Anybody can print a plastic part. What a plastic prototype manufacturer should add is the judgement about what the prototype is for, whether the geometry will survive contact with the production process, and which material will behave the way the design assumes. Those three things are worth more than the part and are what distinguishes suppliers who look identical on a quote.
The manufacturability review is the value
A prototype supplier who will also mould the part has every reason to tell you now that a wall is too thick, a rib will sink or an undercut will need an action. One who only prints has none. Ask explicitly for a review against the intended production process, and treat a supplier who declines as a parts vendor rather than a partner.
Material advice that is not a catalogue
The question is never which plastic is best, it is which plastic survives what this part meets: temperature, load, chemicals, ultraviolet, cleaning, and any regulatory requirement. A good supplier narrows the list by asking about those and then names two or three grades. A poor one sends the full material table and lets you choose.
A route to production that already exists
The strongest reason to pick one supplier over another is that they can take the same file forward: prototype by printing, bridge with a soft tool, produce in steel. Each handover you avoid is a requalification you avoid. Ask what the path looks like and what changes at each step, and get it in writing before the first order.
What to send and what to ask for
Send the model, the intended production process, the quantity now and the quantity later, and the environment the part lives in. Ask for the quote to separate parts from finishing, for a comment on manufacturability, and for a recommendation you did not ask for. The quality of that unsolicited comment is the best supplier test there is.
Questions people ask about plastic prototype manufacturer
What should a prototype manufacturer provide beyond parts?
A manufacturability review against the intended production process, material recommendations narrowed to the application, and a stated route from prototype to production.
Should the prototype supplier also make production parts?
It removes handovers and requalification, which is worth real money. Check they are genuinely equipped for the production process rather than subcontracting it invisibly.
How do I compare two prototype quotes?
Not on price alone. Compare what is included in finishing, what inspection is offered, what the lead time clock starts from, and what each supplier said about the design.
Do prototype suppliers sign confidentiality agreements?
Reputable ones do, and will do it before quoting. Ask up front rather than after the files have been sent.