Numerical and experimental investigation of shrinkage behavior of precision injection molded articles. II. Results and discussion

In the accompanying paper, Part I, presented are the physical modeling and numerical formulation of new lateral motion modelings. In Part II, new models developed in Part I are validated by the successful comparison of calculated residual stress profile with the literature data. The predicted results of the birefringence, residual stress distribution, and shrinkage from new lateral motion modeling are in better agreement with corresponding experimental data than those from the conventional ones. The new model prediction falls between those of two extreme cases corresponding to conventional models. As a result of extensive parametric study of processing conditions, the developed analysis system is found to be capable of successfully predicting the tendency of shrinkage behavior varying with most of processing conditions. In this regard, the new model enables better analysis based design and optimization of precision injection‐molded products. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Residual stress development in neat poly(etheretherketone)

Material property models for poly(etheretherketone) (PEEK) have been combined with a residual stress model to provide a means for investigating the effect of crystallization process on the residual stress development in semicrystalline materials. The analysis shows that crystallization causes an increase in the residual stress levels. This increase is affected through an increase in the resin modulus values and through the resin modulus build-up at higher temperatures. The shrinkage due to crystallization was found to have no effect on the residual stress development in neat PEEK.     

Analytical estimate for curing‐induced stress and warpage in coating layers

A thermoset coating that is applied to an elastic substrate will develop residual stresses during curing because of polymerization shrinkage of the resin. This shrinkage only partly contributes to the residual stresses because, before gelation, the stresses relax completely. In this study, we developed explicit analytical expressions for the curing efficiency factor, the residual stresses, and the resulting warpage. We did this by assuming that after gelation, the material was in its rubbery state and that viscoelastic effects were absent. A difference between the free and constrained warpages during curing was made. The analytical warpage models were shown to give results comparable to those of the numerical calculations with a fully curing‐dependent viscoelastic material model. Furthermore, for the first time, accurate analytical expressions for the stress‐free temperature and stress‐free strain were obtained. With these expressions, the effect of curing shrinkage on the residual stresses could easily be incorporated into existing (numerical) stress analysis without the need for extensive curing‐dependent viscoelastic material models. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012     

Numerical prediction of residual stresses and birefringence in injection/compression molded center‐gated disk. Part I: Basic modeling and results for injection molding

The present study attempted to numerically predict both the flow‐induced and thermally‐induced residual stresses and birefringence in injection or injection/compression molded center‐gated disks. A numerical analysis system has been developed to simulate the entire process based on a physical modeling including a nonlinear viscoelastic fluid model, stress‐optical law, a linear viscoelastic solid model, free volume theory for density relaxation phenomena and a photoviscoelasticity and so on. Part I presents physical modeling and typical numerical analysis results of residual stresses and birefringence in the injection molded center‐gated disk. Typical distribution of thermal residual stresses indicates a tensile stress in the core and a compressive stress near the surface. However, depending on the processing condition and material properties, the residual stress sometimes becomes tensile on the surface, especially when fast cooling takes place near the mold surface, preventing the shrinkage from occurring. The birefringence distribution shows a double‐hump profile across the thickness with nonzero value at the center: the nonzero birefringence is found to be thermally induced, the outer peak due to the shear flow and subsequent stress relaxation during the filling stage and the inner peak due to the additional shear flow and stress relaxation during the packing stage. The combination of the flow‐induced and thermally‐induced birefringence makes the shape of predicted birefringence distribution quite similar to the experimental one.     

