Crack During Mold Release

Puffing. Puffing is an irreversible expansion of molded graphite which occurs during graphitization when volatile species, such as sulfur from the coke, are released. Puffing is detrimental as it causes cracks and other structural defects. It can be eliminated (or at least considerably reduced)... 

The mold was sprayed with a release agent and a circular piece of Kapton was placed in the bottom of the mold. A sufficient number of PFA/carbon fiber disks were loaded into the mold to obtain a height of 15 cm. Another circular piece of Kapton was placed on top of the stack before the top of the mold was closed by the ram. The heaters were turned on, each set at 349°C the mold was pressurized to 22 MPa and held at temperature for 30 minutes. The power input to the base heater was reduced first during the cool-down period, which resulted in a temperature decrease to 260°C after 32 minutes. The same reduction was made to the heater adjacent to the base and after another 32 minutes its temperature reached 260°C. This was repeated for all the heaters. At the end, the power to all the heaters was turned off and the composite rod was removed upon reaching 32°C. The rod was inspected visually and under a microscope at 25X magnification no cracks or voids were observed. 

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