THIN-WALLED

Figure 9.4 A thick-walled pressure vessel might be economical when compared with a thin-walled vessel and its relief and venting system.

Gammagraphic weld inspection in the lower range of steel thicknesses has been done with Iridium and Ytterbium isotope sources throughout the past. The large majority of applications has been using Iridium due to the unfavourable economical parameters of Ytterbium, obviously with non-optimal results at thin wall inspections. 

The project demonstrated that it is possible to measure and evaluate eccentricity of aluminum tubes at drawing velocities around 25 km per hour. Not all demands regarding small diameter tubes and thin walls were tested, and the calibration procedures were not finalized. Optimizing the system should make it possible to expand the limits... 

It has been found that the contrast in film density caused by very small local variations in mass density of the concrete is considerable, e g. S D = 0.12 for a 6 mm diameter hole in a 250 mm thick concrete beam. The image quality provided by fine-grained films (Agfa Gaevert D7) was sufficient to distinguish the thin walls of a pre-stressing duct in a 750 mm thick concrete bridge slab. 

Slurry or slip casting provides a relatively inexpensive way to fabricate unifonn-thickness, thin-wall, or large cross section shapes [4o, 44, 45, 46, 42 aiid 48]. For slip casting, a slurry is first poured into a porous mould. Capillary suction then draws the liquid from the slurry to fonn a higher solids content, close-packed, leather-hard cast on the inner surface of the mould. In a fixed time, a given wall thickness is fonned, after which the excess slurry is drained. 

Teflon TFE-Fluorocarbon Resins Extrusion of Thin-Walled Tubing, buUetia, E.I. du Poat de Nemours Co., lac., Wilmington, Del., 1957. 

PCTFE plastic is available in products that conform to ASTM 1430-89 Type 1 (Grades 1 and 2) and is suitable for processing into parts that meet MIL-P 46036 (Federal Specification LP-385C was canceled 1988). Standards for fabricated forms are available for compression molded heavy sections (AMS-3645 Class C), thin-walled tubing, rod, sheet, and molded shapes (AMS-3650). PCTFE plastics have been approved for use in contact with food by the FDA (55). 

Supports. The spaciag of supports is governed by the hot allowable stress of the piping materials stabiUty, ia the case of large-diameter thin-wall pipe deflection to avoid sagging or pocketing and the natural frequency of the unsupported length to avoid susceptibiUty to undesirable vibration. 

With a simple parison, the large-diameter sections of the botde have a thin wall and the small-diameter sections have a thick wad. Certain modifications of the die can control the thickness of the parison wad along its length, which results in a bottle with improved wad thickness distribution and better strength. High density polyethylene (HDPE) is the most common blow mol ding resin used to produce containers ranging in size from 30 cm to 200 L. 

Rotomolding. Nylon-6, nylon-11, and nylon-12 can be used in rotomolding and are generally suppHed for these appHcations as a powder or with a small pellet si2e. The process involves tumbling the resin in a heated mold to form large, thin-walled mol dings. Nylon-11 and nylon-12 use mold temperatures of 230—280°C and nylon-6 is processed at over 300°C. An inert gas atmosphere is preferred to avoid oxidation. 

The bellows is formed from a length of thin-walled tubing by extmsion in a die. The metals used in the constmction of the bellows must be ductile enough for reasonably easy fabrication and have a high resistance to fatigue failure. Materials commonly used are brass, bronze, beryllium copper, alloys of nickel and copper, steel, and Monel (5). 

Rubber processed in latex form accounts for about 10% of new mbber consumption. Rubber latex is a Hquid, oil-in-water emulsion which is used to make foam or thin-walled mbber articles. The same accelerators and antidegradants used in dry mbber are used in latex, with longer-chain versions preferred for greater oil solubiHty. To prepare these and other additives for addition to latex, they must be predispersed in water and the surface of the powder or oil droplet coated with a surface-active agent to prevent destabilization (coagulation) of the latex. 

Fast, low temperature curing mbber compounds can be prepared by initial heat prevulcanisation of the Hquid latex and are marketed commercially (Revultex, Doverstrand Corporation). Rubber deposited from these often needs tittle more heat than that required to dry the deposit, to achieve optimum tensile strength and elongation. Such compounds are often used by small companies manufacturing thin-wall dipped medical latex products, such as examination gloves, as few compound preparation facilities are needed by the dipping company. 

Many mechanical properties of wood, such as bending and cmshing strength and hardness, depend upon the density of wood denser woods are generally stronger (6). Wood density is determined largely by the relative thickness of the cell wall and by the proportions of thick-walled and thin-walled cells present. 

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