This is a good starting point, but the most important factor is to know the 1st Stage Fill Time and 1st Stage Short Shot Weight. If these are matched, then the outputs match which is critical.
Question: Say i have a process with .94sec fill time and 36mm
screw. When i change to a 50mm screw, what is the formula to
match the same fill/shear rate/?
My Response: This is based off of a ratio between the effective surface are of the screw. The basic formula would be as follows:
Machine B Speed = (Machine A Speed) * [(Machine A Radius)^2 / (Machine B Radius)^2]
For your example:
Machine B Speed = (Machine A Speed) * [(18)^2 / (25)^2] = (Machine A Speed) * 0.518
Additional Thoughts: It is typically easier to match the fill time and short shot weight from the previous process. This will not be exaclty correct because it will not compensate for aspects such as decompression and check ring wear.