Graphite molding powder and method of preparation

Vermiform graphite is compressed to produce a preform with a density of about 0.03 to 0.8 g/cc, followed by comminuting the preform to produce a graphite molding powder that will have a typical mold fill-ratio between about 7 and 10 to 1 for making molded graphite parts with a density of at least 1.1 g/cc.

There are many uses for molded graphite parts made from vermiform graphite 
without binders. Such parts can be made directly by compressing expanded 
or vermiform graphite in a mold, but the fill-ratio, which is the ratio of 
the height of the column of graphite before compressing to the height of 
the molded part after compressing, is on the order of 280 to 1. Tooling to 
produce parts this way is prohibitively costly and large in size. 
An alternate way of molding the graphite parts is to first comminute 
graphite foil that has been made by compressing expanded graphite. 
Graphite foil has a density greater than 1.0 g/cc, usually about 1.1 g/cc. 
The resulting powder is then compressed in a mold to produce the desired 
part. In order to obtain a practical fill-ratio, the powder must be very 
fine, but molded parts made from such powder are considerably weaker than 
parts molded directly from expanded graphite . 
It is among the objects of this invention to produce a molding compound 
which has a better fill-ratio than expanded graphite, which has a better 
fill-ratio than comminuted graphite foil, and which is capable of being 
molded into parts with higher strength than those molded from comminuted 
graphite foil.

In accordance with this invention, vermiform graphite is compressed with 
sufficient pressure to obtain a preform having a density from about 0.03 
to 0.8 g/cc, but preferably from 0.06 to 0.1 g/cc. The fill-ratio for this 
pressing is approximately 12 to 1. The dimensions of the preform are not 
critical as long as the degree of compaction results in a density within 
the range just given. A typical preform, for example, can be 
12".times.25".times.1/4". 
The preform, after being formed in the manner just described, is comminuted 
in any suitable manner to make a powder. This powder constitutes the new 
graphite molding compound. The comminuting is such that substantially all 
of the graphite particles will pass through a 48 mesh screen, and a 
maximum of about 60% of the particles will pass through a 100 mesh screen. 
The compound can be molded into the desired parts or objects by 
compressing it in molds without the necessity for using a binder. For 
example, no binder is required for molding ring-shaped parts having a 
typical density of 1.6 g/cc by using a molding pressure of 6.5 tons psi. 
Satisfactory parts with a density as low as 1.1 g/cc or with a density 
greater than 1.6 g/cc can also be molded from the compound without a 
binder. 
An advantage of this method of making the molding compound is that if the 
preform and graphite foil are comminuted in the same equipment, separately 
of course, under identical conditions and for the same length of time, the 
fill-ratio of the new compound is about 7 to 1 when the powder has been 
made from a preform having a density of 0.03 g/cc, as compared to a ratio 
of about 13.8 to 1 when using a powder formed by comminuting graphite foil 
having a density of 1.1 g/cc. Furthermore, the crushing strength of the 
above-mentioned ring-shaped parts having a typical density of 1.6 g/cc is 
approximately 3.7 pounds for such parts made from the new compound, as 
compared to a strength of 3.0 pounds for like parts made from the 
comminuted foil. 
When the graphite powder was obtained by comminuting for five minutes a 
preform having a density of 0.08 g/cc, the strength of the molded parts 
was 3.4 pounds and the fill-ratio was about 10 to 1 as opposed to a 
typical fill-ratio of about 13.8 to 1 for a molding compound made by 
comminuting for a much longer time graphite foil having a density of 1.1 
g/cc. Although the fill-ratio using a molding compound made from 
comminuted foil might be improved by futher comminution of the foil to a 
still finer powder at additional cost, it is unlikely that a fill-ratio 
equivalent to that of this invention could ever be obtained. Also, parts 
molded from such powder would have even less strength, because the higher 
density comminuted foil particles lack the compactability and the 
mechanical bonding between the individual particles attainable with the 
compound made by this invention. Thus, this invention improves the 
manufacturing economics of molding strong graphite parts. 
According to the provisions of the patent statutes, we have explained the 
principle of our invention and have illustrated and described what we now 
consider to represent its best embodiment. However, we desire to have it 
understood that, within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically illustrated and described.