There are five honest ways to get a custom plastic part and the right one is almost entirely a function of quantity, with size and material as the tie breakers. Getting this decision right at the start saves more money than any amount of negotiating on price afterwards, because the wrong route is usually wrong by a multiple rather than a margin.
One to ten parts
Printing or machining. Printing is fastest and cheapest for shape; machining from stock gives real material properties. Neither needs tooling, so the whole cost is the part. For anything sculptural or with internal detail, print. For anything that has to behave like the production material under load or heat, machine it.
Ten to a few hundred
Cast urethane if the parts must look moulded, machining if they must behave correctly, and a soft aluminium tool once you are near the top of the range. This is the band where the decision is genuinely difficult and where asking two suppliers to quote both routes is worth the fortnight it takes.
A few hundred to a few thousand
Soft tooling, almost always. An aluminium injection tool produces real moulded parts, and the tool cost divided across a thousand parts is usually less than the extra cost of making them any other way. It also de risks the production tool by showing where the part fills and warps.
Thousands upward, and the material question
A production injection tool in steel, with cavity count chosen from the annual volume. At this point the material decision dominates everything else, because resin cost times volume outweighs the tool. Choose the cheapest resin that meets the requirement rather than the most capable one, and confirm it with a moulded sample before the steel is cut.
Questions people ask about custom plastic parts
What is the cheapest way to make a custom plastic part?
For one part, printing. For a thousand, a soft injection tool. The cheapest route changes completely with quantity, which is why the quantity is the first question.
Can I use one process for prototypes and another for production?
Yes, and most programmes do. The risk is that a printed or machined prototype hides moulding problems, so run a manufacturability review against the production process early.
How do I choose the plastic?
Start from what the part must survive: temperature, load, impact, chemicals, sunlight and any regulatory requirement. Narrow to a family, then choose the grade on cost and processing behaviour.
Do I need a drawing?
For printing, often not. For machining, as soon as a dimension matters. For moulding, yes, because the drawing is what the tool is inspected against.