Choosing a process
How to pick a manufacturing process for a real part: material, shape, size, precision and how many you need.
Engineers choose a manufacturing process by weighing six things: the material, the quantity, the shape, the size, the precision needed and the cost. No process wins on all six, so every choice is a trade-off. This page helps you narrow the field, then points you to the lessons to read.
The six questions
- Material. Each process works only on some materials, because it must melt, flow, bend or cut them in its own way.
- Quantity. How many parts you need decides whether an expensive mold or die will pay for itself.
- Shape. Thin walls, hollow insides, deep pockets and fine holes are easy for some processes and impossible for others.
- Size. Every machine and mold has limits, so a part the size of a rice grain and one the size of a car door need different processes.
- Precision. Tight tolerances (the allowed error in size) and smooth surfaces often need machining or grinding after the first shaping step.
- Cost. The aim is the lowest total cost for your quantity: tooling (the molds, dies and fixtures made for your part), machine time, material and finishing.
Decision guide
Quantity and material narrow the field fastest, so this guide starts with them.
Quick picks
Most everyday objects go through a chain of processes, listed here in order.
| I want to make… | Usually made by | Lessons |
|---|---|---|
| Plastic drink bottle | A small test-tube shape (the preform) is injection-molded, then reheated and blown into the bottle | 2.1, 2.3 |
| Phone case | Injection-molded plastic; cases with a hard shell and a soft rim are overmolded | 2.1, 2.2 |
| Garden hose | Soft PVC or rubber pushed through a ring-shaped die, often in layers around a woven mesh | 3.3 |
| Car body panel | Stamped from steel or aluminum sheet, spot-welded into the body, then painted | 4.2, 7.2, 11.1 |
| Drink can | A disc of aluminum sheet is drawn into a cup, its wall ironed thin and tall, then printed | 6.1, 4.2 |
| Kitchen sink | Deep-drawn from one sheet of stainless steel, then trimmed and brushed or polished | 4.2, 5.9 |
| Aluminum window frame | Aluminum profiles are extruded, cut and joined at the corners, then anodized or powder-coated | 3.3, 11.6, 11.2 |
| Bike frame (aluminum or steel) | Drawn or extruded tubes, often shaped by hydroforming, then TIG-welded; aluminum frames are then heat-treated | 3.3, 4.5, 7.1, 12.1 |
| Screw | Steel wire is cold-headed to form the head, then the thread is rolled; often hardened and zinc-plated | 3.5, 3.6, 12.1, 11.5 |
| Crankshaft | Forged steel (sometimes cast iron), then turned, ground and surface-hardened | 3.1, 5.1, 5.7, 12.2 |
| Gear | Teeth cut by hobbing, then case-hardened and ground; small gears are also sintered or molded | 5.5, 12.2, 5.7, 9.1, 2.1 |
| Engine block | Iron or aluminum cast in sand molds, or aluminum die-cast, then machined and honed | 1.1, 1.4, 5.2, 5.8 |
| Jet engine turbine blade | Investment-cast as a single crystal of nickel superalloy, cooling holes drilled by EDM or laser, then coated | 1.2, 1.8, 6.6, 11.10 |
| Titanium hip implant | Forged and machined titanium alloy; porous surfaces are often 3D-printed or plasma-sprayed | 3.1, 5.2, 8.4, 11.10 |
| Carbon bicycle wheel | Carbon prepreg (fiber cloth soaked in resin) laid up in a rim mold, then cured under heat and pressure | 10.1 |
| Printed circuit board (PCB) | Copper foil on a glass-fiber board is patterned with light and etched; holes are drilled and plated; parts are soldered on | 13.2, 6.8, 5.3, 11.5, 7.3 |
| Silicon chip | A silicon crystal is grown and sliced into wafers, then built up by hundreds of patterning, deposition and etching steps | 13.1, 13.2, 13.4, 11.9 |
Cost versus quantity
Every process has two kinds of cost. Tooling cost is paid once, before the first good part: the mold, die or fixture made just for your part. Running cost is paid again for every part: material, machine time, energy and labor.
Cost per part = tooling cost ÷ number of parts + running cost per part
The more parts share the tooling, the less each one carries: over 10 parts, each pays a tenth of it; over 100,000 parts, almost nothing. So a process with costly tooling, like injection molding, is very expensive for a few parts and very cheap for many. A process with little tooling, like machining or 3D printing, costs roughly the same per part whatever the quantity.
Rules of thumb
- Count first. For a handful of parts, choose a process with no custom tooling, such as machining or 3D printing; for tens of thousands, pay for a mold or die.
- Get close, then finish. Cast, forge or mold the part close to its final shape (near-net shape), then machine only the surfaces that must be precise.
- Ask for precision only where it matters. Each tighter tolerance adds machining, inspection and cost, so keep it for surfaces that fit, seal or slide.
- Keep walls even. In molding and casting, thick spots cool last and can sink, warp or trap holes; ribs add stiffness without thick walls.
- Add draft, avoid undercuts. A slight taper and no hooked features let the part come straight out of the mold, so the mold stays simple and cheap.
- Need strength under repeated loads? Consider forging. Forging makes the metal's grain follow the part's shape, so it resists fatigue better than a casting.
- Hollow and long shapes have their own processes. Bottles and tanks: blow or rotational molding (2.3, 2.5). Long constant profiles: extrusion (3.3) or pultrusion (10.3).
- Plan the whole chain. Most parts are shaped, then machined, heat-treated and finished, and every step adds cost and time.
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