Injection molding vs vacuum forming

What injection molding is

Molten plastic is injected under pressure into a steel or aluminum tool cut with the negative of the part. The tool closes, the shot fills and cools, the tool opens, the part ejects. Once the tool exists, the cycle repeats in seconds to minutes with almost no variation between one part and the next, which is what makes molding a production process rather than a one-off process.

• The tool is the cost, not the part. A production tool runs 4 to 8 weeks to design, cut, and sample here, and that cost has to be paid once regardless of whether the run is 500 parts or 500,000.
• Per-part cost drops hard with volume. Once the tool is paid off, a molded part is mostly material and a few seconds of machine time, which is why molding wins decisively at production volumes and loses badly on a single large panel.
• Tolerance is tool-limited, then repeatable. We hold ±0.05 mm on critical, CMM-verified features and ±0.1 mm typical, with uncalled dimensions to DIN 16742, and every part off that tool holds the same number, not just the first one.
• Part size is press-limited. Our press range runs 25 to 3,000 tons with an 8 kg shot capacity in-house and to 15 kg through partner presses; a part beyond that shot weight needs a press, and a tool, this site does not size for.

What vacuum forming is

A sheet of thermoplastic is heated until it sags, then drawn down over a single-sided tool by vacuum, cooled, and trimmed. Because the tool only has to withstand atmospheric pressure rather than injection pressure, it can be cast epoxy or machined aluminum instead of hardened steel, which is the entire reason it costs a fraction of a mold.

• The tool is still real money, just far less of it. A forming buck runs the same 4 to 8 weeks to build here as a mold, but in epoxy or aluminum rather than hardened steel, so the up-front number is a fraction of an injection tool for a part of comparable size.
• Per-part cost stays higher than molding at any volume. Heating and cooling a sheet takes longer than a molding shot, and a production run typically takes 5 to 15 working days once tooling exists, against 3 working days for a molded reorder; forming wins on part size and tooling cost, not on unit economics at scale.
• Wall thickness is a consequence, not a spec. The sheet starts uniform and thins wherever it stretches furthest; on a deep draw the thinnest section can reach 40 to 70 percent of nominal, so thickness is designed around rather than called out as a tolerance the way a molded dimension is.
• Part size reaches far past a press. Heavy gauge forms up to 2,500 × 1,800 mm in 0.5 to 12 mm sheet; thin gauge to 1,200 × 800 mm up to 1.5 mm, in ABS, HIPS, PETG, HDPE, PC, PC-ABS, TPO, and acrylic-capped or co-extruded sheet.

Injection molding against vacuum forming

Which one, for which part

• A part larger than a press can practically shoot is vacuum forming's case outright. A machine cover a metre across would need an enormous press and an enormous, expensive tool to mold; it is routine to form.
• Thousands of identical small-to-mid parts is molding's case, and the per-part cost gap over forming widens every time the tool runs again.
• Hundreds to low thousands of a larger, simpler part is where forming wins on total cost: tooling that a molding program at that size and volume would never earn back, against a per-part cost forming can absorb.
• A tight, CMM-verified tolerance on a small feature that also needs volume is molding's territory. Forming has no equivalent to a called-out ±0.05 mm dimension, because the sheet's own thinning sets the limit before the tool does.

Where the crossover actually sits

The tool is the entire reason molding costs more up front and less per part later, and the same logic runs in reverse for forming: a cheaper tool that never earns back molding's per-part economics. The crossover is not one number, it moves with part size as much as volume. A small part in the hundreds of units usually crosses toward forming or machining before it ever justifies a mold; a large panel rarely crosses toward molding at all, because the press and tool needed to shoot it outgrow what makes molding economical in the first place. That is exactly why we ask for part size, material, and annual volume before quoting either route rather than defaulting to one.

Our status

Trumould offers both processes on the same quoting desk: 25 to 3,000 ton presses for molding, and cast-epoxy and machined-aluminum tooling for forming. Send the part, its size, and the expected annual volume, and the quote states which process is recommended and why, not just a price for whichever one you asked about first.