Screw-channel depth ratio

Screw Section Flight Radii to Channel Depth Ratio... 

D. The channel depth ratio varies from less than 2 to 4. The compression ratio, CR, has been defined as a ratio of the screw flight volume... 

Channel-depth ratio n. In an extruder screw, the ratio of the depth in the first turn of the screw at the feed end to the depth in the last turn at the dehvery end. If the lead of the screw is constant, the channel-depth ratio is shghtly larger than the chaimel-volume ratio (which follows). 

Compression ratio n. In an extruder, the ratio of the volume of the first turn of the feed section to that of the last turn of the metering section. This ratio is a rough indication of the total compaction performed on the feedstock. More precisely called the channel-volume ratio or, for a screw of constant pitch, channel-depth ratio. 

However, for all four screw temperatures the predicted melting rate in the numerical simulation is still significantly higher than the experimental data in Fig. 2. For simulation results as well as for the experimental data in Fig. 2, the sohd fi action presented in Fig. 7 was measured as a ratio of the sohd bed width to the width of the serew ehannel. The sohd fi action thus obtained was further normalized by multiplying it with the ratio of the current screw ehannel depth to the screw channel depth at the entrance of the compression section. 

In the simplest and most often used form, the screw has a free channel cross-section that diminishes at a steady rate from the feed to the delivery end. The ratio of the channel depths from feed to die region along the screw is usually referred to as the compression ratio, since it gives a crude indication of the relative conveying capacities at feed and discharge. 

Due to the complicated helical screw geometry and the assumption that the down-channel drag flow was a result of matching the screw core velocity to the modeled barrel velocity, the literature assumption that the flow occurs in a rectangular channel is reasonable only if the ratio of channel depth to width is small, that is, a channel with a small aspect ratio (H/W). A schematic of the channel depth to... 

The injection-molding press was producing a part and runner system that had a mass of 2.15 kg. The mass was plasticated using a 120 mm diameter, 8L/D screw. The screw used for the process had a barrier melting section that extended to the end of the screw, as shown by the specifications in Table 11.9. That is, the screw did not have a metering channel. Instead, the last sections of the barrier section were required to produce the pressure that was needed to flow the resin through the nonreturn valve and into the front of the screw. The specific rotational flow rate for the screw for the IRPS resin was calculated at 9.3 kg/(h-rpm) based on the depth of the channel at the end of the transition section. The screw was built with an extremely low compression ratio and compression rate of 1.5 and 0.0013, respectively. For IRPS resins and other PS resins, screws with low compression ratios and compression rates tend to operate partially filled. The compression ratio and compression rate for the screw are preferred to be around 3.0 and 0.0035, respectively. The flight radii on the screw were extremely small at about 0.2 times the channel depth. For IRPS resin, the ratio of the radii to the channel depth should be about 1. 

The lead length was 133 mm for the main flight of the screw. The main flight width and clearance were 12 and 0.12 mm, respectively. The compression ratio was 2.7 and the compression rate was 0.0090. The ratio of the flight radii to the local channel depth was 1.0 in all sections. The specific rotational rate was calculated at 8.5 kg/(h-rpm). 

The compression ratio for a high-performance screw is typically based on the axial distance-averaged depth, while the compression rate is based on the channel depth at the entry to the section. The screw described above has a feed section with a depth of 10.9 mm, and thus the compression ratio is about 2.5. 

In its simplest form, the ratio of screw diameter is the basis for scaling. The ratio of the large diameter D2 of the large scale unit to the small diameter Z>i of the lab unit is represented by the lower case d as shown in Table 4. The primary scaling variables are channel depth H, screw length L, helix angle cj), and screw speed N. The ratio of the primary variables of the two scales is then expressed as a power of the screw diameter ratio, d. 

The scale-up factors depend on the specific event being scaled-up in extrusion. The cube rule for mixing (23) states that at constant screw speed, output and power consumption increase with ct when H/D ratio is constant. The square-root rule for conveying (24) of material states that when channel depth is increased and screw speed decreased with x/d, the output rate increases with S, while power consumption increases by One could obtain... 

Fig. 9.37 Simulated and measured pressure profiles for an LDPE extruded in a 2.5-in-diameter, 26.5 length-to-diameter ratio extruder, with a metering-type screw having 12.5 feed section with channel depth of 0.37 in and 4.5 turns of metering section of depth of 0.1275. The flow rate is 136 lb/h, the screw speed 60 rpm, and the barrel temperature was set at 300°F. The SBP is shown in Fig. 9.24. The screw geometry is shown at the top of the figure. Simulation was carried out by the first computer simulation package for plasticating extrusion developed by the Western Electric Princeton Engineering Research Center team (17). [Reprinted by permission from Z. Tadmor and I. Klein, Engineering Principles of Plasticating Extrusion, Van Nostrand Reinhold, New York, 1970.]...

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