2422:43 (is this a planetary gearbox? European manufacturer?) represents a ratio of 56.326:1, and yields these final screw speeds at 60 Hz (actually, a bit lower once slip is factored in).
2 pole, 3600 RPM motor, 63.9 RPM screw
4 pole, 1800 RPM motor, 32.0 RPM screw
6 pole, 1200 RPM motor, 21.3 RPM
40 HP is fairly small for an extruder, so my guess would be it is driving something on the order of a 1-1/2" to 2" diameter screw.
A 32 RPM final screw speed seems low for full output with the screw designs I'm familiar with, and am guessing you might have a 3600 RPM motor - 64 RPM final output is closer to what I've seen on such machines.
There are places where a forced air ventilated AC motor is the only option, typically on larger extruders where the motor is located directly under the barrel, and forces a hard limit on motor size, but my preference is the same as DickDV's - whenever possible use a TEFC motor and select motor RPM (in my case, usually going from a 1750 RPM DC motor to 1150 RPM AC motor) so the lower limit of normal processing motor speed is 60 to 70% of this, but more typically have it running close to or above base speed.
A second consideration in favor of TEFC motors is their ubiquity. Even if you can't get an exact replacement inverter duty motor during an emergency you'll be able to get something that will bolt into place, and operate successfully until the failed motor can be repaired or replaced. AO "air over" cooled inverter duty AC motors are also becoming very common, but forced air AC motors (at least in the larger sizes) are usually quoted on a build-to-order basis, and tend to incur greater lead times.
There isn't anything wrong with the concept of extruder motor forced air cooling (especially in smaller motors like yours), but most plastics plants I've been in have dust, volatiles from die burn-off, venting, and other such crap in abundance.
Intake filter servicing tends to take a back seat, so, unless you duct in clean air from elsewhere this stuff eventually builds up in the motor blocking cooling passages and softening winding insulation. At 40 HP this isn't too bad because the volume of air through the motor is at most only a couple of hundred CFM, but becomes more of a consideration in higher power motors where cooling requirements may be on the order of 1200-2500 CFM.
Even when filter maintenance is performed regularly I'd just as soon eliminate the need for it in the design stage, and enjoy the reduction in ongoing maintenance costs.
Incidentally, I've found an article from Control Magazine which goes over many of the points of DC to AC extruder drive conversion,
and a terse, yet useful overview of extrusion fundamentals at