2.1. Die and Tooling Defects
As mentioned above, most of the die and tooling defects can be removed through operations known as die corrections. Some die defects are so critical in nature that they cannot be repaired and directly lead to die failure. In other cases, further repair is not possible after various die corrections have been done, resulting in the scrapping of the die. Many researchers have attempted to investigate or analyze individual die defects [26
]. Arif et al. [17
] conducted a more comprehensive study on die failure modes and mechanisms and presented a statistical analysis of the main die failure types and their sub-divisions. The focus in the current paper is not die failures but die corrections.
Five major categories of die defects are fracture, wear, deflection (plastic deformation), design/manufacturing, and hardness. The first three (fracture, wear, and deflection) relate to the structural and geometrical condition of the die after being used for some time and can be generally seen distinctly without the aid of a measurement device. The last two (design/manufacturing flaws and hardness problems) usually require an instrument for their detection.
Fracture defects appear as uneven crevices on the die surfaces and are caused by the large thermo-mechanical stresses combined with stress concentration locations in the profile. [15
]. Rib cracks can lead to some minor deflections in the mandrel which are correctable in nature. Initiated cracks may propagate under fatigue loading over multiple extrusion cycles. The crack propagation mostly leads to ultimate brittle failure in the form of fracture of any feature of the die. Fracture is also caused by the grain boundary severity of carbides in the steel die, by reducing the ductility and resistance to temperature variations. Gradually, the inhomogeneity and segregation increase in the die material. It also occurs as a consequence of poor tooling selection, resulting in insufficient support to the die. Cracks are usually seen on bearing, mandrel, and ribs (Figure 5
) that take load from the flowing metal. To prevent fracture defects, proper die hardness levels should be maintained by routine nitriding; mandrels should be designed so that they are free from high unbalanced mass variations and sharp edges, and ribs must be designed for minimum temperature increase and smoother flow.
Wear defects refer to the wear and tear of critical surfaces during service (such as the die bearing) and are typically of two types. Erosion is the deterioration or degradation of the surface, whereas washout (Figure 5
) appears as craters or linear depressions accumulated in certain regions [8
]. If the bearing washout is severe, the dimensions of the profile can change and lead the profile to go off geometry and beyond the tolerance limits. The causes of the wear defects include hard inclusions in the billet, improper setup parameters, loss of hardness, and high temperature rise due to friction. To reduce the wear problems, the die surfaces can be coated with a wear-resistant metal. This coating can reduce the uptake of hard inclusions, adhesion, thermal fatigue, and friction. Other benefits of coating include a higher oxidation temperature up to 750 °C and high corrosion resistance. The polishing of dies before each cycle can facilitate the smooth metal flow over the ribs and bearing. Wear can also be minimized by ensuring the hardness levels of critical regions of the die and mandrel.
Deflection defects are caused by the plastic deformation of the die or its features due to thermal or mechanical stresses or a combination of both [30
]. Deflection in the mandrel can happen for two reasons. First, a plastic deformation can happen because of thermo-mechanical stresses occurring on the ribs and mandrel [17
]. Second, when a crack or fracture occurs on the rib, the mandrel gets bent to an angle from the central axis. This will result in flow-related product defects due to the bend in the cavity. The mandrel deflection can also lead to a dimensional change (angle and wall thickness) of the profile. A feature deflection can alter the dimensions of features in the extruded product, and sometimes this dimensional change (linear or angular) may exceed the designated tolerance limit, and the product will have to be scrapped. To reduce deflection defects, enough support and rigidity should be provided to the mandrel section through a proper selection of tooling. The rib regions should be polished for a smooth metal flow. To prevent feature deflection, additional tooling may be included, such as an insert-bolster attached to an insert-holder to match the aperture of the die backer. Another measure is to use an alternative custom backer, suitable for the die backer aperture.
