Vacuum casting is usually better when you need 10-100 polymer prototypes with consistent surface finish and lower unit cost, while CNC machining is better when you need tighter tolerances, harder plastics, or stronger functional parts. The right choice depends on whether your priority is appearance, strength, speed, or dimensional accuracy. For many teams, the smartest path is a CNC master pattern plus vacuum casting for duplicates.
What Is the Real Difference Between the Two?
Vacuum casting makes parts by pouring liquid polyurethane into a silicone mold, while CNC machining cuts the part directly from a solid plastic block. That means vacuum casting is a replication process and CNC machining is a subtractive process. In practice, casting wins on repeatability for small batches, while machining wins on precision and material authenticity.
When I evaluate a prototype job, I start with the end use, not the process name. If the part needs to look like production plastic for review, casting often makes more sense. If it needs to survive load, thread engagement, or heat, direct machining is usually the safer route. 6CProto often uses both approaches in the same development cycle: a CNC master pattern first, then casting if the batch size justifies it.
The core trade-off
When Does Vacuum Casting Win on Cost?
Vacuum casting usually becomes cost-effective once you need multiple identical parts and the shape is stable enough to justify a silicone mold. A single CNC prototype can be cheaper at first, but the cost per part stays relatively flat. Vacuum casting has an upfront mold cost, then the unit cost drops for each duplicate.
The turning point is rarely just quantity. Part geometry matters more than people expect. Deep undercuts, thin walls, and large polished faces are often easier to replicate with casting than to machine repeatedly. 6CProto typically recommends casting when the customer wants a short run of visually consistent parts without paying for hard tooling.
Cost logic in plain terms
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1-3 parts: CNC is usually simpler and safer.
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4-10 parts: either method may work, depending on geometry.
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10-100 parts: vacuum casting often becomes attractive if the design is stable.
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Revisions expected: CNC is better because the CAD can change without remaking a mold.
A useful rule from the shop floor is this: if you expect one design change after first sample approval, CNC tends to stay cheaper overall. If the design is frozen and the batch is mostly a duplication job, casting starts to pull ahead. That is especially true when the master pattern has already been machined cleanly.
Which Process Gives Better Strength?
CNC machining usually gives better structural strength because the part is made from solid stock, not cured resin. Machined ABS, nylon, PC, POM, and similar plastics keep the base material properties more predictably. Cast polyurethane can be tough, but it usually does not match the stiffness, heat resistance, or long-term load behavior of solid engineering plastic.
That does not make polyurethane weak. It can be very practical for housings, covers, ducts, cosmetic assemblies, and fit-check parts. But if the part needs threaded inserts, repeated fastening, snap-fit retention, or load-bearing brackets, machined polymer is often the better bet. 6CProto frequently advises clients to choose machining for functional validation and casting for appearance validation.
Strength trade-off at a glance
If the prototype is going to be assembled, disassembled, and tested repeatedly, I lean toward machining. If it will be handled, shown, and checked for fit and finish, I lean toward casting. That small difference in intent saves a lot of wasted iteration.
How Does Surface Finish Affect the Choice?
Vacuum casting usually delivers a smoother, more production-like surface straight out of the mold. CNC machining can also produce excellent finish, but tool marks, witness lines, and edge cleanup may still need attention. For cosmetic prototypes, that distinction matters a lot because surface texture changes how a part is judged by clients and engineers.
There is a hidden factory-floor truth here: finish quality is not just about looks. A good cast surface can make a prototype feel more mature, even when the geometry is still in development. A machined surface can make the part feel more honest, because it reveals every design line and wall transition. 6CProto often uses this difference strategically, depending on whether the customer wants design approval or engineering approval.
Finish selection by use case
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Choose vacuum casting for enclosures, display models, dashboard mockups, and trim parts.
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Choose CNC machining for parts where edges, bores, bosses, and interfaces matter.
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Choose a CNC master pattern first when the cast surface must match a polished reference.
The best finish is not always the smoothest one. Sometimes a slightly more visible machined texture is useful because it exposes sink-risk areas, wall transitions, and parting-line problems before a design moves forward.
How Should You Decide for 10-100 Parts?
