The Drawbacks and Challenges of Using 3D Printed Molds for Injection Molding

3D printed molds present an option for limited run injection molding without investing in conventional metal tooling. However, additive manufacturing has distinct limitations relative to CNC machined injection molds.

Factors like part size constraints, faster mold degradation, lower accuracy, and material compatibility must be considered when evaluating 3D printed molds. For any substantial production volumes, traditional machined tooling remains ideal.

This guide examines the key drawbacks and restrictions of using 3D printed injection molds compared to conventional manufacturing methods. Understanding these differences allows properly weighing where additive mold fabrication does or does not make sense.

Size Limitations of 3D Printed Molds

One major restriction of 3D printed molds is the smaller maximum part size compared to traditional tooling. Metal molds CNC machined from tool steel or aluminum alloys can accommodate very large injection molded components.

In contrast, the build volumes of industrial 3D printers top out at around 1 cubic foot for stereolithography (SLA) and 1.5 cubic feet for powder bed fusion. This puts practical limits on the maximum size of injection molded parts from 3D printed molds, typically no greater than 164 cm3. Larger components will require metal tooling.

Multi-part assemblies with connectors or fasteners can work around size constraints to some degree. But 3D printed molds are inherently better suited for smaller injection molded parts.

Faster Degradation Compared to Metal Molds

Another downside of 3D printed injection molds is faster degradation compared to traditional hardened tool steel. 3D printing uses thermoplastics, photopolymers, or sintered metals which cannot match the durability of CNC machined tools.

In best cases, 3D printed molds may withstand hundreds of shots before wearing out. More complex geometries or aggressive filling can shorten lifespan further. In contrast, steel molds often endure tens of thousands of cycles or more.

This makes 3D printed molds viable for tens to hundreds of injection cycles. The faster degradation ultimately limits cost-effectiveness for larger production runs where metal molds amortize favorably. For short runs, the lower initial cost helps justify the replacement rate.

Temperature and Pressure Limitations

Most 3D printing materials have much lower melting points and structural strength than H13 tool steel at over 1000°F. As a result, 3D printed injection molds have reduced tolerance for the heat and pressure inside injection molding machines.

High temperature engineering resins like PEEK (650°F) risk damaging or warping 3D printed tooling. Elevated injection pressures also increase chances of mold failure and component distortion.

These thermal and mechanical property limits put practical constraints on which plastics can be molded with 3D printed tools. More demanding applications often require traditional metal tooling that withstands injection molding conditions.

Limited Production Run Size

Given their faster degradation rate, 3D printed injection molds are inherently better suited for short production runs. Generally speaking, additive molds are most viable for 25-100 injection cycles or fewer, after which metal tools become more cost effective depending on part size and complexity.

It is possible to use 3D printed “bridge tooling” as an interim step for initial market testing before finalizing steel production molds. But for full-scale manufacturing, machined or EDM molds last exponentially longer.

Lower Accuracy and Surface Finish

Despite advancements in additive manufacturing accuracy, CNC machining and EDM still surpass 3D printing for precision, surface finish, and geometric complexity. Machined molds can achieve micron-level tolerances, excellent surface finishes, and intricate details unmatched by 3D printing.

This gives machined molds greater capability for holding tight dimensions and imparting high-quality surface textures on complex plastic parts. 3D printed molds may fall short of the end product quality standards in some applications.

Material Compatibility Challenges

Not all 3D printing resins possess the necessary thermal and mechanical properties for withstanding injection molding processes. Materials like standard photopolymers will fail immediately in the hot, high-pressure environment.

Specialized high-temp resins are required, such as epoxy-based photopolymers, glass/carbon-filled nylons, or metal powder infused plastics. Even then, material properties lag far behind hardened tool steel. Careful design for the expected molding conditions is essential.

Conclusion – Weighing the Trade-Offs

3D printed injection molds provide an accessible and affordable alternative to machined metal tools, but only for short-run production. Size constraints, faster wear, lower accuracy, and material limits make them impractical for large manufacturing volumes.

However, for prototypes and bridge tooling, 3D printed molds can offer huge time and cost savings. The technology continues advancing to push the boundaries further. But for full-scale production, CNC machined molds maintain a clear advantage in durability and performance. Understanding these trade-offs allows properly leveraging the strengths of each mold fabrication method.