Closed Die Forging

Closed die forging is a metal forming process in which a heated billet is placed between two dies — an upper and a lower — that together carry a cavity in the shape of the finished part. When the dies close under hammer blows or press force, the metal flows plastically to fill that cavity. The part leaves the die already close to its final shape, which is why the process is described as near-net.

The name is slightly misleading. The dies are not sealed shut. A narrow gap is deliberately left around the cavity at the parting line, and a small amount of metal is squeezed out through it as flash. That escape route is not a flaw in the process — it is the mechanism that makes the process work, because the restriction builds the pressure needed to push metal into ribs, bosses and sharp corners.

Impression die, closed die, drop forging: untangling the names

You will see the same process called impression die forging, closed die forging, and sometimes drop forging. Impression die and closed die are true synonyms — both describe the tooling. Drop forging describes the equipment instead, meaning a hammer whose ram falls onto the work. Most drop forging uses closed dies, which is why the terms blur in everyday use. Our page on die forging versus drop forging goes through the distinction in detail.

It is also distinct from open die forging, where metal is shaped between flat or simple tools with no cavity at all, and from true flashless precision forging, where billet volume is controlled so tightly that no flash forms. Closed die forging with flash is by far the most widely used of the three for structural steel components.

Why engineers specify it

• Directional grain flow. Deformation elongates the metal's internal structure along the direction it moves, so fibre follows the contour of the part — around fillets, into bosses, along arms — instead of being cut through as it would be in a machined part. See grain flow explained.
• No porosity. The metal never melts, so there are no gas pockets or shrinkage cavities of the kind that can occur in castings. There is nothing inside the section for a fatigue crack to start from.
• Repeatability. Every piece is formed by the same cavity, so weight, geometry and grain pattern are consistent across tens of thousands of parts.
• Material efficiency. A near-net forging typically uses 75–90% of the purchased steel in the finished part, against 35–50% for the same component machined from bar.
• Less machining. Only functional features need cutting, which shortens cycle time and removes whole operations from the routing.

The trade-off is tooling. A closed die is a one-time investment that must be amortised across the order, which is why the process makes commercial sense from a few hundred pieces upward and why very small quantities are usually better served by open die forging or machining.

A closed die forging passes through ten controlled stages between raw bar and a packed, certified component. The sequence below is the one we run at our plant; most competent forges follow a broadly similar route, although the number of preforming stages varies with part complexity.

1. 01Material verification and billet cutting
2. 02Induction heating
3. 03Preforming — fullering, edging or bending
4. 04Blocking
5. 05Finishing
6. 06Trimming, piercing and coining
7. 07Controlled cooling and heat treatment
8. 08Shot blasting
9. 09CNC machining
10. 10Inspection, testing and dispatch

1. Material verification and billet cutting

Every incoming heat lot of steel is tested by optical emission spectrometry against its mill certificate before it enters production, and lots are physically segregated by heat number. Grade substitution is a real and recurring risk in the steel supply chain, and this is the only point at which it can be caught cheaply.

Verified bar is then sheared or sawn into billets cut to a calculated length, held to about ±1 mm. Billet weight matters more than most buyers realise: too light and the cavity under-fills, too heavy and the surplus becomes flash that you pay for on every piece. Our forging weight calculator shows how billet weight relates to finished weight.

2. Induction heating

Billets are heated to between 1,150°C and 1,250°C for carbon and alloy steels. We use induction heating rather than a fuel-fired furnace because it heats only the billet, reaches temperature quickly and keeps soak time short. Short soak time matters for two reasons: it limits scale formation, which costs metal and damages dies, and it limits decarburisation, which leaves a soft surface layer that cannot be hardened properly.

Billet temperature is verified by infrared pyrometer and recorded against the batch. Temperature is the single process variable that touches every outcome — flow, fill, die life, grain size and surface quality — so it is controlled rather than estimated.

3. Preforming

Complex parts cannot go straight into a finishing cavity. Metal first has to be redistributed roughly to where the part will need it. Fullering thins a section and pushes metal lengthwise; edging gathers metal into regions that will become heavy sections; bending forms an initial curve for parts such as crankshaft preforms or curved levers. A connecting rod, with its thin shank and heavy ends, is the textbook example of a part that needs this stage.

4. Blocking

The blocker impression forms the workpiece into an intermediate shape with generous radii and approximately the final volume distribution. Skipping it on a complex part is the most common cause of laps — folds of metal pressed shut without bonding — because the finisher cannot distribute metal and form fine detail in the same blow.

5. Finishing

The finisher impression gives the part its final shape. The cavity is cut oversize by the shrinkage allowance, roughly 1.2–1.5% for steel, so that the forging measures correctly once it has cooled from forging temperature. This is the stage where flash forms around the parting line.

