Plastic injection moulds: what drives the cost

The injection mould is a custom tool, machined for a single part. Its cost is added to the price of each moulded part: price per part = mould cost ÷ total quantity + moulding cost per part, the latter covering cycle time on the press, material, finishing, inspection and packaging. The mould is an upfront investment, often the biggest item in a plastics processing project.

A mould is made of two halves: the fixed half, on the nozzle side of the press, and the moving half, on the ejection side. Between them, the cavity gives the part its shape; ejector pins push it out of the mould and cooling circuits remove the heat. After machining, the mould is tried out on a press, corrected and then validated: allow for at least one round of rework, which has a cost and a lead time.

As a guide, a mould costs anything from a few thousand euros for a small, simple tool to several tens of thousands of euros, or more, for a multi-cavity steel mould with slides and hot runners. Lead time follows the same logic: from a few weeks to several months, trials and rework included. These orders of magnitude do not replace a quote: only the part, the material and the target volume can pin them down.

The table ranks the factors that weigh on the price of the tool; the following sections detail the most expensive ones.

The number of cavities

A multi-cavity mould produces several parts per cycle: the moulding cost per part falls, but the tool is more expensive and requires a press with a higher clamping force. For a modest volume, one cavity is almost always enough; multi-cavity tooling is justified by high annual volumes.

Steel or aluminium

Steel, pre-hardened or hardened depending on the target tool life, withstands long runs and abrasive glass-fibre-filled materials. Aluminium machines quickly, dissipates heat well and costs less: it suits prototypes and small runs, with a shorter life of a few hundred to a few thousand parts, sometimes more for a simple part in an unfilled material. The SPI classification, a common reference, runs from class 105 (prototype, a few hundred cycles) to class 101 (over a million cycles).

Tool life

Tell the toolmaker your total volumes over the life of the product: they guide the choice of steel, treatments and wear parts. An oversized mould is expensive for nothing; an undersized one wears out before the end of the run. The choice of steel also affects rework: hardened steel is harder to modify than pre-hardened steel, which makes corrections after trials more expensive.

A mould opens in a single direction. Any shape that prevents demoulding along that axis (side hole, notch, hook, thread) is an undercut. To release it, the toolmaker adds a slide (sliding core), an angled lifter or an unscrewing mechanism: more parts to machine, fit, guide and maintain, and often a longer cycle. Each slide also leaves a visible parting line on the part.

Before paying for a slide, ask the moulder whether the shape can change: orient an opening along the mould opening axis, replace a closed hole with an open notch, move a clip. Draft, that is, the taper of the walls in the demoulding direction, is the basic rule: around 1° to 2° is common, more on textured surfaces. Without it, the part sticks or gets scratched.

The material reaches the cavity through runners. With cold runners, it solidifies in those runners and is ejected with the part: the tool is simpler and cheaper, but the sprue and runners represent a loss of material, which is ground up and reused in some cases and scrapped in others. With hot runners, a heated block keeps the material molten up to the cavity: no more runner waste and sometimes a shorter cycle, but the system is equipment that has to be bought, controlled and maintained.

Hot runners are mainly justified for high volumes, costly resins and multi-cavity moulds. For a small run, cold runners are generally still the right choice. Heat-sensitive materials and frequent colour changes complicate the use of hot runners.

The following example is fictional: it illustrates the mechanism, not a price list. Assumptions: a €12,000 mould and a moulding cost of €0.60 per part (cycle, material, packaging).

To compare with another method, divide the mould cost by the saving made on each part. If, in this fictional example, a 3D-printed part cost €8, the saving would be €7.40 per part and the mould would pay for itself at around 1,600 parts. Also allow for maintenance (cleaning, replacing ejector pins and seals, polishing), which is billed over the life of the mould. Finally, state in writing who owns the mould, who maintains and insures it, where it is stored and how you can get it back: see protecting drawings and tooling.

3D-printed mould inserts also exist for very small runs, with limits on material, injection pressure and tool life. A prototype validated in a material close to the series material confirms the shape and the assembly, but not the behaviour in injection moulding (shrinkage, weld lines, warping): only a mould shows that. See from prototype to series production, 3D printing or CNC machining and the choice of engineering plastic.

• The 3D model (STEP) and a drawing with the functional dimensions and tolerances.
• The material: designation, fibre content, colour (RAL or Pantone), special requirements (food contact, fire resistance, UV).
• The volumes: quantity per run, annual volume and product life, as they determine the size of the mould.
• The appearance: visible faces, polishing, texture, marking.
• The mould: ownership, maintenance, storage, return.
• The trials: number of samples expected and acceptance criteria for the mould (dimensions, appearance).
• Two price lines: the mould, a non-recurring cost, and the part by quantity break, with the lead times for the mould and then for the first parts.

On Meaflow, posting a request is free and commission-free: plastics processors can send you their quote, mould included, which you compare before discussing through the messaging system. For small runs, also look at 3D printing, CNC machining and the glossary entry on plastic injection moulding.