Sliding interface

Using friction attaclnnents (see section (bl.20.2.4)). many remarkable discoveries related to tiiin-film and boundary lubrication have been made with the SEA. The dynamic aspect of confined molecules at a sliding interface has been extensively investigated and the SFA had laid the foundation for molecular tribology long before the AFM teclnhque was available. 

As the temperature of the sliding interface increases, the coefficient of friction varies. This variation is unpredictable, and there exists no general trend except that at extremely high temperatures the coefficient may become very low (<0.15). This temporary loss in friction is referred to as fade (25). 

In this chapter the physical properties of resins related to processing will be described. The chapter and Appendix A4 include physical properties for many resins. These properties include bulk density and compaction, lateral stress ratio, stress at a sliding interface, melting flux, heat capacity, thermal conductivity, and melt density. Some of these properties are easy to measure by many laboratories while others such as the melting flux and stress at a sliding interface can be measured in only a few places using highly specialized equipment. 

This section wiil compare the modified Campbeii-Dontula modei to the experimen-tai data for the LDPE resin. The physicai properties for this resin were provided in Section 5.3.2. The dynamic coefficients of friction are avaiiahie in Appendix A5 and are reported as stress at a sliding interface. The coefficient of friction is related to the stress at a sliding interface as follows ... 

The coefficients of dynamic friction need to be determined first for this analysis. The average pressure and temperatures are specified, but the velocities at the sliding interfaces need to be determined. The sliding velocities need to be calculated based on screw rotation physics. The sliding velocity at the barrel interface is as follows ... 

Figure 13.6 Stress at a sliding interface for TPU resin at a pressure of 1.4 MPa...

Appendix A4 Shear Stress at a Sliding Interface and Melting Fluxes for Select Resins... 

Table A4.1 Data for Shear Stresses at a Sliding Interface for Select Resins in this Book...

Figure A4.1 Shear stress at a sliding interface for a PC resin with an MFR of 3 dg/min (300 °C, 1.2 kg) as a function of temperature and sliding velocity at a pressure of 0.7 MPa...

The stress at the sliding interface was measured using a Screw Simulator as outlined in Section 4.3.1 for the LDPE resin and is shown in Fig. A5.17 at a pressure of 0.7 MPa and shown in Fig. A5.18 at a pressure of 3.4 MPa. At temperatures of less than about 110 °C, the stress at the interface is due to a frictional force mechanism. At temperatures higher than 110°C, the forces are from a viscous mechanism. The coefficients of friction were reported as stresses so that the data from both mechanisms can be reported on the same figure. The coefficient of friction is related to the stress at the interface r as follows ... 

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