Donggang Yao

Simulation of the filling process in micro channels for polymeric materials

There is some evidence indicating that polymeric flows in micro channels differ significantly from those in macro geometries. As micro molding is attracting more attention these days, efforts need to be made to identify the significant factors that influence microscale polymeric flow behaviors and to develop new simulation schemes for micro molding. In this study, we have investigated the consequences of microscale phenomena, particularly size-dependent viscosity, wall slip and surface tension, on the filling process of polymeric materials into micro channels. The standard scheme of two and half dimensions for injection molding simulation was modified to include these microscale effects. With data currently available for polystyrene, the simulation results indicate the importance of employing size-dependent viscosity and wall slip to predict micro filling behaviors. It appears that wall slip should always occur in channels downsized to several micrometers or less, because the wall stress would otherwise be enormous. The surface tension effects turn out to be less important and can be neglected in micro injection molding in which high injection pressure is employed.

Scaling Issues in Miniaturization of Injection Molded Parts

Miniaturization of injection molded parts causes changes in the relative contribution of relevant design and process parameters. As a result, scaling-related size effects occur. Size effects can be either of the first order or of the second order. First-order size effects can be predicted using standard modeling, while second order ones cannot. This paper deals with first-order size effects encountered in injection molding miniaturized parts. Through the scaling analysis of the heat transfer and flow process in injection molding, the size effects on the change of molding characteristics including viscous heating, freezing time, capillary force effect, and moldability were identified. Strategies were consequently proposed to alleviate or eliminate the scaling-related molding difficulties in molding ultrathin-wall parts and microparts. Particularly, a scalable filling process was presented, with experimental verification.

Osteogenic Cells Derived From Embryonic Stem Cells Produced Bone Nodules in Three-Dimensional Scaffolds

An approach for 3D bone tissue generation from embryonic stem (ES) cells was investigated. The ES cells were induced to differentiate into osteogenic precursors, capable of proliferating and subsequently differentiating into bone-forming cells. The differentiated cells and the seeded scaffolds were characterized using von Kossa and Alizarin Red staining, electron microscopy, and RT-PCR analysis. The results demonstrated that ES-derived bone-forming cells attached to and colonized the biocompatible and biodegradable scaffolds. Furthermore, these cells produced bone nodules when grown for 3–4 weeks in mineralization medium containing ascorbic acid and beta-glycerophosphate both in tissue culture plates and in scaffolds. The differentiated cells also expressed osteospecific markers when grown both in the culture plates and in 3D scaffolds. Osteogenic cells expressed alkaline phosphatase, osteocalcin, and osteopontin, but not an ES cell-specific marker, oct-4. These findings suggest that ES cell can be used for in vitro tissue engineering and cultivation of graftable skeletal structures.

INCREASING FLOW LENGTH IN THIN WALL INJECTION MOLDING USING A RAPIDLY HEATED MOLD

Injection molded parts are driven down in size and weight especially for portable electronic applications. While gains are achieved via cost reduction and increased portability, thinner parts encounter more difficulty in molding due to the frozen layer problem. To increase moldability in thin wall molding, a rapid thermal response (RTR) mold was investigated. The RTR mold is capable of rapidly raising the surface temperature to the polymer melt temperature prior to the injection stage and then rapidly cooling to the ejection temperature. The resulting filling process is done inside a hot mold cavity and formation of frozen layer is prohibited. Concepts of scalable filling and low-speed filling are discussed in the article to address the benefit of this molding method. Simulation results showed that significant reduction in injection pressure and speed can be achieved in RTR molding. In contrast to the filling behavior in conventional molding, the injection pressure in RTR molding decreases as the injection speed decreases, and therefore, extremely thin parts can be molded at lower injection speeds. Filling lengths of both RTR and conventionally molded polycarbonate samples, with two levels of thickness, under two levels of injection speed were experimentally studied. The experimental results demonstrated the advantage of the new molding method.

Controllable Growth of Gradient Porous Structures

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.

