Guides / Process
Open Die vs Closed Die Forging: Which Process Fits Your Part
September 18, 2026
Open die and closed die forging are the two processes most buyers choose between. They can make parts from the same alloy with similar strength, yet the cost, tolerance and supplier base are very different. Picking the right one before you send an RFQ gets you better quotes from the right shops.
How each process works
Open die forging heats a billet and works it between flat, V or simple shaped dies. The operator or a manipulator turns and moves the workpiece between blows, drawing it out, upsetting it or punching it. The dies do not enclose the part, so the final shape comes from skill and sequence rather than from a cavity.
Closed die forging, also called impression die forging, presses or hammers the hot billet into a cavity cut into the top and bottom dies. Metal fills the impression and the excess squeezes out between the dies as flash, which is trimmed off afterward. The result is a near-net shape that repeats from piece to piece.
Side by side
| Open die | Closed die | |
|---|---|---|
| Typical shapes | Shafts, blocks, discs, sleeves, step bars | Rods, flanges, levers, yokes, gear blanks |
| Typical weight | About 5 kg to many tonnes | About 0.1 to 250 kg |
| Typical tolerance | Machining allowance of a few mm per side | About ±0.5 to ±1.5 mm plus draft |
| Die cost | Low, standard dies | High, one die set per part |
| Sensible quantity | 1 to a few hundred | A few hundred and up |
| Machining after forging | Heavy | Light to moderate |
These are typical ranges. Large closed die presses make much heavier parts, and some open die shops hold tighter allowances.
Cost: die now or chips later
The main economic difference is where you pay.
With closed die forging you pay for the die before the first part. Die cost depends on part size, complexity and whether trim and pierce dies are needed. Once it is paid, each piece uses less material and needs less machining, so the piece price is lower.
With open die forging there is almost no tooling cost, but each piece starts from more material and needs more machining to reach final shape. The piece price is higher and stays higher at any volume.
So the question is how many pieces you will buy over the life of the design. Few pieces favor open die. Many pieces favor closed die. The crossover moves with part weight, alloy price and how much machining the open die version needs.
Tolerance and machining
Closed die parts come out close to final shape. Surfaces that do not mate with anything can often stay as forged. Critical faces and bores still get machined, but with a small allowance.
Open die parts are forged oversize and machined all over. That is fine for shafts and blocks that need machining anyway, and it keeps design changes cheap because there is no die to rework.
Closed die parts also need draft angles on walls so the part releases from the die, typically a few degrees. If your drawing has vertical walls that must stay vertical, expect those faces to be machined.
Grain flow and strength
Both processes refine the structure of the metal and close internal porosity, which is why both beat castings for demanding parts. The difference is in grain direction. In a closed die part the grain follows the contour of the finished shape. In an open die part the grain follows the forged shape, and machining away the extra stock cuts through it at shoulders and flanges. For parts under heavy fatigue load, such as crankshafts, hooks and connecting rods, this is one reason closed die forging is preferred once volume allows.
A rule of thumb
- One piece to a few dozen, or a part above a few hundred kilograms: start with open die.
- A few hundred pieces a year or more, under a few hundred kilograms and with a shape worth putting in a die: ask for closed die quotes.
- A ring or large flange: ask ring rolling suppliers as well, since seamless rolled rings often beat both.
- Unsure: send the drawing, weight and yearly quantity and ask suppliers to propose the process.
What to put in the RFQ
Whichever process you lean toward, include the finished weight, yearly quantity, alloy and spec, heat treatment, required tests and certificates and how the part should be delivered. If you are open to either process, say so. Suppliers will often quote both, which is the easiest way to see the real crossover for your part. Our guide on how to choose a forging supplier covers the rest of the checklist.
Open die against closed die is the first fork among the types of forging.
See also the reference pages on open die forging and closed die forging.
Frequently asked questions
Is closed die forging stronger than open die forging?
Not by default. Both refine the grain and close internal voids. Closed die parts often perform better in fatigue because the grain follows the final shape, while open die parts have their grain cut when the extra stock is machined away.
What quantity makes closed die forging worth it?
It depends on die cost and part weight, but a few hundred pieces a year is a common starting point. Below that, open die forging plus machining is often cheaper once the die cost is spread over the order.
Can a part start as open die and move to closed die later?
Yes. Many parts are open die forged and machined for prototypes and early batches, then moved to closed die once the design is fixed and volume grows. Tell suppliers on the RFQ if you expect that path.
What is impression die forging?
It is another name for closed die forging. The dies carry an impression of the part and the metal is pressed or hammered into it. Excess metal escapes as flash, which is trimmed off after forging.
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