From Prototype to Production
Design for manufacture and assembly, tooling cost against volume, pilot runs, documentation and compliance.
Design for manufacture and assembly
A prototype made by hand may be impossible or expensive to make a thousand times. Design for manufacture (DFM) makes the parts easy to produce; design for assembly (DFA) makes them easy to put together. The biggest saving is fewer parts.
- Reduce the part count: combine parts, use snap-fits (see Mechanisms, Fasteners and Snap-Fits) and integral features instead of separate fasteners.
- Standardize: the same screw everywhere, standard parts and materials, common PCB parts.
- Make assembly foolproof: parts that can only go in one way, symmetric parts where orientation does not matter, guides and chamfers.
- Assemble from one direction (top down) so that nothing needs turning over.
- Design for the process: draft angles and uniform walls for molding, bend radii for sheet metal, tool access for machining.
- Avoid tight tolerances and special finishes unless the function needs them (see Mechanisms, Fasteners and Snap-Fits).
Worked example: the Boothroyd–Dewhurst assembly efficiency
- The index is E = 3 × Nmin ÷ ta, where Nmin is the theoretical minimum number of parts and ta the total assembly time in seconds (3 s is the “ideal” time per part).
- The first design has 18 parts at about 6 s each: ta = 108 s. If 7 parts are the theoretical minimum (those that must move, be a different material or allow assembly), E = 3 × 7 ÷ 108 = 19 %.
- The redesign combines parts and uses snap-fits: 10 parts at 5 s: 50 s and E = 42 %. It saves 58 s per unit, $0.48 of labor at $30 an hour: $4 833 over 10 000 units, plus fewer parts to buy and fewer faults.
Tooling and volume
Mass production processes need tooling (molds, dies, fixtures) that costs a lot up front but makes parts cheaply. The right process depends on the volume.
Worked example: when does an $8 000 mold pay?
- Injection molding: tool $8 000 plus $0.80 per part. 3D printing: $6.00 per part, no tool. Molding is cheaper in total when 8 000 + 0.80 q < 6.00 q, i.e. q > 8 000 ÷ (6.00 − 0.80) = 1 538 parts.
- Against urethane casting ($400 for the molds and $3 per part) the break-even is 3 455 parts. So: print up to about a hundred, cast up to a few thousand, and mold beyond that.
- The tool cost spread over the quantity is shown in the last column: at 1 000 parts it adds $8 to each, at 100 000 only $0.08.
| Quantity | 3D print | Urethane casting | Injection molding | Tool cost per part |
|---|---|---|---|---|
| 100 | $6.00 | $7.00 | $80.80 | $80 |
| 500 | $6.00 | $3.80 | $16.80 | $16 |
| 1 000 | $6.00 | $3.40 | $8.80 | $8 |
| 5 000 | $6.00 | $3.08 | $2.40 | $2 |
| 20 000 | $6.00 | $3.02 | $1.20 | $0 |
| 100 000 | $6.00 | $3.00 | $0.88 | $0 |
- Bridge tooling (soft aluminum molds, 3D-printed molds) covers the gap of 100 to 5 000 parts at lower cost.
- Design the part for the process before you cut the tool: changing a steel mold costs far more than a CAD model.
- Ask the toolmaker for a DFM review early (see Sourcing, Services and Outsourcing).
- Sheet metal and extrusions have their own tooling: dies and brakes; soft tooling is cheap for small runs.
Process capability and tolerances
A production process varies from part to part. Its capability compares the spread with the tolerance: Cp = tolerance half-width ÷ (3σ), and Cpk allows for an off-center mean (see Statistical Tolerancing and Process Capability).
Worked example: a pilot batch measurement
- A dimension has a tolerance of ±0.20 mm. The pilot parts have a standard deviation σ = 0.05 mm and the mean is 0.05 mm off the nominal.
- Cp = 0.20 ÷ (3 × 0.05) = 1.33; Cpk = (0.20 − 0.05) ÷ (3 × 0.05) = 1.00.
- The fraction out of tolerance is the upper tail beyond 3.0 σ plus the lower beyond 5.0 σ: about 1 350 ppm (0.14 %). If the process were centered it would be 63 ppm (Cpk = 1.33): centering the process is worth a factor of 21.
- A common target is Cpk ≥ 1.33 for important dimensions. Where it is lower, tighten the process, widen the tolerance if the function allows, or inspect every part.
- In a pilot run of 50 units you would expect 0.07 bad parts at this rate for this dimension: with several dimensions the total adds up.
Cost of goods and price
Worked example: from bill of materials to selling price
| Item | Per unit |
|---|---|
| Parts (BOM) | $18.00 |
| Assembly labor and overhead | $6.00 |
| Packaging | $2.00 |
| Test and calibration | $1.50 |
| Warranty allowance (3 %) | $0.83 |
| Cost of goods | $28.33 |
- Yield matters: if four process steps have yields of 99 %, 98 %, 99.5 % and 97 %, the overall yield is 93.6 %, and the cost per good unit is $28.33 ÷ 0.936 = $30.25.
