Particle Packing in Ceramic Injection Molding

The shrinkage of ceramic injection molding suspensions caused by pyrolytic removal of the organic vehicle was measured for initial ceramic volume fractions from 0.48 to 0.64. Shrinkage was inversely related to initial ceramic volume fraction V , and maximum volume fraction after pyrolysis V *,_max was 0.65. The factors which restrict shrinkage are discussed. The maximum volume fraction, obtained from semiempirical equations relating the viscosity of suspensions to volume fraction of powder ( V _max) was 0.73 to 0.76. Both the viscosity of the suspensions and the shrinkage on removal of the organic vehicle can be interpreted in terms of a free volume of fluid over and above that needed to fill interstices between contacting particles. Thermomechanical measurements also show that the free-volume concept helps to interpret the transition from fluid to quasi-solid properties as the organic vehicle is removed by pyrolysis from a molded body.     

Shrinkage anisotropy in fiber reinforced injection molded products

This paper includes a systematic study of the effect of fiber concentration and molding conditions on fiber orientation and shrinkage in injection molded composites. Closed-form expressions were derived to relate shrinkage and internal stresses to the molding pressure and fiber orientation. The shrinkage predictions were seen to agree well with experimentally measured shrinkages.     

Investigation of shrinkage control in Ni–Al2O3 (Metal–Ceramic) functionally graded materials

Nickel–alumina (metal–ceramic) graded composites were fabricated based on empirical relationships that were used to predict shrinkage expected during the sintering process. Compositions ranged from pure Al₂O₃ to nickel. Compositions with different sintering behaviors were composed of Ni powders having 3-μm particles and Al₂O₃ powders having 0.16-μm particles that were mixed in specific volume percentages. The green and sintered densities were measured for each layer of the multi-layered samples. Based on these measurements, an empirical formulation for shrinkage was derived to express the relationship between green and sintered densities. Shrinkage was predicted from the observed decrease in porosity during sintering. This was used to establish a relationship between shrinkage and composition. Dramatic shrinkage, which can lead to cracking of the sample, was resolved by minimizing shrinkage differences within the functionally graded materials using the derived empirical formulation for shrinkage; internal cracking was significantly reduced by maintaining a consistent shrinkage gradient.     

Factors Influencing Anisotropic Sintering Shrinkage in Tape-Cast Alumina: Effect of Processing Variables

The effect of the processing variables shear rate, solids loading, and sintering temperature on the anisotropy of sintering shrinkage of aqueous tape-cast alumina was studied. Higher shear rates and higher solids loading resulted in higher in-plane shrinkage anisotropy, whereas the shrinkage anisotropy in the thickness direction was higher for low solids loadings. The in-plane shrinkage anisotropy was found to be fairly constant above a certain critical shear rate (∼100 s⁻¹) independent of the solids loading. The shrinkage anisotropy through the thickness was higher than in-plane directions. A higher thickness direction sintering rate was observed and attributed to a greater number of interparticle necks in the thickness direction because of the platy nature of alumina particles and the greater thickness direction strains associated with binder removal. The binder did not significantly affect the in-plane sintering shrinkage but significantly affected the shrinkage in the thickness direction. It was suggested that emulsion binder particles occupy sites in between layers of particles in the thickness direction. The degree of anisotropic shrinkage was quantified using edge orientation polarograms and a direct correlation was obtained between the processing variables, shrinkage anisotropy, and the edge orientation index.     

DE-AIRING AS CORRECTIVE IN DRYING1

The physics of mechanical removal of water from clay involves character of pores, adsorption capillarity and it also involves air filled cavities. Theory of clay particle system when plastic is set forth and argument is made for the effect of air on water removal and on drying shrinkage.     

Effect of stretching conditions on the properties of polyester textile yarns

Conclusions By varying the temperature of the heating roll and, correspondingly, the magnitude of the natural stretch ratio, one can obtain a required level of change in shrinkage in a definite sequence; and by periodicity of removal of the yarn being stretched from the heater, one may obtain lengthwise variation in shrinkage of polyester textile yarn.Translated from Khimicheskie Volokna, No. 2, pp. 37–39, March–April, 1987.     

The mechanical behavior of polymer coatings: Solvent removal in polyamic acid films

The development of the mechanical properties and stresses during the formation of polyamic acid (PAA) coatings has been studied using methods such as a “propagating wave technique.” The shrinkage stress due to solvent removal in PAA coatings under a one dimensional constraint was determined to be 8 MPa. A coupling between the residual stress and solvent removal process has been observed.     

