Concept renderInjection mold tooling is the machined metal mold whose core and cavity shape each plastic part. The tool material, cavity count and SPI class set how long it lasts and what each part costs. Commit to steel after the design is stable and ownership is in writing.
The enclosure tool is a large, early purchase in a hardware pilot, and it is hard to change once it is cut. This guide covers what the tool is made of, how its life is rated, how many cavities to ask for, what drives its cost, and when to move from aluminum to steel. It contains no prices.
Figures come from manufacturer design guides read in November 2026. They sometimes disagree, and we say so where they do. Confirm every value with the toolmaker who quotes your part.
What is injection mold tooling?
An injection mold is a precision metal tool. Plastic is melted, pushed into the closed tool, cooled, and pushed out as a finished part, over and over. For a wearable or a tracker, tools usually make the top and bottom shells, button caps and any light pipe or window. Every tool has the same main parts.
| Tool part | What it does | Why a device team cares |
|---|---|---|
| Core and cavity | The shaped sections in each half of the tool that give the part its final shape. | Decide which half forms the outside of the enclosure, since that face carries the finish buyers see. |
| Gate | The opening between runner and part, often narrower than the runner so the runner breaks away cleanly. | It leaves a mark. Keep it off cosmetic faces, logos and gasket surfaces. |
| Runner, hot or cold | The channel that carries plastic to the part. A hot runner costs more, gives more freedom in where the gate goes and wastes less plastic. | On a small shell the runner can weigh a lot next to the part. Ask which type is planned. |
| Ejector system | Pins that push the part out, with guide pins and bushes that keep the halves aligned. | Pins press on the part as it ejects. Keep them off sealing lands and label areas. |
| Side actions | Moving sections needed when an undercut would stop the part from ejecting. | Side USB-C ports, side buttons and snap hooks often create undercuts. |
Sources: Toolcraft for core, cavity, gate, runner and ejector; Protolabs cost tips for undercuts and side actions. Both as of November 2026.
Why the parting line matters
The core and cavity meet along the parting line, and 3D Systems notes that every molded part has one (3D Systems, as of November 2026). On a small enclosure it shows as a faint line. Agree where it runs before tool design starts, so it does not cross a seal, a logo or a face that touches skin.
Aluminum, P20 or H13: which mold material fits your volume?
Tool material sets how many parts the tool can make and how long it holds a finish. Softer metals cut faster and cost less to change. Harder steels last longer and polish better. Shot counts below are from Fictiv and hardness from Protolabs, as of November 2026.
| Material | Hardness as published | Shot life as published | Typical fit |
|---|---|---|---|
| Aluminum | Not stated. 3D Systems names QC-7 and QC-10 aircraft aluminum as grades in use. | Usually 10,000 shots or fewer (Fictiv). Harder 7000-series aluminum may last much longer, and 3D Systems says QC-7 and QC-10 can be economical for hundreds of thousands of parts. | Prototype and bridge builds, designs still changing. |
| P20 pre-hardened steel | About 28 to 32 HRC (Protolabs). | 50,000 to 100,000 shots. | Moderate runs on a mostly settled design. |
| H13 steel | 45 to 55 HRC (Protolabs). Holds strength through repeated heating and cooling, and can take a mirror polish. | 500,000 to 1 million shots. | High volumes and glossy cosmetic faces. |
| S136 stainless steel | Not stated. | 1 million shots or more. | Very long programs. |
The sources do not agree on hardness. Protolabs gives about 28 to 32 HRC for P20, while 3D Systems gives 38 to 45 HRC for pre-hardened steel as a group and 50 to 60 HRC as typical for hardened steel. A grade name alone does not fix the hardness, so ask for the core and cavity hardness in writing.
Fictiv also ties tool life to care: gentle cleaning, lubrication of ejector pins, slides and cores, scheduled inspections, and a record of each service. Ask to see that record for any tool you pay for.
How do SPI mold classes rate tool life?
The SPI classes give buyers and toolmakers a shared way to say how long a tool must last. This summary is from Xometry, checked against Fictiv, as of November 2026. We did not read the original SPI document, so ask your toolmaker which version they build to.
| Class | Expected cycles | Main requirements |
|---|---|---|
| Class 101 | Over 1 million | Cavities and cores at least 48 RC, structure at least 28 RC. Guided ejection, slides with wear plates. |
| Class 102 | 500,000 to 1 million | Most Class 101 properties. Plated cavities and corrosion-resistant temperature control channels not mandatory. |
| Class 103 | Under 500,000 | Cavities and cores at least 28 RC, structure at least 18 RC. |
| Class 104 | Under 100,000 | Base, cavities and cores can be aluminum or mild steel. |
| Class 105 | Under 500 | Prototype class. Can be cast metal or epoxy. |
For a pilot of a few hundred to a few thousand units, a low class can be enough. Pick the class from expected lifetime volume plus a margin, and write the margin down. Some molders sell their own tiers instead. Protolabs lists prototyping tools as guaranteed for at least 2,000 shots and its production tier as unlimited (Protolabs tiers, as of November 2026). Whatever the label, get the shot count in the quote.
How many cavities should the tool have?
Cavity count is how many parts come out of each cycle. More cavities make more parts per hour, but Protolabs notes that tool price and part cost do not always cross over in your favor. The table follows the Protolabs multi-cavity guide, with the family mold conditions from Protolabs cost tips, as of November 2026.
| Layout | What it is | Upside | Watch for |
|---|---|---|---|
| Single cavity | One part per shot. | The typical starting point. | Fewer parts per hour. |
| Multi-cavity | Copies of one part per shot, such as two, four or eight. | More parts per cycle once volume is real. | More cavities do not necessarily mean lower cost, and a manual step such as hand-loaded pick-outs gets slow on an eight-cavity tool. |
| Family mold | Different parts in one tool, such as left and right halves. | One tool for a matched set, which Protolabs says can lower the price. | All parts share one plastic and should be roughly the same size. |
The top and bottom shells of an enclosure look like a natural family pair. If the bottom shell later needs a different resin or color, though, a shared tool makes that change harder. Decide resin and color for each part before you choose a family layout.
