Differential thermal shrinkage

The bowing of polyvinyl chloride extrusions caused by differential thermal shrinkage associated with different cooling rates was modelled. The extrusion was considered to consist of a number of rectangular sections, and a graphical solution for the differential equations for... 

At the other extreme to the restrained layer theory is the deformable layer theory where a flexible, deformable phase seems desirable to accommodate stresses set up at the interface due to differential thermal shrinkage between resin and filler when the composite is cooled. There is still considerable confusion and debate over what morphology might be desirable in resin adjacent to treated mineral and metal substrates. The amount of coupling agent in a typical finish is probably insufficient to provide a low-modulus layer at the interface. However, Erickson... 

Stresses from welding result principally from the effects of differential thermal expansion and contraction arising from the large temperature difference between the weld bead and the relatively cold adjacent base metal. Shrinkage of the weld metal during solidification can also induce high residual stresses. Unless these residual stresses are removed, they remain an intrinsic condition of the weldment apart from any applied stresses imposed as a result of equipment operation. 

The radial (compressive) stress, qo, is caused by the matrix shrinkage and differential thermal contraction of the constituents upon cooling from the processing temperature. It should be noted that q a, z) is compressive (i.e. negative) when the fiber has a lower Poisson ratio than the matrix (vf < Vm) as is the normal case for most fiber composites. It follows that q (a,z) acts in synergy with the compressive radial stress, 0, as opposed to the case of the fiber pull-out test where the two radial stresses counterbalance, to be demonstrated in Section 4.3. Combining Eqs. (4.11), (4.12), (4,18) and (4.29), and for the boundary conditions at the debonded region... 

Increased tendency for drying shrinkage and differential thermal cracking. 

The principle that is the basis for bicomponent fibers usually is likened to that which underlies the bicomponent metal strips often used in temperature controllers. With the latter, differential-thermal expansion of the two joined components results in a bending of the thermal element. With fibers, moisture usually is the agent that acts upon the two side-by-side portions. Differential swelling or shrinkage causes the fiber to be brought into... 

The pressure referred to here arises from differential thermal contraction during the cooling stage of manufacture, and from solidification shrinkage of the pofymer (6.N.6). 

Microvoids are formed in the phases due to the differential in thermal shrinkage between the phases as the molded parts cool. There is some evidence that the number and size of these microvoids influences shrinkage control. 

Each of the above four factors has a strong influence on the joint performance. Therefore, the joint must be designed to eliminate stress concentrations, which are not always apparent and may occur as a result of differential thermal expansion of adhesive-adherend, and shrinkage of adhesive on curing. Air can also become entrapped at the interface if the adhesive does not flow easily during curing or does not wet the substrate. The types of stress found in adhesive j oints are compression, shear, tension, peel, and cleavage (see Fig. 

Phenolic glass and a diallyl phthalate glass material are available with very low shrinkage. Glass and other mineral fillers minimize the thermal expansion differential problem. Phenoxy and polyphenylene oxides are examples of being low in shrinkage and thermal expansion. 

Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4>5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history. 

Thermosets are often used in intimate contact with materials of much lower coefficient of linear thermal expansion p. A thermoset film may be coated on a surface, or sandwiched between two surfaces as an adhesive. A thermoset matrix may be filled with high-modulus fibers in aerospace structural materials and in other composite materials. A residual stress other material(s) to changes in temperature. The value of or typically reflects the balance between the driving force to produce residual stresses due to differential shrinkage upon cooling and the temperature-dependent capacity to relax these stresses. The worst such effects... 

For prediction of mold filling and flow-through dies, computer modelling is increasingly used. In addition to viscosity, measurements may be needed for specific heat, shrinkage, thermal conductivity, and pressure- volume -temperature (PVT) relationship. A differential scanning calorimeter (DSC) can be used for specific heal measurements. 

The variation in coefficients of thermal expansion between polymers gives difficulties where materials are composites or where several different material types are in close contact On cooling, the differential shrinkages may generate stress between the materials, causing them to fail or to come apart from each other. 

---