Approximate model for analyzing frozen-in strains and shrinkage of extruded PVC lineal profiles

A mathematical model, based upon utilization of an “approximate” time-temperature superposition for estimating stress-strain properties of PVC, was derived for analyzing shrinkage of extruded PVC profiles. The model is composed of a single deformation, followed by a strain recovery (shrinkage) at the same or a different temperature. Profile shrinkage at any given time and temperature can be reduced by decreasing the magnitude of the imposed frozen-in strain, increasing the temperature at which the strain is imposed, and/or increasing the time that is required to impose the strain. The model provides good estimates of the shrinkage of extruded profiles.     

Constitutive Model for Predicting Ultimate Drying Shrinkage of Concrete

A constitutive model is derived from theory of elasticity for predicting ultimate drying shrinkage of concrete. The model was extended by incorporating the semiempirical composite model proposed by Hirsch and Dougill for predicting Young's modulus of concrete. Their composite model is the geometric mean of Paul's upper and lower limit boundaries of a two-phase composite. According to the shrinkage model the parameters needed for predicting ultimate drying shrinkage of concrete at any relative humidity of drying are the following: ultimate shrinkage of a paste of same water-to-cement (W/C; ratio and degree of hydration as the concrete, relative volume of aggregates and unhydrated cement, and the elastic properties of hydrated paste and the particles. The shrinkage model was tested on shrinkage results obtained in this study and by Pickett. Three different W/C ratios were covered together with a wide range in aggregate contents. Excellent agreement with the results was found.     

Effect of particle shape and size on the shrinkage of stainless steel compacts

The effects of particle size and shape on the shrinkage of commercial 316 L stainless steel compacts are evaluated. Emphasis is placed on the effect of changes in surface area and the anisotropy in inter-particle contacts brought about by the irregular powders. The effects of fine particle addition on this anisotropy and surface area are presented and plained. An empirical relationship is derived which can be used to predict changes in the axial, radial and volume shrinkage when spherical particles are substituted into a commercial blend. The effect of spherical particle substitution on the shrinkage ratio is also discussed. It is shown how the combined effects of fine particle additions and substitutions of spherical particles for irregular particles can be used to produce a compact which exhibits a shrinkage ratio (radial to axial shrinkage) of unity.     

Properties of PET fibers with high modulus and low shrinkage (HMLS). I. Yarn properties and morphology

Classical morphological analysis has been performed on novel PET fibers of high modulus and low shrinkage (HMLS). As expected, amorphous orientation controls the degree of shrinkage and tenacity. The uniqueness of these materials is derived from a high “effective” crosslink density which results in a high retractive force during elevated temperature shrinkage and significant stress-amplification during room temperature extension. Although the morphological origin of the high effective crosslink density is unknown, it is speculated that the interfibrillar regions contribute to the observed behavior by suppressing yielding.     

Shrinkage of Tape Cast Products During Binder Burnout

During sintering of tape cast products, anisotropic shrinkage occurs, which can be attributed to an anisotropic green tape structure concerning particle and pore orientation. Little is known about the shrinkage during binder burnout (BBO) and its relation to the microstructure of green tapes including the binder–plasticizer phase. Therefore, the article determines the shrinkage behavior of green tapes derived from alumina powders with different particle shape during binder burnout and prefiring in all spatial directions. The shrinkage after prefiring relative to the green and the debindered states is also discussed. The interrelation between shrinkage behavior and microstructure is investigated in dependence on different process parameters and specifically on the thermal behavior of the binder–plasticizer phase in the green tapes. It is shown that the subtraction of the BBO shrinkage from the total shrinkage results in completely different data for the sintering shrinkage anisotropy in z direction.     

Sensitive Resonant Frequency Technique for the Study of Sintering Kinetics

A technique is described for following the progress of sintering based on changes in the flexural resonant frequency of a prismatic bar. The relations involved are defined and a simple relation between the linear shrinkage and the resonant frequency is derived. These two are related by a multiplier which is shown to be a function of the initial porosity and the elasticity-porosity relation of the material. Typical values of the multiplier are large (>25), making this a very sensitive method of detecting shrinkage. Sintering-related shrinkage was observed in yttrium oxide, erbium oxide, dysprosium oxide, and aluminum oxide at very low temperatures, often as low as two tenths of the absolute melting temperature.     

EQUATIONS AND TABLES FOR SHRINKAGE,EXPANSION AND DESIGN CALCULATIONS1

Attention is drawn to an error in the equation for calculating linear shrinkage (per cent of dry length) from volume shrinkage (per cent of dry volume) which was given in the Report of the Committee on Standards, American Ceramic Society , Year Book, 1921–22. In order to avoid errors due to the use of incorrect equations, in future work, the more useful equations for making the ordinary shrinkage, expansion, and design calculations are derived and tabulated. To facilitate numerical computation, four figure tables are given by means of which the labor involved in the more usual ceramic calculations involving shrinkage and expansion can be greatly reduced. The use of the equations and tables is explained and illustrated by means of examples.     

