There are many possible ways of optimising the pressing process for increased efficiency while minimising costs. A major influence factor in any process, these costs can be divided as follows:
- Initial cost: Investment, interest, depreciation, tax aspects; required infrastructure, buildings, special constructional measures (‘water-proof pit');
- Operating costs: Productivity, availability, set-up times, personnel required, energy wage; and
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Repair/maintenance costs: Downtime for maintenance, time for replacement and procurement of spare parts, availability of specialised maintenance staff.
Like any other machine, a powder compacting press is most efficient when it is operated at 100% of its capacity during its whole time of use and produces parts of perfect quality to 100%. This is the theoretical outset, which in practice is not really feasible. There are further factors of influence described as follows:
- The start of a new part on an automatic press requires the employment of personnel and time which it is difficult to estimate;
The set-up time required, when changing from one job to the next can not be fixed definitely;
- The duration of product quality optimisation;
- The optimisation of pressing cycles to shorten the process and thus increase the stroke rate; and
- Maintenance and repair work at regular intervals.
To begin with, each pressed part is developed by tool engineering. This is done with focus on the ease of manufacture and on the structure of the tool components as such. It is at this point that the pressing process is already defined with its effect on all the subsequent steps.
And it is exactly at this point that the programming support by the Intelligent Programme Generator (IPG®) begins:
The same program which runs on the production equipment is used already in the design for creating the process cycle. A plausibility check for travels and forces is carried out, and a first indicative value for possible stroke rates is calculated. This means, the engineer is given a software tool that provides valuable, practically oriented support for production of the green part. For this, the operation of the machine in production need not be interrupted, because the IPG may be installed on a computer in the office.
In addition, the production test with the machine can be shortened, because a ready-prepared programme can be loaded on the machine control system. Better use is made of the resources in engineering and manufacturing — with the result of increased efficiency. The employment of personnel and machine can be planned better.
The IPG also supports the operator for the restart of a compact that has been in production before. Normally, certain difficulties will occur in this, due to deviating conditions, such as temperature, humidity, a new batch of powder or a re-machined tool. This may also be the case when an already proven and well-tested program is being used.
The IPG contributes to a quick start of production with its optimisation function.
The first compact produced will be measured, and the thereby determined actual data of density, height and weight are entered in the respective fields of the program. Based on this, the IPG calculates the necessary readjustments for achieving the desired values. Within a few steps of iteration, the optimum pressing cycle for the given circumstances is found, and the production can be started.
Systems for die set exchange with set-up stations for preparing the next tool while the production of another green part is still running are being widely used today. In many factories they ensure the quick change from part A to part B.
Dorst Technologies optimises this procedure further and offers for big hydraulic presses, a fully automatic die set exchange system. After the production-stop of part A, the mode of die set exchange is started by selecting it on the screen, and the operator has to carry out just a few steps as prompted by the screen. This includes changing the gripper jaws, inserting and removing the ram-blocks and opening and closing the safety guard. In all other operations, in particular coupling and uncoupling all supply lines (hydraulic, electric and pneumatic) the die set are carried out automatically by the press system. After the loading of the new tool program, the production can start without delay, so only one man is required on the machine for the change from part A to part B. The time for change is reduced to about 20 minutes, and due to automation, mistakes, such as wrong coupling of hydraulic lines, are excluded. The result is higher efficiency as a consequence of noticeably reduced set-up times as well as increased precision and security in the correct execution of each individual action. In addition, an inventory can be minimised, as ‘just-in-time' production becomes feasible.
The IPG also helps in the optimisation of actually running programs. Modifications entered by hand are incorporated in the pressing cycle in that the IPG recalculates and optimises the complete program. Thus all modifications become an integral part of the process and are subject to control and monitoring by the IPG. This means that different versions of demoulding for complex geometries can be tested quickly and easily, and can be checked with regard to their effect on the compact. If the desired effect could not be achieved, one can return to the original process by simply pressing a button.
