Sheet stretching

The speed of closure to the form is a function of machine condition and sheet thickness, with the thicker sheets being more difficult to move rapidly. The designer should indicate this to the sheet former when form stability at elevated temperatures is critical and the process must be tailored to improve this condition. One should also select a material which is intrinsically more stable and easily formed to minimize the possibility of unmolding. These factors must be considered in conjunction with the design of the product to minimize sheet stretching differentials in the part. 

Cast and extruded sheets, cell-cast sheets, stretched sheets, films down to 50 microns laminated protective surfaces on ABS, PVC or other plastic sheets that are thermo-formed into parts requiring resistance to outdoor weathering for residential siding and transformer housings... 

Some plastic sheets stretch as much as 600%, others as little as 15%. This behavior directly influences what shapes can be formed and their quality. Those with a putty-like appearance respond to very small pressures others, which tend to be stiff, require heavier operating equipment. The pressure response is somewhat related to the ability to be stretched while hot. 

Another kind of fault block mountains comes from stretching of Earth s crust. A model of this kind of mountains could be made by compacting a 6-in (15-cm) thick layer of moist sand on top of a rubber (not rubberized) sheet. When the sheet stretches, mimicking the elastic properties of the lower crust, the sand will crack along lines perpendicular to the direction the sheet is being pulled. Some of the surface will remain the same height, and some blocks will slide down the sides of the blocks which remain stable. This is particularly noticeable if the top surface of the compacted sand has been dusted with powder. This is a model of the process that formed the mountains in the Basin and Range province. 

Imagine that the A coordinate system is fixed to the background and that the B coordinate system and the composition vector are attached to a rubber sheet. Stretch and shear are applied to the sheet until the axes of the B coordinate system are orthogonal as shown in Fig. 37(b). Looking back into the A coordinate system in the background, the composition vector has changed to... 

Keeler SP (1961) Plastic instability and fractine in sheet stretched ovct rigid punches. PhD Thesis. Massachusetts Institute of Technology, Boston... 

Also in 1963 Raumann reported measurements on specimens prepared from amorphous sheets stretched below the glass transition temperature. The experimental method was identical with that reported earlier for polyethylene. Major features were that as the result of orientation q increased to over five times the isotropic value, and the shear modulus at 90° dropped to about half the isotropic shear modulus other moduli showed only small variations from the isotropic values (Ladizesky and Ward have commented that Raumann s torsional moduli are probably in error, owing to her assumptions of the validity of the St. Venant theory, and an inadequate correction for tensile stress). 

SP Keeler. Plastic Instability and Fracture in Sheet Stretched over Rigid Punches. ScD thesis, Massachusetts Institute of Technology, Cambridge, MA, 1961. 

Although elastic stress-strain behavior describes sheet stretching for most thermoforming processes, sheet deformation during plug-assist prestretching is... 

Uniaxial extension is not the only kind of stretching flow that one can visualize. If Cj = 2 = Cfi, 3 = 2 b, and 0, one has equal biaxial extension physically this can be realized in a sheet-stretching experiment. If one dimension of the sheet is constrained not to change so that 2, say, is zero, then 3 = i. This is called planar extension or pure shear. 

Brake forming Dimpling Beading Drop hammer Rubber press Sheet stretching... 

The sheet stretching takes place at the high-shear zones, whereas the filaments breakup occurs at low-shear zones via Rayleigh instabilities. 

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