Microinjection molding (lIM) is a fast-developing technology which is used to produce polymeric microcomponents or components with micro/nanoscale features, such as are used in many fields including microfluidic diagnostics, microneedle drug delivery devices, microgears, and microswitches. The capabilities and performance of the microinjection molding process can be improved by incorporating a variotherm system. This leads to improved component quality, especially for high aspect ratio features. It can also help to increase the polymer flow path, improve feature replication, reduce residual stresses and molecular orientations, and also can eliminate weld lines. This article reviews the use of different variotherm systems in lIM, and describes how simulation of its use can provide insight when designing a mold cavity or a component with challenging microfeatures. The article highlights important problems, challenges and areas for further research. An increased understanding of these issues will provide opportunities to enhance further developments in the lIM process.

This article studied the demolding of an array of injection molded micro‐structures based on a design of experiments (DOE) method. The demolding force (Fd) to eject an array of 4 × 5 micro‐ridges from the cavity of a mold was calculated indirectly. It was found that mold temperature had a significant effect on the demolding force: the demolding force decreased as mold temperature increased and as the part substrate thickness also increased. The demolding force is a combination of the adhesion force and friction force that exist between a molded feature and the microcavities of a mold. When the processing parameters were optimized to minimize the demolding force during the ejection process, it was found that the adhesion force had a bigger influence than the friction force. POLYM. ENG. SCI., 56:810–816, 2016. © 2016 Society of Plastics Engineers

Miniaturized parts weighing single or tens of milligrams represent a large category of microinjection moulded products. Both miniaturization and extreme processing under microinjection moulding subject material to high shear rates and high cooling rates, and cause the same material to have different morphologies and final properties when used in conventional injection moulding. It also makes mold design challenging. This study investigates the effects of micro gate design (opening and thickness) and cavity thickness (100-500μm) on filling, morphology and the mechanical properties of miniaturized dumbbell parts. It is found that a reduction of gate size has two conflicting effects, namely, increased shear heating increases flow length, and increased cooling rate reduces flow length. Filling increases significantly with an increase of cavity thickness. In addition, the thickness of the skin layer reduces from ~70% to ~10% when part thickness increases from 100μm to 500μm. This oriented skin layer determines molecular orientation and broadly influences Young's modulus, elongation and yield stress. Natural aging at room temperature induces an increase of modulus and yield stress, and a decrease of strain at break. The mechanical properties of microinjection moldings is significantly different from those of conventional injection moldings and measurement at microscale is required for miniaturized product design. 1 Effect of Gate Design and AbstractMiniaturized parts weighing single or tens of milligrams represent a large category of microinjection moulded products. Both miniaturization and extreme processing under microinjection moulding subject material to high shear rates and high cooling rates, and cause the same material to have different morphologies and final properties when used in conventional injection moulding. It also makes mold design challenging. This study investigates the effects of micro gate design (opening and thickness) and cavity thickness (100-500μm) on filling, morphology and the mechanical properties of miniaturized dumbbell parts. It is found that a reduction of gate size has two conflicting effects, namely, increased shear heating increases flow length, and increased cooling rate reduces flow length.Filling increases significantly with an increase of cavity thickness. In addition, the thickness of the skin layer reduces from ~70% to ~10% when part thickness increases from 100μm to 500μm. This oriented skin layer determines molecular orientation and broadly influences Young's modulus, elongation and yield stress. Natural aging at room temperature induces an increase of modulus and yield stress, and a decrease of strain at break. The mechanical properties of microinjection moldings is significantly different from those of conventional injection moldings and measurement at microscale is required for miniaturized product design.

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