When a customer hands over a part drawing, the first question I ask isn't about material or tolerance—it's about volume and geometry, because those two factors usually settle the process before anything else. Injection molding remains the default for high-volume, complex parts: it handles tight tolerances, inserts, and multi-cavity tooling well, but the mold cost (often a five- or six-figure investment for a steel tool) only pays back at volume. Compression molding is a different animal, typically used for thermosets like SMC and BMC—think large, thick-section parts such as electrical enclosures or automotive panels. The tooling is simpler and cheaper than injection, but cycle times run longer and flash control becomes a real design headache.
Extrusion is for continuous profiles—pipe, sheet, film, cable jacketing—where the "mold" is a die rather than a cavity, so wall-thickness uniformity and die-land length are the details that make or break the run. Transfer molding sits between compression and injection and is still common for encapsulating electronic components with thermosets; the key tooling point is getting the transfer pot and runner geometry right so the melt fills cavities before it cures. Blow molding covers hollow parts like bottles and tanks, and here mold design hinges on pinch-off geometry, cooling-channel placement, and parison control—get the pinch-off wrong and you'll fight weld-line failures all day.
In practice, the boundaries blur: a part might start as compression and migrate to injection as volumes climb, or a die head might be shared across several profile sizes to spread tooling cost. The rule I follow is to match process to the part's thinnest section, required surface finish, and annual quantity—then design the mold around those constraints rather than the other way around. For more mold sourcing and process-selection references, visit MoldSo at www.moldso.com.