DROP FORGING Drop forging is a mass production techniquewhich hammers the metal between two dies. Half ofthe die is attached to the hammer (upper section)and half to the anvil (lower section). The hot metal isplaced in the lower half of the die and struck oneon more time with the upper die. This forces themetal to flow in all directions, filling the die cavity.Excess metal squeezed out between the die faces iscalled flash or flashing. After the forging iscompleted the flash is cut off in another press with atrimming die.
A metal shaping process, the metal to beformed is first heated then shaped by forcing it intothe contours of a die, this force can be in excess of2000 tons. The drop forging process can beperformed with the material at varioustemperatures;Hot Forging During hot forging the metals are heated toabove their recrystallization temperature. The mainbenefit of this hot forging is that work hardening isprevented due to the recrystallization of the metalas it begins to cool.
Cold Forging Cold Forging is generally performed with metal atroom temperature below the recrystallization temperature.Cold forging typically work hardens the metalTWO TYPES OF DROP FORGINGOPEN DIE Drop forging requires the operator to position thework piece while it is impacted by the ram. The dieattached to the ram is usually flat or of a simple contour,most of the shaping is achieved by the operatorphysically positioning the work piece before each strokeof the ram. There are also special dies which can beused to cut the metal, form holes or notches.
CLOSED DIE (Impression die) Drop forging comprises of a die on the anvilwhich resembles a mould, the ram which falls andstrikes the top of the metal billet can also beequipped with a die. The heated metal billet isplaced on the lower die while the ram drives downforcing the metal to fill the contours of the dieblocks.
Process detailsClosed-die forgingA heated blank is placed between 2 halves of adie
A single compressive stroke squeezes the blank intothe die to form the part. In hammer or drop forgingthis happens by dropping the top of the mould froma height. An alternative is to squeeze the mouldstogether using hydraulic pressure.
Once the die halves have separated, thepart can be ejected immediately using anejector pin.
Advantages1. Parts of sizes up to 25 tons can beproduced with closed die forging2. Closed die forging can produce near netshapes that will require only a small amountof finishing3. Economic (profitable) for large runs ofproduct
4. Forging provides superior mechanicalproperties over castings due to theinternal grain structure formation inforged parts5. Makes strong products.6. Many metals are forged cold, but ironand its alloys are always forged hot.
Disadvantages1. Not very economical for short runs due to the highcost of die production2. The business set up cost of drop forging is veryhigh, not only are the machines and furnaces costlybut special building provisions must be in place tocope with the powerful vibrations caused by dropforging. A special foundation must be laid to dealwith this environment3. Drop forging presents a dangerous workingenvironment
Materials and shapes1. Any metal can be forged, provided theblank is hot enough (( 60% of the meltingtemperature).2. Typical possible sizes for closed dies rangefrom 10g to 10kg, depending on complexity.3. The part is left with good surface andmechanical properties, although cold-forgingcan perform even better.
4. Complex parts can be formed using aseries of forging dies with increasing levels ofdetail.5. A draft (taper) angle has to beincorporated to allow easy removal of thepart.6. Any waste material squeezed betweenthe die halves, called flash, is readilyrecycled.
EconomicsProduction rate is limited by the insertion andremoval of the blank, so some form ofautomation is often used.As a result, machines can cost £100,000+, butcan produce many parts a minute (if small).As both the machines and the dedicated diesare costly, production runs in excess of 50,000are often needed to produce small partseconomically.Large parts can be produced economically atsmaller batch sizes, because there is lesscompetition.