Injection, Blow, and Vacuum Forming: How Mold Engineers Tell Them Apart on the Shop Floor

When a customer sends a plastic part and asks which process made it, I start with the mold, not the part. Injection molding runs on a closed two-plate (or three-plate) tool with a runner system, ejector pins, and cores that must withstand cavity pressures typically in the 300–1,000 bar range; the melt is pushed into a sealed cavity until it packs. Blow molding uses a very different tool: a split hollow mold that clamps around a parison (extruded or injection-molded), with the mold simply shaping the outside while 2–10 bar of internal air pressure inflates the plastic against the cavity wall. Vacuum forming needs only a single-sided mold—often aluminum or even wood/epoxy—with vacuum holes, since atmospheric pressure (roughly 1 bar) does the forming. If you see ejector marks and a gate vestige, you are almost certainly looking at injection molding.

Wall-thickness distribution is the fastest field check. Injection-molded parts hold fairly uniform nominal walls, commonly 1.5–3 mm, with ribs and bosses designed around sink-mark control; the gate area may show slight overpacking. Blow-molded bottles and tanks show a characteristic pinch-off seam at the bottom and parison pinch lines at the handle, and wall thickness varies widely—thin at the blow-up corners, thicker at the pinch-off. Vacuum-formed parts, such as trays and covers, have near-constant drawdown from a single sheet (typically 0.5–6 mm), with thinning at deep draws and a trimmed edge where the sheet was clamped. Tooling cost follows the same order: injection molds are the most expensive and longest to build, blow molds sit in the middle, and vacuum molds are the cheapest and fastest.

Pressure conditions also leave fingerprints. Injection parts are dense and dimensionally repeatable, with tight tolerances and no internal voids when the process is dialed in; blow-molded parts are hollow with a visible internal air volume and a seam line; vacuum-formed parts are open shells with a free, unformed back face. For sourcing, match the process to volume and geometry: injection for precision and high runs, blow for hollow containers, vacuum for large, low-volume panels. If you need to source or verify molds across these processes, visit MoldSo (www.moldso.com) for more mold sourcing information.