There are four honest ways to get a plastic prototype and they answer different questions at different prices. Printing is fastest and cheapest and least representative. Machining gives real material properties in small numbers. Cast urethane gives molded appearance in tens. A soft tool gives genuinely molded parts. The right answer depends on the question and the quantity, and it changes as the project moves.
Printing: plastic rapid prototyping for shape
Best for early geometry, clearance and ergonomics. Filament parts cost almost nothing and arrive in a day. Sintered nylon parts behave well enough for snap fits and clips. Resin parts look good. None of them have the fibre orientation, weld lines or shrinkage behavior that a molded part will have, so they answer shape and not process.
Machining: real material, small numbers
Cutting the part from a block of the production polymer gives correct stiffness, correct chemical behavior and correct thermal performance. It is the right route when the test is functional and the quantity is small. The limits are geometry, because internal features a mold would form easily may be unreachable with a cutter, and cost per part, which does not fall much.
Cast urethane: molded appearance in tens
A silicone tool made from a finished master produces parts that look molded, in a range of hardnesses and colors, in quantities from a handful to around a hundred. It is the standard route for launch runs, photography, user testing and clear parts. Material properties approximate rather than match the production resin.
Soft tooling: genuinely molded
An aluminum tool produces real molded parts in the real resin, so everything the process does to the part is visible: weld lines, sink, warp, shrinkage. It costs the most to start and the least per part, and it is the only route that de risks the production tool. Move to it as soon as the geometry stops changing.
Questions people ask about plastic prototyping
Which rapid prototyping plastic route is cheapest?
Filament printing, by a wide margin, and it is the right answer whenever the question is about shape rather than behavior.
When should I stop printing and start moulding?
When the questions become about how the process affects the part, or when the quantity passes the point where each printed part costs more than a molded one plus its share of the tool.
Can I test a printed part for strength?
With care. Sintered nylon is reasonably representative; filament is not, because it fails between layers. Machining from the production polymer is the safer route for a load test.
How many plastic prototypes should I order?
More than one. Spares cost little once the setup or tool exists and save a whole lead time when a part is destroyed in testing.