The easiest way to decide is to ask three questions: how critical is strength, how important is finish, and how locked is the design? If strength and tolerance matter most, CNC is usually the better route. If visual consistency and low batch cost matter most, vacuum casting is often the smarter choice.
For 10-100 polymer prototypes, I usually break the decision into three scenarios. If the part is functional and likely to change, machine it directly. If the part is stable and needs a production-like appearance, machine one master and cast the rest. If the part has mixed requirements, use CNC for critical interfaces and casting for outer shells.
A practical decision guide
This is where 6CProto adds value beyond basic manufacturing. We do not just quote the process; we help match the process to the stage of development. That matters because the wrong process often looks cheaper on paper and becomes expensive after rework.
Why Do Master Patterns Matter So Much?
A good CNC master pattern is the difference between a clean cast part and a frustrating batch of inconsistent copies. The master defines the surface quality, sharpness of detail, and dimensional accuracy of the silicone mold. If the master is flawed, every cast part repeats those flaws.
In real production, I treat the master like a tool, not just a sample. Edges need to be crisp, draft needs to be intentional, and surface defects must be corrected before molding. A rushed master produces flash, mismatch, and surface noise in the final polyurethane parts. That is why 6CProto’s CNC machining step is often the most important part of the whole casting workflow.
What a strong master should do
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Hold the critical dimensions cleanly.
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Carry the intended finish without extra polishing.
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Avoid sharp undercut traps that damage the silicone mold.
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Represent the final product accurately enough for approval.
If the master is right, vacuum casting becomes a very efficient replication system. If the master is wrong, the process just multiplies the problem.
Can Hybrid Workflow Reduce Risk?
Yes, and this is often the smartest approach for 10-100 parts. A hybrid workflow means CNC machining the first article or master pattern, then vacuum casting only after the design is proven. That reduces revision risk and keeps the final batch closer to the approved geometry.
This approach is especially useful when the part has both functional and cosmetic zones. For example, a housing may need a machined sealing face but only a visually clean outer shell. In that case, 6CProto can support a workflow where the precision surfaces are validated in CNC first, then the replicated parts are cast for quantity and appearance.
Hybrid workflow benefits
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Faster early validation.
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Lower tooling risk than committing to hard tooling.
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Better confidence before batch replication.
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More flexible path from prototype to pilot run.
A hybrid method is not a compromise. In many cases, it is the most disciplined way to get both technical certainty and batch efficiency.
6CProto Expert Views
“On the factory floor, the best process is the one that matches the part’s real job. If the prototype must survive repeated assembly, heat, or load, I trust CNC machining first. If the geometry is frozen and the customer needs 20, 50, or 100 identical polymer parts with a production-like finish, a CNC master plus vacuum casting is often the smarter route. At 6CProto, we use that choice to reduce waste, shorten iteration loops, and protect the customer from paying for the wrong kind of certainty.”
Conclusion
For 10-100 polymer prototypes, vacuum casting usually wins on cost and finish once the design is stable, while CNC machining wins on strength, accuracy, and revision flexibility. The best choice depends less on the process itself and more on the stage of development, the part’s mechanical demands, and how many changes you still expect.
If the prototype is for fit, appearance, or short-run duplication, vacuum casting is often the better value. If the prototype must function like the final part, CNC machining is the safer path. In many projects, the most efficient answer is a CNC master from 6CProto, followed by vacuum casting for the batch.
FAQs
Is vacuum casting cheaper than CNC for 10-100 parts?Usually yes, once the silicone mold cost is spread across multiple parts. For very small quantities or changing designs, CNC can still be cheaper overall.
Are cast polyurethane parts as strong as machined ABS?Not usually. Machined ABS is generally more predictable in strength and durability, while cast polyurethane is better when finish and repeatability matter more.
Can vacuum casting handle functional prototypes?Yes, but only for moderate-duty functional prototypes. If the part needs threads, repeated fastening, or higher heat resistance, CNC machining is the better option.
Does vacuum casting give better surface finish?In most cases, yes. Vacuum casting often produces a cleaner, more production-like finish directly from the mold, which is useful for presentation parts.
Who should choose a hybrid CNC plus casting workflow?Teams that need both validation and quantity. It works well when the design is nearly frozen, the first article must be precise, and the final batch needs a consistent cosmetic finish.