Some factors influencing cooling rates of rotationally molded parts

Recently, rotational molding engineers, concerned with warpage and uneven cooling in parts, have been “pre-cooling” the mold in forced draft air after removal from the oven and prior to water quenching to removal temperature. In this paper, we analyze some of the factors that influence the rate of heat removal from an amorphous plastic in a metal mold. We find that mold thickness and thermal diffusivity, convection heat transfer coefficient of the cooling fluid, the thermal properties of the plastic and the initial, final and “freezing” temperatures of the plastic influence this cooling rate and the corresponding rate of volumetric shrinkage. We illustrate our analysis with several examples and discuss some guidelines in detail.     

Effect of Temperature on the Shrinkage Rate of Compacts Composed of Nanoparticles

An expression describing the shrinkage rate of compacts composed of nanoparticles is obtained under the assumption that nanoparticles undergo fluctuation melting. The expression derived has the following form: dY/dt = Aexp(−(T _M + q)/T), where dY/dt is the shrinkage rate; T _M is the melting temperature of nanoparticles; T is the sintering temperature; and A and q are constants that depend on the material, the initial porosity, and the size of nanoparticles in the compact. The experimental data on the shrinkage rate of compacts composed of copper and nickel nanoparticles 70 nm in size are compared with the shrinkage rates calculated according to the derived expression. This comparison shows that the experimentally measured shrinkage rates are close to the calculated values.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Stepanov, Alymov, Evstratov.     

High-Temperature Reactions of Clay Mineral Mixtures and Their Ceramic Properties: III, Shrinkage and Porosity in Relation to Initial Mineralogy

Little exact information exists on the relation of mineralogical composition of clay materials to their properties after firing. Property-composition diagrams are constructed for linear shrinkage and apparent porosity of quartz-kaolinitemica mixtures fired at 1100° and 1200°C.; contours of equal shrinkage and equal porosity are derived. Sixty natural clays and shales are analyzed semiquantitatively for mineral content, and their shrinkage and porosity values are obtained after firing to 1100° and 1200°C. The ceramic properties of the natural materials are compared with those of the prepared mixtures. Conclusions are drawn regarding the influence of mineralogical composition on the ceramic properties. The mica content is shown to be particularly important because of its role in the for mation of a glassy component.     

400m~3球罐焊缝组对几何尺寸控制

介绍了 4 0 0m3 球罐分带装组法的组装程序及控制纵焊缝角变形和吊装变形的方法 ;对带板纵焊缝间隙影响直径收缩量进行了探讨 ,得出了组对平均间隙与直径总收缩量的关系。实践证明 ,对容积较小的球罐运用分带组装法组装 ,可以减少高空作业和较容易地控制变形 ;从整体成形分析 ,只要找出准确的组对间隙和直径收缩量的关系 ,就可以控制各带之间的组对几何尺寸和成形质量 ,使球罐都达到标准     

Separation of drying strains and the calculation of drying stresses considering the viscoelasticity of red pine wood during drying

The objective of this study was to separate drying strains into elastic, viscoelastic, and viscous strains using free shrinkage and recovery from the deformation of slices at stress relief. The apparent shrinkage deformation was obtained by measuring the change in width of drying specimens during drying. Using the slice method, elastic and viscoelastic deformation were defined as the instantaneous change in width of a slice right after cutting and the change in the width of a slice with constant moisture content during 48?h, respectively. Viscous deformation, permanent and nonrecoverable deformation of wood, was defined as the difference in deformation between free shrinkage and the sum of the apparent, elastic, and viscoelastic deformations. These elastic, viscoelastic, and viscous strains were applied to a viscoelastic model, and coefficients of viscoelasticity and viscosity were derived. The drying stress and deformation of red pine wood at specific times during the drying process can be predicted using each coefficients and modulus of elasticity obtained by experiment.     

Thermal conductivity of ceramic green bodies during drying

The thermal conductivity of alumina and kaolin green bodies has been studied as a function of the water loss during drying. Experimental measurements show strong variations with 3 distinct regimes. In the first regime, thermal conductivity increases during shrinkage. When shrinkage stops, a decrease in thermal conductivity with water loss is observed which becomes even stronger during the last phase of drying. This can be explained by the variations in the volume fractions of each phase and the effective thermal contacts between grains. Using analytical relations, the thermal resistance of an equivalent plane of small area grain-grain contacts is shown to increase strongly at the end of drying due to the removal of water. Finally, in certain drying conditions, if a portion of the heat required for drying, is supplied by conduction through the green body, then the rate of water evaporation increases with higher thermal conductivity.