6. Trimming, piercing and coining

Flash is sheared off on a dedicated trimming press immediately after forging, while the part is still hot. Hot metal shears cleanly at low force; trimming cold raises the shear load and risks cracking along the flash line. Where the part has a through hole, the thin web left in the centre is pierced out in the same operation. Some parts are then coined — given a final high-pressure squeeze that flattens a face or sizes a thickness more accurately than forging alone can.

7. Controlled cooling and heat treatment

A forging cools unevenly: thin sections faster than thick ones. Left alone, that produces mixed grain size and hardness that varies across a single part. Depending on the grade and the specification, parts are normalised to even out the structure, quenched and tempered to develop specified strength, or — in the case of micro-alloyed grades — cooled at a controlled rate so they reach full strength without any further treatment. Every furnace charge carries a recorded chart. Our heat treatment services page covers the cycles in more depth.

8. Shot blasting

Steel shot strips scale from every recess of the forged contour. This is a quality operation, not a cosmetic one: it lets dimensions be measured against the true surface, exposes surface defects for inspection, and leaves a uniform finish that coatings adhere to.

9. CNC machining

Functional features — bearing seats, bores, threads, sealing faces, mounting pads — are finished on CNC turning centres and vertical machining centres, using fixtures designed to locate on stable forged features despite draft angles and the parting line. Routine work holds ±0.02 mm; ground features reach ±0.01 mm and Ra 0.4 µm. See precision CNC machining.

10. Inspection, testing and dispatch

Parts are checked dimensionally, hardness tested, and where the specification requires it, examined by ultrasonic testing for internal soundness or magnetic particle inspection for surface cracks. Material certificates, heat treatment records and inspection reports travel with the batch under a single heat number reference, and parts are packed with corrosion protection matched to the delivery route.

Almost everything a closed die forging line can or cannot achieve is decided when the die is designed. A well-designed die fills reliably, lasts a long time and produces parts with grain flow where the load is. A poorly designed one under-fills, laps, cracks early or places weak fibre ends at the worst possible location. This is why we keep die design and tooling inside our own plant.

Parting line placement

The parting line determines where flash forms — and therefore where grain fibre ends surface on the part, because fibre terminates where metal is expelled. Placed on the largest cross-section and away from stressed fillets, it is harmless. Placed across a highly loaded fillet, it puts exposed fibre ends exactly where a fatigue crack would start, quietly cancelling the benefit of forging the part at all. A flat, single-plane parting line is also cheaper to machine and less prone to die mismatch than a stepped or curved one.

Draft angles

Vertical walls must taper so the forging can be released from the cavity. External surfaces normally need 3–7° of draft; internal surfaces need 5–10°, because the forging shrinks onto internal die features as it cools and grips them. Draft-free walls are possible with ejector tooling, but it is almost always cheaper to accept draft on the forging and machine it away where a feature genuinely needs to be parallel.

Fillet and corner radii

Metal does not flow around sharp internal corners — it folds. Sharp corners also concentrate stress in the die and crack it. A minimum internal radius of about 3 mm is a practical floor, and larger is better wherever the design allows: generous radii improve fill, extend die life and reduce stress concentration in the finished part simultaneously.

Flash land and gutter

The flash land is the narrow, controlled-thickness restriction around the cavity. Make it thinner and back pressure rises, filling detail better but loading the die harder and wearing it faster. Make it thicker and die load falls but the cavity may under-fill. Beyond the land sits the gutter, a relief cavity that simply gives the flash somewhere to go without adding further restriction.

Die material and life

Closed dies are machined from hot work tool steels such as H11 and H13, heat treated to a working hardness that resists both thermal fatigue and abrasion at contact temperatures above 1,000°C. Detail that cutters cannot reach is finished by EDM. Typical die life on medium components runs from 20,000 to 60,000 pieces, after which the worn impression is re-sunk — recut slightly deeper to restore dimensions — a process that can usually be repeated several times before the die block is exhausted.

Closed die forging can be carried out on hammers, which deliver energy by impact, or on presses, which apply force over a stroke. Neither is universally better; the right choice depends on section thickness, required consistency and how many different part numbers are being run.

A hammer's energy concentrates near the surface and dissipates before it reaches the core of a heavy section. A press keeps the metal plastic for longer, so strain penetrates the whole section. On parts thicker than roughly 60 mm this is the difference between uniform properties and a coarse, lightly worked core. Presses also have a mechanically fixed stroke, which gives tighter weight control and better statistical capability. Hammers, in turn, change over quickly and tolerate variation in billet temperature and size, which suits mixed-model production.

We run both pneumatic and friction screw hammers and friction screw and hydraulic presses, so a part is matched to the equipment that suits it rather than to whatever a supplier happens to own. The comparison is covered further in drop forging versus press forging.

Most closed die forgings are made from a relatively small group of steels. The right one is chosen by working back from the part's failure mode — fatigue, wear, overload or corrosion — and then checking that the grade can actually develop its properties through the section size involved. The table summarises the grades we forge most often.

