Among the various parameters of a venting screw, the channel depths H₁ and H₂ are the two most important, as they directly influence the operating behavior of the venting screw.
Once the screw parameters D (diameter), φ (helix angle), and rotational speed n are fixed, the throughput of the venting screw is determined by the channel depth H₁ of the first metering section. The method for determining this value does not differ greatly from that for a conventional screw. However, because the L/D ratio of a venting screw is relatively large, a larger H₁ may be selected. According to Eq. (8-10), H₁ = KD, it can be seen from the statistical data in Table 11-3 for existing screws that, for the same diameter, a larger L/D ratio corresponds to a larger K value and thus a larger channel depth H₁.
Once H₁ has been determined, H₂ cannot be chosen arbitrarily. It is known from hydrodynamic theory that the channel depth directly affects the slope of the screw characteristic curve, and the relative inclination of the characteristic curves of the two screw sections influences the likelihood of surging or vent flooding.
For this reason, the concept of pump ratio Ω is introduced. The pump ratio is defined as the ratio of the channel depth of the second metering section to that of the first metering section, i.e.:
Ω = H₂ / H₁ (11-10)
The pump ratio Ω is a very important parameter for venting screws. The closer Ω is to 1, meaning H₂ and H₁ are nearly equal, the closer the two screw characteristic curves lie to each other, and the greater the possibility of vent flooding. As Ω increases, although the likelihood of vent flooding decreases, the tendency for extrusion instability at low die pressures will increase.
When the melt is a Newtonian fluid under isothermal conditions, the viscosity η remains constant. Setting dp_max/dΩ = 0 gives the theoretical optimum pump ratio of Ω = 1.5, which yields the maximum working pressure. Table 11-2 lists the calculated p/pmax values for different pump ratios. If the output Q₁ of the first metering section is represented by a set of lines with different slopes, it can be analyzed that the maximum working pressure Pmax is exactly the pressure at the intersection of the envelope of these sloping characteristic lines and the horizontal line representing Q₁.
When the plastic is treated as a non-Newtonian fluid, for polyethylene with a melt index (MI) of 0.2–5.0 it can be demonstrated that the maximum die pressure is obtained when the pump ratio Ω is 1.75. According to the statistical compilation of venting screws from various countries in Table 11-3, the pump ratio for most venting screws lies in the range of 1.5–2.0. When a smaller pump ratio Ω is adopted, throughput fluctuations are smaller and extrusion quality is high, but the risk of vent flooding also increases.
Once the pump ratio Ω is determined, the channel depth of the second metering section can be calculated from Eq. (11-10). Some references suggest the existence of a "second compression ratio" for venting screws, but this view is incorrect. As is well known, the compression ratio is defined as the ratio of the volume of the first screw channel in the feed section to the volume of the last screw channel in the metering section. In the venting section of a venting screw, the screw channel is not fully filled, so a volume ratio based on filled channels is meaningless.
