Screw output

Type of Screw Output, Produc- Drive Screw Dimensions Total... 

Example 11.3 A plasticizing screw driven by a 50-hp motor is used to raise the temperature of the polymer feed from 25°C to the processing temperature. Assuming a mechanical efficiency of 60% and neglecting the energy required to pump the polymer and that from the heating bands, estimate the screw output if ... 

The following table shows some thermal properties of three thermoplastic polymers. Compute the screw output for each polymer material if the screw drive power input is 60 hp and the mechanical efficiency is 75%. Assume that the heat requirement for molding the materials is satisfied by the viscous dissipation... 

Each test included screw speeds of 25, 50, 75, 100, and 125 rpm. Melt temperature was checked at each screw rpm using a hand held IR gun, melt probe, and immersion probe. Three one-minute sheet samples were taken at each set rpm to calculate screw output rate. The barrel pressure, immersion probe, screw speed and motor anqis were all monitored and recorded at one-second intervals on the NetDAQ. 

Care must be taken when designing for an integral barrel configuration, because of the excess power required from the motor resulting from the increase in solids conveying and screw output. 

A web of molten plastic is pulled from the die into the nip between the top and middle roUs. At the nip, there is a very small rolling bank of melt. Pressure between the roUs is adjusted to produce sheet of the proper thickness and surface appearance. The necessary amount of pressure depends on the viscosity. For a given width, thickness depends on the balance between extmder output rate and the take-off rate of the pull roUs. A change in either the extmder screw speed or the puU-roU speed affects thickness. A constant thickness across the sheet requires a constant thickness of melt from the die. The die is equipped with bolts for adjusting the die-gap opening and with an adjustable choker bar or dam located inside the die a few centimeters behind the die opening. The choker bar restricts flow in the center of the die, helping to maintain a uniform flow rate across the entire die width. 

A common use of screw extruders is in the forming and compounding of plastics. Table 20-54 shows typical outputs that can be expected per horsepower for various plastics and the characteristics of several popular extruder sizes. 

For ease of illustration we will consider the characteristics and behaviour of a centrifugal pump which is similar in behaviour to radial/axial flow fans and centrifugal/screw compressors. Figure 63 shows the mechanical connection of a flow valve to control the output of the pump or the discharge of the fluid through the throttle of the valve. Figure 6.39 illustrates the characteristics of the pump ... 

A turbine-driven screw compressor might be applied to a catalyst regeneration process. The nature of the process then will require constant volume control to maintain a required output temperature in the regenerator. The arrangement is shown in Figure 8-40(b) and would occur as follows ... 

The output shaft in this case is the lead screw, which is assumed to have zero moment of inertia /q and viscous friction Cq. The free-body diagrams of the machine-table and lead-screw are shown in Figure 4.30. 

As plastics can have quite different viscosities, they will tend to behave differently during extrusion. Fig. 4.3 shows some typical outputs possible with different plastics in extruders with a variety of barrel diameters. This diagram is to provide a general idea of the ranking of materials - actual outputs may vary 25% from those shown, depending on temperatures, screw speeds, etc. 

In commercial extruders, additional zones may be included to improve the quality of the output. For example there may be a mixing zone consisting of screw flights of reduced or reversed pitch. The purpose of this zone is to ensure uniformity of the melt and it is sited in the metering section. Fig. 4.4 shows some designs of mixing sections in extruder screws. 

Equation (4.12) enables the die characteristics to be plotted on Fig. 4.12 and the intersection of the two characteristics is the operating point of the extruder. This plot is useful in that it shows the effect which changes in various parameters will have on output. For example, increasing screw speed, N, will move the extruder characteristic upward. Similarly an increase in the die radius, R, would increase the slope of the die characteristic and in both cases the extruder output would increase. 

In the intermittent processes, single or multiple parisons are extruded using a reciprocating screw or ram accumulator. In the former system the screw moves forward to extrude the parisons and then screws back to prepare the charge of molten plastic for the next shot. In the other system the screw extruder supplies a constant output to an accumulator. A ram then pushes melt from the accumulator to produce a parison as required. 

If the extruder is coupled to a die which is used to produce two laces for subsequent granulation, calculate the output from the extruder/die combination when the screw speed is 100 rev/min. Each of the holes in the lace die is 1.5 mm diameter and 10 mm long and the viscosity of the melt may be taken as 400 Ns/m. ... 

An extruder is coupled to a die, the output of which is given by (KP/ij) where P is the pressure drop across the die, i] is the visco.sity of the plastic and is a constant. What are the optimum values of screw helix angle and channel depth to give maximum output from the extruder. 

Prior to blending, the LCP was dried at 155°C for 5 h. The melt blending of the materials was carried out with a Berstorff ZE 25 x 33D corotating twin-screw extruder at a melt temperature of 290°C, with a screw speed of 200 rpm, and an output of 6.4 kg/h. The extrudate was immediately quenched in a water bath and repelletized. 

A distinction should be made between machine conditions and processing variables. Machine conditions are basically temperature, pressure, and processing time (such as screw rotation/rpm, and so on) in the case of a screw plasticator, die and mold temperature and pressure, machine output rate (lb./hr), and the like. Processing variables are more specific such as the melt temperature in the die or mold, melt flow rate, and pressure used. 

B-type screws for high homogenization requirements C-type screws for high performance D-type screws for ultrahigh output High-performance intake design... 

Fig. 9.3. Optical alignment of the fiber-optic output with respect to the microscope axis (black line). A close up is shown of the side-port of the Axiovert 200 microscope and fiber-optic coupling of a modulated 514 nm laser source. Left the fiber output (coming from the right) is aligned onto the microscope axis enabling wide-held excitation. Right the fiber output is aligned slightly off axis, but sufficient to induce TIRF. The scale of the picture can be inferred from the optical table M6 screw mounts separated by 1 inch.

The main problem with a feed controlled condition is that throughput is determined entirely by what the feed end manages to pull in and this is very sensitive to the conditions in the feed area. For this reason it is better to keep the machine running full so that the conveying characteristics of the screw determine output rather than the combined characteristics of screw, feed roll, feed opening and feed strip. The compression designed into a screw should therefore be sufficient to achieve complete filling at the lowest likely head pressure. 

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