The five variables that decide it
Engineers rarely choose a process on cost alone. Five variables interact: part size, annual volume, tooling budget, dimensional tolerance, and feature complexity. Get those on the table and the answer usually resolves itself.
Injection molding excels at small, high-volume, high-precision parts with complex internal geometry. Heavy-gauge thermoforming excels at large parts at moderate volume where tooling cost and lead time matter. The middle ground is real, but the extremes are clear.
Part size: the first filter
Injection molding is bounded by clamp tonnage and tool size. Past a certain envelope, the press simply does not exist, or the tool cost becomes prohibitive. Heavy-gauge thermoforming runs in the opposite direction, its sweet spot is large. Floe Thermoforming forms parts up to 25 feet long, 10 feet wide, and 6 feet deep.
If the part is larger than roughly a cubic yard, injection molding is usually off the table on tooling cost alone, and thermoforming becomes the default.
Volume and tooling cost
Injection tools are steel, multi-cavity, and expensive, often six figures, because they are built to amortize across hundreds of thousands of shots. Thermoforming tools are single-sided and a fraction of that cost, which is why thermoforming wins at the hundreds-to-low-thousands annual volumes typical of OEM equipment, trucking, and medical programs.
The break-even is a volume question: below it, thermoforming's lower tooling cost dominates; far above it, injection's lower per-part cost wins back the tooling premium.
Tolerance and detail
Injection molding holds tighter tolerances and reproduces intricate features, bosses, ribs, snap fits, thin internal walls, that thermoforming cannot. When a part needs that, injection is the right tool.
But many OEM parts are panels, enclosures, covers, and fairings where moderate tolerance and a clean cosmetic surface are enough. Pressure forming closes much of the detail and finish gap when appearance matters.
A simple decision rule
Large part, moderate volume, cost-sensitive tooling: thermoform it. Small part, very high volume, tight tolerance, complex features: injection mold it. When the part is large but needs an injection-molded look, pressure forming is the bridge.
The cleanest way to settle a borderline case is a manufacturability review against your actual drawing, which is what Floe Thermoforming's engineering team does before quoting.