PTFE has an unusually high melt viscosity, which prevents use of common processing technologies, such as extrusion and injection molding. The extremely high molecular weight is responsible for the rubberlike behavior of PTFE melts. For this reason, alternative processing technologies like sintering have been developed. In this process, PTFE fine powder is compressed inside a steel mold; a block is obtained, which is then heated above melting temperature for a time sufficient to allow for the particles to coalesce. During heating, the material is neither confined nor subjected to any applied load.
A typical application of PTFE is as thin film, skived from sintered cylindrical blocks. ptfe teflon sheet The very low thermal conductivity of the material is responsible for high temperature gradients during sintering, which in turn initiate thermal stresses. As a consequence, out-of-plane deformations are observed in the skived film. Thermal stresses may even induce fracture during sintering, but this is an uncommon event. To reduce them, heating and cooling rates must be very slow, and in industrial practice a few intermediate isothermal steps are applied to reach thermal equilibrium; industrial sintering cycles may last as long as one week.
A simulation of the sintering process may lead to an optimization of processing conditions. In fact, knowledge of the effective thermal history at every site of the block may ensure overall good sintering, selecting the best compromise between cycle duration and residual stresses. Jog et al. (1) proposed a computer simulation of temperature profiles during the sintering cycle of PTFE billets using heat transfer equations.
An extensive literature deals with theoretical predictions of residual stresses. The existing theories are based on constitutive equations of linear elasticity (2) and linear viscoelasticity (3). The first mathematical analyses of thermal-stress development during cooling based on linear elasticity were reported long ago (4, 5). Transient thermal stresses were predicted in both thin plates and cylinders. Later, various authors (6, 7) calculated the transient thermal stresses along the longitudinal, radial, ptfe teflon sheet and angular directions from the temperature field in a cylinder. According to linear elasticity, transient thermal stresses become equal to zero as soon as the temperature in the sample becomes uniform. In reality, however, there are residual thermal stresses frozen in the sample at this point. To avoid this deficiency, the theories based on linear elasticity have incorporated the thermoelastic response of the medium together with some assumptions regarding material behavior above and below glass-transition temperature (8-11). Some authors (9, 10) propose that [T.sub.g] separates the fluid and elastic behavior of the medium: above [T.sub.g], the medium is plastic and strains may develop instantaneously with no internal stresses, while below, the medium is elastic, thermal strains are residual, and internal stresses are unrelaxed.
The theories based on linear elasticity cannot predict the transient thermal stresses. Thus, during the 1960s, the theories of thermal stresses based on linear viscoelasticity evolved (12-15). Nonisothermal viscoelasticity with time- and temperature-dependent relaxation modulus was employed.
In the present study a finite element model has ptfe product been used to predict temperature, deformations, and stresses during sintering of cylindrical PTFE blocks. The parameters necessary to describe the material behavior were determined experimentally. Their determination, which has proven to be quite complex, has revealed interesting phenomena related to orientation and memory effects in PTFE. These effects have been reported by several authors, notably Yamaguchi et al. (16). Finally, different experimental techniques were used to verify data obtained from simulation. In particular, residual stress measurements were performed using a layer-removal technique (17), a variant of a method that has been applied to plastic tubes (18, 19).
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