Design for manufacturing (DFM) means shaping a part around the process that will make it, so that it can be made efficiently while still doing its job. In practice that comes down to features like corner radii, wall thickness, and draft (the slight taper that lets a part release from a mold), each of which a process limits in its own way. Some guides write the term as design for manufacturability.
A part can look right in a computer-aided design (CAD) model and still be hard to make, because each process has its own limits. Many customers who come to us need guidance on how their part will be made, and design for manufacturing is where that guidance starts. Six principles apply to every process, and the sections after them give the specific rules for CNC (computer numerical control) routing, 3D printing, and molding and casting.
What Are the Design for Manufacturing Principles and Examples?
Six principles run through most design for manufacturing work, and the first one drives the other five: choose the process before you finalize the geometry. Routing a plastic panel, printing a housing, and molding an enclosure each reward different features, and a part optimized for one needs changes for another. Protolabs Network notes that no industry-wide standard exists for CNC design limits, because machine and tool makers keep expanding what is possible, so treat every number below as a starting point to confirm with your manufacturer.
For the Guggenheim Museum, the planning models were CNC routed from Palboard, a lightweight, sturdy board that machines easily. A channel routed into the board where it needed to bend made a live joint, so the material folded like origami and one continuous piece replaced several joined sections. The Abu Dhabi skylights went the other way and were printed in resin, because routing was a poor fit for them.
- Choose the process first, before the geometry is final.
- Use standard features, such as standard drill sizes.
- Specify only the tolerance the part needs, because leaving a tolerance at the machine’s standard saves time and money.
- Use fewer parts where you can: in a board material like Palboard, a routed channel can let one piece bend and replace several joined sections.
- Keep wall thickness uniform wherever the process builds in layers or cools in a mold, because uneven walls warp.
- Get a manufacturer’s review before any tooling is cut.
These design for manufacturing examples show the principles at work. Each row pairs a common problem with the process it belongs to and the design change that removes it.
| Problem in the design | Process | Design change |
|---|---|---|
| Sharp inside corner | CNC cutting | Use an inside radius larger than the cutter’s own (Xometry advises 130 percent of the cutter radius) |
| Cavity deeper than three to five times its width | CNC cutting | Shorten it or widen it |
| Plastic wall thinner than 1.5 mm | CNC cutting | Thicken it to 1.5 mm or more |
| Overhang the printer cannot print cleanly | Filament printing | Keep it within what the printer prints cleanly (Prusa gives 45 to 60°, depending on nozzle and settings) |
| Fillet on an edge that faces the print bed | Filament printing | Use a chamfer (an angled edge) instead |
| Sealed hollow part | Resin or powder printing | Add escape holes of at least 5 mm (Xometry’s recommendation) |
| Vertical wall with no draft | Injection molding | Add 1 to 2° of draft |
| Feature that locks the part in the mold | Injection molding | Redesign it, or plan a slide or lifter and accept the added tooling cost |
| Part split into several joined sections | CNC routing | In a bendable board, let a routed channel replace the seam |
| Tight tolerance on every dimension | Any process | Reserve tight tolerances for features where fit depends on them |
Which Rules Apply When Designing for CNC Routing?
Protolabs Network’s CNC design guide and Xometry’s CNC design tips, both written for milling, agree on the basics: plastic walls of at least 1.5 mm, metal walls of about 0.8 mm, and holes no deeper than four times their diameter. CNC routing works plastics, wood, and soft metals with a spinning cutter, and a spinning cutter leaves its own shape behind: Protolabs Network explains that a cylindrical cutter transfers its shape to the part, so the internal corners of a cut part always have a radius no matter how small the cutter is. Both guides are written for milling, so treat their numbers as a starting point and confirm the limits for your material and cutter before you finalize a routed design.
Xometry advises inside radii of 130 percent of the cutter radius to avoid tool wear, and it keeps cavity depth to three to five times the width because deeper, narrower cavities cause tool deflection, poor chip evacuation, and broken tools. Holes that must be accurate should also match standard drill sizes. In a board that will fold, we would ask whether a routed channel can replace a seam before splitting a part into pieces.
