Felt, board & the meaning of purity

Three descriptions, three different questions.

Rayon identifies the fiber precursor. Carbonized or graphitized describes thermal processing and structure. Purified addresses impurity control. None of those words, on its own, specifies the finished insulation. SGL describes higher-temperature treatment of cellulose-derived felt as creating a more graphite-like structure. SGL Carbon · SIGRATHERM soft felt

Soft felt and rigid board have different jobs.

Soft felt can conform around penetrations. Rigid felt offers shape stability and machinable forms; SGL lists both plates and cylinders, with optional protective facings. Those are commercial forms worth evaluating for a cylindrical build. SGL Carbon · SIGRATHERM rigid felt

Ipsen describes an all-felt package protected by C/C in its furnaces, including edge protection where needed. That is a documented system choice, not proof that every board-and-felt package should be replaced. Ipsen · Graphite felt and board in vacuum furnaces

Is lower ash better?

For an ash specification, lower means less residue after the stated test. That is generally desirable when residual contamination matters. It does not identify every contaminant or measure vacuum outgassing, moisture, particle shedding or the finished assembly’s cleanliness.

For example, SGL’s GFA sheet lists typical ash of 1,000 ppm and a purified option below 20 ppm. Preserve that distinction: a typical value and an upper bound are different kinds of evidence. Neither is a lot-specific certificate. SGL Carbon · GFA data sheet

Make the comparison fair.

Request conductivity versus temperature at the same pressure, gas, density, compression and heat-flow direction. Match installed thickness. Record facing and binder details. A cold-face temperature from one test is not interchangeable with a material conductivity value.

For HeatGuard or another named board, begin with the exact grade and current data sheet. The HeatGuard brochure is linked for vendor follow-up; this first edition does not assign it a performance ranking. Graphite Machining Inc. · HeatGuard brochure

Ask before ordering.

• What are the precursor, thermal treatment and purification route?
• Which values are typical, guaranteed limits or actual lot results?
• What thickness and compression will the installed package have?
• How are hot faces, edges, joints and penetrations protected?
• What should incoming inspection and storage control?