Injection Molding Nanoscale Features with the Aid of Induction Heating

During injection molding of micron or submicron scale features, incomplete filling frequently occurs, resulting from premature freezing of the polymer melt in contact with a cold mold. In order to overcome the filling difficulty without increasing the total cycle time, the mold surface temperature was raised rapidly by induction heating. A prototype mold insert with cooling channels was fabricated and integrated with a nickel stamp having nanoscale-grating structures. The nickel stamp surface was successfully heated from 25 to 258°C in 2.7 sec. Four different mold surface temperatures, 100, 150, 200 and 250°C, were tested to determine if the nanograting structures can be replicated with an optical quality cyclic olefin copolymer. Experimental results indicate that the nanocavities were successfully filled when the surface temperature reached 250°C, but mold release caused drag damages on the nanogratings. Further, coupled thermoelectromagnetic analyses were carried out to simulate the induction heating process of the nanostructured mold insert. The predicted surface temperature responses in general agree with the experimental ones and the simulation model can be used in the further development of process control and mold design in micro/nano molding.

Replication of Microstructures by Roll-to-Roll UV-Curing Embossing

The characteristics of pattern replication and releasing in a roll-to-roll ultraviolet- (UV) curing embossing process were investigated. The roll embossing system was designed for large-area continuous embossing, with employment of a fast curing resin, a heat damage protector, and a surface energy reducing coating. A 60° V-groove pattern with a groove period of 30 µm was embossed. It was found that the replication quality was profoundly influenced by the pattern geometry, the pattern direction, and the mold surface energy. In particular, the pattern direction significantly affected the edge sharpness and the surface topography of replicated features. In the parallel groove mode, a significant amount of tearing and sliding occurred, whereas in the transverse groove mode, biting marks were observed on the side wall of the V-groove. A simple mechanical model was used to explain the difference in pattern releasing with different pattern layouts. The replication quality was found to be significantly improved with the application of a fluorinated coating on the roll mold.

ELIMINATING FLOW INDUCED BIREFRINGENCE AND MINIMIZING THERMALLY INDUCED RESIDUAL STRESSES IN INJECTION MOLDED PARTS

Eliminating flow-induced birefringence and stresses and reducing thermally induced stresses in the injection molded parts have been studied using rapid thermal response (RTR) molding technique. In the RTR molding, mold surface temperature can be rapidly raised above T g in the filling stage, while the normal injection molding cycle time is still maintained. Therefore, the melt can fill the cavity at temperatures above T g, which enables the flow-induced stresses to relax completely in a short time after filling and before vitrification. Residual stresses and birefringence in a RTR molded strip specimen are compared with the conventional molded parts by applying layer removal method and retardation measurement. For the material (Monsanto® Lustrex Polystyrene) and process conditions chosen, the birefringence level decreased as the RTR temperature approached and exceeded the glass transition temperature until it almost disappeared at a RTR temperature of 180°C. Reduction of magnitude and shift of peak location were observed in the gapwise stress profile for RTR molded specimen. *Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation.

A reflow process for glass microlens array fabrication by use of precision compression molding

A novel low-cost, high-volume fabrication method for glass microlens arrays was developed by combining compression molding and thermal reflow processes. This fabrication process includes three major steps—i.e., fabrication of glassy carbon molds with arrays of micro size holes, glass compression molding to create micro cylinders on a glass substrate, and reheating to form microlens arrays. As compared to traditional polymer microlens arrays, glass microlens arrays are more reliable and therefore may be used in more critical applications. In this research, microlens arrays with different surface geometries were successfully fabricated on a P-SK57 (Tg = 493 °C) glass substrate using a combination of the compression molding and thermal reflow processes. The major parameters that influence the final lens shape, including reheating temperature and holding time, were studied to establish a suitable fabrication process. A numerical simulation method was developed to evaluate the fabrication process. Finally, both surface geometry and optical performance of the fabricated glass microlens arrays were analyzed.

Numerical simulation for injection molding with a rapidly heated mold, Part I: Flow simulation for thin wall parts

The rapid thermal response (RTR) injection molding is a novel process developed to raise the mold surface temperature rapidly to the polymer melt temperature prior to the injection stage and then cool rapidly. The resulting filling process is achieved inside a hot mold cavity by prohibiting formation of frozen layer so as to enable thin wall injection molding without filling difficulty. The present work covers flow simulation of thin wall injection molding using the RTR molding process. Both 2.5-D shell analysis and 3-D solid analysis were performed, and the simulation results were compared with the prior experimental results. Coupled analysis with transient heat transfer simulation was also studied to realize more reliable thin-wall-flow estimation for the RTR molding process. The proposed coupled simulation approach based on solid elements provides reliable flow estimation by accounting for the effects of the unique thermal boundary conditions of the RTR mold.