3D printing has taken the manufacturing world by storm and is often portrayed as a revolutionary technology capable of producing everything from prototypes to end-use products at the push of a button. Contrasting this image of a flexible, all-purpose solution is injection molding – a proven, high-precision process that has formed the backbone of plastic mass production for decades, but is currently often perceived as less innovative.
However, the question of whether 3D printing will replace injection molding is misleading. The truth is far more complex and reveals a coexistence rather than a rivalry. In practice, especially in regulated industries such as medical technology, the distinction is so clear that an injection-molded part cannot be replaced by a 3D-printed part without comprehensive, prior validation. Instead of a competition, we at RKT see it much more as a strategic partnership where each technology leverages its unique strengths.
The Underlying Manufacturing Principles
The injection molding process is a high-precision manufacturing process in which thermoplastics in granular form are melted using heat. This molten mass is then injected under high pressure into a specially manufactured mold (an injection mold tool), into the so-called cavity. The cavity defines the exact geometry of the desired end-product.
Inside the cavity, the plastic cools and solidifies, thereby taking on the shape of the cavity. After cooling, the mold opens, and the finished part is removed. This process enables the mass production (or serial production) of complex plastic parts with high repeatability and efficiency.
As a side note: A tool at RKT typically has multiple cavities; often, these are high-cavity tools with up to 128 cavities, which we manufacture and operate.
In 3D printing, often also referred to as an ‘additive manufacturing process’, workpieces are built up layer by layer. Depending on the process (e.g., SLS, SLA, FFF, …), powdery, liquid, or filament-like materials are processed. Some 3D printers are designed to process thermoplastics, such as those also used in injection molding.
Partners, Not Rivals
3D printing should be viewed as a complement to injection molding, rather than an alternative. Their respective strengths illustrate why this is the case:
- Injection Molding: The injection molding process is optimized for mass production. It is characterized by extremely high repeatability and enables the efficient production of thousands of identical parts with consistent high quality and minimal unit costs.
- 3D Printing: Here, the focus is on customization and speed in development. The technology excels in manufacturing parts with complex geometries, undercuts, and internal voids that are difficult or impossible to demold using injection molding.
A perfect example of this synergy is the development process: 3D printing allows for the quick and cost-effective production of prototypes of parts that are later intended for high-volume production via injection molding. This accelerates iteration cycles and reduces the risk of expensive mold modifications.
Same Material, Different Properties
A component made from the same plastic can exhibit completely different mechanical properties depending on the manufacturing process. The reason lies in the difference in the manufacturing principle.
In injection molding, the part is formed from a single, homogeneous melt that is injected into a mold. This results in a solid body with very high internal strength. By contrast, the inter-layer bonding (the adhesion of the layers applied in 3D printing) is generally, and depending on the specific 3D printing process, weaker than the strength of a part produced entirely from a single melt. This is because the successive printed layers essentially only “adhere” to one another. These differences become even more pronounced with fiber-reinforced plastics, where the orientation of the fibers massively influences the part’s strength.
The Hidden Costs of Geometric Freedom
One of the greatest advantages of 3D printing is its seemingly limitless geometric freedom. However, this freedom can come at an often-overlooked price: the necessity of support structures.
Many complex geometries, especially those with overhangs, require temporary support scaffolds that are built up concurrently during the printing process. After the print is complete, these structures must be removed. This process requires more or less labor-intensive post-processing, which is usually done manually.
This manual step refutes the simple notion of a “print-and-done” process and adds hidden costs that must be factored into the cost-benefit analysis.
Quality Assurance
In industrial manufacturing, particularly for medical, diagnostic, and life science products, consistent quality and its verification are crucial. This is where clear differences in process control become apparent.
Injection molding is a highly monitored process. Methods such as Design of Experiments (DOE) are employed to define the permissible range of injection parameters required to meet quality specifications. This stable process window guarantees that all parts within a series exhibit identical mechanical and physical properties and very low dimensional variation.
In 3D printing, by contrast, it is difficult to verify and document manufacturing parameters and specific part quality characteristics. Often, parts must be inspected for internal defects using X-rays, which nevertheless may only allow for limited conclusions about the actual load-bearing capacity. The surface finish also differs: While injection-molded parts exhibit a smooth surface from being molded from a melt, the surface of 3D-printed parts is inherently rougher due to the layer-by-layer construction – although this also depends on the specific 3D printing process.
Technology Plus Economic Viability
Ultimately, the comparison between injection molding and 3D printing is not a question of “better” or “worse.” It is about selecting the right tool for the specific task. The decision depends on the specific requirements: If the goal is validated mass production with the highest level of repeatability, injection molding is the first choice. If the focus is on rapid prototypes, individualized single pieces, or geometrically highly complex parts, 3D printing plays to its strengths. RKT utilizes 3D printing for internal tasks, such as producing manufacturing aids, as well as for development projects by manufacturing prototype parts for customers.
The cover image shows an in-house designed robotic gripper for removing injection-molded parts from the machine. The vacuum air channels were integrated using 3D printing. This gripper is one of many examples of manufacturing aids that we at RKT produce using our in-house 3D printing capabilities.
