When a customer sends a 3D file and asks for "a plastic part," the first thing I check is wall thickness and geometry. Injection molding handles complex, tight-tolerance parts—think 0.5–3 mm walls, ribs, bosses, and snap fits—using hardened P20 or 718H steel cavities rated for 500,000+ cycles. Blow molding is different: the parison inflates against a two-half aluminum or beryllium-copper mold, so you get hollow, double-walled parts like 5 L jerry cans or 200 L drums. Pinch-off edges and cooling channels matter more than core-cavity precision here. Vacuum forming is the low-cost cousin—a single-sided aluminum or epoxy tool, 0.5–1.5 mm sheet, and pressure down to -0.08 MPa. No core, no injection unit, but you inherit webbing and thin corners.

From a tooling cost perspective, the gap is steep. A single-cavity vacuum-forming tool for a 600×400 mm tray might run $800–$2,000 and cut in three days. A blow-molding mold for a 1 L bottle with handle: $3,000–$8,000, plus a die head and parison controller. An injection mold for the same bottle neck finish and cap? Easily $15,000–$40,000 with a hot runner and 1.2 s cycle. That is why packaging engineers pick blow molding for hollow HDPE containers, vacuum forming for refrigerator liners and automotive trim, and injection molding only when dimensional repeatability or cosmetic Class A surfaces are non-negotiable. Material choice follows: PP and HDPE dominate blow and vacuum forming; ABS, PC, PA66+GF30, and POM are injection-only.

The selection rule I give junior engineers is simple: if the part is solid and needs features on both sides, go injection. If it is hollow and you can blow air through it, go blow molding. If it is a large, shallow shell with one finished face, vacuum forming wins on speed and tooling cost. Always check shrinkage—injection 0.4–2.0%, blow 1.5–3.0%, vacuum forming 0.5–1.5%—and design draft angles of at least 1–2° per side. For mold sourcing, supplier vetting, and real-world tooling quotes across these processes, visit MoldWorld at www.moldw.com.