Semisolid metal processing is a well-known technology that can be used to enhance manufacturing product quality in broad industries. The technology controls the thixotropic properties of alloys ...within their solidus and liquidus temperature ranges. In general, most known alloys can generate semisolid slurries at high solid fractions, and the remaining can form slurries at low solid fractions. This has provided opportunities for many metals and alloys to be processed as semisolid slurries in today's casting industry. However, only a few researchers studied this technology potentially for a new metal-based additive manufacturing or 3D printing process. This article reviewed literature and findings from thixotropy rheology and semisolid metal processing methods, finally to thixotropic metal 3D printing. The survey shows that more future work is needed, including the investigation of thixotropic metal flow mechanics, the modelling and simulation of semisolid metal extrusion and further development of a fully thixotropic 3D printing system.
Embryonic stem cells (ESCs) are an ideal source for chondrogenic progenitors for the repair of damaged cartilage tissue. It is currently difficult to induce uniform and scalable ESC differentiation ...in vitro, a process required for stem cell therapy. This is partly because stem cell fate is determined by complex interactions with the native microenvironment and mechanical properties of the extracellular matrix. Mechanical signaling is considered to be one of the major factors regulating the proliferation and differentiation of chondrogenic cells both in vitro and in vivo. We used biocompatible and elastic polydimethylsiloxane (PDMS) scaffolds, capable of transducing mechanical signals, including compressive stress in vitro. ESCs seeded into the PDMS scaffolds and subjected to mechanical loading resulted in induction of differentiation. Differentiated ESC derivatives in three-dimensional (3-D) PDMS scaffolds exhibited elongated single cell rather than round clonal ESC morphology. They expressed chondrogenic marker, Col2, with concomitant reduction in the expression of pluripotent marker, Oct4. Immunocytochemical analysis also showed that the expression of COL2 protein was significantly higher in ESCs in 3-D scaffolds subjected to compressive stress. Further analysis showed that compressive stress also resulted in expression of early chondrogenic makers, Sox9 and Acan, but not hypertrophic chondrogenic markers, Runx2, Col10, and Mmp13. Compressive stress induced differentiation caused a reduction in the expression of β-Catenin and an increase in the expression of genes, Rhoa, Yap, and Taz, which are known to be affected by mechanosignaling. The chondroinductive role of RhoA was confirmed by its downregulation with simultaneous decrease in the transcriptional and translational expression of early chondrogenic markers, SOX9, COL2, and ACAN, when ESCs in PDMS scaffolds were subjected to compressive stress and treated with RhoA inhibitor, CCG-1432. Based on these observations, a model for compression induced chondrogenic differentiation of ESCs in 3-D scaffolds was proposed.
An important theory on the dynamics of complex interfaces is the Doi and Ohta theory where the interfacial contribution to the Cauchy stress tensor is determined from an interfacial conformation ...tensor. For a uniform deformation field in the Eulerian framework, Doi and Ohta adopted a decoupling approximation to reduce a fourth-order tensor into two second-order tensors and derived a differential equation governing the evolution of the interfacial conformation tensor. In this paper, a different formulation is presented for establishing the Cauchy stress tensor based on a path-independent interfacial energy function. By differentiating this interfacial energy function against a Lagrangian strain tensor, a nearly closed-form solution for the stress tensor was determined, involving only elementary algebraic and matrix operations. From this process, the stress-conformation relation proposed by Doi and Ohta is also confirmed from a thermodynamic perspective. The testing cases with uniaxial elongation and simple shear further showed improved fitting to the analytical or exact solutions.
Cocontinuous phase structures of immiscible polymers can be developed under appropriate melt-blending conditions. Because of the presence of interfacial tension, such cocontinuous structures start to ...coarsen when heated to a temperature higher than the melting/softening temperature of both phases. In this study, a method for controllable growth of gradient porous structures utilizing variable coarsening rates in a gradient temperature field was investigated. The phase structure coarsens at a higher rate in higher temperature regions but at a slower rate in lower temperature regions, resulting in the generation of a gradient phase morphology. Subsequent dissolution of one phase in the binary blend yields a gradient porous structure made of the remaining polymer component. A polystyrene/poly(lactic acid) (PLA) blend was used as a model system. By designing proper thermal boundary conditions and introducing different thermal gradients during annealing, different types of gradient porous structures of PLA were created.
The injection molding process has several inherent problems associated with the constant temperature mold. A basic solution is the rapid thermal response molding process that facilitates rapid ...temperature change at the mold surface thereby improving quality of molded parts without increasing cycle time. Rapid heating and cooling systems consisting of one metallic heating layer and one oxide insulation layer were investigated in this paper. Design issues towards developing a mold capable of raising temperature from 25°C to 250°C in 2 seconds and cooling to 50°C within 10 seconds were discussed. To reduce thermal stresses in the layers during heating and cooling, materials with closely matched low thermal expansion coefficient were used for both layers. Effects of various design parameters, such as layer thickness, power density and material properties, on the performance of the insert were studied in detail with the aid of heat transfer simulation and thermal stress simulation. Several rapid thermal response mold inserts were constructed on the basis of the simulation results. The experimental heating and cooling response agrees with the simulation and also satisfies the target heating and cooling requirement.
