In this article we will discuss about Impression Die Forging describing all the processes involved and what are the advantages and disadvantages of impression-die forging.
Impression die forging is a forging process in which dies move towards each other and covers the workpiece in whole or in a part. The heated raw material is placed at the bottom of the die. The dies then move towards each other until the raw material is formed into its required shape and size.
The operations may be so closed together that it is unnecessary to heat between them. The number of forging steps necessary to produce the finished forging in the impression die is dependent on the size and shape of the forged part, on production quantities, and on the kind of material being forged.
Impression Die Forging Process
Impression dies generally contains preliminary shaping steps to permit the change from the original forging stock to the finished forging without mechanical defects. Simple symmetrical parts may be forged directly in the finished impression (finishing die cavity) without preliminary shaping. The more difficult or complex shapes may require several different steps to produce a finished forging.
The most-used preliminary forging step is the edger, which serves to proportion the cross-sectional area along the length of the flowing metal from a section being reduced to a section being enlarged. The fullering step or fuller, reduces the cross-sectional area between the ends of the forging stock without appreciable change to the end sections. The bending step or bender forms the length of the forging stock to a shape for the finishing impression.
The preform may be further shaped to bring it closer to the final configuration in a so-called blocker die which assures proper distribution of material but not the final shape. Excess material is allowed to run out between the flat die surfaces and this flash is sometimes removed or trimmed prior to forging in the finishing die. The excess material is again allowed to escape into a flash, which must now be thin to assure die filling and close tolerances. The flash is reduced to its minimum thickness over only a small width called flash land, and the rest is allowed to flow freely into a flash gutter.
The production-design engineer must exercise care in determining the amount of material to be inserted in the die. If there is not enough material, the part will not fill up the die. If there is too much material, the flash will be excessive, resulting in wasted material and greater die wear. In general around 10 per cent excess material over calculation is allowed.
Dies must, however, be heated before the first forging is made and often gas or electric heaters are used to keep the dies hot so that the forgings will not cool too fast, shrink, and be difficult to eject. Many mechanical forging presses have lower die ejectors, and upper die strippers can be furnished.
After the forging operation the part must be trimmed to remove the flash. The flash is removed hot or cold. If the carbon content is low and the forging small, it is usually removed cold. Most medium-sized and large forgings are trimmed hot. Subsequent operations to remove the scale or oxide include shot blasting, or pickling.
Advantages of Impression Die Forging
- Superior mechanical properties and a better surface finish
- For large production runs, it is cost-effective.
- Reduced or no machining
- Dimensions with tighter tolerances and a variety of shapes are possible.
- More accurate and consistent impressi-ons
- Capability to replicate almost any shape and/or size
Generally, when large quantities of identical forgings of greater accuracy are required as is necessary in mass production, impression die forging is commonly employed, and where forgings can be made by either of the several methods, the quality is comparable, and the choice is made for economic reasons.
- Not typically economical for short or small production runs due to the cost of die production
- Higher setup cost due to costly machines and furnaces
Applications of Impression Die Forging
Because of its high precision, closed die forging is preferred for small critical parts with safety considerations. It is widely used in the production of small products such as forged fittings, forged lifting and rigging hardware, forged automotive parts, and so on. Closed die forging is used in a variety of industries, including oilfield, automotive, forestry and agriculture, and mining.