Pressure extrusion

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

In mercury porosimetry it was proposed that the pore potential prevents extrusion of mercury from a pore until a pressure less than the extrusion pressure is reached. Similarly, the pore potential, when applied to gas adsorption, is used to explain desorption at lower relative pressures than adsorption for a given quantity of condensed gas. 

Materials Minimum Section Thickness, (in.) Extrusion Temperature (°F) Extrusion Pressure (ksi) Extrusion Exit Speed (fpm) Extrusion Ratio InlAo/A,)"... 

The extrusion pressure could be further modulated by choice of dies or screens with appropriate L/D ratios. The pressure differential between points of die entry and exit due to viscosity for a Newtonian material moving within a cylinder is represented by Equation (4), derived from the Hagen-Poisuille expression for pressure drop in a pipe of constant diameter as (41)... 

The addition of tellurium has a deoxidizing action and confers on lead useful work and precipitation-hardening properties. It also improves resistance to wear, vibration, and mechanical breakdown. These properties, along with improved corrosion resistance, are utilized in the sheathing of power communication and marine cables, in chemical equipment (especially those exposed to sulfuric acid), or where resistance to fatigue is important. Tellurium has successfully replaced tin (1—3% Sn) in the sheathing. Thinner sheaths can be extruded, and the extrusion pressure for 0.05% Te-beafing lead is about the same as for a 3% Sn alloy. 

The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity. The pressure developed in the extruder system is largely a function of die resistance and dependent on die geometry and melt viscosity. Extrusion pressures are lower than those encountered in injection molding. They are typically 500 to 5000 psi (3.5 to 35 MPa). In extreme cases, extrusion pressures may rise as high as 10,000 psi (69 MPa). Variants on the single screw include the barrier or melt extraction screw and the vented screw (Chapter 3). 

Fig. 18. Best analytical fits to experimental extrusion pressure — extrusion ratio Rj, data...

Fig. ZO. Extrusion pressure v. extrudate velocity for polyethylene - large scale R40 polyethylene extrusion (Tj, = 90 °C, product diameter 15.5 mm)...

PTFE Resin Reduction Ratio Range Specific Gravity Average Particle Size, pm Thermal Instability Index Extrusion Pressure, MPa Reduction Ratio Bulk Density... 

As expected from the monodispersity of pore size for such model materials, both intrusion and extrusion pressure ranges are relatively small. Values of pressure dispersion AP n,/Pi t are reported in Table 2. 

Table 2. Chain loading, pore radius, intrusion and extrusion pressures and calculated contact angles 0a and 0,...

The addn of certain nonionic surfactants (Tergicol XC or Arlacel C) to the molten mix of AN thermoplastic cellulose esters reduces the consistency of the molten mixr and the extrusion pressure needed to form a proplnt grain) K)P.O.Marti, L SP 2946672 (I960) CA 54, 25831 (I960) (The use of PVC in the binder of a comp proplnt improves the stability of the grain to thermal shock) L)F.B.Cramer, USP 2949352 (I960)... 

The value of tq can be determined by plotting the extrusion pressure against the extrudate velocity V for extrusion through dies of constant value of L. The extrapolated value of extrusion pressure at E = 0 gives the value Pqvo at zero velocity, as shown by Eq. (15) ... 

Low Density Polyethjdene (LDPE) was extruded by Buckley and Long at room tempera-ture They reported satisfactory extrusion for nominal draw ratios of 3,5, and 7. The observed diametral die sweU amounted to 20%, 12%, and 11%, respectively. For the same set of esq riments the extrusion pressure iiKreased from 31.5 MPa to 94.5 MPa. These values are about W% lower than for the extrusion of HDPE under similar conditions. 

As shown in Fig. 2 for two homopolymer samples and one copolymer, the polymer grade had a strong influence on the extrusion behavior. At an extrusion temperature of 110 C the highest extrusion pressure was needed for the high molecular weight homopolymer sample. The difference in extrusion pressure increased with increasing draw ratio. The copolymer was more easily extruded than the homopolymers. Consequently, the authors observed a significant variation of the maximum extrusion ratio... 

Fig. 3. Variation of extrusion pressure with extrudate velocity. Extrusion temperature M °C, nominal draw ratio IS, die diameter 15.5 mm, sample R (for sample description see Table 2). R mes at lower and higher velocities are isothermal and adiabatic, respectively ...

Most experiments were run at an extrusion rate of 0.4 mm /s. At this rate, little influence of the extrusion temperature on the extrusion pressure was observed as the data in Table 6 indicate for material C. 

Attempts have been made to relate the extrusion pressure and the strength of the green body to the water content of the plastic mixture. For a wide range of clays, the extrusion pressure p has been observed to depend on the water content w according to the equation ... 

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