- A common multiplier from cost of goods to retail price is 3 to 5 (to cover design cost, marketing, distribution, support and profit): at 4× the price is $113.30 and the gross margin 75 %.
- Non-recurring costs (design, tooling, certification, say $2 000 for a first batch of 500) add $4.00 per unit: spread them over the whole run.
- Build the BOM early: every part with supplier, part number, quantity, price breaks and lead time; check availability (see Assembly).
- Target cost from the start: decide what the unit may cost and design to it.
- Look at the biggest items first: a few parts are usually most of the cost.
The pilot run
- Build a small batch (20 to 100) with the production parts, tools and people, and not by the prototype maker.
- Time every step and record the problems: they show what to fix in the design or the process.
- Measure the critical dimensions of every part and calculate the capability (above).
- Test every unit and keep the results: the failure rate and the causes.
- Run the product tests (reliability, environmental, user) on pilot units, not on prototypes (see Testing, Iteration and Feedback).
- Freeze the design only when the pilot meets the requirements; then control changes.
Regulations and certification
| Item | Where | What it covers |
|---|---|---|
| CE / UKCA marking | EU / UK | Safety, EMC, RoHS (and radio, if wireless) |
| FCC Part 15 | USA | Unintentional and intentional radiators |
| UL / IEC 62368-1, 61010-1 | Worldwide | Product safety (mains, batteries, hazards) |
| RoHS, REACH | EU and others | Restricted substances |
| Battery transport (UN 38.3) | Worldwide | Lithium cells and packs |
| IP rating (IEC 60529) | Worldwide | Water and dust ingress |
- Plan and budget for it at the start: testing takes weeks and costs thousands, and failures mean redesign.
- Use certified modules (power supplies, radios) where you can: they carry much of the burden.
- Design for the tests: EMC (see Signal Integrity and EMC), safety distances (see Traces, Current, Spacing and Clearance) and the labeling.
- Machine safety and robot cells have their own standards (see Industrial Robotics and System Integration).
- Check the rules for your market and product type: this table is a map, not advice.
General information, not legal advice: get a test lab or a consultant to confirm the requirements for your product.
Documentation and change control
- Drawings and models with revisions; the bill of materials; assembly instructions with photographs; test procedures and limits; software versions and programming steps.
- Engineering change control: once the design is released, every change gets a number, a reason, a decision and a date; the factory builds only the released revision.
- Traceability: serial numbers and batch codes of parts and units, so that a fault can be traced and a batch recalled.
- User documentation: manual, safety warnings and labels (a requirement for most markets).
- Keep the prototype records (what failed and why): the next product starts from them.
Supply chain
- Second sources: for every critical part, an alternative with the same fit and function.
- Lead times and minimum quantities: some parts take months; plan orders early.
- Lifecycle: check that a part is not obsolete or “not recommended for new designs”.
- Contract manufacturer: choose by capability, quality system and willingness to work at your volume; visit or audit if you can.
- Inventory and cash: a minimum order of 10 000 resistors is cheap; one of 1 000 custom parts is not.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Molded part warps or has sink marks | Uneven wall thickness, cooling | Uniform walls, ribs not thick sections; ask the toolmaker |
| Parts do not assemble at the factory | Tolerance stack, poor capability | Check the stack with real measurements; tighten or add adjustment |
| Unit cost higher than target | Part count, expensive parts, low yield | DFA review; look at the biggest items; fix the yield loss |
| Fails the EMC or safety test | Late design for compliance | Pre-compliance tests; fixes on the board and case; allow time for a retest |
| Part is out of stock or obsolete | Single source | Qualify an alternative; buy a stock ahead |
| Field failures after launch | Reliability not tested at volume | Analyze failures; update the design under change control |
Related
- Prototyping Techniques: the overview of this guide.
- Rapid Fabrication: Choosing a Method: cost against quantity.
- Project Management: Budget, Time and Risk: planning the move to production.
- Fits and Tolerances: capability and statistical tolerancing.
FAQ
When should I switch from 3D printing to injection molding?
When the tool cost is repaid by the lower cost per part: with an $8 000 tool and $5 saved per part, above about 1 500 parts. Check the break-even with real quotes.
What is DFM?
Design for manufacture: shaping the design so that it is easy and cheap to make with the intended process, mainly by reducing parts and following the process rules.
How do I estimate the selling price?
Add up the cost of goods (parts, assembly, packaging, test, warranty) and multiply by 3 to 5 for a retail product, adjusted for your channel and volume.
What is a pilot run for?
To prove that the product can be made repeatably with production tools, and to find the process and design problems before committing to volume.
Do I need certification before I sell?
Most products need some: safety and EMC at least, and radio approval for wireless. The rules depend on the market and the product, so check early.
References
- Boothroyd, G., Dewhurst, P. and Knight, W. A., Product Design for Manufacture and Assembly, CRC Press.
- Montgomery, D. C., Introduction to Statistical Quality Control, Wiley (process capability).
- IEC 62368-1, IEC 61010-1, FCC 47 CFR Part 15, EU directives for CE marking (LVD, EMC, RED, RoHS).
Last reviewed: October 2026. Costs, break-evens and capability figures are illustrative; regulations change: confirm with the relevant bodies.