“Design for Manufacturing” (DFM) is frequently mentioned in engineering meetings, yet it’s often misunderstood or oversimplified. To some, it means ensuring the design is easy to machine. To others, it is a step that should be ignored until the overall design is “done.” In reality, DFM is not a final checkpoint. It’s a mindset that should guide decisions from the very first sketch.
Design for manufacturing is about aligning your design intent with the realities of how a part will actually be made. Materials behave differently, manufacturing processes have limits, and tolerances are expensive. In product development, skipping DFM early can lead to complications later.
DFM Starts With Manufacturing Method Selection
One of the biggest DFM mistakes engineers make is designing a part without committing to a realistic manufacturing process. A design that favors CNC machining may be completely unsuitable for injection molding, and vice versa.
For example, CNC machining is best suited for parts with sharp internal corners, variable wall thicknesses, and relatively loose design constraints, especially in low production volumes. Injection molding, on the other hand, demands draft angles, uniform wall thickness, proper gate locations, and careful control of shrinkage. Designing a generic part without considering these differences almost guarantees a redesign later.
Good DFM means asking early:
The answers should directly influence the geometry you create in CAD.
Tolerancing Is a Design Decision, Not a Guess
Tolerances are one of the clearest places where DFM succeeds or fails. Over-tolerancing is incredibly common, especially in early-stage designs. Engineers often default to tight tolerances to be safe, without considering that every extra decimal place can significantly increase manufacturing cost and lead time.
Different processes have very different tolerance capabilities:
Design for manufacturing means applying tight tolerances only where they are functionally required. Critical interfaces, sealing surfaces, or alignment features usually justify tighter tolerancing. Cosmetic surfaces or non-functional features usually don’t include tolerancing requirements at all.
A well-designed drawing communicates intent clearly.
Designing for Injection Molding: More Than Just Draft
Injection molding is one of the most common production methods for medical devices, but it also has some of the most misunderstood design rules.
First, draft angles are the most well-known requirement. Parts need sufficient draft on all vertical faces to allow ejection from the mold. Without appropriate draft angles, ejector pins are unable to exert enough force to push the part from the mold, or may result in poor part quality due to damage from friction between the plastic part and the mold. Ignoring this can lead to stuck parts, damaged tooling, or forced design changes after the mold is cut.
Uniform wall thickness is another major DFM principle. Thick sections cool more slowly than thin ones, leading to sink marks, warping, and internal stresses. Gradual transitions, ribs instead of solid walls, and consistent thicknesses improve part quality and cycle time.
Engineers must also think about:
These considerations are often overlooked in early concept designs.
Real-World Example: Disposable Medical Housings
Consider a disposable handheld medical device, such as a glucose meter housing. Early prototypes may be 3D printed or CNC machined, allowing for thick walls, sharp edges, and complex internal geometries. The device functions perfectly in testing.
When the team transitions to injection molding, problems emerge. Wall thickness isn’t uniform, leading to sink marks on the exterior and a reduction in aesthetic value. Internal bosses crack due to stress concentration. Sharp corners require redesign to add radii and draft.
A DFM-driven approach from the start would have accounted for molding constraints early, reducing tooling revisions and accelerating time to market.
Designing for CNC Machining: Don’t Overcomplicate It
While CNC machining is more forgiving than molding, DFM still matters. Designs that fail to consider tool access, standard cutter sizes, or fixturing realities can inflate costs.
Internal corners are a common issue. Square internal corners require small-diameter tools, increasing machining time and production costs. Adding fillets that match standard end mill radii simplifies machining and lowers cost.
Deep pockets with tight tolerances can also be problematic. If a feature doesn’t require extreme depth or precision, relaxing those requirements can dramatically improve manufacturability.
DFM here is about respecting how tools physically remove material.
Medical Device Example: Implantable Components
In implantable medical devices, DFM becomes even more critical. For example, a titanium bone implant often relies on CNC machining due to material properties and precision requirements.
Designers must balance tight tolerances for bone interfaces with manufacturability and inspection capability. Features that are impossible to measure reliably can introduce compliance risks during validation and FDA submission.
In this context, DFM isn’t just about cost. It’s about repeatability, inspection, and regulatory defensibility.
Final Thought
Design for manufacturing is not a box to check at the end of a project. Whether you’re designing a disposable plastic housing or a precision-machined medical component, DFM determines whether your product moves smoothly from idea to production, or stalls under the weight of redesigns, delays, and cost overruns.
The earlier you think about how something will be made, the better your design will be.
If you have questions about the development process, feel free to reach out for help. We do hundreds of free consults every year to help guide innovators along their path of device development.