Injection molding vs sheet metal fabrication

What injection molding is

Molten thermoplastic is injected under pressure into a machined 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, and every cycle after the first repeats the same cavity almost exactly.

• The tool is the cost, paid once before part one exists. A production tool runs 4 to 8 weeks to design, cut and sample here, by tool class and complexity, whether the run is 500 parts or 500,000.
• A repeat order is fast because the tool already exists. Off existing tooling, a reorder runs about 3 working days here, material and cycle time only.
• Tolerance is set by the tool, then holds. We hold ±0.05 mm on critical, CMM-verified features and ±0.1 mm typical, to DIN 16742, and part 100,000 off the tool holds the same number as part one.
• Material choice is a production resin, and the part carries its own features. ABS, PC, PC-ABS, PP, PA6, PA66, glass-filled PA, POM, PBT, PMMA, TPE and TPU run in-house, across a 25 to 3,000 ton press range with an 8 kg shot capacity in-house and to 15 kg through partner presses. Ribs, bosses, snap fits and cosmetic texture come off the tool as part of the shot, not as a separate step.

What sheet metal fabrication is

Flat metal stock is cut to a developed pattern, then bent to shape between a punch and die on a press brake, or rolled into a continuous curve. Because the material starts flat and stays a constant thickness through the part, nothing has to be built before the first part exists for laser-cut, punched or brake-formed work; only a hard progressive-die tool, used at very high volumes, carries the kind of upfront tooling cost molding always does.

• No mandatory tool, so no wait before part one on standard work. Typical lead time here is 5 to 10 working days, by process mix and finish, for laser cutting, punching and press-brake forming, our standard route for enclosures and brackets.
• Tolerance is set by the cut, then loosened by every bend. Cut features hold a tighter tolerance than formed ones, and bend angle varies with material springback, which itself varies by alloy, temper and grain direction; a dimension crossing three bends carries the stacked variation of all three. There is no single figure that covers every feature the way a molded part's tolerance does.
• Material choice is real sheet stock, not resin. CRCA mild steel, galvanized steel, stainless 304 and 316L, aluminum 5052 and 6061, brass and copper, in thicknesses from 0.5 to 12 mm on our standard laser and to 25 mm in mild steel, on sheets up to 3,050 x 1,525 mm.
• The part is joined and finished, not molded whole. Multi-piece assemblies are TIG, MIG or spot welded, hardware such as self-clinching nuts and standoffs is pressed in, and the finish (powder coat, wet paint, anodizing, plating, passivation) is applied after forming, not built into a single shot.

Injection molding against sheet metal fabrication

Which one, for which part

• An enclosure, bracket or chassis in metal, at almost any volume, is sheet metal's case: no tool to pay for, and the constant-thickness, cut-and-bend construction is exactly what the process is built for.
• A part that needs molded-in ribs, bosses, snap fits or cosmetic texture straight off the tool is molding's case; sheet metal can only add those as separate pressed-in hardware or a secondary weld, not as part of the base shape.
• Thousands of identical parts in a thermoplastic or elastomer still favors molding once the tool is paid off, the same crossover the cost page linked below works through in numbers.
• A design still changing, or a one-off enclosure, favors sheet metal over molding regardless of the eventual production material, because no one should pay for a production tool before a design is frozen.
• A part needing both, a metal chassis carrying a molded plastic bezel or gasket, is common enough that we quote the two halves together rather than asking a buyer to source them from separate shops.

Where the difference actually sits

The comparison against CNC machining turns on a single crossover volume, because both processes trade a fixed cost against a per-part cost the same way. Sheet metal fabrication doesn't reduce to one number the same way: for laser-cut, punched and brake-formed work there is no tool to amortize, so per-part cost tracks material, sheet utilization and bend count from the first part to the thousandth, closer to how CNC machining scales than to how molding does. The real decision is therefore about material and geometry class before it is about volume: a shape that unfolds to a flat pattern in a real engineering metal is sheet metal's territory at almost any quantity; a shape that needs a molded-in feature, or a volume large enough to amortize a production tool in plastic, is molding's. Where a project genuinely straddles both, we quote each half on its own economics rather than forcing a single process to do a job it isn't suited to.

Our status

Trumould runs both processes in-house on the same quoting desk: 25 to 3,000 ton presses for molding, and laser cutting, punching, waterjet, press-brake forming, rolling and welding for sheet metal. Send the part, the material and the expected volume, and the quote states which process is recommended and why, including a two-piece quote where the part genuinely needs both.