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Boyang Hardware Precision CNC Machining

CNC machining vs die casting vs investment casting vs injection moulding

Direct answer

The deciding factor is quantity against tooling cost. CNC machining needs no tooling and is the cheapest route from 1 part up to roughly 500; investment casting becomes economical from about 500 parts and suits complex geometry; die casting becomes economical from about 2,000 parts for metal components; and injection moulding from about 5,000 parts for plastics. Tolerance follows the opposite order: CNC machining holds 0.01 mm and better, die casting and injection moulding about 0.05 mm, and investment casting about 0.10 mm. The two are usually combined — cast or mould to near-net shape, then machine the critical features.

Reviewed by the Boyang Hardware engineering team Page last updated:

Which process should you use?

Two questions settle most cases: what the part is made of, and how many are needed per year.

Decision flow for choosing a manufacturing process A flowchart. Question one: is the part a thermoplastic produced at volume? If yes, use injection moulding, with a CNC machined prototype first. Question two: what is the annual quantity of the metal part? Under 500 pieces use CNC machining. Between 500 and 2,000 pieces use investment casting or CNC. Over 2,000 pieces use die casting. A closing note says to check geometry before quantity, because draft and uniform wall thickness favour casting while sharp internal corners, undercuts and tight tolerances favour machining. Q1 Is the part made of plastic? Produced at volume, from a thermoplastic Yes Injection moulding, from about 5,000 pcs Machine the prototype first; cut the tool once the design is fixed Q2 What is the annual quantity? Answer for the first two years of production Under 500 pcs CNC machining No tooling cost, no tooling lead time. Any machinable material. 500 – 2,000 pcs Investment casting Handles complex internal geometry. Machine after casting for tight features. Over 2,000 pcs Die casting Tooling amortised over volume. As cast, about 0.05 mm. Check the geometry before the quantity. Draft and uniform wall thickness favour casting and moulding. Sharp internal corners, undercuts and tolerances under 0.05 mm favour machining.

Full comparison

Table 1 — CNC machining, die casting, investment casting and injection moulding compared. Quantities are typical industry break-even points, not a quotation for a specific part.
Factor CNC machining Die casting Investment casting Injection moulding
Tooling cost None to low High Medium High
Tooling lead time None 4–10 weeks 3–8 weeks 4–10 weeks
Cost per part High Low at volume Medium Low at volume
Economical from 1 piece ~2,000 parts ~500 parts ~5,000 parts
Tolerance, typical ±0.01 mm ±0.05 mm ±0.10 mm ±0.05 mm
Tolerance, best ±0.001 mm (grinding) ±0.03 mm ±0.05 mm ±0.02 mm
Surface finish, as formed Ra 1.6 µm typical Ra 1.6–3.2 µm Ra 3.2–6.3 µm Finish from the tool, Ra 0.8 µm and up
Minimum wall thickness 0.5 mm, if supported 0.9–1.5 mm (zinc thinner) 1.0–1.5 mm 1.0–1.5 mm, material dependent
Draft angle Not required 1–3° 0–1° 0.5–2°
Internal corners Cannot be sharper than the cutter radius Radius from the die Sharp possible Radius from the tool
Undercuts and internal cavities Possible with special tooling Needs side-action slides Yes, from the wax pattern Needs slides or lifters
Materials Any machinable metal or plastic Aluminium, zinc, brass Wide metal range, including steels Thermoplastics and over-moulded inserts
Design change after tooling Drawing revision only Tool modification or new tool New pattern Tool modification
Strength Wrought material properties retained As-cast; porosity possible As-cast; finer grain than die casting Dependent on material and wall uniformity
Lead time to first part 3–7 working days Tooling plus production Tooling plus production Tooling plus production
Best suited to Prototypes, low volume, tight tolerance, changing designs High-volume housings and structural parts in light alloys Complex geometry at medium volume, including steels High-volume plastic components

Break-even: when does tooling pay for itself?

A tool is only worth cutting when the savings per part recover its cost. The calculation is simple:

Break-even quantity

break-even parts = tooling cost ÷ (CNC unit price − cast or moulded unit price)

Only the tooling cost and the unit price difference go into this sum. The next section lists the costs that are usually left out and that move the real break-even later.

Table 2 — Break-even quantity in parts, by tooling cost and saving per part. Read down a column: the same saving recovers a larger tool over more parts.
Tooling cost Saving 5 / part Saving 10 / part Saving 25 / part Saving 50 / part
10,0002,0001,000400200
25,0005,0002,5001,000500
50,00010,0005,0002,0001,000
100,00020,00010,0004,0002,000

Figures are in currency units of your choice; the ratios are what matter. Worked example: a tool costing 25,000 that saves 25 per part pays back at 1,000 parts. If the annual volume is 400, the tool never pays back and machining stays the cheaper route — unless the volume is expected to grow.

