Injection molding vs vacuum casting
Both give you parts that look molded. One needs a hard metal tool; the other uses silicone molds that last about twenty parts each. How they compare, where the costs cross, and which suits your run.
The short answer
Vacuum casting (also called urethane casting) pours liquid polyurethane into a silicone mold made from a master part. The molds are cheap but wear out after roughly 15 to 25 parts, so each part costs much more than a molded one. It suits tens to a few hundred parts, quickly.
Injection molding needs a metal tool that costs thousands up front, then makes each part for a fraction of the price. It wins once you need more than a few hundred, and the crossover for your part is in the calculator below.
Find the crossover for your part
Set your part's size and drag the quantity. Compare the casting line with the two mold lines, and look at the tooling each one needs.
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Side by side
| Vacuum casting | Injection molding | |
|---|---|---|
| Tooling | A master part plus silicone molds, each good for roughly 20 parts | A metal mold, paid once, good for thousands of parts or far more |
| Cost per part | High, and it stays roughly flat as the run grows | Low, and the tool's share falls as the run grows |
| Time to first parts | Often days to a couple of weeks | Usually several weeks for the tool |
| Materials | Polyurethanes that imitate ABS, PC, PP or rubber | The real production plastic |
| Surface | Copies the master, so finish the master well | Set by the mold's polish or texture |
| Best for | Samples, beta units, small batches, testing before tooling | Production runs from a few hundred upward |
Look-alike materials aren't the same material
Cast urethanes can match the look and feel of production plastics closely, but not every property. Heat resistance, UV stability, chemical resistance and long-term strength can differ. If you need certification that depends on the material, such as flammability ratings or food contact, check whether cast parts will be accepted, or plan to test molded parts.
Why the costs cross
With casting, every batch of about twenty parts needs a new silicone mold, plus the labor of pouring and curing each part. So the cost per part barely falls as you make more. With molding, the tool is the big cost, and spreading it over more parts makes each one cheaper. For small parts the lines often cross in the low hundreds; an aluminum mold usually moves the crossover lower than a steel one.
A common path
- 3D-print prototypes until the design stops changing.
- Vacuum-cast tens to a couple of hundred parts for testing, beta users or early sales.
- Cut an aluminum mold for the first production run.
- Move to steel once volume proves out.
Pilotrun shows this path for each of your parts, with the quantity where each step starts to pay off. More on low-volume molding.
What to ask a casting supplier
- How many parts each silicone mold is good for, and how many molds the quote includes.
- Which resin they'll use, with its datasheet.
- How they'll finish the master, and whether color is pigmented in or painted.
- What tolerances they can hold on your critical dimensions.