Thermal Stresses in Nonlinearly Viscoelastic Solids

1992 ◽  
Vol 59 (2S) ◽  
pp. S43-S49 ◽  
Author(s):  
Giancarlo U. Losi ◽  
Wolfgang G. Knauss
Keyword(s):  
Glass Transition ◽  
Residual Stresses ◽  
Free Volume ◽  
Thermal Stresses ◽  
Morphological Changes ◽  
Polymer Chains ◽  
Transition Range ◽  
Rheological Models ◽  
Molded Parts ◽  
Injection Molded

Three different rheological models are applied to the study of transient and residual thermal stresses in amorphous polymers cooled across the glass transition. The models differ mainly in their treatments of the nonequilibrium (time-dependent) portion of the morphological changes in the polymer and their influence on the relaxation process. The interstitial volume between polymer chains (free volume) is found to play an important role in the residual stresses; they are affected by the relative time scale of thermal diffusion and thermoviscoelastic relaxation/creep. This result has implications for injection molded parts of different section dimensions and for extrusion products. This fact must also be accounted for in determining the thermomechanicalproperties in the glass transition range. The step cooling ofPVAc spheres (1 and 20 mm dia.) and a cylinder (20 mm dia.) have been considered; most of the results presented apply to the sphere(s). Residual stresses can vary by as much as 100percent depending on whether the interstitial molecular (free) volume is counted or not. It is also demonstrated that residual stresses can be higher than an elastic analysis based on the glassy properties would suggest; thus the “stressfree temperature” is found to be significantly above the glass transition.

Prediction and measurement of residual stresses in injection molded parts

2001 ◽  
Vol 2 (4) ◽  
pp. 203-211 ◽  
Author(s):  
Young Il Kwon ◽  
Tae Jin Kang ◽  
Kwansoo Chung ◽  
Jae Ryoun Youn
Keyword(s):  
Residual Stresses ◽  
Molded Parts ◽  
Injection Molded

Applicability of Free Volume Concept to Relaxation Phenomena in the Glass Transition Range

1968 ◽  
Vol 41 (3) ◽  
pp. 555-563
Author(s):  
A. J. Kovacs
Keyword(s):  
Glass Transition ◽  
Free Volume ◽  
Excess Entropy ◽  
Heat Content ◽  
Configurational Entropy ◽  
Wide Distribution ◽  
Dielectric Constants ◽  
Experimental Point ◽  
Transition Range ◽  
Glass Transition Range

Abstract Except for discrepancies mentioned with respect to pressure investigations, which need future clarification, we can conclude in a general way, as follows. As far as only average parameters of macroscopic specimens are considered (complex moduli, or dielectric constants, volume or heat content etc...), the free volume concept can relate variations of molecular mobility to changes of an average free volume in a semiquantitative way. This average free volume can no longer fully characterize the wide variety of molecular motions involved in the kinetics of redistribution of holes in the liquid during recovery experiments. These kinetic processes involve a wide distribution of retardation times, which may be associated with local distribution of holes, or with that of cooperating groups of molecules, or molecular segments. On the other hand, free volume is not necessarily the fundamental molecular parameter which controls rate of configurational changes, characterized by variation of entropy of the liquid. Even if this is the case, most of the above discussion may be applied to any other average excess parameter, as far as the Doolittle equation is formally adopted, in which f/b is expressed in terms of the new parameter, rather than that of free volume. However, since relaxational free volume, as determined from the WLF equation, and independently measured volume changes are often in close agreement, this means that variations of excess entropy, or those of configurational free energy, and changes in volume are closely related. This conclusion is in agreement with that of Eisenberg and Saito, who found that the Gibbs—Dimarzio theory, based on configurational entropy, is practically equivalent with the free-volume approach. Thus, the free volume concept remains still a valuable tool for unifying different kinds of rate processes from both a theoretical and an experimental point of view, especially in the glass transition range.

Residual stresses determination in injection molded virgin and recycled HDPE blends: mechanical properties and morphology

e-Polymers ◽  
2008 ◽  
Vol 8 (1) ◽  
Author(s):  
Mirigul Altan ◽  
Mehmet Emin Yurci ◽  
Nihan Nugay
Keyword(s):  
Elastic Modulus ◽  
Impact Strength ◽  
Residual Stresses ◽  
Thermal Stresses ◽  
Mold Temperature ◽  
Morphological Properties ◽  
Time Layer ◽  
Melt Temperature ◽  
Injection Molded ◽  
Recycled Hdpe

AbstractAn experimental study of residual thermal stresses has been carried out in injection molded virgin and recycled high density polyethylene (HDPE) blends. Effects of blend concentrations on residual stresses were investigated under different injection conditions such as melt temperature, mold temperature and cooling time. Layer removal technique was used for measuring residual stresses. In order to determine the relation between the residual stresses and material characteristic of HDPE blends, mechanical and morphological properties of the blends were also investigated. Elastic modulus and impact strength were important key factors for determining the blend characteristics. As a result, it was found that HDPE blends gave higher residual stresses but lower impact strength with higher elastic modulus when recycled concentration was increased. Furthermore, it was seen that shape and size of the crystallites were also effective on residual stresses. Small and spherulitic crystallite structured blends such as 30 % recycled HDPE induced reduction in residual stresses due to easier relaxation with lower elastic modulus and higher impact strength while lamellar crystallite structured blends such as 50 % recycled HDPE gave higher elastic modulus but lower impact strength with higher residual stresses.

Analysis of polymer crystallization and residual stresses in injection molded parts

2014 ◽  
Vol 15 (1) ◽  
pp. 89-96 ◽  
Author(s):  
Roberto Spina ◽  
Marcel Spekowius ◽  
Rainer Dahlmann ◽  
Christian Hopmann
Keyword(s):  
Residual Stresses ◽  
Polymer Crystallization ◽  
Molded Parts ◽  
Injection Molded

Comparative Analysis of Different Methods in Residual Stress Measurement for Polymeric Parts

2013 ◽  
Vol 837 ◽  
pp. 175-178
Author(s):  
Alexandra Raicu
Keyword(s):  
Residual Stress ◽  
Residual Stresses ◽  
Stress Measurement ◽  
Polymeric Materials ◽  
Injection Molding Process ◽  
Uniform Temperature ◽  
Molding Process ◽  
Molded Parts ◽  
Injection Molded ◽  
Technological Constraints

This paper presents the measurements of the residual stresses for polymeric parts using different methods. The residual stresses are usually introduced during manufacturing and are caused by processes such as molding. In order to optimize injection molding process with polymeric material, it is important to predict the internal stress development during molding. The residual stresses are caused mainly by non-uniform temperature profile in the cavity during filling, packing and cooling stages. This research offers information and a methodology which may be applied in practical conditions for a large number of parts manufactured from the different polymeric materials and for several technological constraints. The author confirmed that all this methods which measure the residual stresses can be applied to injection molded parts.

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