For tens to low hundreds of parts, vacuum casting is usually cheaper and faster than injection moulding, because a silicone mould costs a fraction of a machined tool. Past a break-even quantity, which depends on the part, the hard tool wins: each silicone mould only lasts a limited number of castings, while an injection mould produces parts for cents each. The worked example below finds that crossing point for one part.
Key points
- Vacuum casting has low fixed cost and high per-part cost. Injection moulding is the opposite.
- A silicone mould wears out after a limited number of castings, so casting cost rises in steps.
- Casting is usually quicker to first parts; moulding is far quicker per part once the tool exists.
- Cast polyurethanes imitate production plastics; they are not the same material.
- The break-even is a calculation, not a rule of thumb. Run it with your own quotes.
How does vacuum casting work?
Vacuum casting starts with a master pattern, usually a high-resolution resin print that has been sanded and finished to the surface the final part needs. The master is suspended in a box and liquid silicone is poured around it. When the silicone cures it is cut open along a planned parting line and the master is removed, leaving a cavity.
Two-part polyurethane resin is then mixed and poured into the silicone mould under vacuum, which pulls out air bubbles, and cured, often in an oven. The flexible mould is opened and the part peeled out. Then the next one is poured.
Because silicone is flexible, the mould can release mild undercuts that would need a side action in a hard tool. Because silicone wears, each mould has a limited life. The vacuum casting technology page covers the process in more detail.
How does injection moulding differ?
Injection moulding (injection molding in the US) injects molten thermoplastic into a machined metal mould under high pressure. The mould costs far more to make and takes longer, but each shot takes seconds and the tool lasts for thousands to hundreds of thousands of parts or more. The injection moulding cost guide breaks that price down line by line.
Side by side
| Vacuum casting | Injection moulding | |
|---|---|---|
| Mould | Silicone, cast around a master | Aluminium or steel, machined |
| Fixed cost | Low: master plus one or more silicone moulds | High: the tooling |
| Cost per part | High: hand pouring, curing, demoulding | Low: resin plus seconds of machine time |
| Mould life | Limited, typically tens of castings per mould | Thousands to a million or more shots, by tool type |
| Time to first parts | Often days to a couple of weeks | Often several weeks, mostly the tool |
| Materials | Polyurethanes that imitate ABS, PP, PC, rubber; clear and coloured grades | The actual production thermoplastic |
| Undercuts | Mild ones release from flexible silicone | Need side actions or lifters, each adding tool cost |
| Draft | Helpful but less critical | Needed on every face that slides out of the tool |
| Best fit | Prototypes, pilot runs, marketing samples, bridge production | Frozen designs at volume |
Where is the break-even?
The structure of each cost is what decides it.
Vacuum casting cost = master + (number of moulds × mould cost) + (parts × cost per casting). The number of moulds is the quantity divided by castings per mould, rounded up. So the total climbs in steps: every time you exceed a mould’s life, you pay for another mould.
Injection moulding cost = tool + setup + (parts × cost per part). The tool is a big step at the start, then the line is almost flat.
Plot both and they cross. Below the crossing point, cast. Above it, mould.
Worked example: a 50 mm handheld enclosure half. Every figure is an example for the arithmetic, not a market price.
Vacuum casting: master pattern €150; each silicone mould €400 and good for 20 castings; resin and labour €18 per casting.
Injection moulding: aluminium tool €6,000; setup €400; €1.50 per part.
| Quantity | Moulds needed | Vacuum casting total | Injection moulding total | Casting per part | Moulding per part |
|---|---|---|---|---|---|
| 20 | 1 | €150 + €400 + €360 = €910 | €6,400 + €30 = €6,430 | €45.50 | €321.50 |
| 50 | 3 | €150 + €1,200 + €900 = €2,250 | €6,400 + €75 = €6,475 | €45.00 | €129.50 |
| 100 | 5 | €150 + €2,000 + €1,800 = €3,950 | €6,400 + €150 = €6,550 | €39.50 | €65.50 |
| 160 | 8 | €150 + €3,200 + €2,880 = €6,230 | €6,400 + €240 = €6,640 | €38.94 | €41.50 |
| 200 | 10 | €150 + €4,000 + €3,600 = €7,750 | €6,400 + €300 = €6,700 | €38.75 | €33.50 |
| 500 | 25 | €150 + €10,000 + €9,000 = €19,150 | €6,400 + €750 = €7,150 | €38.30 | €14.30 |
At 160 parts casting is still €410 cheaper. At 161 parts a ninth mould is needed: casting becomes €150 + €3,600 + €2,898 = €6,648, against €6,400 + €241.50 = €6,641.50 for moulding. So in this example the break-even sits at about 160 parts, and the step is caused by the ninth silicone mould.
