Column: Can You Do That With Plastic, Too? Injection molding—even with a plastic… mold?

Do injection molding tools have to be made of steel? Additively manufactured resin inserts enable near-production prototypes and small-batch runs made of plastics such as PP or PA6 in just a few days.

Anyone who works in plastics processing is usually familiar with the question posed in this column series from one perspective: Which metallic or inorganic component can be replaced by a smart polymer? But today we’re turning the tables and looking at the issue from the other side: Is injection molding actually possible with plastic—specifically, on the mold side, where the mold can be additively manufactured from a filled resin?

The initial reaction from the field is usually cautious: Why on earth would anyone use resin tool inserts? They can't hold up to anything!

Just in Time and Just Enough

The real advantage here lies not in the tool itself, but in the component. Instead of additively manufacturing a prototype using filament, powder, or resin, a component is produced from the exact same material that will later be used in mass production. This is particularly useful when working with materials that are difficult or impossible to produce using additive manufacturing, such as PP, POM, or PA6. At the same time, the surface finish, shrinkage, and undercuts are much more consistent with the subsequent mass production process than those of an additively manufactured component.

3D-printed resin inserts are no substitute for steel, but they fill gaps in prototyping and small-batch production.

All of this is achieved at a fraction of the cost of a metal mold and with drastically reduced lead times: development cycles are shortened, and design changes can be evaluated within a few days instead of several weeks. Experience at SKZ demonstrates just how significant this time savings can be: in one instance, it took just four days from a customer inquiry on Monday morning to the first manufactured injection-molded parts. This makes multiple mold iterations economically feasible, brings products to market faster, and makes even small-batch production viable using injection molding. This applies from the early stages of series production all the way to the cost-effective manufacturing of replacement parts and discontinued products in small quantities. For such applications, a conventional steel mold would often be economically unfeasible.

Material-Specific Challenges 

Nevertheless, the use of resins presents material-specific challenges. While steel quickly dissipates heat from the part, resin acts more like an insulating layer and quickly becomes a limiting factor in cycle times. In addition, high injection pressures and ejection forces require precise design and tailored injection molding process control: anyone who applies the standard process parameters for a steel mold on a one-to-one basis will quickly be in for a nasty surprise.

Additively manufactured tools will not replace steel, and that is not the goal anyway. Rather, they bridge the gap between additively manufactured prototypes and production-ready tools. This can significantly accelerate development, testing, and small-batch production. Perhaps that is why the answer to this column’s question is not simply “yes.” It also shows that plastic sometimes proves its worth precisely where you least expect it—even in an injection-molding tool.