When you choose exhausting type screw...
1）L/D Ratio: between 24-33,bigger length than single-step screw.
The section before exhausting part will be 52%-58% of total length of screw, that ensure plasticizing before materials running into exhausting section.Generally speaking,when L/D is 30,length of exhausting section is about 4D(D=diameter of screw). The second compressing section should be less than 2D, but the second metering section is longer and better,15-25% OF total length,which is good for stable extrusion.
2）Pump ration: Ratio(X) of depth of screw slot on second metering section(h2) and first metering section(h1), X=h2/h1
Usually, X is between1.6-1.7, that reduces the possibility of materials overflow out of screw.If extruder is for mixing color, X could be big. If for extrusion pipe profile,small.