Notice how flat the casting cost per part is: it barely moves from 20 parts to 500, because every casting carries its own labour and a share of a mould. Moulding per part falls by a factor of more than 20 over the same range, because the tool is being spread thinner. That is the whole comparison in two columns.
What else should decide it?
Price is not the only question. These often matter more:
- Is the design frozen? A change to a cast part means a new master and new moulds, a few hundred euros in the example above. A change to a hard tool can mean welding and re-machining, and some changes cannot be made at all. If the design is still moving, casting buys flexibility.
- Do you need the production resin? If the part needs a flammability rating, a food contact declaration or a specific grade for certification, a cast polyurethane will not do. Testing needs the real material, which means the real tool.
- How soon do you need parts? Casting usually delivers first parts much sooner. A common pattern is to cast a pilot batch while the injection tool is being made: casting as bridge production.
- What finish do you need? A cast part copies the master’s surface, so a well-finished master gives a well-finished part. Colour can be pigmented into the resin. The surface finish guide covers what each finish adds.
- How large is the part? Large castings use a lot of resin and long cure times, which moves the break-even down. Very small parts can often be cast several to a mould, which moves it up.
What about 3D printing instead of either?
For very small quantities, or for parts that do not need a moulded material or surface, printing each part directly can beat both: no master, no mould, no tool. SLS and MJF in particular produce functional nylon parts at a per-part price that is flat with quantity, much like casting. The 3D printing vs injection moulding guide runs the same break-even arithmetic for printed parts, and the tool amortisation entry explains how the fixed cost is spread.
How to get comparable quotes
Ask each supplier for the same things, so the numbers line up:
- The fixed cost, separately: master and moulds for casting, tool and setup for moulding.
- The castings per mould, or the tool life in shots, that the price assumes.
- The per-part price at three or four quantities, not one.
- The lead time to first parts and to the full quantity.
- The exact material, by name and grade.
With those five, you can redo the table above with real numbers in a few minutes.
Quotometry prices both today from the same uploaded file: vacuum casting with the master, the silicone moulds the run needs and each casting, and injection moulding as a budgetary estimate the shop always reviews. So a shop can show a buyer both columns of the table above for a real part.
Questions people ask
How many parts can you get from one silicone mould?
It varies with the resin, the geometry and how much the part has to be pulled out of the mould. A commonly quoted working range is around 15 to 25 castings before the surface and edges degrade, with simple shapes lasting longer and parts with deep ribs or sharp details wearing the mould faster. Ask your caster what they assume, because it sets the price.
Is a vacuum cast part as strong as an injection moulded one?
Not exactly. Cast polyurethanes are formulated to imitate ABS, polypropylene, polycarbonate or rubber, and they can come close on stiffness, hardness and look. They are a different chemistry, though, so impact strength, heat resistance and long-term ageing can differ. For fit, look and user testing they are usually fine; for certification you normally need the production resin.
When should I skip vacuum casting and go straight to injection moulding?
When the quantity is past the break-even with the tool, when you need the exact production resin for testing or certification, or when the design is frozen and you will need thousands more parts soon. If the design may still change, casting first is cheap insurance: a new master and new moulds cost far less than modifying a hard tool.
How long does vacuum casting take compared with injection moulding?
Vacuum casting is usually quicker to first parts because the mould is made from a printed master in days rather than machined over weeks. Injection moulding takes longer up front for the tool, then produces parts far faster per hour. For a few dozen parts needed soon, casting usually wins on time; for thousands, the moulding press does.
Quotometry prices every part from your own machines and rates, on your website, in seconds.