The project investigated the formation and mechanical performance of seams that arise when a thermoset resin is injected into a mold and the melt fronts meet. Seams are unavoidable in many injection‑molding operations, such as the filling of inserts or the creation of wall‑thickness variations, and they can act as weak points that reduce the load‑bearing capacity of the final part. By gaining a detailed understanding of how these seams develop and how their strength depends on material, process settings and filler orientation, the study aimed to reduce the need for over‑engineering and thereby improve resource efficiency.

To explore seam behaviour, the team produced test specimens from epoxy resins containing either glass fibres or glass beads, and from phenolic resins with varying filler contents. Two seam types were examined: stagnant seams, where the melt fronts collide and stop, and flowing seams, where a flow‑obstruction splits the front and the melt continues to flow behind it. Tensile bars were cut along the seam line from a plate that incorporated a removable flow‑obstruction, allowing a direct comparison of the mechanical properties of the seam and the surrounding material.

A statistical design of experiments was employed to quantify the influence of key process parameters—back pressure, injection speed and mold temperature—on the resulting seam strength. The analysis revealed that for flowing seams the reduction in tensile strength relative to the bulk material ranged up to 10 %, whereas for stagnant seams the loss could reach 30 % in phenolic resins. In addition, the orientation of the filler particles within the seam was found to correlate with the observed strength loss, indicating that controlling the melt flow pattern can mitigate the adverse effects of the seam.

Based on these findings, the team formulated practical recommendations for process settings and part design. For example, increasing the back pressure and mold temperature can promote a more continuous melt front, reducing the likelihood of a stagnant seam. The recommendations were validated on a complex, real‑world part that incorporated multiple seams; the measured tensile performance matched the predicted values within a 5 % margin, confirming the reliability of the guidance.

The project was carried out from 2021 to 2024 at the Institute for Plastic Processing (IKV). It was funded by a German research grant and involved close collaboration with industry partners, including material suppliers, mold makers, injection‑molding machine manufacturers and plastic processors. The advisory group for injection molding, which brings together representatives from these companies, served as a forum for presenting interim results and gathering user feedback. The final results were presented to the advisory group in November 2023 and will be discussed again in the 2024 meeting.

Throughout the project, the knowledge generated was transferred to industry through regular dialogues, advisory sessions and the inclusion of the findings in the institute’s teaching programmes. Dual students from the Ecole Mines‑Telecom IMT – Université de Lille completed practical phases at IKV, and their reports are being incorporated into the curriculum. After the project’s conclusion, the advisory group will continue to host discussions to explore follow‑up projects and to disseminate the recommendations to a wider industrial audience.