Roll Forming vs Extrusion vs Injection Molding

Start with the part, not the machine

I'm Iris Xu, a Sales Manager in ZTRFM. The question I hear most from new importers is not which machine to buy, but which process should make the part at all. Roll forming, aluminium extrusion, and plastic injection molding each make long or shaped sections, and on paper they look interchangeable. On the floor they are not. Picking wrong costs tooling money and gives you a part that fights its own material.

Roll forming shapes a steel coil at room temperature by pulling it through a series of rolls. Extrusion forces heated aluminium through a die. Injection molding injects molten plastic into a closed mold and pops out a discrete part. The first two make continuous length; the third makes one piece at a time. That single difference reshapes every other choice below.

Where each one wins

Roll forming wins on volume and length. A line runs 10 to 60 metres per minute and never stops for a billet. For a steel stud, a purlin, a roof panel, or a shelving upright, nothing else matches the unit cost once the rolls are paid for. It is cold, so the steel keeps its strength and the coating stays intact. The catch is that it only makes open sections, U, C, Z, angle, and the like. A closed tube needs a weld seam added after.

Extrusion wins on shape freedom. An aluminium press pushes the metal through a die and out comes a hollow, multi-chamber profile in one pass, something roll forming cannot do. The die is cheap, so you can afford to try several shapes. The cost is speed and temperature: 1 to 6 metres per minute, and the part must cool and age before it is stable. Extrusion suits aluminium, not steel strip. Steel is extruded only as hot solid bar or tube, never as thin coil strip, so do not spec extrusion for a thin steel section.

Injection molding wins on 3D detail. Threads, snaps, ribs, and textures land in one shot, and tolerances of plus or minus 0.05 mm are routine. The mold is the expensive, slow part, and it only makes discrete pieces, not metres of rail. For a bracket that clips together, plastic is hard to beat. For a beam that holds a roof, plastic is the wrong call.

Why metal still beats plastic

Two facts move engineers back to metal. First, fire. Thermoplastics soften and then burn; steel and aluminium hold shape and only lose strength at hundreds of degrees. Building code after building code therefore bans plastic for structural and escape-route members. Second, load. A steel purlin carries orders of magnitude more than a plastic one of the same weight, so the structure needs less material overall. Metal is also recycled without loss of property, while most plastics downcycle.

That is not to say plastic loses. For enclosures, trims, and light clips where nobody stands on the part, injection molding is fast and cheap. The mistake is asking plastic to do a steel job, or asking extrusion to make a thin steel rail.

A cost comparison on one real job

Numbers make the choice concrete. A customer wanted 50,000 metres a year of a 1.2 mm steel shelving upright, 200 mm wide with two 25 mm lips. Roll forming the steel cost roughly 0.40 USD per metre in converted coil, with rolls priced at about 30,000 USD and paid off inside the first year at that volume. Quoting the same shape in aluminium extrusion came to about 1.10 USD per metre in material plus a 60,000 USD press, and the part was heavier and softer. Injection molding could not make a 6-metre rail at all, the mold would cap near 1 metre. At that volume and length, steel roll forming was the only process that closed the business case.

The reverse case is just as clear. A short, detailed 150 mm plastic clip made 200,000 times a year is perfect for injection molding at roughly 0.05 USD a piece, far below the cost of a formed and assembled steel bracket. The rule is simple: long and structural goes to metal, short and detailed goes to plastic, and hollow light goes to aluminium. Volume sets which metal process wins.

One more factor: surface and finish

The table above leaves out surface. Roll forming keeps coil finish, so a prepainted or galvanized strip leaves the line already finished. Extrusion needs a separate anodize or powder coat. Injection molding can mould colour in one shot but shows flow lines and gate marks that metal never does. If the part is visible and must look clean with no secondary operation, that can tip a borderline call toward molding or toward pre-finished coil.

Lead time is the quiet factor. A roll forming line is built in weeks and the rolls are cut in two or three. An extrusion die is similar, but the press itself is a six-figure commitment. A steel mold for injection can take three to four months from drawing to first shot, which kills a product launch on a tight clock. When a buyer says we need samples in six weeks, molding is often off the table before the cost is even discussed.

There is also the question of what happens after. Steel off the roll line can be punched, notched, and cut to length inline, then packed and shipped. Extruded aluminium usually needs a separate cutting and machining cell. Molded parts need degating and sometimes assembly. The process you choose decides your whole downstream cell, not just the forming step, so we always sketch the full line, not just the machine, before advising.

What we tell buyers choosing a process:

• Start from annual volume and part length. Below a few thousand metres, tooling cost dominates and molding or extrusion may win.
• Match the material to the fire and load rule first, then pick the process.
• If the section is open and thin, roll forming will almost always be cheapest at volume.
• Send the drawing and volume together. A number without the other is half a question.

Takeaway

Choose the process by the part, not by the catalog. Need kilometres of thin steel section at low cost: roll form it. Need a hollow aluminium shape with internal chambers: extrude it. Need a detailed discrete clip: mold it. Match the material to the load and the fire rule, then match the process to the geometry.

At ZTRFM we build roll forming lines for the steel side of that decision, and we are happy to tell you when a different process is actually the better fit. Send us your profile drawing and annual volume and we will say plainly which way we would make it.