The body of the Ferrari F355 GTS subwoofer enclosure was designed as nine 3/4 inch MDF (medium-density fiberboard) parts, CNC routed and then glued and nailed together at the joints, with the nine-way split chosen so the box looks like part of the car’s interior. Its fit was checked digitally in CAD against the scan and then test-fit in the car, which confirmed the placement was right and that the sub did not interfere with anything. Our guide to CNC routing for custom parts covers what the process is and when it fits.
Which Limits Matter Most When 3D Printing a Part?
Start with the minimum walls in Xometry’s general 3D printing guide, 1.0 mm for self-supporting features and 1.5 mm for unsupported or load-bearing ones, then adjust for the technology you choose. Prusa says a filament printer, meaning an FDM (fused deposition modeling) machine, can cleanly print overhangs at angles between 45 and 60 degrees, depending on nozzle and settings, and on a standard 0.4 mm nozzle a wall thinner than one perimeter, about 0.45 mm, cannot print at all. Printing with SLA (stereolithography) resin or SLS (selective laser sintering) powder adds limits of its own, such as the escape holes below. Surfaces printed above supports come out rougher than ones printed on the bed or a side wall, and because a fillet on an edge that faces the bed creates a steep overhang, Prusa recommends a chamfer there when finish matters.
Orient structural parts so loads do not pull layers apart, since the layer-to-layer bond in an FDM part is weaker than the strength within a layer. We would settle orientation before finalizing a design, because it decides where the supports and the weak directions land. Keep wall thickness uniform, since uneven walls cool unevenly and warp, and where a part needs strength Xometry suggests ribs, coring, or lattice structure before added thickness.
On William Nelson’s Reason sculpture series, the figures were sectioned for print optimization and each was given a skeletal structure for rigidity and mounting. Fasteners joined the pieces throughout, and the seams were post-processed and blended to be invisible.
Size features to the process. Hollow parts need escape holes, and Xometry recommends at least 5 mm, noting that a cavity deeper than about 50 mm with a single opening may need more than one.
How Do Molding and Casting Change the Design Rules?
Molded and cast parts add a new constraint: the part has to leave the mold. In injection molding, surfaces parallel to the direction of release need draft, and Protolabs puts draft at 1 to 2 degrees for most parts. Walls should be uniform because thick sections keep shrinking after the surface freezes and cause warpage and sink.
Any feature that locks the part in an injection mold, called an undercut, usually calls for a slide or lifter, which Xometry notes adds tooling cost. Our guide to injection molding DFM covers draft, wall thickness, ribs, and gates, and our breakdown of injection molding undercuts explains how undercuts drive tooling cost.
Cast parts, such as urethane resin poured into silicone molds, follow different rules. A flexible silicone mold can release undercuts that an injection mold usually handles with a slide, and Smooth-On’s one-piece cut block mold shows the technique on a deeply undercut original. The mold also reproduces whatever surface the master pattern, the original it is made from, has, flaws included. On the replacement handles for Barcade’s Quick & Crash controllers, 3D-printed prototypes were fit-checked on a cabinet already in service before any mold was committed, and the internal ribs were redesigned to aid even resin flow in the silicone molds.
What Will a Design for Manufacturing Review Catch Before Tooling?
A design for manufacturing review flags what cannot be made as drawn and looks for changes that cut complexity without costing function. Get one before any tooling is cut, because aPriori points out that design changes get harder to make once tooling has begun.
Guiding people through how their part will be made is part of what we do. Before tooling is cut, send our Brooklyn team the model, or a description of the job the part has to perform, through the contact page, and you can reach us directly at 718-557-9578. We will point out the features that are ready and the ones that are not.
Sources: Protolabs Network, CNC design guide; Xometry, 10 Tips to Improve Your CAD for CNC Design; Xometry Pro, General Best Practices for 3D Printing Design; Prusa, Modeling with 3D printing in mind; Protolabs, Draft Angle Guidelines for Injection Molding; DuPont, General Design Principles, Moulding Considerations; Xometry, Types of Injection Molding Side Actions; Protolabs, Designing for Moldability: Complex Features; Formlabs, Guide to Vacuum Casting; Smooth-On, Making a One Piece Cut Block Mold; Modus Advanced, What Happens During a DFM Review?; Formlabs, How to Cast Silicone Parts in 3D Printed Molds and What Is the Strongest 3D Printing Material?; UltiMaker, Design for FDM 3D printing; aPriori, A Guide to Design for Manufacturability.
