Micro-injection molding How Boy and Ewikon Reduce the Residence Time to Four Minutes

At Fakuma 2026, the companies Boy and Evonik demonstrated the processing of a bioresorbable polymer using microinjection molding. Using an 8-mm plasticizing unit and a micro hot-runner system, two medical microscrews, each weighing 0.065 g, were produced without a sprue. The material residence time was approximately four minutes.

Today, many medical products are manufactured using micro-injection molding. In addition to the very low part weights, the reproducibility of the process and the dwell time of the plastic in the machine play a particularly crucial role.

In practice, however, standard injection molding machines—which were not originally designed for such small shot weights—are often used to manufacture micro-injection-molded parts. To ensure stable production nonetheless, a large portion of the shot volume is often used for the sprue. This not only makes it more difficult to control the process but also impairs the effectiveness of monitoring and control functions. At the same time, significant material losses occur.

Practical experience shows that, for shot weights of less than one gram, often less than five percent of the material used actually ends up in the molded part. While this approach is often considered economically acceptable for low-cost, standard plastics, it quickly reaches its limits when it comes to high-priced materials where dwell time is critical.

Designed specifically for the lightest shot weights

To meet these requirements, Boy developed an 8-mm micro-injection molding screw. It operates according to the classic three-zone first-in, first-out principle. Pre-metering was deliberately omitted in order to keep the material’s residence time as short as possible and to avoid stagnation zones.

Due to the small screw diameter, the 8-mm unit also offers significantly higher process resolution. Compared to a 14-mm screw, this represents a 3.1-fold improvement. In addition, the backflow preventer closes more quickly due to its design, which further increases the reproducibility of the injection molding process.

Short dwell times thanks to optimized system technology

When combined with the L2X-Mikro micro hot-runner system from Ewikon , very short residence times can be achieved thanks to the compact design and low material volumes. This enables gate-free processing of thermally sensitive plastics, such as the bioresorbable and biocompatible polymer Resomer LR 704 S from Evonik Industries AG.

The particular challenge in processing this material lies in the degradation of the polymer chains. Even exceeding the permissible dwell time can lead to undesirable material degradation. Since bioresorbable plastics are associated with high material and regulatory costs, eliminating sprues offers significant cost-saving potential.

Practical Demonstration at Fakuma 2026

At Fakuma 2026, the concept’s capabilities were demonstrated in a joint application by Boy, Ewikon, and Evonik. At Boy’s booth, a medical microscrew was manufactured on a Boy XS E equipped with an 8-mm plasticizing unit.

The material used was Resomer LR 704 S. Two micro screws, each weighing 0.065 g, were injection-molded directly into the part. The total shot weight was thus only 0.13 g. With a cycle time of 12 seconds, a total material residence time of approximately four minutes was achieved in the plasticizing unit and hot runner system.

By comparison, the residence time would have been about 11 minutes under identical process parameters and using a 14-mm unit. In that case, a significantly greater degradation of the polymer chains would have been expected. The results of the experiment were confirmed by Evonik’s application engineering team.

Conclusion

The results presented at Fakuma 2026 demonstrate that even high-cost plastics with critical dwell times can be processed economically and reliably in micro-injection molding, provided that the machine and hot-runner technology are consistently designed to meet the requirements of the smallest shot weights. The combination of the Boy XS E’s 8-mm plasticizing unit and the L2X micro hot-runner system enables short residence times, high reproducibility, and a significant reduction in material loss. This opens up new possibilities for cost-effective series production in micro-injection molding, particularly for bioresorbable and medical-grade specialty plastics. (sl)