 RPM, feedrate 0.007 IPR and 0.05" depth of cut (no finish pass).
• Large radius inserts.
• Boring can hold +/-0.003.
• May be threaded, but no longer than one times the diameter.
• Small diameters work best from the bottom of the billet in the "Z" direction.

Milling

• Suggest 3000 RPM, feedrate 0.010" cut to size (no finish pass).
• May be thread milled or tapped (min. 1/4 NC)
• Boring can hold +/-0.003".
• Two fluted mills work best.
• Slitting saws allow increased feed rate (100 IPM)
• Max. Slot depth: 1" with 1/16" wall thickness.
• Preferred to hold width/length/height on a bandsaw (+/-0.02")
• Minimum thickness for 10" x 10" plate: 1/4"
• Minimum thickness for 0.5" x 0.5" plates: 0.1"

Fixturing

Due to the low strength of these materials, they are easily crushed. However, parts with moderate areas can be carefully clamped in a vise.

Smaller parts need the use of P.E.G. (poly-ethylene-glycol) to "wax down" the parts to the graphite backing plates. The recommended procedure is to preheat the material and backing plate to 130-140°F, put a small amount of wax on the backing plate then apply the graphite part. When assembling the material and the backing plate on a heat sink, an additional cold heat sink on the foam will ensure the wax sets quickly. Due to the porous nature of these materials, excess wax does not usually cause a thicker bone line; instead it flows out of the bond area or into the pores. Testing has not been completed to ensure that PEG does not cause reduction in material conductivity as it solidifies in the pores.

Removal of waxed parts from backing plates can be done in an oven set to 460-500°F for 10 hrs. to ensure complete removal of the PEG. Waxed parts can also be separated by immersion in hot (160°F) water. Note that water does not readily wet either material. These materials are less dense than water and will float.

Cleaning

Ultrasonic cleaning has several difficulties: the materials float and prolonged exposure to ultrasonic energy may damage the foam ligaments or the pore walls of the porous graphite.

Alternate, less severe methods of cleaning PocoFoam and POCO HTC have proved to be very effective. Air blown off or through the pores will remove most of the machining dust. Use of cellophane tape pull off on the surface will remove surface dust in the pores.