This possibility allows for the quick implementation of ideas without losing production time through time-consuming programming; and also the valuable working hours of the press specialists may be made use of in an optimum way.
Automatic readjustment of individual pressing parameters in the course of a large quantity production run can be carried out by the monitoring of various parameters. Internal parameters of the press that are measured, such as pressing force or positions of punches, are processed automatically in the control system without any further operator action required.
In addition, externally measured values also can be processed on the featured automatic presses, such as the weight of the part determined by a balance, or height values from a laser measuring instrument. For this purpose, respective pages in the machine control system are included, on which the desired values can be programmed. According to the external data made available by the balance or by height measurement, the control system will always aim for the desired value that has been put in. It is not necessary to store tool specific programme data in the external measuring system or to programme it in the peripheral equipment. All data is processed exclusively in the machine control system. The operator only needs to have programming knowledge of the powder compacting press' control system and not of the peripheral equipment.
In this way, optimum use is made of the efficiency of the additional equipment and of the operating staff of the press.
Greatest importance has to be attached to the maintenance and repair of production systems running in continuous operation, often seven days per week at 24 hours per day. For this, Dorst Technologies offers service contracts ensuring inspection and maintenance by qualified specialists at regular intervals. Under such a contract, the company will organise and fix in time the dates of service visits in agreement with the user of the machine. The required wear parts for replacement will be available at this visit, and can be installed if required. All machine functions will be checked, and any deviations will be corrected and documented making it possible to anticipate imminent failures of components and take preventive action. Unexpected machine downtime and loss of production could be considerably reduced, and the efficiency of the plant as a whole could increase.
Lowering forces
To make optimum use of the components' quality, the software for the pressing cycle has been optimised. The number of components actuated during the cycle, i.e. which are activated once or several times during each pressing cycle, have been reduced to less than half when compared to previous press generations. Components that are not installed can not fail. Also the remaining components are activated less frequently, and consequently their service life is considerably longer. The efficiency of the individual components and thus of the system as a whole is increased. This, of course, at the same time means less frequent maintenance taking less time. As a result, the effect of efficiency is noticeably increased.
It is not just since the drastic increase of gas prices — starting even in the USA a trend towards vehicles of reduced fuel consumption — that industrial production has aimed for maximum efficiency rates. Hydraulically operated powder compacting presses are normally equipped with strong drive systems to be able to generate the required high pressing forces and stroke rates. In the pressing cycle, however, these forces are required only at the moment of compaction. For the rest of the cycle, drastically lower forces are sufficient. Dorst Technologies makes use of this knowledge in two ways:
- All die sets have positive stops, and thus do not need any energy in pressing position. In addition, no capacity and compacting time is occupied in the control system for settling of the position control, with the result of less time required for the cycle and an increase in the stroke rate; and
- The number of motors has been reduced to one main motor, generating all the movements. An intelligent control system with integrated energy-management system makes sure that this motor is always used in an optimum way. The maximum power is pulled only immediately before the pressing position is reached. During the rest of the cycle the press runs at considerably lower power and additionally accumulated energy is fed in from a pressure accumulator. This accumulator is charged at times when excess energy in the form of hydraulic pressure is available.
As a result of this technology, the featured powder compacting presses only need about 30% of the amount of the energy that has been required in the past. For a powder compacting press of 5000 kN pressing force, this may mean energy savings of up to €50 000 (in electric current costs in Central Europe) per year.
Even with hybrid presses it is not possible to achieve such a result. The benefit of modern hydraulic plants is clearly in the noticeably improved efficiency. All of the above factors noticeably increased efficiency of today's pressing systems. While taking into consideration these measures, we should look at productivity. The standard for evaluating the efficiency of a powder compacting press is not so much the number of strokes per minute, but the output of good parts per month.
The author
This article is drawn from Efficiency Increase by Optimized High-Tech Forming Concepts, a paper by Lutz Lackner of DORST Technologies, presented at Euro PM2008, Mannheim.
