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New Hope for Rising Slug Detection in Stamping Dies

By: Thomas Kopka and Anton Schwer, of Schwer + Kopka GmbH
Topics: Rising Slug Detection, Force Sensors, Acoustic Sensors, Eddy Current Sensors, Metal Stamping, IMPAX-SK

Rising slugs are common in today's complex stamping dies, but new sensors and algorithms offer reliable detection

Most 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 the rejection of entire lots, or even the loss of the whole order. Many stamping companies are facing the fact that product complexity and demand for quality are consistently rising, while the increasing complexity of stamping dies also multiplies the risk of getting rising slugs.

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

Unfortunately, rising slugs occur randomly, and this means that the defective parts caused by rising slugs cannot be reliably detected and segregated with conventional means such as statistical sampling. But there is help, developed by IMPAX-SK: a new sensing technology in combination with modern monitoring algorithms is now able to detect rising slugs during the stamping process, and thus prevent costly rejects and expensive inspection and sorting work. This article explains this new technology, and describes the benefits gained by a manufacturer of stamped parts in the automotive industry, Ernst Klimmer, from using the monitoring system on their stamping machines.

Bruno-Michael Albrecht, Klimmer's technical manager
Technical manager Bruno-Michael Albrecht (above)
thinks highly of IMPAX-SK monitorings. Stamped
automotive parts are his company's main business.

What causes rising slugs?

Slugs are actually scrap pieces which typically are produced during cutting and piercing operations. Normally, the punches push these slugs through the cutting plate, after which they exit the die area through provided scrap channels, or they are transported out of the die when it opens. There are quite a few reasons why this does not work all the time, and why slugs can 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 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, regarding the everyday problems he faces 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 inside the die, hidden from outside views. Statistically, it is not a question of 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 refuse to accept the respective batch and reject it. The automotive industry in particular applies extremely high quality standards to supplied parts as is manifested in their zero defect (0 ppm) policies.

"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", says Albrecht, justifying his decision to implement IMPAX-SK process monitoring systems for all of his stamping presses.

IMPAX-SK Process Monitor
One of the IMPAX-SK die monitoring systems
installed at Klimmer's stamping machines

The state of technology in monitoring slugs

Three different sensing techniques have been established for detecting these unpredictable rising slugs in real-time 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

1) Eddy current sensing: registers mainly large slugs

Eddy current measurement typically has two or four sensors placed on each die segment. The sensors register the distance between the upper and lower die plate when the die is fully closed in lower dead center. If a slug happens to be caught inside the die, there is often a tilting effect between the upper and lower die, which in turn produces different eddy current sensor readings. Unfortunately, the tilting effect is less visible for smaller parts: the thinner the strip material is, the smaller the slug is, and the more the slug is located towards the center of the die. In addition, the momentum of the closing die can squeeze the slugs fully home into the material, and in this case the eddy current sensor 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 one side of the strip.

2) 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, particularly the strain and 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 center 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 the detection of the defect.

3) Acoustic emission sensing: 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 noise which is produced by the closing of the die, especially when the stripper plate hits 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. This makes acoustic emission sensing an ideal solution for slug detection. On the other hand, acoustic emission sensors can also react 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.

However, a new type of sensor especially designed by IMPAX-SK for the detection of rising slugs in stamping dies now solves the conflict between the need for a stable signal on the one hand, and a good detection of errors on the other. The combined measurement of force and acoustic emission with the same sensor brings 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.

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. Selection 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 even 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 these 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, and temperature. Ideally, to maintain optimum monitoring results, every die intervention, every coil change, or even every machine stop, should be followed by verification of these 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.

IMPAX-SK envelope monitoring, in manual mode
Manually adjusted limits are often too wide (green band).
Small defects such as rising slugs may not be detected.

Prozzy-logic automatically adjusts monitoring parameters and relieves the operator

Fortunately, modern IMPAX-SK monitoring systems today offer anticipation logic (called Prozzy) which is capable of fully automating the proper selection of all monitoring parameters, without the need for any operator intervention. Unwanted machine stops are avoided because the Prozzy-logic automatically includes past and present process variation into the calculation of the monitoring limits.

IMPAX-SK envelope monitoring, in Prozzy (automatic) mode
Prozzy-logic continuously observes the process variation and
adjusts the limits automatically to the best possible range.

While conventional monitoring methods rely on human inspection and reaction, these Prozzy 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 in high speed applications. The IMPAX-SK Prozzy-logic keeps a permanent eye on the relevant parameters and adjusts all channels to give optimum monitoring results under the given conditions of process repeatability and stability.

Conclusion: more quality through better error detection

The ever increasing complexity of stamping dies has led to more frequent occurrences of rising slugs. These slugs are unavoidable, and their effects are difficult to detect without the use of in-die process monitoring systems. Hence, stamped parts with slug markings often get through to the end customer despite costly final inspections. Conventional sensing techniques are typically not able to provide reliable relief.

However, IMPAX-SK's new combi-sensor measures force and acoustic emission at the same time, combining the benefits of signal stability and sensitivity, and offering 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 IMPAX-SK Prozzy technology. The quality of the monitoring limits has moved from a subjective trial-and-error method to a set of electronically calculated, objective rules. This combination of new sensing techniques with innovative monitoring logic offers reliable detection of large and small rising slugs in stamping dies. This was also the driving force for the stamped parts manufacturer Ernst Klimmer, to equip all of their stamping machines with IMPAX-SK process monitoring systems. The investment paid for itself in a few months time, not least of all because the high inspection and sorting cost could be reduced drastically.

IMPAX-SK Process Monitor on the shop floor at Klimmer
Operator attention is reduced to a
minimum on modern die protection systems

Company and Product Profile

IMPAX-SK Monitors use process enveloping to provide the highest quality process monitoring. They also have advanced production tracking capabilities, an easy-to-use visual interface, and many enhanced software features.
More information: IMPAX-SK Monitors

Process Technologies Group, Inc. (PTG) designs and manufactures process monitors, efficiency monitors, sensors, and data collection software for all production environments.
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