“Which process should I use?” is the wrong first question. The right one is “what volume, what material, and what tolerance does this part actually need?” Answer those three and the process usually chooses itself. Injection molding wins on unit cost at volume and on repeatability; 3D printing wins on speed and complexity; CNC machining wins on low-volume tight tolerance and on parts too large or too few to justify a mold. This guide covers injection molding and the processes it is most often compared against.
Key takeaways
- Below roughly 500 parts, 3D printing or CNC is normally cheaper than injection molding once tooling is counted.
- Between 500 and 10,000 parts the decision depends on part complexity and how much redesign is still expected.
- Above 10,000 parts, injection molding almost always wins on unit cost.
- Insert molding and overmolding are not alternatives to injection molding — they are variants of it.
- Rotational molding competes with injection molding only for large, hollow, low-volume parts.
Injection molding vs 3D printing vs CNC
The crossover is driven by tooling amortisation. A mold costs money up front and then produces parts for a few cents each; printing and machining cost roughly the same per part whether you make one or one thousand. The practical crossover point for a hand-sized part is usually a few hundred pieces — below that, printing or machining is cheaper; above a few thousand, injection molding pulls decisively ahead.
Other differences matter too: 3D printing gives design freedom that molding cannot (internal channels, lattice structures) but limited material properties and a rougher surface; CNC holds tight tolerances on hard materials but struggles with complex internal geometry and wastes material. Our comparison of injection molding, 3D printing and CNC machining sets out cost, tolerance, material and lead time side by side.
Insert molding and overmolding
Both processes combine two materials in one part, and buyers often treat them as synonyms. Insert molding places a pre-formed insert — threaded metal, an electrical contact, a bushing — into the cavity and molds resin around it, eliminating a secondary assembly step and locking the insert mechanically. Overmolding molds a second resin layer over an existing substrate, typically to add grip, sealing or a soft-touch surface.
The design implications differ. Insert molding is about insert retention, thermal expansion mismatch and gating that does not displace the insert. Overmolding is about chemical and mechanical bond between the two materials — some resin pairs simply will not adhere without a mechanical interlock.
See our guide to insert molding for when the process pays for itself, and insert molding vs overmolding for the design rules that separate them.
Rotational molding
Rotational molding suits large, hollow, double-walled parts — tanks, kayaks, play equipment — at low volume. It has almost no tooling pressure, which allows inexpensive aluminum molds, and it produces seamless hollow parts that injection molding cannot make without welding two halves. Against that: cycle times measured in tens of minutes rather than seconds, a narrow material range dominated by polyethylene, and looser tolerances.
It competes with injection molding only at the large-and-few end. Our rotational molding vs injection molding comparison sets out where each process is the right answer.
Machining: when it beats molding outright
For a handful of parts, or for a part so large that a mold is uneconomic, CNC machining is the answer. It also holds tighter tolerances than molding on many features and needs no tooling investment, which makes it the natural choice for functional prototypes and bridge production before a mold exists.
Cost in machining is dominated by machine hours and setup, so complexity and tolerance drive price far more than material does. Our CNC machining cost guide breaks down how machining quotes are built, and precision machining vs CNC machining clarifies a distinction that causes a lot of avoidable confusion in RFQs.
Low volume: the awkward middle
Between 500 and 20,000 parts, none of the standard answers is obviously right. The options are aluminum tooling, bridge tooling with a shorter design life, family molds that share a base across several parts, or simply machining. Each trades tooling investment against unit cost differently. Our low volume injection molding guide works through the decision for each band.
Start here: the full process library
- Injection molding vs 3D printing vs CNC — the cost crossover
- Insert molding guide — what it is and when it pays
- Insert molding vs overmolding — key differences explained
- Rotational molding vs injection molding — where each wins
- Precision machining vs CNC machining — terminology that affects RFQs
- CNC machining cost guide — how machining quotes are built
- Low volume injection molding — small-batch economics
- Automotive injection molding guide — automotive plastic parts end to end
- Phone case mold manufacturing — a worked product example
- Rapid prototype injection molding — prototype tooling, cost and lead time
Tell us the volume and we will tell you the process
Send the part, the annual quantity and the tolerance requirement. We will tell you honestly whether injection molding is the right process for it — and if machining or printing is cheaper at your volume, we will say so. Get a process recommendation.