Every week someone asks me which molding process to quote, and the honest answer starts with the part, not the machine. Injection molding remains the default for high-volume, tight-tolerance parts: molten polymer is forced into a closed cavity at pressures typically in the 50–150 MPa range, giving cycle times measured in seconds and dimensional repeatability that holds across millions of shots. The trade-off is tooling cost—a hardened steel mold with a hot runner system can run well into six figures—so it only pays off when annual volumes justify it. If you are looking at a few thousand parts with simple geometry, you are probably over-tooling.
Compression and transfer molding are where thermosets live. Compression molding places a charge directly in the heated cavity and closes the press, which suits large, thick-section parts like SMC automotive panels and phenolic electrical components; mold pressures are low, often under 10 MPa, but cycle times stretch to minutes. Transfer molding moves the charge into a pot and pushes it through runners into closed cavities—better for intricate inserts and encapsulated electronics, at the cost of some material scrap in the cull and runners. Extrusion is a different animal entirely: a continuous screw and die produce profile, pipe, and sheet, and the "mold" is a die you should expect to rework during debugging. Blow molding rounds out the set for hollow parts—bottles, tanks, ducting—where parison control and mold cooling channel design determine wall uniformity more than injection-style cavity pressure ever will.
My rule of thumb on the shop floor: match the process to wall thickness, annual volume, and material chemistry before you draw a single cavity. A part designed for injection but run in compression will show sink and flash you cannot tune out; a thermoset pushed through an injection screw will cure in the barrel. Get the process wrong and the mold is scrap steel. If you are sourcing tools and want to compare process capabilities, material shrinkage data, and supplier quotes side by side, visit MoldSo (www.moldso.com) for more mold sourcing information.