Amorphous polymers are generally useful only if ther Tg is above ambient temperature; otherwise they creep in service. This means that strains imposed during thermoforming need not be plastic since quenching from the forming temperature, which is above Tg, can freeze the viscoelastic deformation. so amorphous thermoplastics are thermoformed just above Tg and cooled in the die to 'fix' the shape. this enables relatively low temperatures and low cycle times to be used.
(Edwards, L., Endean M., MAterials in Action Series: Manufacturing With Materials,1995,pg.194)
Neutron Diffraction
Neutron Diffraction (ND) is capable of measuring the elastic strains induced by residual stresses throughout the colume of relatively thick stess components with a spatial resolution as small as 1 mm^3. Such capabilities provide for residuals-tress measurement inside components without sectioning or layer removal. Principal ND methos, as with XRD methods, measure the spacing between crystallographic planes in a component, and this spacing is affected by residual and applied stress. The spacing between a selected set of a crystallographic planes is related to the angle of incidence and diffraction of where teh neutron radiation, which are equal, and the wavelength of the monochromaric radiation by Bragg's Law:
λ= 2 d sin θ
The elastic strain induced bt the residual stress perpendicular to teh diffracting crystallographic plane then is related to d by:
ε= (d-d0)/do= sinθ0(sinθ-sinθ0)
(Totten, G., Howes, M., Inoue, T., Handbook of Resifual Stress and Deformation, 1st Printing, March 2002, pg.113)
Springback
The stress state is extremely complex in bending. The complete tensile and compressive stress-strain curves of the material are traversed on rthe tensile and compressive sides of the bend, respectively. This means that around the neutral plane the stresses must be elastic. When the forming tool is retracted, the elastic components, of the stress cause springback, and a residual stress pattern. Springback increases both the angle and radius of the bent part. The elastic zone is more extensive for a relatively gentle bend(large Rb/h ratio) and for a material with high ratio of yield strength σ0.2 to elastic modulus E; therefore springback also increases according to the approximate formula
Rb/Rf=1-3(Rb*σ0.2)/(h*E)+4((Rb*σ0.2)/(h*E))^3
where Rb is the radius of the bending die and Rf is the radius obtained after the forming spressure is released.
Springback established a new force equilibrium with a residual stress distribution typified by a compressive stress on the outer and tensile stress on the inner surface
Several techniques are used to combat springback:
1. If springback for a given material is known and if the material is of uniform quality and thickness, compensation for springback is possible for overbending.
2. Alternatively, the elastic zone can be eliminated at the end of the stroke by one of two means. First, the two ends of the sheet may be clamped before the punch bottoms out, so that the end of the stroke involves stretching of the part, causing tensile yielding in the entire sheet thickness. In the second method the punch nose is shaped to indent the sheet, so that plastic compression takes place throughout the thickness
3. If a counterpunch is used with a controlled pressure, compressive stress are maintained in the bend zone during the entire process. Since this also has the effect of imposing a hydrostatic pressure on the bend zone, bending beyond the limits given by
Rb=h(1/2q-1) for q<0.2
Rb=h((1-q)^2)/(2q-q^2) for q>0.2
4. Less ductile materials may have to be bent at some elevated temperature; because the yield strength is lower, springback is also less.
(Schey, J.A., Introduction to Manufacturing Process, 2nd Edition,pg.299,300,301)
ISO 14000
ISO 14000 is a family of standards first published in September of 1996 and pertaining to international Environmental Management Systems (EMS). It concerns the way an organization's activities affect the environment throughout the life of its products. These activities (a) may be internal or external to the organization (b) range from production to ultimate disposal of the product after its useful life, and (c) include effects on the environment, such as pollution , waste generation and disposal, noise, depletion of natural resources, and energy use.
A rapidly increasing number of companies in many countries ( with Japan leading) have been obtaining certification for this standard. The ISO 14000 family of standards has several sections: Guidelines for Environmental Auditing, Environmental Assessment, Environmental Labels and Declarations, and Environmental Management.
(Kalpakjian S., Schmid S.R.,Manufacturing Engineering and Technology, 5th Edition, pg.1119)
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