注塑工艺参数对制品残余应力和收缩的影响

注塑成型工艺参数对制品的最终残余应力和收缩有着直接的影响。基于线性黏弹性模型模拟计算了注塑成型过程中由温度和压力引起的残余应力和收缩。以无定型材料PS和ABS为例,系统地研究了不同成型工艺条件下平板制件的最终残余应力和收缩,并和实验结果进行对比验证。结果表明：在流动方向上无定型材料的收缩基本保持不变,残余应力沿壁厚分布的形状也基本相同,但流动末端处的应力值稍大于流动入口处;保压压力是影响制品收缩的关键因素,提高保压压力和注射温度可以降低制品的最终收缩,而模具温度对收缩的影响较小。     

Assessment of residual stresses during cure and cooling of epoxy resins

A new stress monitoring technique, a stress-tracking device, is described here. It has been used to study some important properties of epoxy resin. Residual stresses, including a curing shrinkage stress and a cooling shrinkage stress, were measured automatically and continuously during curing and cooling. Simultaneously, information such as an apparent gelation time and glass transition temperature were obtained directly during the experiment. These epoxy resin properties were related to the extent of cure. Varying cure temperature produced changes of cure behavior, which resulted in different residual stresses.     

Numerical prediction of residual stresses and birefringence in injection/compression molded center‐gated disk. Part II: Effects of processing conditions

The accompanying paper, Part I, has presented the physical modeling and basic numerical analysis results of the entire injection molding process, in particular with regard to both flow‐induced and thermally‐induced residual stress and birefringence in an injection molded center‐gated disk. The present paper, Part II, investigates the effects of various processing conditions of injection/compression molding process on the residual stress and birefringence. The birefringence is significantly affected by injection melt temperature, packing pressure and packing time. However, the thermally‐induced birefringence in the core region is insignificantly affected by most of the processing conditions. On the other hand, packing pressure, packing time and mold wall temperature affect the thermally‐induced residual stress rather significantly in the shell layer, but insignificantly in the core region. The residual stress in the shell layer is usually compressive, but could be tensile if the packing time is long, packing pressure is large, and the mold temperature is low. The lateral constraint type turns out to play an important role in determining the residual stress in the shell layer. Injection/compression molding has been found to reduce flow‐induced birefringence in comparison with the conventional injection molding process. In particular, mold closing velocity and initial opening thickness for the compression stage of injection/compression molding have significant effects on the flow‐induced birefringence, but not on the thermal residual stress and the thermally‐induced birefringence.     

N‐(2‐hydroxypropyl)methacrylamide‐based polymer conjugates with pH‐controlled activation of doxorubicin. I. New synthesis,physicochemical characterization and preliminary biological evaluation

New method of synthesis of water‐soluble polymer‐drug conjugates, exhibiting remarkable anticancer activity in mice models, has been developed. In the conjugates, an anticancer drug doxorubicin (DOX) is attached to a polymer carrier based on N‐(2‐hydroxypropyl)methacrylamide (HPMA) copolymer via a hydrolytically labile hydrazone bond. New methacrylamide‐type comonomers, containing either hydrazide group or hydrazon of DOX, were used for copolymerization with HPMA. In contrast to the synthetic procedure described earlier the new method is simpler, cheaper, and results in a better‐defined conjugate structure. The conjugates are fairly stable in buffer at pH 7.4 (model of blood stream) but release DOX under mild acid conditions modeling the tumor microenvironment. The conjugates showed significant in vivo antitumor activity in treatment of T‐cell lymphoma EL‐4 bearing mice with up to 100% long‐term survivors. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Multi-parameter models of the viscoelastic/plastic mechanical properties of coatings via combined nanoindentation and non-linear finite element modeling

Coatings deteriorate from a variety of failure mechanisms. To improve coating durability and/or enable structure-performance correlations, it is necessary to develop more advanced methods of mechanical characterization. For complex multifunctional coatings, multi-parametric constitutive models that simultaneously account for elastic, viscoelastic, and plastic mechanical properties should be used, especially when mechanical properties in the (macro-scale) bulk state differ from properties that occur as a result of thin-film application, post-treatment processes, or aging effects. The nanoindentation creep experiment combined with non-linear finite element modeling of nanoindentation is an effective tool for characterizing the properties of such coatings. Three- and four-parameter viscoelastic/plastic finite element models, implemented using the ABAQUS™ commercial finite element software, have been developed to simulate the isotropic indentation response of coatings. Unified constitutive models where both plastic and viscoelastic deformation are considered simultaneously have not been published previously within the indentation modeling literature. The parameters are determined by an optimization program that automatically matches the load vs. indentation deformation plot from the nanoindentation experiment, with the load vs. indentation deformation plot obtained by the finite element simulation. The computed parameters become a unique “thumbprint” for a particular coating. These parameters may then be used as input data for more complex simulations, for example, capable of computing stress and strain fields, strain energy dissipation, residual stress, and residual strain during particulate scratching; or various other forms of mechanical loading.     

