Additive manufacturing vs. injection molding: Selection criteria for plastic parts

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Surface Texture and Quality Assurance

In injection molding, as mentioned, parts are molded in a single operation from the melt, creating a smooth surface—depending on how the surface of the cavity is textured or polished. But the surface of 3D printed parts is typically not as smooth due to the layered construction of the walls, as it can be with injection molded parts. In injection molding, the machine settings and process parameters are also well-known and can be very well monitored (for example, the temperature of the melt).

In terms of quality assurance, RKT mostly relies on statistical experimental design such as DOE (Design of Experiment) to define the range of permissible injection parameters for production in regard to quality requirements. Due to this approach in determining process parameters, all injection-molded parts in a production run essentially have the same mechanical-physical properties. Moreover, the dimensions of these plastic parts exhibit very low variability.

Gallery

This insert was additively manufactured using MSLA (Masked Stereolithography Apparatus) from a photoreactive polymer. It is used at RKT in a base tool for the production of initial prototypes. Thus, injection molding and 3D printing complement each other here.(Image: RKT)

At RKT, injection molding tools with many form nests, called cavities, are also designed and built. Here is a 32-cavity tool for the production of sealing rings.(Image: RKT)

A view of a single cavity in an RKT tool. Using collapsible cores, undercuts on the plastic injection-molded part are demolded in a 32-cavity tool.(Image: RKT)

This component also relates to the topic of operating resources at RKT. It is a 3D-printed connector piece for a cavity-separating part tray for injection molding.(Image: RKT)

Gallery with 6 images

In 3D printing, on the other hand, it is difficult to check and verify manufacturing parameters and details of part quality. Therefore, high-quality 3D printed parts are often examined with X-rays to check for defects. However, this provides only limited conclusions about the mechanical-physical properties. In individual cases, special methods would need to be defined to verify whether the printed plastic parts meet the specified quality requirements.

This is How 3D Printing Complements Injection Molding

The consideration of production times and material costs necessary for a part reveals that 3D printing processes are not ideal for mass production. Therefore, they are no alternative to injection molding, but rather an interesting complement. As mentioned, 3D printing can produce parts that are difficult or impossible to manufacture with injection molding machines. However, 3D printing allows for the rapid and cost-effective production of prototypes, accelerating development processes. Additionally, patterns of parts that will later be injection molded in large quantities can be produced quickly and flexibly. It is also possible to individually change the geometry and structure of parts for study and demonstration purposes or to represent parts that are originally tiny in greatly enlarged form. For such tasks, it is not even necessary to use the same plastic intended for the subsequent injection molded parts.

Conclusions and Summary

While injection molding produces identical parts in large quantities, each part is individually designed in 3D printing. Thus, 3D printing or additive manufacturing is justified when it comes to the individuality of components. The choice between injection molding and 3D printing thus depends on the specific requirements and constraints of a project. For mass production, injection molding is usually the better choice.

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Date: 08.12.2025

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