Lubricated compression

The normal stress difference that drives the flow is 

Schematic of lubricated squeezing geometry (a) the initial sample disk is smaller than plate radius (b) sample and plate radius are the same. 

The varying sample radius can be determined from the thickness, assuming a constant volume sample 

The working equations for lubricated compression are summarized in Table 7.3.1. 

Secor (1988) extended their work and established criteria for lubrication 

---

This section addresses the design of immediate-re-lease powder formulations for hard gelatin capsules. In general, powder formulations for encapsulation should be developed in consideration of the particular filling principle involved. The requirements imposed on the formulation by the filling process, such as lubricity, compressibility and/or compactibility, and fluidity can vary between machine types. Furthermore, the... 

In the life cycle phases of product manufacturing, the focus of resource efficiency moves from the material applied per unit to resources used in the various production phases, for example, cooling lubricants, compressed air or hydraulic oil and on the energy requirements of the production processes [24]. Process relevant information is based on equipment energy consumption curves. Each curve is specific to a production equipment item and enables an accurate determination of the energy consumption of the item over the production time. 

Rigid specimens (e.g., apple, cheddar cheese) often exhibit a sudden decrease in force (stress) after a certain amount of deformation (maximum strain). At this point the specimen has fractured. Maximum stress and strain values may vary depending on the chosen specimen. Specimens that are weakly structured and tend to flow under lubricated compression (e.g., mozzarella cheese, marshmallow) demonstrate squeezing flow. As a result, the force (stress) continually increases as the specimen deformation (strain) increases. These materials do not fracture, but continue to stretch radially while under compression. Both rigid and soft specimens of the same material may exhibit varying characteristics depending on the deformation rate and the aspect ratio of each specimen. 

The graft polymers were diluted with an SAN copolymer to 20 and 30 wt % substrate. The dry blends were mixed on a two-roll mill for a maximum of 5 minutes (165° 175°C) without added lubricants. Compression-molded samples from the milled slabs were evaluated for impact and tensile strength. Figure 5 is a schematization of this procedure. 

Lubricated compression Simple sample preparation, grip Easy to generate small displacement Need lubricant High 1] e < 2... 

The application of this new method Is outlined, and applied to a model which Includes both lubricant compressibility and pressure viscosity effects. Solutions are then presented which cover a wide range of material and lubricant combinations, contact loads and rolling speeds. These results are found to exhibit good agreement with previously reported solutions. 

The underlying solution technique used In this paper Is the Newton-Raphson scheme, refined by Houpert and Hamrock [3], which Incorporates lubricant compressibility, the Roelands pressure-viscosity relationship, an Improved elastic calculation and variable mesh spacing. 

Recently, an improved version of Okamura s approach was developed by Houpert and Hamrock (1986) than enables solutions to eiastohydrodynamic lubricated line contacts to be made with no load limitations. Successful runs were reported to have been obtained at high pressure (to 1.8 GPa) with lower CPU times. The new approach presented by Houpert and Hamrock (1986) allows for lubricant compressibility, the use of Roelands pressure viscosity, a general mesh (nonconstant step), and accurate calculations of the elastic deformations. This approach enables eiastohydrodynamic lubrication calculations to be performed at the operating conditions normally experienced in non-conformal conjunctions such as those that exist in rolling element bearings and gears. 

U.l The Influence of lubricants compressibility on pressure and film thickness... 

Fig. 3 presented the results of pressure and film shapes obtained by considering or neglecting the lubricants compressibility. We can see from the figure that the Influence of compressibility are ignorable in engineering use except the slight diminishes of pressure spike and minimum film thickness. 

The apparatus used for lubricated compression also can be readily adapted to give lubricated planar flow, as shown in Figure 7.3.7. Only one normal stress difference is measured, (th — 133) = FjA. Thus only one of the two planar viscosities is available, /zi. However, as shown elsewhere (Figures 4.2.7 and 7.4.6), this viscosity should be the more strongly thickening one. 

Lubricated compression is probably the simplest extensional method. If samples can be made in a solid form, they will be easy to load and test. Temperature can be readily controlled over a wide range. Strain appears to be limited to 1-1.5 by loss of lubricant, and samples with viscosity ri > 10 Pa-s seem necessary to create enough difference between sample and lubricant. The advantages and limitations of lubricated compression are summarized in Table 7.3.1. However, the method has not been widely studied, and further work is needed to accurately determine its limitations. 

Figure 7.4.4 shows data from the rotating clamp device for the transient equibiaxial viscosity at three different extension rates. For comparison, the linear viscoelastic viscosity and the uniaxial viscosity are shown. Results for the biaxial viscosity compare well to those measured in lubricated compression on the same polyisobutylene sample as in Figure 7.4.4 (Chatraei et al., 1981). So far, only results with the rotating clamp method have been reported for this sample. Maximum strains were 2.5 in the biaxial and multiax-ial tests and k < 0.1 s. Friction on the talcum powder may limit the total strain and the detectable stress values. Much larger, more homogeneous samples are required than were used in the lubricated squeezing experiments. However, because the rotating clamps can...

B.2 Lubricated Compression Molding. For the compression molding process shown in Figure 10.11 the plates are lubricated either with mold-release agent or by using Teflon sheets. Calculate an expression for the force required to close the plates similar to Eq. 10.50 when the polymer is assumed to exhibit complete slip. Do this first for a Newtonian fluid and then a polymer melt with rheological properties described by the PTT model. 

---