Analysis of the development of isotropic residual stresses in a bismaleimide/spiro orthocarbonate thermosetting resin for composite materials

In this article, we extend our model of isotropic residual stress development in thermosets to a novel thermosetting resin system: bismaleimide/spiro orthocarbonate. In this system, the cure shrinkage and resulting isotropic residual stresses are reduced through a ring‐opening reaction that occurs independently of the addition reaction. The modeling effort includes a parametric analysis of the effects of various parameters, including the volume changes involved in the reactions, the relative rates and orders of the reactions, the cure history, and the values of the bulk moduli and thermal expansion coefficients. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 227–244, 2003     

High‐Pressure Treatment of Wood – Combination of Mechanical and Thermal Drying in the “I/D Process”

Thermal drying of materials with internal pores is always a time‐consuming and energy‐intensive step within a production process. For chemical and pharmaceutical mass products and, in particular, for wood as an important raw material it is desirable to reduce the water content before thermal treatment by mechanical operations. The wood‐processing industry, facing a rising stress of competition, is forced more than ever to offer high‐quality products at lowest prices. Today, drying of timber is mostly done by air drying or by technical drying in kiln dryers. In any case, drying is necessary to prevent deterioration in quality by shrinkage, formation of cracks, discoloration or infestation. A new process of dewatering wood by combining mechanical and thermal means has been developed at the University of Karlsruhe. Compared to conventional drying processes, short drying times and a low residual moisture content can be achieved and, thus, energy consumption and costs can be reduced. In industrial wood drying only thermal processes (e.g., convective kiln drying, vacuum drying, etc.) have been established because so far no method has been known for removing liquid by mechanical force without significant change in wood structure. With the new I/D process chances for alternatives to conventional thermal drying or for mechanothermal applications are offered.     

填充环氧树脂界面残余应力研究——Ⅳ.界面残余应力形成的分子机理

本文针对橡胶改性环氧树脂体系,从微观分子运动角度对界面残余应力形成原因进行了初步探索,发现复合材料界面残余应力的生成是界面处分子运动不平衡所导致的。同时界面残余应力有效体积是一个重要参数,它和材料温度以及界面键合强弱有关。     

In-mold shrinkage and stress prediction in injection molding

In-mold shrinkage may occur for product parts that solidify under low holding pressure and are not restricted by ribs or flanges. It not only affects the final product dimensions but in addition may have a large effect on the residual stress distribution. A simple elastic model is used to study the effect of in-mold shrinkage on final product dimensions and residual stress distributions. Friction between polymer surface and mold wall as well as deformation of the mold cavity are taken into account. The model uses local values for temperature, pressure, and crystallization, which belong to the standard output of most simulation codes.     

Experimental investigation of the cure‐dependent response of vinyl ester resin

The objective of this research is to understand the influence of the thermochemical and thermomechanical material response of low temperature cured vinyl ester resin (Dow Derakane 411‐C‐50) on the development of residual stress and warpage during processing. The primary experimental technique is the bimaterial specimen experiment, in which the warpage of a bimaterial beam is used as a measure of residual stress. The bimaterial specimen experiment was developed to isolate the chemical and thermal contributions to curvature. Existing material models for shrinkage, modulus, and glass transition temperature as a function of cure were evaluated. These material models were used as input into the bimaterial equation for curvature prediction. The predicted curvatures were used along with the experimental curvatures to evaluate the material models and their ability to accurately describe the material response of the vinyl ester resin. Results showed that the model captured the overall experimental trend in curvature buildup during processing but overestimated the curvature from chemical effects during isothermal cure. Improved correlation was achieved by incorporating a time shift in the model to account for viscoelastic stress relaxation of the resin.     