Costs that move the real break-even later

  • Finish machining. Cast and moulded parts usually need critical features machined afterwards, which adds a machining operation back into the unit cost.
  • Tool maintenance. Dies wear and need refurbishment; stamping tools are reworked as the edge dulls.
  • Minimum order commitment. Tooling is often quoted with a minimum production quantity, so the first order is larger than the demand.
  • Sampling rounds. First-off samples may need two or three iterations before the tool is signed off.
  • Change cost. If the design is still moving, a tool change can cost more than the tool saved.
  • Inventory. Volume production means holding stock, and stock ties up cash and can become obsolete.

Which materials each process handles

Table 3 — Material availability by process
Material CNC Die casting Investment casting Injection moulding
AluminiumYesYesYesNo
ZincYesYesLimitedNo
Brass and bronzeYesYesYesNo
Carbon steelYesNoYesNo
Stainless steelYesNoYesNo
TitaniumYesNoSpecialist onlyNo
Engineering plasticsYesNoNoYes
Carbon fibre compositeYesNoNoNo

Combining processes: the usual answer at volume

The comparison above is usually a false choice. Most production parts use two processes in sequence:

  1. Prototype by CNC machining. The design is tested as a real part, in the real material, before any tool is cut. Iterations cost a drawing revision.
  2. Tool once the design is frozen. Die casting, investment casting or moulding brings the unit cost down.
  3. Finish machine the critical features. Bores, sealing faces, bearing seats and threads are machined after forming to the tolerance the assembly needs.
  4. Continue in parallel. Low-volume variants, spares and revised versions stay on the machining route after the main volume has moved to tooling.
Why this matters when choosing a supplier If the prototype shop, the caster and the finishing shop are three companies, the tolerance chain and the schedule are split across three parties and nobody owns the assembly fit. Machining, casting, moulding and finishing under one roof in Zhongshan keeps the process route, the tolerance stack and the responsibility in one place.

Questions about process selection

Is CNC machining or die casting cheaper?

At low volume, CNC machining: there is no tooling cost, so the first part and the hundredth part cost roughly the same. At high volume, die casting: the tool is expensive but the cost per part is far lower, so the tool cost is recovered and then beaten. The crossover is tooling cost divided by the saving per part — a 25,000 tool saving 25 per part breaks even at 1,000 parts.

At what quantity does die casting become cheaper than CNC?

Typically 1,000 to 5,000 parts per year for a medium component, but the number is specific to the part. It moves later if the casting needs significant finish machining, if the tool needs maintenance, or if the tooling quotation carries a minimum production quantity. Ask for both routes to be quoted at your actual volume — a comparison built on real numbers beats a rule of thumb.

Which process holds the tightest tolerance?

CNC machining, by a wide margin: 0.01 mm is routine and 0.001 mm is achievable with grinding. Die casting and injection moulding hold about 0.05 mm, investment casting about 0.10 mm. This is exactly why cast and moulded parts have their critical features finish-machined after forming. See the tolerance capability table.

Investment casting or die casting?

Investment casting uses a sacrificial wax pattern in a ceramic shell, so it can produce complex internal cavities and undercuts with little or no draft, and its tooling is cheaper than a die. It becomes economical from roughly 500 parts, and it casts steels and stainless steels that die casting cannot. Die casting needs draft and uniform wall thickness, but above roughly 2,000 parts in aluminium, zinc or brass it is the cheaper route per part.

Can a part be cast and then machined?

Yes, and it is normal at volume. Casting brings the part to near-net shape cheaply; machining then brings the critical features to tolerance. Doing both in one facility avoids the tolerance stack and schedule risk of shipping castings between suppliers.

What if the quantity is uncertain?

Machine the prototype and the first production batch, and hold the tool decision until the demand curve is real. A machined part can be revised for the cost of a drawing revision; a tooled part cannot. Where the volume is genuinely uncertain but the part is near a break-even, an investment casting pattern is usually the cheaper way to hedge.

Next step Send the drawing and the annual quantity, and both routes come back priced so the comparison is made on your part rather than on this table: evelyn_lin@machiningservice.cn.

Get both routes priced on your part

Send the drawing and the expected annual quantity. Machining and tooling options come back together, so the decision is made on your numbers.

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