The question behind the question
"Should this part be machined, die cast or injection molded?" is really a question about quantity, timing and how settled the design is. Each process has a different balance of upfront tooling cost and per-part cost. At low volume, the process with no tooling usually wins. At high volume, the process with the lowest per-part cost wins, even if its tooling is expensive. The crossover point depends on the part, so this guide explains the factors rather than quoting fixed numbers.
The three processes in brief
CNC machining
Cuts the part from solid stock. No part-specific tooling beyond fixtures, so the first part can be made in days. Per-part cost is driven by machine time and material, and it stays fairly flat as quantity grows.
Die casting
Injects molten metal, usually aluminum or zinc alloys, under high pressure into a hardened steel die. Die cost is substantial and lead time for the die is measured in weeks. Once the die exists, parts come out quickly, with low per-part cost.
Injection molding
Injects molten plastic into a steel or aluminum mold. Like die casting, it needs a tool first and then makes parts fast and cheaply. Tooling ranges from simpler aluminum prototype molds to multi-cavity hardened steel production molds.
How the cost curves behave
Total cost for a run equals tooling cost plus per-part cost times quantity. Divide by quantity to get cost per part.
- Machining starts with low or no tooling, so cost per part is reasonable even for one piece, but it falls only slowly with quantity
- Die casting and molding start with a large tooling cost, so the first few parts are very expensive per part, but cost per part drops steeply as tooling is spread over more pieces
- Somewhere along the quantity axis, the curves cross; beyond that point the tooled process is cheaper
The crossover point moves with part size, complexity, material and tolerance. A small, simple plastic clip may favor molding at a few hundred pieces, while a large, complex aluminum housing may stay machined into the thousands.
Factors that move the crossover
| Factor | Pushes toward machining | Pushes toward tooled processes |
|---|---|---|
| Annual quantity | Low, uncertain or one-off | High and steady over years |
| Design maturity | Still changing | Frozen and validated |
| Tolerances | Very tight on many features | Moderate, with a few machined features |
| Part size | Large parts with long cycle times are expensive to machine; large dies are also costly | Medium and small parts |
| Material needs | Specific wrought alloys or high-performance plastics | Standard casting alloys or molding resins |
| Lead time to first parts | Days | Weeks to months for tooling |
| Geometry | Deep pockets accessible from few sides | Thin walls, ribs and complex shapes in one shot |
Material differences that matter
Die cast alloys are not the same as wrought alloys. Common aluminum die casting alloys, such as A380, contain more silicon for fluidity, have lower ductility than wrought 6061 and usually contain some porosity. They anodize poorly, giving a gray, uneven finish, so cast parts are more often painted or powder coated. Zinc die castings give fine detail and excellent surface finish for small parts.
Molded plastic grades differ from machined plastic stock, too. Many resins are available only as molding grades, while some high-performance materials are machined more easily than molded. Fillers such as glass fiber improve stiffness but affect surface finish and anisotropy.
If a machined prototype will lead to a cast or molded production part, test with the production material as soon as you can.
Geometry differences
Machined parts can have nearly any wall thickness, sharp outside edges and features in many directions, but internal corners always have the cutter radius, and deep narrow pockets are slow.
Cast and molded parts need draft angles on walls parallel to the tool opening, uniform wall thickness to avoid sink and porosity, generous radii and, ideally, no undercuts. Undercuts require slides or lifters that add tool cost. Ribs replace thick solid sections.
A part designed for machining usually needs redesign before it can be molded or cast efficiently. Plan that redesign rather than discovering it when tooling is quoted.
Hybrid approaches
The choice is not always one process or another.
Cast and machine
Die cast the near-net shape, then machine critical bores, faces and threads. This is very common for housings and brackets.
Bridge tooling
Use a simpler prototype mold or die for early production volumes while the design settles, then move to a production tool.
Machine first, tool later
Launch with machined parts, gather field feedback, then invest in tooling once demand is clear. Machining avoids betting on tooling for a design that may still change.
Family molds and multi-cavity tools
Combine several related parts into one tool, or make several cavities for higher output, to spread tooling cost.
A volume-based starting point
These ranges are only rough starting points and can shift widely with part specifics:
- Prototypes and very low quantities: CNC machining, or 3D printing for form checks
- Low to mid quantities with an unsettled design: machining, possibly with bridge tooling for plastics
- Steady mid to high quantities of plastic parts: injection molding
- Steady mid to high quantities of metal housings: die casting, usually with secondary machining
- Very high quantities of small metal parts: die casting, or other processes such as stamping where geometry allows
Always ask for quotes at the actual planned quantities. A quote for 100 pieces cannot be scaled to 10,000 by simple multiplication.
Questions to settle before choosing
- What quantity is needed now, and what is the realistic annual volume over the product life?
- How likely is the design to change in the next year?
- Which features need machining-level tolerances?
- Does the material need to be a specific wrought alloy or engineering plastic?
- How soon are first production parts needed?
- Can the part be redesigned for draft and uniform walls?
Hidden costs to include
Tooling is not the only upfront cost. Molded and cast parts usually need first article inspection and sample approval before production, tool maintenance over the life of the program, and possibly tool modifications if the design changes. Machined parts have their own setup and programming costs per batch, which matter for small reorders. Count these when comparing options, not just tool price and part price.
Ownership and storage of tooling are also worth clarifying in advance, especially for long-running programs.
Quality and consistency over a run
Process choice also changes what quality looks like. Machined parts are measured feature by feature, and each batch depends on setup and tool condition, so first-piece and in-process checks matter. Once a die or mold is approved, parts tend to be very repeatable from shot to shot, but tool wear and process drift still need monitoring, and cast parts may need porosity checks such as X-ray on critical areas.
Molded plastic parts can shrink and warp differently as process settings change. Production molding usually locks process settings after sample approval so that later parts match the approved first articles.
Sustainability and waste
Machining turns a large share of the stock into chips, which are normally recycled but still represent material and energy. Casting and molding produce near-net shapes with less waste per part, plus runners and gates that can often be reground or remelted. At high volume, this efficiency is part of why tooled processes win.
Summary
Machining is fast to start and flexible, ideal for prototypes, low volumes and changing designs. Die casting and injection molding cost more to start but make high volumes cheaply. Use realistic quantities, design maturity and tolerance needs to choose, and do not hesitate to start with machining and switch when demand justifies tooling.