Formation of cracks on photodegraded nylon 6 filaments

Cracks were found on the surface of drawn nylon 6 filaments irradiated with ultraviolet (UV) light from a mercury lamp under various humidities at room temperature. The cracks were formed perpendicular to the fiber axis and were of varying sizes. No cracks were observed on undrawn filaments or drawn filaments exposed to UV light in a dry atmosphere. Considerable shrinkage was found in drawn samples by thermomechanical analysis indicating the presence of residual stress in the material. The cracking is explained in terms of the residual stress and plasticization by moisture.     

In situ monitoring of residual strain development during composite cure

Internal (residual) stresses build up in a thermosetting composite as the matrix shrinks during cure, and again as the composite is cooled to ambient from its elevated processing temperature. These stresses can be significant enough to distort the dimensions and shape of a cured part as well as initiate damage in off‐axis plies, either during fabrication or under the application of relatively low mechanical loads. The magnitude of these stresses depends on a number of factors including constituent anisotropy, volume fraction and thermal expansion, ply orientation, process cycle, and matrix cure chemistry. In this study, embedded strain gauges were employed to follow, in situ, the buildup of residual strains in carbon fiber‐reinforced laminates during cure. The data were compared to those from volumetric dilatometer studies to ascertain the fraction of resin shrinkage that contributed to residual stress buildup during cure. Based on earlier studies with single‐fiber model composites, the process cycle in each case was then varied to determine if the cycles optimized to minimize residual stresses for isolated fibers in an infinite matrix were applicable to the reduction of residual stresses in conventional multifiber composites. The results of these studies are reported here.     

Studies on suspension and emulsion. LIV. Shrinkage of polymer emulsion film having asymmetric porous structure

It was found that the film cast on a glass plate from emulsifier-free ethyl acrylate–methyl methacrylate copolymer emulsion when separated from there shrank and curled immediately. This film had an “asymmetric” porous structure. It was discussed the relationship between the porous structure and the shrinkage due to the residual internal stress produced during film formation.     

Suspension flow modeling for general geometries

The flow of concentrated suspensions is well known to be accompanied by irreversible bulk migrations. One approach to modeling the migration of particles relative to the bulk motion is based on a particle flux induced by spatial variation of the particle normal stresses. A frame-invariant formulation of the mixture stresses is presented in a general form and used in this approach; specific forms of the constitutive model for the stress are considered. These forms either include only the isotropic shear-induced particle stress (particle pressure) or include both normal stress differences and the particle pressure. The predicted flow and migration behavior are considered for complex geometries, using the flows in a contraction and an expansion as the primary examples. Results are limited to two-dimensional flows.     

Control of volume shrinkage and residual styrene of unsaturated polyester resins cured at low temperatures. II. Effect of comonomer

The effect of a comonomer, methyl methacrylate (MMA), on volume shrinkage and residual styrene content of an unsaturated polyester (UP) resin with low profile additives (LPAs) cured at low temperature was investigated by an integrated reaction kinetics-morphology-property analysis. MMA affects the volume shrinkage and residual styrene content differently depending on MMA to styrene (St) CC bond molar ratio. At low MMA/St ratio, residual styrene decreases and the volume shrinkage of the resin system remains unchanged. At high MMA/St ratio, residual styrene can be substantially reduced, but the resin system suffers poor volume shrinkage control. Reactivity of the comonomer MMA and its compatibility to other components in the resin system can explain the observed results. A series of Seemann composites resin infusion molding process (SCRIMP) were conducted to study the relationship among materials, processing, and properties of molded composites in low temperature curing processes.