A common problem when stamping parts: markings and surface defects caused by rising slugs or scrap

Process Monitoring Systems for Metal Stamping Machines

No chance against rising slugs in stamping dies? Two thirds of all stamping companies are familiar with the problem of rising slugs that can cause surface defects and marks on stamped parts. The way this problem is being tackled, however, is quite different depending upon the type of product. While producers of “noncritical“ stampings typically do not care much about surface defects, parts for automotive applications must be perfect all around. Any imperfections such as slug markings can result in rejecting entire lots, or even in loosing the complete order. Many stamping companies are facing the fact that product complexity and demand for quality are consistantly rising, while the also increasing complexity of the stamping dies multiplies the risk of getting rising slugs. It is typical and unpleasant at the same time, that rising slugs are occurring only randomly. This prevents that defective parts caused by rising slugs can be reliably detected and segregated with conventional means such as statistical sampling. But there is help: a new sensing technology in combination with modern monitoring algorithms is now capable to detect rising slugs already during the stamping process, and to prevent costly rejects and expensive inspection and sorting work. The following article explains this new technology, and describes which benefits are gained by a German supplier of stamped parts for the automotive industry, Ernst Klimmer GmbH of Burgau, from using the monitoring system on their stamping machines. Written by: Thomas Kopka and Anton Schwer, Schwer + Kopka GmbH

What is causing rising slugs? Slugs are actually scrap pieces which typically are produced during cutting and piercing operations. Normally, the punches will push these slugs through the cutting plate, after which they exit the die area through provided scrap channels, or will be transported out of the die when it opens. There are quite a few reasons why this is not working all the time, and why slugs remain inside the die. A very common cause is that punches wear, and the slugs tend to stick to the bottom of the punch. When the punches pull out of the strip, the slugs will be wiped off and remain inside the die’s working area. During the next stroke, the slugs are being pressed onto the stamping and leave visible surface marks. It now depends on the further degree of forming done to the part, whether or not these marks remain visible from the outside. „Cutting operations can typically be found at the beginning of progressive stamping dies, and, as a result, slugs are very likely to occur in that area. The following forming operations often cover the marks such that they become very difficult to see” describes Bruno-Michael Albrecht, the technical manager of Klimmer, the everyday problems he is facing as a stamped parts manufacturer.

High sorting and inspection cost Problems with rising slugs are hard to figure out and very difficult to detect by means of visual inspections. They happen randomly, can be located anywhere on the stamping, and often are hidden inside the die from outside views. Statistically, it is not a question if but rather when and how often stamped parts with slug markings are going to be delivered to the final customer, and how he will react to the fact. A single defective part that is found by the customer usually initiates costly sorting and inspection operations for the entire batch. The customer in turn will debit the supplier with these cost, or refuses to accept the respective batch and rejects it. The automotive industry in particular applies extremely high quality standards to supplied parts as is manifested in their known zero defect (0 ppm) policy. „If in addition the customer happens to be far away from our own plant, and insists on immediate delivery of substitute parts, we are faced with yet more cost for express shipments and internal organizational efforts. With a view to this, our monitoring systems have paid for themselves in a very short period of time”, justifies Klimmer’s technical manager his decision for implementing process monitoring system to all of his stamping presses.

State of technology in monitoring for slugs in stamping operations Three different sensing techniques have been established for detecting those unpredictable rising slugs already during the actual stamping process: 1. Eddy current sensors which measure the distance between the upper and the lower die section when the die closes (shut height measurement) 2. Force sensors located inside the die 3. Acoustic emission sensors located inside the die parameters. The necessary fine-tuning is not carried out, and the desired monitoring performance may not be reached.

Eddy current sensing registers mainly large slugs Eddy current measurement typically has two or often four sensors placed on each die segment. The sensors register the distance between an upper and a lower die plate when the die is fully closed in lower dead centre. If a slug happens to be caught inside the die, there should be in theory a tilting effect between upper and lower die which in turn will produce different eddy current sensor readings. It is quite obvious that the tilting effect is becoming less visible the thinner the strip material is, the smaller the slug is, and the more the slug is located towards the centre of the die. In addition, the momentum of the closing die can squeeze the slugs fully home into the material and the eddy current

sensor in this case may not see any difference in the closing gap of the die. Consequently, eddy current sensing has limited applications and is mainly suitable when large slugs are expected which ideally are located more to the side of the strip.

Bruno-Michael Albrecht, Klimmer’s technical manager, thinks highly of his monitoring systems

Stamped parts for the Automotive Industry is the main business of the German specialist company Ernst Klimmer GmbH

Force sensing supplies stable signals Force sensors for detecting slugs are typically mounted onto the stripper plate of the die. Normally, one or two sensors per plate are sufficient. They measure the stamping forces respectively the strain and the deformation of the stripper plate when the die closes. If a slug is caught between the stripper plate and the material, the deformation of the plate will be quite different if compared to trouble free operation. Unlike eddy current sensing, force measurement registers slugs located on the side as well as in the centre of the die. Force sensors inherently deliver strong and stable signals and can be monitored with relatively tight limits. Nevertheless, the bigger and coarser the slug is, the better is the detection of the defect.

