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.

High‐frequency proximity heating was used to rapidly heat injection molds. The principle is based on the proximity effect between a pair of mold inserts facing each other with a small gap and forming a high‐frequency electric loop. Because of the proximity effect, the high‐frequency current will flow at the inner surfaces of the facing pair, thus selectively heating the mold surface. With this method, the electrical insulation layer beneath the mold surface can be eliminated, resulting in a mold insert made of a single metal. A mold with a cavity of 25 × 50 mm2 was constructed with careful design on its electrical, structural, and thermal performance. Air pockets with reinforcing ribs were embedded right beneath the mold surface for enhancing the heating performance. The resulting mold cavity can be rapidly heated from room temperature to about 240°C in 5 s with an apparent heating power of 93 W/cm2. The new mold heating method was applied to thin‐wall molding and micromolding, and in all testing cases, short cycle times less than a minute were achieved. POLYM. ENG. SCI. 46:938–945, 2006. © 2006 Society of Plastics Engineers

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.

A heated mold with temperature above the polymer-softening temperature is highly desired in precision injection molding. The elevated mold temperature reduces unwanted freezing during the injection stage, thus improving moldability and enhancing part quality. The resulting advantages include, but are not limited to, longer flow path, improved feature replication and

An approach for making poly(lactic acid) (PLA) single-polymer composites (SPCs) on the basis of PLA's slowly crystallizing characteristics was investigated. As a slowly crystallizing polymer, PLA can be processed with standard polymer processing techniques into end-use products with varied crystallinities, from amorphous films to highly crystalline fibers. In this study, amorphous PLA sheets and crystalline PLA fibers/fabrics were laminated and compression-molded to form an SPC at a processing temperature substantially lower than PLA's melting temperature. The effects of the major process conditions on the performance of the SPC were studied. The processing temperature played a profound role in affecting the fiber-matrix bonding properties. As the processing temperature increased, a drastic improvement in the interfacial bonding occurred at a temperature of around 1358C, which indicated the lower boundary of the process window. The compression-molded SPC exhibited enhanced mechanical properties; particularly, the tearing strength of the fabric-reinforced SPC was almost an order higher than that of the nonreinforced PLA.