Pressure recession method

The pressure recession method consists of rapidly subjecting the cylinder to hydrostatic test pressure, then immediately cutting off the pressure supply and observing the recession of pressure in the cylinder due to permanent ex... 

The pressure recession method may be used for testing cylinders that require a test pressure of 2000 psi (13 790 kPa) or more. The pressure gauge must be observed for at least 2 minutes to determine whether or not pressure in the cylinder decreases. If the pressure does not decrease, this indicates that the cylinder does not have a measurable permanent expansion at the test pressure. 

Fig. 10-12. Typical schematic diagram of pressure recession method testing equipment.

The pressure recession method consists of subjecting the cylinder rapidly to hydrostatic... 

An alternative approach is to measure two stresses. Lodge (1988) has shown how the pressure hole method can be used as an in-line rheometer. N is measured in slit flow by two opposed pressure transducers as shown earlier (Figure 6.3.5). A third, recessed transducer downstream is used to get the shear stress. Together this gives N (r), which is very sensitive to polymer concentration and molecular weight. Padmanabhan and Bhattacharya (1993) have proposed a combination of entrance pressure drop and wall shear stress. 

The cleaning may be carried out by solvent vapours, by immersion in the liquid phase—and by both methods in combination. Immersion also should overcome static attraction, but if this phase is not used a gun emitting deionized air under low pressure may be employed to clean individual parts and to take away debris. Agitation of components by ultrasonic means can be helpful in removing contamination from blind holes and recesses. 

A variety of standard and specialized applicators exist for pressure-sensitive labeling. Standard portable head applicators provide top, side, and bottom labeling. These applicators use one of several basic methods available for application of automatic pressure-sensitive labels. In a tamp-blow method of application, the label is dispensed from the backing web and held in position by vacuum on a tamp pad. The tamp pad is indexed 0.8-1.5 cm away from the product, and then the label is applied to the product by a gentle blast of air. Tamp-blow applications are used for a large array of packages that have irregular, curved, or recessed surfaces. 

Compression molding is the most common method hy which thermosetting plastics are molded [ 1-3]. In this method the plastic, in the form of powder, pellet, or disc, is dried by heating and then further heated to near the curing temperature this heated charge is loaded directly into the mold cavity. The temperature of the mold cavity is held at 150°C—200°C, depending on the material. The mold is then partially closed, and the plastic, which is liquefied by the heat and the exerted pressure, flows into the recess of the mold. At this stage the mold is fuUy closed, and the flow and cure of the plastic are complete. Finally, the mold is opened, and the completely cured molded part is ejected. 

In practice, there are three variants of the method, depending on the geometry of the mold and pattern size, the thickness of the polymer film, polymer properties, and on the process conditions (temperature, pressure). If the depth of the recessed features is large compared to the thickness of the spin-coated film, the latter carmot planarize the relief features, leaving a large undulation on the coated polymer surface [9]. On the other hand, when the polymer thickness is comparable to the height of mold features, the polymer forms a planarized layer over the whole mold. 

Despite the ability of the hole pressure method to measure normal stresses reliably at high shear rates, it has not yet seen wide use. Some of the reasons may include problems with maintaining and cleaning the pressure slots for routine tests. Melt samples especially may degrade in the recess. Materials with a yield stress may... 

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