Shell-Mold Additive Manufacturing with Injection-Filled Cores

Extrusion-based additive manufacturing gives polymer processing a form of geometric freedom that conventional molding cannot easily match and can build customized parts directly from digital toolpaths, produce internal features without dedicated molds, and adapt quickly from one geometry to another. However, the same layer-by-layer and road-by-road deposition that gives the process its flexibility also limits performance when dense, load-bearing polymer parts are needed. A printed component made by fused deposition or fused filament fabrication is not simply a shaped polymer body; it is an assembly of deposited roads whose interfaces record the thermal and mechanical history of printing. The weakest direction is often the build direction, where tensile loading must be carried through interlayer bonds rather than through a fully consolidated material volume. A second challenge is production rate and increasing extrusion speed alone cannot fully resolve it because the melt must still be generated, delivered, deposited, and bonded along a sequential path. Higher flow rates also affect nozzle pressure, material residence time, and deposition stability. The trade-off between resolution and throughput therefore remains deeply connected to the physics of material extrusion, not only to machine settings.

In a recently published research paper in Journal of Manufacturing Processes, Zexin Wang, Shilin Wang, Jiawei Zhu and Professor Ranran Jian from Qingdao University of Science and Technology working together with Dr. Xiang Cheng from China Aerospace Science and Technology Corporation developed an injection-based additive manufacturing process in which an extrusion-printed thermoplastic shell serves as an intrinsic mold for a subsequently injected core. The work is unique in the unified reciprocating screw mechanism, which performs shell printing by rotation and core filling by axial injection within the same material system. They also developed a coupled simulation strategy to analyze shell-core interfacial melting, pressure-driven deformation, cooling shrinkage, and debonding.

During shell fabrication, screw rotation controls the extrusion and deposition of successive layers. The same unit then shifts to axial motion and injects molten polypropylene into the cavity enclosed by the printed shell. This dual operation replaces the slowly deposited internal infill with a dense core formed through rapid volumetric filling. The researchers treated the new mechanism as a coupled thermal, fluid, and structural problem. They examined flow simulations and how nozzle diameter and injection speed influence shear heating during core delivery. The authors found for polypropylene, small nozzles sharply restricted the safe injection rate because the melt temperature rose too quickly. Larger nozzle diameters provided a better balance between extrusion stability and injection capacity, while very small nozzles generated higher pressure and stronger velocity fluctuations during shell printing. The nozzle used for validation was therefore selected to perform reliably in both phases of the process.

The research team paid good attention to the shell-core interface  and found that molten polypropylene entering the cavity transferred heat to the colder printed shell and partially re-melted its inner surface. This local re-melting promoted fusion across the interface rather than leaving the core in mechanical contact with the shell alone. At the same time, injection pressure could deform the shell, and uneven cooling could generate shrinkage stresses between the two regions. The co-simulation therefore considered interfacial melting, shell deformation, structural collapse, and debonding. Shell thickness and injection flow rate proved especially important. Thicker shells resisted deformation and retained greater thermal stability, whereas slower filling prolonged heat transfer into the shell.

The investigators printed tensile specimens with the build direction aligned with the loading direction. This placed the comparison in the mechanically weakest orientation for conventional extrusion-based manufacturing and made the test particularly sensitive to interlayer bonding. The reference specimens contained fully printed grid infill, whereas the hybrid specimens combined a printed shell with an injected core. The injection-based parts showed an approximately 80% increase in ultimate tensile strength and an approximately 89% increase in elongation at break. These changes indicate that the hybrid process improved both resistance to failure and the capacity for deformation before fracture. They also examined fracture morphology and noticed the extrusion-based specimens retained the layered structure and local voids associated with deposited roads, and failure occurred mainly along weak interlayer boundaries. Their fracture surfaces also showed stress whitening and localized crazing, consistent with greater plastic deformation. The improvement arose from two related effects: injection replaced much of the road-by-road internal structure with a consolidated core, and the heat of the incoming melt strengthened bonding where the core met the printed shell.

The findings of Professor Ranran Jian and colleagues point to practical engineering use wherever polymer components need the geometric flexibility of additive manufacturing but also require better consolidation, higher build-direction strength, and shorter production time than conventional extrusion-based printing can usually provide. The shell-mold printing and core-filling strategy is especially relevant for customized structural polymer parts in which the outer geometry must be digitally defined, but the interior does not need to be built slowly as a printed grid. By replacing printed infill with an injected polymeric core, the process creates a denser internal structure and reduces the number of weak interlayer interfaces that normally control failure in the build direction. This makes the approach useful for load-bearing or semi-load-bearing thermoplastic components where conventional FDM or FFF parts may be limited by anisotropy. Housings, brackets, fixtures, tooling aids, protective casings, and customized mechanical supports could benefit from the combination of printed shape control and injection-like core consolidation, provided the part can be designed with cavities that can be filled through suitable gates and venting paths. The work also suggests value for small-batch manufacturing, where traditional injection molding may be too costly or slow because a dedicated mold is required. Here, the printed shell becomes the mold, allowing mold fabrication and part fabrication to merge into one additive route.

In the authors’ analysis, the time saved becomes more substantial as the volume of printed infill that would otherwise be deposited line by line is replaced by rapid cavity injection. This is important for industrial additive manufacturing, where production time often determines whether a process is practical beyond prototyping. From a design standpoint, we can think of the new study gives engineers useful guidance  and shell thickness, nozzle diameter, injection speed, cavity height, and filling pressure must be chosen together because they control thermal penetration, shell deformation, interfacial fusion, and shrinkage behavior. Controlled shell deformation may even help compensate for cooling shrinkage, but excessive pressure or heat must be avoided.