None of the benefits of impression die forging, including reduced secondary machining and part-to-part variation, can be achieved without paying close attention to every aspect of the process, including tooling design.
Each tooling piece, or die, contains a cavity the shape of half the part being produced. In production, a hot metal billet is placed between the two halves. The billet is then compressed, and the molten metal flows into the two cavities.
These dies are designed to optimize material flow in ways that achieve the geometric goals. A key element of this is managing the formation of flash around the parting line between the two dies. This blog looks at how and why flash is an essential element of forging, and how flash design affects the final results.
Material Flow
Compression of the preform begins before the die is closed, causing metal to extrude laterally. If there’s no resistance to this movement, the vertical regions of the die won’t fill, and the part will be incomplete. So, to ensure consistent filling, the dies are designed with features that control and restrict flash formation.
The challenge for tool designers is to optimize flash-controlling features to ensure cavities are filled while still tolerating some sideways flow. In addition, the preform must be sized to accommodate the expected amount of flash formation.
Part Complexity
The challenge of filling the cavities becomes harder as the part gets taller and the cavities correspondingly deeper because it increases the amount of vertical deformation needed, which is achieved by increasing resistance to sideways displacement.
Resistance to flash formation can be varied around the perimeter of die cavities to handle increased part complexity in specific regions. When metal must be deformed into smaller areas to produce the required geometries, more careful control of flash formation is needed.
Forging Load
Forging more complex and taller parts requires more force from the forging press or hammer to restrict flash growth and generate vertical movement within the die cavity.
As a result, taller parts need more powerful equipment, which can limit the number of forging businesses able to handle a given design. When designing parts to be forged, reducing overall height also reduces costs.
Grain Flow
The initial compression as the die first contacts the preform pushes the metal outwards, giving the grains a more horizontal orientation. Subsequent flash restriction then moves the material upwards, altering grain flow.
When designing tooling for a part, achieving the desired grain flow is often a top priority, and effective flash management is a key factor in achieving the desired mechanical properties.
Temperature Loss
Flash cools much faster than the material remaining in the dies, increasing flow resistance and supporting cavity filling. However, by acting as a heat sink, flash also pulls heat away from the preform, creating temperature gradients and uneven cooling that can cause residual stress and distortion.
By optimizing flash dimensions, particularly flash thickness, engineers can control both cavity filling and part cooling rates.
Material Waste & Trimming Costs
After forging, flash is trimmed away from the part, which can increase project costs, especially when a significant amount of flash needs to be removed.
Trenton Forging recycles flash. While recycling helps reduce waste, it still uses additional energy, which is why engineers try to minimize flash whenever possible.
In addition, when flash is larger, the billet is also larger, which requires more energy to heat and compress to the final shape.
Die Life
Flash influences die wear rates and can even cause cracking. Temperature gradients are one factor, and another is the increase in forces on the die as flash raises resistance to sideways material flow.
Work With Forging Experts
Flash on a forged part can appear as an unfortunate consequence of the compression process. As discussed here, though, the reality is more complex. The way flash is managed influences many aspects of the forging process, including part quality, material waste, and die life, which is why our engineers at Trenton Forging consider flash design carefully when designing tooling for projects.
We’ve specialized in impression die forging for over 50 years, and during those years, we’ve optimized our process to ensure exceptional part quality, reduce waste, and improve cost efficiency.
Visit our website for more forging resources, or contact us today to learn more about our capabilities or to request a first article sample.