Some die defects are due to flaws or miscalculations in designing the die geometry, to die material issues, or to manufacturing errors. These problems are therefore categorized as design/manufacturing defects. Sometimes, they cannot be detected until after at least a few trial or actual extrusion runs [31
]. Inaccurate angles can result in metal flow variations in different regions of the profile and may result in concave/convex product defects. The ribs control the flow of metal through the cavities of dies. The ribs may have excessive material (distended), and this will cause obstruction to the metal flow in the cavities. Once the flow is obstructed, the pressure will rise and may cause flashing defects (the metal passes over the die) or blocked metal flow. Distended ribs can be easily corrected by machining off the excess material. This will reduce the pressure build-up.
The two types of hardness-related die problems are low-hardness and high-hardness (Figure 6
). Low hardness makes the die softer than specified, resulting in dimensional and other errors in the extruded product. High hardness makes the die over-hard and brittle, increasing the chances of chip-off and other types of breakages. Routine grinding and polishing procedures may chip off the die bearing as tiny fragments because of its high brittleness. Once the bearing is chipped, it is usually not correctable. Common causes of these defects are either over-usage or improper re-hardening operations during die corrections [32
]. In such cases, a recalculation of the time required for periodic nitriding of the dies may be needed. The orientation of the dies while they are placed in the carbonitriding chamber must be checked for a uniform heat distribution. The preheating time of the dies should not be too long.
Defects that cannot be categorized into any of the above types are named other defects. These could be a combination of the major defects or defects due to setup errors or inefficient correction operations during die maintenance (Figure 6
). If the die is not corrected properly, this can cause various flow-related problems, a temperature rise, and pressure build-ups at the die–billet interface. Correction beyond the tolerance limits can result in scrapping the die. The supervision of correction works by a co-worker or supervisor is usually practiced. Training workshops can be held from time to time to instruct the workers and improve their die correction knowledge and practical experience. The die inspection tools (discussed later) must be checked periodically for errors and recalibration requirements. One of the most common setup errors is the malfunctioning of the die preheating oven (also called die furnace). Usually, the dies are preheated at 450–500 °C for a time period of two hours. If any of these parameters are not maintained satisfactorily, thermo-mechanical stress variations may occur. Billet preheating is another factor which is done at 420–450 °C. The die furnace and billet furnace should be monitored and controlled appropriately. Another setup error is related to the choice of the right type of bolster, feeder plate, and die backer, which are essential for ensuring the rigidity of the die during extrusion and avoid cracks or fracture defects. A proper and rigid setting of the tool-stack should be ensured in the die slide. The performance of selected die sets and tool-stacks should be monitored from time to time, evaluating the appearance of crack defects on the die. A classification of the defects into common sub-categories is given in Table 1
There are three ways by which a need for die corrections is identified. First, the new dies go through trial runs before being used for the actual production. If there are problems during these trial runs or if the extruded product from these trial runs is defective, die corrections may be needed. Second, the dies are inspected after every production run. Routine correction operations, such as die cleaning etc., are always required. Some other die corrections are done periodically, such as die re-hardening. Third, a poor product quality during actual production runs may suggest certain die corrections.
2.7. Specific Die Correction Operations
These die corrections need to be carried out for specific problems observed by the quality control or die-shop personnel. Problems in the extruded product could be due to die-related issues, wear and tear etc. of the die and tooling itself. Described below are all the major die correction operations generally carried out. Table 3
lists the die defects and product defects for which each corrective operation is carried out [1
]. Later, a brief frequency-based statistical analysis of die defects is also presented, based on three-year defect data from an actual medium-to-large size commercial extrusion plant. It should be pointed out here that die corrections can be performed only a finite number of times. After repeated die corrections of the same type, the die has to be scrapped because of issues regarding its strength, dimensions, etc.
The bearings of dies and mandrels need to be shortened when there is a requirement to increase the flow at certain regions of the profile. This “shortening of the bearings” is done by milling and grinding for corrections and hand/machine polishing for small corrections, resulting in a decrease of the frictional area of the bearing land. Product defects such as speed difference, concavity/convexity, angle-out, etc. can be rectified using this operation. An increased metal flow can also require a further fine tuning by filing the choke or relief [41
The operation of “choking” is performed when there is a requirement to reduce the metal flow at certain portions and is mostly done on hollow dies. It is the process of giving an angle to the bearing or increasing the angle of the bearing on the inner web, to decrease the metal flow. Angle openings are very minute, and so this operation is usually done by hand-filing and not by grinding. Once the cap-bearing is choked, the same choke angle should be given to the mandrel bearing. Since it is meticulous, this operation is avoided whenever possible and is performed only by highly skilled workers when necessary. However, it is the most appropriate measure to take when mismatch occurs after shortening. Hence, choking is usually employed when there is incorrect shortening, or shortening can no longer be done.
