Manufacturing process selection is the engineering task of choosing how a part will be made so that it meets performance requirements at an acceptable cost. The best process depends on the material, part shape, required tolerances, surface finish, production quantity, and schedule. A choice that is ideal for one prototype may be too slow or expensive for a million parts.
Good process selection prevents redesign, scrap, delays, and unnecessary tooling costs.
Engineers often compare processes with a selection matrix that scores options such as machining, casting, forging, injection molding, sheet forming, and additive manufacturing. Each process has limits on geometry, material compatibility, accuracy, repeatability, and setup cost. Cost per part usually decreases as quantity increases because fixed tooling and setup costs are spread over more units.
The best process is often found at the crossover point where one process becomes cheaper than another for the required production volume.
Key Facts
- Total cost = fixed cost + variable cost per part × quantity
- Cost per part = fixed cost / quantity + variable cost per part
- Break-even quantity = (fixed cost B - fixed cost A) / (variable cost A - variable cost B)
- High fixed-cost processes can be economical at high volume if their variable cost per part is low.
- Tighter tolerances usually increase cost because they require better machines, slower processing, inspection, or secondary finishing.
- Process selection must match material behavior, geometry complexity, tolerance needs, surface finish, quantity, and budget.
Vocabulary
- Manufacturing process
- A method used to transform raw material into a finished or near-finished part.
- Tolerance
- The allowed variation from a specified dimension, such as 25.00 mm ± 0.05 mm.
- Fixed cost
- A cost that is paid before or during setup and does not depend strongly on the number of parts made.
- Variable cost
- A cost that increases with each additional part, including material, machine time, labor, and energy.
- Break-even quantity
- The production quantity where two manufacturing processes have the same total cost or cost per part.
Common Mistakes to Avoid
- Choosing the lowest prototype cost, because the cheapest process for one part may have high labor or machine time and become expensive at production scale.
- Ignoring tolerance requirements, because a process that can form the shape may still be unable to hold the required dimensions without secondary machining.
- Comparing only material cost, because tooling, setup, scrap rate, cycle time, finishing, and inspection can dominate the final cost per part.
- Assuming additive manufacturing is always best for complex geometry, because it may have slow build rates, limited materials, rough surfaces, or poor economics at high volume.
Practice Questions
- 1 Process A has a fixed cost of 12 per part. Process B has a fixed cost of 4 per part. Find the break-even quantity.
- 2 A machined aluminum bracket costs 18 per part. A die-cast version costs 3 per part. Calculate the cost per part for each process at 100 parts and at 2000 parts.
- 3 A company needs 20 complex plastic prototypes this week, then may later need 100,000 identical parts per year. Explain why the best process for the prototypes may differ from the best process for full production.