Closed Die or Open Die Forging

Last updated: 2026-10-28

Both routes start with a heated billet and end with a part that has grain flow instead of cut fibres. The difference is what shapes the steel: a die cavity cut to the part, or a pair of flat dies worked by an operator and a manipulator. That single difference decides tooling cost, tolerance, how much material is left for machining, and what quantity makes sense.

This page is the decision, not the process description. If you want the closed-die process itself, our closed die forging page covers it, and the forging vs casting page covers the other comparison buyers usually make at the same time. Here the question is narrower: which of the two forging routes should the drawing go to.

The two routes side by side

The table is the short version. The three questions after it are how the decision is actually made.

Question 1: How many parts are you ordering?

This is the question that settles most cases, and it is arithmetic rather than judgement. The break-even is the die cost divided by what the closed-die part saves per piece.

Two things are saved per piece: machining, because a closed-die forging comes out close to the finished contour, and material, because an open-die forging is machined all over and the billet has to carry allowance on every surface.

The answer runs the opposite way to what most buyers expect. A die is designed and cut for one part, and its cost rises with part weight and press class, but it rises more slowly than the saving per piece does, because a heavy part gives back far more steel and machine time on every piece. So the small parts are the ones that need volume: a part in the 5–30 kg band can take close to a thousand pieces before the die pays for itself, while a part above 150 kg pays it back in the low hundreds. That is why we quote the die as a separate line instead of burying it in the piece price, and why it is returned once the agreed volume is reached: the customer does not carry the break-even on the first order.

Question 2: What shape is it, and does it need draft?

Every face of a closed-die part that is parallel to the direction of die movement needs draft so the part can be ejected, and internal corners need radii so the die does not crack. Undercuts cannot be formed at all in a two-part die: the shape would lock in the cavity. Where the part needs to be formed from more than one direction, as with a valve body that has a bore through two axes, that is a press capability question, not a die question. Our presses are multi-directional, which is how we form valve bodies and covers as single forgings instead of welding them up from pieces.

Open die is the opposite trade: simple solids, worked progressively, with the complex geometry created afterwards by machining. That is efficient when the shape is basically a block, disc, shaft or sleeve and the finished part is dominated by turning and boring. It is wasteful when the finished part is a cavity, because the cavity has to be cut out of solid steel rather than formed.

One more geometric factor belongs here: forging ratio. On open-die work for shafts and heavy sections, specifications often call for a minimum reduction ratio from the starting billet, typically 3:1, to make sure the centre of the section has been worked. Closed-die forging achieves its deformation differently, through the die cavity, so the requirement is usually written as a property requirement rather than a ratio.

Question 3: How big is it?

Weight and size usually rule one route out before cost is discussed. Our die-forging range is 5 kg to 520 kg: the 6,000 T press covers 150–520 kg, the 3,000 T covers 30–150 kg and the 2,000 T covers 5–30 kg, and the equipment can reach 800 kg subject to a review of the part structure. Largest machining envelope is Ø600 × 1000 mm.

Above that range the part goes to open die. In our case it is supplied by another plant in our group rather than here, which means one enquiry can cover both routes and the answer comes back as a comparison rather than a refusal. Parts that are large and repeated are the interesting middle: a heavy part with a high annual volume can sometimes be re-designed into the die-forging range, and that conversation is worth having before the drawing is frozen.

Where the difference shows up in inspection

Closed-die parts are traceable by heat lot and inspected close to finished shape, so NDT covers the surfaces that will remain in service and the dimensional report is short. Open-die parts start from a much larger section: ultrasonic testing follows the standard and acceptance level the order names, the forging ratio is recorded, and the dimensional report is written against a rough shape with machining allowance on every face.

That changes what to ask for in the enquiry. On a closed-die part, ask for the heat lot, the NDT coverage on the finished contour and the product analysis. On an open-die part, ask for the reduction ratio, the UT standard and the test coupon location, because the properties at the centre of a large section are the ones that matter and they are not the same as the properties at the surface.

When to ask for both quotes

Some enquiries sit on the line and are worth quoting both ways:

• Annual volume in the low hundreds, near the break-even point for die tooling, where a few percent either way decides it.
• Part weight near 500 kg, where the die-forging route is possible but the press choice is not obvious.
• Simple geometry with a high volume, where a re-design into a closed-die shape could remove most of the machining.
• A drawing that is not frozen yet: the die-forging option may change the design, and that is cheaper to hear before the drawing is released than after.

What we run

Longrui runs closed-die forging: three multi-directional presses of 6,000 T, 3,000 T and 2,000 T, parts from 5 kg to 520 kg, 15,000 tonnes a year, with dies designed and cut in house and five granted tooling patents. Closed-die forging is also what our API Spec 20C licence and NORSOK M-650 qualification cover, which is why oil and gas valve bodies and covers are the bulk of what leaves the plant.

Open-die forging is supplied by another plant in our group. Send the drawing and the annual quantity, and the answer comes back with the route each specification points to, the tooling it needs and the price difference between them.

Related Reading

• Closed Die Forging
• Forging vs Casting
• Capability and Presses
• Forged Valve Bodies and Covers

Send a drawing for a route recommendation

Frequently Asked Questions

Is closed die forging always more expensive per part?

No. The die is a one-off cost, and the part itself is usually cheaper: less material, less machining and a shorter cycle on the finished part. It becomes the cheaper route once the die is spread over enough pieces, which runs from a few hundred pieces on the large presses to close to a thousand on the small ones.

Can an open-die forging be made to the same tolerance as a closed-die one?

Not as-forged. Open-die work is a rough shape with machining allowance on every face, and the tolerance is achieved by machining. Closed-die parts come out closer to final form, with as-forged tolerance around ±0.8 mm on lengths and widths up to 150 mm, widening as the part grows.

What decides the press for a closed-die part?

Part weight, projected area and how much of the shape has to be formed from more than one direction. Our 6,000 T press covers 150–520 kg, the 3,000 T covers 30–150 kg and the 2,000 T covers 5–30 kg.

Do you do open die forging?

Not in this plant. Die forging is what our presses, dies and certifications cover; an open-die forging is supplied by another plant in our group, so a single enquiry can still cover both routes.