Microinjection molding (µIM) appears to be one of the most efficient processes for the large-scale production of thermoplastic polymer microparts. The microinjection molding process is not just a scaling down of the conventional injection process; it requires a rethinking of each part of the process. This review proposes a comparative description of each step of the microinjection molding process (µIM) with conventional injection molding (IM). Micromolding machines have been developed since the 1990s and a comparison between the existing ones is made. The techniques used for the realization of mold inserts are presented, such as lithography process (LIGA), laser micromachining and micro electrical discharge machining (µEDM). Regarding the molding step, the variotherm equipment used for the temperature variation is presented and the problems to solve for each molding phase are listed. Throughout this review, the differences existing between µIM and conventional molding are highlighted.

The skin–core crystalline morphology of injection-molded semi-crystalline polymers is well documented in the scientific literature. The thermomechanical environment provokes temperature and shear gradients throughout the entire thickness of the part during molding, thus influencing the polymer crystallization. Crystalline morphologies of a high-density polyethylene (HDPE) micromolded part (μpart) and a classical part (macropart) are compared with optical, thermal and x-ray diffraction analyses. Results show that the crystalline morphologies with regard to thickness vary between the two parts. While a ‘skin–core’ morphology is present for the macropart, the μpart exhibits a specific ‘core-free’ morphology, i.e. no spherulite is present at the center of the thickness. This result seems to be generated under the specific conditions used in microinjection molding that lead to the formation of smaller and more oriented crystalline entities.

This study investigates the morphology of a highdensity polyethylene processed with microinjection molding. Previous work pointed out that a ''core-free'' morphology exists for a micropart (150-lm thick), contrasting with the well-known ''skin-core'' morphology of a conventional part (1.5-mm thick). Local analyses are now conducted in every structural layer of these samples. Transmission electron microscopy observations reveal highly oriented crystalline lamellae perpendicular to the flow direction in the micropart. Image analysis also shows that lamellae are thinner. Wide-angle X-ray diffraction measurements using a microfocused beam highlight that highly oriented shish-kebab morphologies are found through the micropart thickness, with corresponding orientation function close to 0.8. For the macropart, quiescent crystallized morphologies are found with few oriented structures. Finally, the morphology within the micropart is more homogeneous, but the crystalline structures created are disturbed due to the combined effects of flow-induced crystallization and thermal crystallization during processing.

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