For the purpose of this discussion let's assume you need to make at least 2000 parts. For lower volume prototyping builds rapid prototyping processes are more appropriate.
Here are some example plastic manufacturing processes that are commonly used to produce parts in consumer electronics and industrial automation.
- Injection molding: This is the most common process that yields repeatable parts with the lowest part cost. Appropriate for moderately sized runs. The up front tooling cost is a concern. There are services like Protomold that reduces the up front investment by creating a quick and dirty mold that can't run as fast and won't last through as many parts (but is perfectly fine for a first build if you already know you have design iterations coming in the form of a quick-turn product refresh).
- Thermoforming: This is best for large parts that will be too expensive to injection mold (due to the cost of the mold). Cosmetic parts that are 12" and above in the longest dimension can be good candidates for thermoforming. The benefits are lower cost tooling and much faster turnaround time, at the cost of less precise part tolerances and less options for surface finish and in-mold decorating.
- Rotomoting: This is how rubber ducks are made - appropriate for soft squishable parts that have undercuts.
- CNC machining: Amazingly, this could be a real option for small run parts as you can get good material properties and surface finish. The flip side is intolerably high costs. It could make sense for very small lots for very high priced products.