A practical guide to choosing a process, setting wall thickness, draft, ribs and bosses, and allowing for paint and fit, from a workshop that has made prototypes and small batches since 1986.
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By Tom Davies-Kent, Director, Complete Fabrication. Updated October 2026.
Design for manufacture (DFM) means designing a part so it can be made reliably, at the right cost and in the quantity you need. It covers the choice of process and material, and details such as wall thickness, draft, ribs, bosses, undercuts, fit and finish. Boothroyd Dewhurst, who developed the DFMA method, put the share of a product’s cost that is fixed at the design stage at about 70%, so the earlier these decisions are made, the cheaper they are to get right.
Complete Fabrication checks every file before we make it and tells you if a change would make the part stronger, cleaner or cheaper to make. For a fuller DFM review, send us your CAD, the quantity you expect and the finish you want.
There is no single crossover quantity between processes. It depends on size, geometry, material and finish, so we price the routes side by side. See trial batches for user trials and hall tests.
These are widely used starting points for vacuum casting and injection moulding. Exact values depend on the material and geometry, so check them with us before you commit.
Uneven walls cure and cool unevenly, which causes sink marks, warping and weak spots. For vacuum casting, design guides recommend at least 1.5 mm for full mould filling, with 0.75 mm possible in small, non-load-bearing areas. Where thickness has to change, taper it gradually rather than stepping it.
A slight taper on faces parallel to the mould opening helps the part release cleanly and reduces mould wear. Around 1° to 2° is a common starting point. Textured surfaces need more, often 3° to 5°.
Ribs add stiffness without adding bulk. Make them about half to 60% of the wall they join, and no taller than about three times their own thickness, to avoid sink marks on the show face. Several thin ribs usually work better than one thick one.
Make a boss’s outside diameter at least twice the screw or insert diameter, and support tall bosses with ribs or gussets rather than merging them into a side wall. Threaded brass inserts can be cast into vacuum cast parts, as on the Owlstone casework, which avoids tapping threads into plastic.
Undercuts need extra mould pieces, split lines or inserts. They add cost and wear the mould faster. Moving a parting line or changing a feature’s direction often removes them.
Paint adds thickness. A primer and colour system can add around 0.1 to 0.25 mm per surface, and a full high-build 2K system up to around 0.4 mm. Increase clearances on painted mating faces to suit, and mask threads, bores and snap fits, or re-drill and tap them after painting. Tell us which faces are seen and handled, the A-surfaces, so supports, split lines and finishing effort go where they count. See painting and finishing.
Clearance between mating parts depends on the process. As a starting point for a sliding fit, allow about 0.1 to 0.2 mm per side on SLA parts, 0.05 to 0.15 mm on Carbon DLS parts and 0.02 to 0.05 mm on CNC-machined parts. Add the paint allowance on top if either face is painted.
Our Prototyping Checklist white paper covers file preparation, fit and paint allowances, build orientation and the extra checks for medical prototypes in more detail.
Design for manufacture (DFM) is designing a part so it can be made reliably, at the right cost, in the quantity you need, by choosing the right process and getting details such as wall thickness, draft and fit right before production.
DFM focuses on making each part easy to manufacture. DFMA, design for manufacture and assembly, also looks at how the parts go together, aiming to reduce part count and assembly time.
For a one-off looks-like model, usually not. Once you need batches, plan to move to another process, or are heading for production tooling, a DFM check saves time and money.
Yes. We check every file and tell you if a change would make the part stronger, cleaner or cheaper to make. For a fuller review, send your CAD, quantity and finish.
To ask for a DFM review, email info@compfab.co.uk, call 01223 836837, or use the contact page.
Sources:Boothroyd Dewhurst; design guidance from Xometry, Makerverse and RapidDirect; paint and fit allowances from our Prototyping Checklist white paper.
