Process
Back pressure is the resistance applied against the screw retracting during plasticizing; it compresses and homogenizes the melt, improves color distribution and drives out air. It is expressed on two bases: the hydraulic back pressure set on the machine and the specific back pressure the melt sees in front of the screw. The specific value is about 10 times the hydraulic value (7–15 depending on the machine's intensification ratio). No setting should be made without checking which basis a value in a table is given on.
There are two universal rules. With heat-sensitive materials (PVC, POM) and fiber-reinforced grades, back pressure is kept low: too much generates shear heat and breaks the fibers. For color and masterbatch distribution, on the other hand, back pressure is raised; the price is a longer plasticizing time.
European manufacturer literature gives specific back pressure in the 50–150 bar band, US practice around 34–69 bar; both are valid, and it is a systematic difference in convention. Anyone who sees a 10-fold difference in the same table should look for a mix-up in basis, not a difference in material.
When color streaks, non-homogeneous melt and air bubbles are seen; in small steps, knowing that plasticizing time and shear heat will increase.