Defects in forging can be largely prevented when performed by individuals with expert levels of process knowledge who take a rigorous, detail-oriented approach to tool design, manufacture, and maintenance.
Die shifts, sometimes referred to as die mismatch, is a common type of forging defect that occurs when the upper and lower cavities are offset laterally, so the top half sits shifted relative to the bottom half.
Appearance
Forging die shifts appear as stepped edges around the parting line. This defect can be subtle on symmetrical parts, but highly apparent on parts with bosses, flanges, or bored features that straddle the parting line.
Primary Causes
In impression die forging, die shifts are primarily caused by misalignment, worn gibs, and side thrusts from part geometry.
Misalignment
Dies are designed so that one half is mounted to the stationary anvil of a hammer or bed of a press, while the other is attached to the ram that moves down to bring the dies together. Misalignment occurs when the upper and lower dies are not perfectly matched.
Worn Gibs
Gibs are a high-wear component (by design) that must be regularly adjusted as well as replaced over time. In forging equipment, gibs guide the ram, but as they wear, the clearance between the ram and frame opens up, allowing higher levels of lateral motion that produce mismatch, as well as uneven loading that accelerates wear on the gibs.
Side Thrust
Lateral force is a common risk during asymmetrical forging, which is why process control and die design are so critical. Steep or unequal draft angles and deep cavities increase the risk of lateral force and resulting die shift. Counterlocks can be used to prevent lateral movement.
What Excessive Die Shift Does to Forged Parts
Die shift leads to:
- Dimensional Issues: The offset between upper and lower areas of the forged part can create a stepped line or appearance, and may put key features on upper and lower surfaces out of alignment. For example, many forged parts incorporate bosses for locating mounting surfaces and threaded holes. Excessive mismatch between upper and lower dies can put these outside the acceptable tolerance windows.
- Uneven Section Thickness: Uneven thickness reduces strength, can complicate boring of precise holes, and may reduce part functionality, such as when holes are used for fluid flow.
- Altered Metal Flow: Die shifts cause metal to flow (under compression) differently compared to what may have been predicted during simulation and die design. This difference in metal flow can lead to additional surface defects, including cracks and cold shuts, which reduces overall part strength and integrity.
More importantly, excessive die shifts add time and cost to forging. Additional machining is needed to remove the excess material and bring part features to their required relative positions. And, it may also be necessary to increase the size of the billet to accommodate the extra metal removal.
How Trenton Forging Prevents Die Shifts & Other Defects
Trenton Forging has nearly 60 years of experience creating manufactured components via impression die forging. Through this time, we’ve focused on continuous improvements to our process to ensure quality, repeatability, and cost efficiency.
Given the consequences of die shift, Trenton Forging:
- Ensures high levels of precision when mounting dies to the anvil and ram.
- Performs regular tooling and press inspections to identify areas of wear.
- Conducts routine equipment maintenance, especially gib replacement.
- Emphasizes optimal die design, specifically parting line design.
- Uses counterlocks (steps in the mating die surfaces) to resist lateral movement.
- Supports in-house resinks (removal of material from the die and cavity surface) as necessary to account for die wear.
Learn More From Trenton Forging
Trenton Forging is a leading forging manufacturer in the United States. We use advanced technologies and equipment to deliver high-quality symmetrical and asymmetrical forgings. In addition to impression die forging, we provide value-added capabilities in-house, including die design, reverse engineering, machining, 3D scanning, and die maintenance for high-volume, demanding applications.
Visit our website for more forging resources, or contact us today to request a quote.
