Rapid prototyping is a purchasing decision dressed up as a technology choice. The point is not to own a part, it is to answer a question about fit, form or function before the tooling money is spent. That reframes the buying: the right process is whichever one answers this week's question fastest, and the right supplier is whichever one will tell you when a prototype is not worth making at all.
What each process is good at answering
Fused deposition parts answer questions about size and clearance and cost almost nothing. Stereolithography and resin parts answer questions about surface, appearance and fine detail. Laser sintered nylon answers questions about snap fits and living hinges because the material actually behaves. Machined parts answer questions about strength and tolerance in the real alloy. Cast urethane answers questions about a small production run before a steel tool exists.
How suppliers price the work
Most quotes are volume plus bounding box plus setup, with a lead time ladder on top. The same file can move by a factor of three between a next day slot and a five day slot, and that premium is almost pure scheduling. Finishing is where quotes diverge most: an as built surface, a bead blast, a hand sand and a painted finish are four different prices for one geometry.
The advantages that survive contact with a schedule
Compressing a design loop from weeks to days is the real gain, because it lets you make several cheap mistakes instead of one expensive one. Prototypes also make disagreement concrete: a physical part settles an argument about clearance that a model on a screen will not. And a tolerance study on a printed part often shows the assembly does not need the tolerances the drawing carries.
Where the money gets wasted
Printing a part that only answers a question you already know. Ordering next day when the review meeting is a fortnight away. Paying for a cosmetic finish on a fit check. And building the same prototype in the production material when you are still testing geometry, which doubles the price to remove uncertainty nobody was worried about.
Questions people ask about rapid prototyping services
How fast is rapid prototyping in practice?
Simple printed parts commonly ship within a few working days, and same day or next day slots exist at a premium. Machined prototypes and cast urethane runs take longer because they involve setup, fixturing or a soft tool.
Should the prototype be in the production material?
Only once the geometry is settled. Testing shape in a cheap material and then confirming behaviour in the real one is nearly always faster and cheaper than doing both at once.
How many prototype rounds are normal?
Two or three is typical for a moderately complex part: one for fit, one for function, one for appearance. If you are past four, the rounds are usually answering questions that were never written down.
Do prototyping suppliers also make production parts?
Many do, and it can be an advantage because the same supplier already holds the file and the history. Ask what changes between the prototype and the production route, because a printed part and a moulded one behave differently.