Molded vs. machined-from-solid transmission components: why prefer injection molding

Molded vs. machined-from-solid: what we're talking about

Injection molding is a process in which the polymer material is melted and injected into a closed mold, where it solidifies taking its final shape. It's the reference method for high-volume production: it allows complex shapes, good dimensional accuracy and consistent surface finish. Gears obtained this way add light weight, corrosion resistance and quiet operation.

Machining from solid follows the opposite logic: you start from a solid block of technopolymer and remove the excess material by milling or turning. It's better suited to small runs and prototypes, where flexibility counts more than volumes. However, as you'll see in the following paragraph, this flexibility has a performance cost worth considering in advance.

Why molding pays off economically

Injection molding requires an initial investment for the mold, which can range from 5,000 to over 50,000 € depending on complexity. This cost is amortized quickly with production runs of hundreds or thousands of pieces, bringing the unit cost down to a few cents.

During production you can integrate complex functions directly into the component, reducing the number of subsequent assemblies. You can also choose among different material variants: glass-fiber reinforced technopolymers, carbon fiber for more demanding performance, or specific formulations for wear resistance or food compatibility. In this metal-replacement perspective (replacing metal with technical polymers), some technopolymers approach the mechanical performance of metals with a significantly lower weight.

The structural advantage: what happens to the fibers

All other conditions being equal, a molded transmission component is stronger than the same component obtained by material removal. The reason is structural.

When you CNC-machine a glass-fiber reinforced material, the cutting tools sever the fibers along the tooth profile. The resulting distribution is discontinuous and not favorably oriented relative to the bending stresses: surface fatigue strength drops significantly.

In the molded part the opposite happens: during injection, the fibers follow the material flow and orient themselves naturally along the component geometry, actively contributing to structural strength.

Molding allows class IT10-IT12 tolerances (medium dimensional accuracy, suited to most transmission applications) with high repeatability after mold setup.

When to use CNC machining

CNC machining remains useful as a complement to molding, not as an alternative. Through milling and turning you can adapt holes, keyway seats or other functional features on already-molded components, without having to make new molds and with fast lead times. The same applies to racks intended for automation and robotics: components that convert rotary motion into linear movement and that often require targeted customizations.

Keep in mind, though, that every material removal on a reinforced technopolymer reduces the mechanical properties relative to the molded part. Use CNC machining for localized geometric modifications such as holes, slots or seats. Don't cut the tooth profile from solid if you can avoid it, and leave the technicians to assess how much material can be removed without compromising the component.

When molding is the right choice

Choose injection molding when:

• You produce medium-to-large runs where quality, repeatability and unit cost make the difference
• The geometry is defined and stable
• You need optimized mechanical and surface characteristics
• You work in sectors with stringent safety or hygiene requirements: food, medical, automotive, packaging

The ability to integrate inserts, elastic elements or functional details directly into the mold makes this process flexible even on complex components.