Section size is where most material mistakes happen. EN8 hardens well at the surface but has limited hardenability, so in a section much above 60 mm the core stays soft however the part is quenched — and a tensile test on a small coupon will not reveal it. Above that size, or wherever fatigue governs the design, EN19 is the safer choice. The full reasoning is in EN8 versus EN19.

Stainless grades forge in a narrower temperature window, need more force, and must be handled on tooling kept completely separate from carbon steel. Carbon steel particles picked up in handling embed in the stainless surface and rust weeks later, which looks exactly like a material failure.

The most common drawing error we see is a machining tolerance applied to an as-forged surface. No closed die forging process holds ±0.05 mm on a forged face, because four sources of variation add together: thermal shrinkage as the part cools, progressive die wear across the run, mismatch between the upper and lower dies, and variation in flash thickness. Each is controlled; none can be eliminated.

The practical rule is simple. Put a named tolerance class — IS 3469 or DIN 7526, grade E or F — on as-forged surfaces. Put tight numeric tolerances only on features that will be machined, and mark them as machined on the drawing. Specify fine grade only where a feature must fit or seal without machining, because it genuinely costs more: tighter billet control, shorter die maintenance intervals and more inspection. See forging tolerances for more detail.

Nearly every closed die forging defect traces to one of four root causes: temperature, die design, billet size or lubrication. Knowing which one produced a defect turns a guess into a fix. The table below lists the defects that matter, and the controls we use against each.

Laps and cold shuts cannot be repaired by welding or heat treatment — affected parts are scrap. That is exactly why these defects are designed out at die trial rather than sorted out at final inspection. For a diagnostic sequence on one of the most common problems, see solving forging underfill.

A closed die forging price has two parts that behave completely differently: a one-time die cost, and a per-piece price. Much of the apparent variation between supplier quotations comes from comparing the two inconsistently, or from quotes that include different scopes.

The per-piece price

Material is usually the largest share — typically 40–60% of piece cost — and it is calculated on billet weight including flash, not on the weight of the finished part. The forging operation itself is typically 10–20%, heat treatment 5–10%, machining anywhere from nothing to 40% depending on how many features need cutting, and inspection and packing a few percent more.

How quantity changes the economics

Because the die costs the same whether it makes 500 parts or 50,000, its contribution to each piece falls sharply as quantity rises. The table uses a nominal die cost of 100 units to show the effect independently of any particular rupee figure.

Two consequences follow. First, stating your realistic annual volume — even approximately — lets a supplier price tooling sensibly rather than defensively. Second, the largest cost reductions usually come from design rather than negotiation: a near-net feature that removes one machining operation saves money on every piece for the life of the part.

Closed die forging is not the right answer for every part. It wins decisively on fatigue strength, soundness and repeatability at volume; it loses on geometric freedom, on very small quantities and, sometimes, on tooling cost. The comparison below is deliberately even-handed.

If your part genuinely needs internal passages, casting is the better route and we will say so. If you need twenty pieces, open die forging or machining from a forged blank will cost less. For fatigue-loaded parts made in hundreds or thousands, closed die forging is usually the lowest total cost and the most reliable result. More detail: forging versus casting and closed die versus open die.

Closed die forgings are used wherever a component carries fatigue loading, contains pressure or supports a load over people. The parts differ by sector; the reason for forging them does not.

Quality in closed die forging is a process discipline, not a final gate. Catching drift at the forging stage costs a die adjustment; catching it at final inspection costs the batch. Our controls run through every stage:

• Incoming material — spectro analysis of every heat lot against the mill certificate, with physical segregation by heat number.
• Process records — billet weight, heating temperature, forged weight sampling and die condition, recorded per shift.
• Heat treatment — furnace chart per charge, with hardness verified across the section rather than at one convenient point.
• Dimensional inspection — against the drawing, with first article reports on new parts and sampling or 100% inspection as specified.
• NDT — ultrasonic examination to ASTM A388 and magnetic particle inspection where the application requires it.
• Grain flow verification — macroetch examination to ASTM E381 during die validation.
• Certification — material test certificates to EN 10204 3.1, with PPAP packages for automotive customers.

Every batch carries its heat number from the steel mill to the packing list. If a question arises years after delivery, that chain identifies exactly which billet lot, die, furnace charge and machining setup produced the part.

Six pieces of information let us quote accurately the first time. With all six, a quotation — piece price and die cost shown separately — normally follows within two working days.

1. 01A drawing, 3D model, or physical sample of the part
2. 02The material grade, or the mechanical properties the part must achieve
3. 03Quantity per batch and approximate annual volume
4. 04Any specification or standard the part must meet
5. 05The scope you want: forged blank, heat treated, machined, or fully tested
6. 06Your delivery expectation

Once the quotation is approved, die design takes five to ten working days and die manufacture two to three weeks, after which first article samples are supplied with a dimensional report. On approval, production runs against your schedule, and the die stays in our custody for repeat orders. If you have a sample but no drawing, our reverse engineering service produces one for your approval.