Maintenance of dies for the cold stamping of sheet metal
The effectiveness of a sheet metal cold stamping plant depends on several factors.
In the evaluation of the overall effectiveness of a plant (OEE = Overall Equipment Effectiveness), in addition to machine stops due to breakages or malfunctions, slowdowns caused by the inadequate speed of the machine (compared to the value considered as the reference standard) and the number of defective pieces are of crucial importance.
In the cold stamping of sheet metal there is a specific point in the production process in which these three factors are particularly critical: the die.
The die is the heart of the process, it is the place where the sheet is shaped.
A problem in the die automatically turns into inefficiency due to:
- machine downtime
- lower production speed
- rejected pieces
Thus, incorrect die operation immediately translates into unwanted.
Poor maintenance: the most common reasons
Die productivity often takes a back seat in favour or the urgency of delivery.
Ordinary maintenance of the die requires time and skills, which are not always present in the company. This translates into a real cost which is not always accepted. If the die is still productive, why spend money?
One often forgets that correctly cleaning and greasing the die before storing it in the warehouse after each production may significantly increase its life span. No die maker skills are required to perform this minimal maintenance, but most of the time it is not carried out correctly due to lack of time because: “it is necessary to produce”.
The problem of poor ordinary maintenance is easy to solve, but it is often accompanied by problems in extraordinary maintenance. The need to deliver “on time” leads to the underestimation of possible problems on one of the cavities of a die that produces more pieces per stamping. We often prefer to stamp anyway and discard a piece, rather than stopping to fix the die.
If this is repeated over time in a die with 2 outs per stamping, it will lead to a sharp decrease in the OEE by 50% (one defective piece out of two).
Other times, it may prove to be impossible to fix the die because the technical drawings of the worn component are missing. Few companies are able to support the customer in the correct extraordinary maintenance of the dies.
Why do the dies break?
It is important to understand the most common dynamics that lay behind die breakage. Here, the term breakage involves the physical breakage of the die (which leads to a stop) as well as those problems that do not allow working at the correct design speed, or that highlight possible quality problems on the stamped products (such as scratches or burrs).
A statistical research on the interventions requested by our customers has allowed us to identify the following main reasons for breakages in progressive dies:
In the life of the die there is a need to preserve its initial functionality over time. This functionality can be preserved with maintenance.
The high concentration of breakages due to over-stamping of pieces appears immediately evident.
This type of problem is mainly due to an inadequate removal of the piece after stamping, but the repeated occurrence of the problem during production may be caused by a sub-optimal die design.
Secondly, in addition to breakages caused by over-stamping, we find breakages due to a lack of ordinary maintenance on the dies. This is immediately followed by breakage (or malfunction) due to the loss of the thread on the cutting dies and to the loss of the coatings. We can associate the latter with the lack of extraordinary maintenance and it strongly depends on how frequently the die is used
The analysis found that the problems generated in the dies due to a lack of ordinary and extraordinary maintenance represent approximately 37% of the total criticality.
This is good news for those who use the dies, because it shows that almost 40% of the problems can be eliminated by activating recurring and planned maintenance processes.
F.lli Poli steps in for any other problems.