Method of manufacturing polyimide foam shapes having improved density and cell size uniformity

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG.
1, there is seen a schematic representation of a knife-type chopper 10.
The chopper, of conventional design, used 8 rotating knives 4 1/4 inches diameter having stationary blades spaced between the rotating knives.
A 1 HP electric motor drove the rotating knives at about 5,000 rev.
per minute.
It was usually necessary to chop the material 3 times to get the desired flake size, and to make the flake size more uniform.
Chopper 10 includes a plurality of rotating knife blades 12 driven by a motor 14.
Foam blocks or pieces 16 are fed into chopper through a hopper 18 which also permits air to be drawn into the chopper as indicated by arrows 20.
When foam 16 encounters blades 12 it is chopped into small flakes which exit chopper through hopper 22 into a large diameter duct 24.
Moving chopped foam through 10 and duct 24 is difficult because the chopping action causes a strong electrostatic charge to be built up on the small flakes.
The flakes adhere tenaciously to all surfaces.
In some cases, introducing a water spray mist into hopper 18 with foam 16 to slightly moisten the flakes will help reduce the electrostatic charge.
However, we have found that drawing the flakes from the chopper with a centrifugal blower 26 of the sort used in conventional dust collectors is effective despite the electrostatic charge.
Blower 26, driven by motor 28, draws a strong air current through chopper 10 to pull the flakes from the chopper, then directs them to bag filter 30, generally made from a fiberglass fabric.
This action reduces the electrostatic charge on the flakes.
Bags 30 may then be easily emptied into any suitable container.
Emptying should be done carefully, since rubbing the flakes together will reintroduce an electrostatic charge.
Any suitable anti-static agent compatible with the other materials may be used, if desired.
The methods of forming polyimide foam products of uniform density, using the flaking method and apparatus of FIG