Acoustic emission measures the “sound” of the die Just like force sensors, acoustic emission sensors are placed on the stripper plate of the die. Again, one or two sensors are sufficient per plate. The acoustic emission sensors register the sound or the noise which is produced by the closing of the die, especially when the stripper plate hits onto the material strip. If a slug is found between the stripper plate and the material, the noise of the closing die will change and is detected by the sensor. Unlike the two previously described techniques, even small slugs will cause significant changes in the noise of the closing die which makes acoustic emission sensing an ideal solution for slug detection. On the other hand, acoustic emission sensors will also react sensibly to other noise sources inside the die, and as such, the stability of the acoustic emission signal may suffer.

New combination sensors are suitable for force and acoustic emission Based upon the advantages and disadvantages described above, potential users of slug monitoring systems had to make their choice between the stability of forces sensors or the sensitivity of acoustic emission sensing, depending upon the requirements of the application. A new type of sensor which was especially designed for the detection of rising slugs in stamping dies now solves the conflict between the need for a stable signal on the one side and a good detection of errors on the other side. The combined measurement of force and acoustic emission with the same sensor puts together the advantages of both techniques. The combined signal has the sensitivity to

detect even very small slugs and, at the same time, offers the stability of a pure force signal to prevent unnecessary machine stoppages. .

One of the die monitoring systems installed at Klimmer’s stamping machines

Operator attention is reduced to a minimum on modern die protection systems

Innovative algorithms to process the new sensor signals The reliable detection of process errors such as rising slugs not only depends on a good quality sensor signal but also on the right method to analyze the measured signals. Setting of the relevant monitoring parameters is a direct function of the signal quality. When sensor signals are stable and reproducible, the monitoring limits can be set quite closely to enhance error detection capabilities. If, however, sensor signals are found to be widely spread already during good parts production, users are forced to widen the monitoring limits accordingly. In essence, it’s a constant struggle between a fine setting of monitoring limits to get the desired sensitivity for detecting also small errors, and a coarser setting to prevent unnecessary machine down time. Once those limits have been established, they are only valid as long as other process parameters remain unchanged. In stamping operations, such parameters for example are strip lubrication, stroke rate, material consistency, or temperature. Ideally, to maintain optimum monitoring results, every die intervention, every coil change, or even every machine stop, should initiate a verification of those monitoring parameters. In reality, the situation is quite different. In order to avoid the ongoing verification of correct monitoring parameters, and to prevent frequent machine stoppages, the monitoring limits are often consciously set to very coarse values. If the limits are set coarsely enough, the machine can be re-started after every stop with the same inaccuracy. The operator is no longer “bothered” by his monitoring system, but, of course, error detection is reduced to a bare minimum.

Prozzy-logic automatically adjusts monitoring parameters and releaves the operator Modern monitoring systems today are offering anticipating logics (called Prozzy) which are capable to fully automate the proper setting of all monitoring parameters without the need for any operator intervention (fig 5). Unwanted machine stops are avoided because the Prozzy-logics automatically include past and present process variation into the calculation of the monitoring limits. While conventional monitoring methods rely on human inspection and reaction, modern techniques are designed to verify and update monitoring parameters on an ongoing basis across the entire production process. Even complex systems with multiple monitoring channels are dynamically updated also in high speed applications. The Prozzy-logic keeps a permanent eye on the relevant

parameters and adjusts all channels to give optimum monitoring results under the given conditions such as process repeatability and stability.

Manually adjusted limits are often too wide (see green band). Small defects as rising slugs may not be detected.

Prozzy-logic continuously observes the process variation and adjusts the limits automatically to the best possible range.

Conclusion: more quality through better error detection The ever increasing complexity of stamping dies has led to more frequent occurrences of rising slugs. While those slugs apparently are unavoidable, their effects are difficult to detect without the use of properly working in-die process monitoring systems. Hence, stamped parts with slug markings often get through to the end customer despite of costly final inspections. Conventional sensing techniques are typically not able to provide reliable relief. A new combi-sensor measures force and acoustic emission at the same time, combines the benefits of signal stability and sensitivity, and offers a highly reliable method to detect rising slugs during the actual stamping process. The quality of the sensor signal is supported by new monitoring algorithms, where the monitoring limits are permanently verified and updated using our Prozzy technology. The quality of the monitoring limits has moved from a subjective try-and error method to electronically calculated, objective setting rules. The combination of new sensing techniques with innovative monitoring logics now offers reliable detection of large and small rising slugs in stamping dies. This was also the driving force for the German stamped parts manufacturer Ernst Klimmer GmbH, to equip all of their stamping machine with modern process monitoring systems. The investment paid for itself in a few months time, not at least because the previously high inspection and sorting cost could be reduced drastically.