What are the main injection mold cost factors?
Tooling cost follows the work needed to build the tool. Each driver below can be changed in the design, and most cost little to change in CAD and a lot to change in steel. For snap hooks, side ports and buttons, see how to design out undercuts before you pay for side actions.
| Factor | What the sources say | How to keep it down |
|---|---|---|
| Tool build time | Protolabs calls the time to produce the tooling the main determinant of molding cost. | Simplify geometry before the quote. |
| Undercuts | They can complicate or prevent ejection and may need side actions, sliding shutoffs or pick-outs. | Redesign snaps and side openings to pull straight, or move the parting line. |
| Surface finish | Anything finer than PM-F0 (as machined) needs hand work. Finishes go up to SPI-A2 mirror. | Polish only the faces that need it. |
| Texture and draft | 3D Systems gives 1.5 degrees of draft for each 0.001 inch of texture depth as a rule of thumb. | Pick the texture before the tool is cut. |
| Abrasive resins | Glass-filled materials need more frequent tool maintenance. | Name any glass fill before the steel is chosen. |
| Design revisions | Removing metal from a mold is relatively easy. Adding it back is hard. | Plan likely changes so they add plastic, which means cutting steel away. |
Sources: Protolabs cost tips for build time, undercuts, finish and revisions; 3D Systems for texture draft; Xometry for abrasive resins. All as of November 2026.
Wall design matters too. 3D Systems gives typical walls of 2 to 4 mm and says thickness should not vary by more than 10% in high-shrinkage plastics, as of November 2026. Fixing uneven walls after the tool is cut means more tool work.
When should you commit to a steel tool?
Commit to steel when the design has stopped moving and the volume justifies the life. Before that, an aluminum or bridge tool lets you change the part for less. Our guide to prototype plastic injection molding compares aluminum tools with printing and casting. In a program gated as EVT, DVT and PVT, the production tool usually comes once the design holds up in testing. Use this table as a check.
| Signal | Stay with aluminum or a bridge tool | Move to steel |
|---|---|---|
| Design status | Still changing after the first builds. | Fit, sealing and drop results are stable on near-final parts. |
| Lifetime volume | Inside the published life of an aluminum tool. | Beyond it, with margin. |
| Cosmetic finish | Matte or as-machined is fine for now. | High polish or a texture that must match lot after lot. |
| Resin | Unfilled plastic. | Glass-filled or other abrasive resin. |
| Test builds | Internal fit and function checks. | Formal lab tests or sellable stock that must match production. |
| Ownership terms | Not yet written down. | Signed and tied to payment. |
These signals are engineering judgment, and no source sets a fixed rule. If the design status row is still on the left, wait.
Put tool ownership in writing first
Paying for a tool does not always make it yours. Even one molder's tiers differ: Protolabs lists mold ownership as on request for prototyping tools and included for production tools, as of November 2026. The contract should say who owns the tool, from which payment, where it is stored, who maintains it, and how it ships elsewhere.
Our gated prototype development process states what you keep from the small-batch gate. Yours: Molds and fixtures you paid for, labeled with your name. Certification grants in your name. If you stop there: Move production to a factory of your choice with molds, fixtures and test procedures. Have your lawyer review tooling terms, since this is not legal advice.
What to ask your hardware team
- Where will the gate, ejector marks and parting line land on the enclosure?
- Which features need side actions, and can any pull straight instead?
- What is the core and cavity material and hardness, in writing?
- Which SPI class or shot count is the tool built to, and is it guaranteed?
- How many cavities, and why that number for our volume?
- Which surfaces get polish or texture, and to which finish?
- Which likely design changes are steel safe?
- Who owns the tool, who maintains it, and how does it move to another factory?
What else do buyers ask about mold tooling?
What is the difference between soft tooling and hard tooling?
Definitions vary. Vendors use soft tooling for silicone or composite molds (3ERP) and also for aluminum molds (Fictiv), as of November 2026. 3ERP puts its soft tools at typically fewer than 100 parts and lists aluminum, nickel and steel alloys as hard tooling. Ask the toolmaker to name the material and the shot count.
How long does an injection mold last?
The SPI classes run from under 500 cycles for Class 105 to over 1 million for Class 101 (Xometry, as of November 2026). Real life also depends on cleaning, lubrication and recorded maintenance.
Can I move my mold to another factory?
Only if your contract allows it and the tool fits the new press. Settle ownership and transfer terms before you pay. See also who owns source files under a US contract. This is not legal advice.
How could Duobay help with tooling?
Tooling is planned in the small-batch gate of our gated process, and you can stop at any gate. A Gate 0 Feasibility study is a paid, fixed-fee study that sets the plan. We do not publish prices. Tell us about your device and we will explain how it works.
Where do these numbers come from?
All pages accessed November 5, 2026. Confirm each figure with your toolmaker.
- Toolcraft, injection tool parts explained
- Protolabs, injection molding cost tips
- Protolabs, design considerations for multi-cavity molds
- Protolabs, prototyping and on-demand tool tiers
- Protolabs, tool steel in manufacturing
- Fictiv, injection molding tooling life
- Xometry, plastic injection molding lifespan and SPI classes
- 3D Systems, basics of injection molding design
- 3ERP, soft vs hard tooling
- Fictiv, soft tooling and hard tooling for injection molding
If you are planning an enclosure and want a tooling plan that fits your volume, tell us about your device.
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