The ridges in hollow dies may become blunt because of die wash, mostly resulting from over-usage or out-of-limit service. These ridges can be sharpened by “chiseling” the edges. This is a temporary method of fixing and cannot be generally repeated. Also, once it is done, the corrected feature will not be maintained for a long time. Die problems such as erosion, rib design defects, and correction defects can be rectified with this operation.
“Undercutting” is mostly employed for the correction of mandrels in hollow dies. Apart from blockages etc. due to usage, there may not be sufficient metal flow because of a design or manufacturing error. The flow speed and volume will increase when blockages are relieved and chambers are widened, as shown in Figure 8
. Undercutting can help prevent the product defects known as ripping and flashing, in addition to speed difference and other flow-related problems. Shallow grooves are sometimes created on the die face by grinding to further control the metal flow.
Pockets in the die can be carved out to increase the metal flow. This “increasing of the clearance” will be done after each carving, allowing more metal to flow. This is usually done when the undercut is not sufficient.
The depth of the chamber near the bridge can be increased by milling or other machining techniques, as and where required to regulate the flow. This “increasing of depth” is usually done on the mandrels of dies for hollow profiles.
“Machining/skimming” is mostly employed on the bearings, port-holes, and ribs, for all kinds of wear and washouts and dimensional inaccuracies. Sometimes, this is done intentionally to adjust the metal flow in porthole dies, to improve the product quality (Figure 8
). Because of the high hardness of H13 or similar steels, diamond tip tools are mostly used. The die corrector’s skills are critical in doing a good machining. After this operation, the die will have a unique design, slightly different from the manufactured one.
The ribs and mandrels are always prone to plastic deformations after excessive use or large stresses, especially at high temperatures. These deflections are usually not correctable but can be repaired through “heating and realigning” in some cases. These corrections do not guarantee perfection and efficient functioning but make the die usable to some extent. Feature deflections related to profiles such as projections and cavities are also corrected (realigned) using this operation.
Also known as punching, “peening” involves the use of a punch and a hammer or peen. It is usually employed in the case of minor wear or deflection at the bearing edges of hollow dies, mandrels, and die surfaces. These edges and corners are peened with a hammer and a punch to impart slight adjustments. The dies can be heated (250–300 °C) and peened to prevent minute cracks, possible in cold working. However, this is generally avoided, as wrong punching pressures may destroy the design.
The die surface or edges may gradually develop cracks or tiny fissures during service. These defective regions or spots can be repaired by “welding” and machining (Figure 8
). Gas tungsten arc welding (GTAW) is the method commonly used. Welding should preferably be done after annealing the die (550–650 °C) to minimize cracks from thermal stresses. Another application is to weld a feature (such as tongue) back on the die when the feature is broken off or deflected. The new feature is made of the same material as the die and machined as per the drawings. Then, it is grafted on the die by welding.
Surface “grinding” can be used to rectify wear and geometrical deflections of the cavities (Figure 8
). It can also be used to modify undercuts and clearances, etc. It can also be a secondary correction operation after other procedures (such as welding).
Routine “nitriding” (or carbo-nitriding) has been described above. Here, it is discussed as a task-specific corrective operation. As a consequence of certain working conditions or after repeated usage, some damage or degrading of the top nitrided layer may occur. Low hardness (softening) may also be reported resulting from the thermo-mechanical conditions, especially in the bearing region. Such dies are re-nitrided to ensure sufficient hardness at critical regions. The Rockwell hardness of H13 steel dies after carbo-nitriding should be around 63–64 HRC. In addition to resolving low hardness problems, this process can protect and strengthen the newly welded portions after correcting deflection and fracture defects.