Powder pressing is traditionally a mass manufacturing enterprise. But although large production lots are still the norm in many areas, there is a trend towards smaller quantities in some parts of the PM business. Not quite 20 years ago, a similar trend compelled the hardmetal industry to change its way of thinking in the production of cutting tips.
But current conditions are not just a rerun of history, for although there are similarities, there are also new factors to be considered. Among these is a global demand for the ability to shift production from one location to another in an uncomplicated fashion. These requirements increasingly apply to pressed parts in the range up to 1200 kN pressing force.
This is an area where a new pressing technology can make a contribution to support the manufacturers of pressed parts with practical solutions. Noting market trends, Dorst Technologies developed the TPA-HS series of presses, which has already seen great success over a number of years in the production of hard metal components. It has now been developed further for metal powder applications.
Everyday press experience in the hardmetal industry shows that accuracies within a few μm in positioning of the tool levels are attainable. Such precision has to be made available to the powder metal industry to meet higher accuracy requirements that are becoming normal customer demands. While higher accuracy is one aspect of improved quality standards, the increasingly complex geometry of multi-level compacts are another. Multi-platen technology is required to satisfy demand. These criteria were included in the further development of the TPA-HS series TPA-HS series.
Quick-action clamping
The most striking characteristic of the machine is its extremely rigid, closed frame construction consisting of two plates The frame is over-dimensioned for the nominal pressing force, but this reduces the physically inevitable extension of the press to a minimum. In practice this means that at maximum pressing force the frame extension is infinitesimal.
As a result of the forces acting between upper and lower punch, all machines have a tendency to become “constricted” in the central area of the press frame, which, of course, affects the precision of movements and consequently of the compacts produced. In order to compensate for this effect, the die table of the TPA-HS series presses is mounted directly in the center of the frame. The die table thus contributes considerably to the rigidity of the press frame, because it exactly counteracts the “strangling effect” of the machine, and the precision of the press is therefore enormously increased.
The guides of the tool levels are also mounted directly into the press frame. The components used for the guides are not standard machine elements, but clearance-free, high precision linear guides which are mounted under preload in the frame. Because of the generous dimensioning and the rigidity of the frame, deformation resulting from pressing forces is eliminated. These could otherwise have a negative effect on precision. These special guides require no maintenance and are free from any wear.
It is these clearance-free guides which permit the use of quick-action clamping systems to locate the forming tool punches directly – without tool adaptor or die set – in the press frame.
Each tool adaptor has its manufacturing-related individual tolerances. When used in production, additional deviations from the ideal condition are the result of natural wear. This is the reason for the need to reset and readjust the tool each time when production with another compact is started to achieve and maintain the required tolerances. This also applies to tools that have been used successfully in production before. This setting process consumes valued machine and setting time and generates considerable costs, since each time the individual settings are different to the last production run.
The new press design without tool adaptors makes it possible to eliminate this work and the delays when starting production of a compact. The forming tool punches are ideally mounted directly in the machine in order to avoid additional influences from tolerances in the tool adaptor guides. It is for this reason that in the hardmetal industry standard quick-action clamping systems, assuring precise and positive engagement of the punch to the machine axis have been in use for a long time. So far these systems have been restricted with respect to their maximum pressing force, and they could not be used in multi-platen systems. Dorst Technologies developed its own quick-action clamping system, which can be used for up to three upper and up to three lower levels.
Swift set-up
The system is based on a punch-holder inserted on centreing elements directly on the press. In case of more than one level, the levels are telescopically inserted into each other. The positioning of each individual punch is accomplished with a tolerance of ± 2 μm. This ensures an accurate arrangement of the tool in the press. For installation in the automatic press, the tool is placed directly onto the guide and locked automatically.
When a tool has already been mounted on the press, the complete upper punch set can be fixed together with the die and it can be removed as one unit from the machine. In the same way, the complete lower punch set together with the core rod can be removed. The next time the tool is installed, each complete set will be inserted on the machine, the axes will be moved to the upper or lower part, and the individual levels will be locked. This setting operation takes only a few minutes, and – due to the fact that there are no additional adjustments – production can be started immediately with the existing program. Only the given powder characteristics have to be taken into consideration, and this is accomplished quickly by the optimisation function of the Intelligent Program Generator®. This technology, without tool adaptor or die set, but with quick-action clamping systems opens up new ways for problem solving:
- Greater flexibility: Each tool can be run on each TPA-HS machine with the existing program, without having to carry out a new setting operation.
- Repeatability/reproducibility: The tool is located exactly like in the last production run, i.e. all parameters are identical, and consequently also the compacts will be identical to those produced during the previous run.
- Availability: As a result of the extremely short set up times the availability of the automatic presses for production will be increased. Times consumed for “test runs” are eliminated and production time can be increased.
- Punch length management: Replacement or length readjustment of punches can be accomplished outside of the press in the tool shop. The new dimensions have to be introduced in the program, which then automatically carries out the necessary corrective calculations. After that production can start immediately.
- Automation – A view into the future: This system opens up the way to automated compact production, with the tool and the powder automatically provided to the machine. The punches are automatically clamped by the quick-action clamping system, and the program for the tool is identified and read by a bar code. The production run can be started immediately, without any further intervention by the operator.
The Dorst visualisation system/Dorst control system (DVS/DCS®) has been used successfully for many years on all hydraulic automatic presses with closed-loop control. The hardware of this completely freely programmable control system consists of two independent computers, the DVS as “man-machine interface” on the screen from where all process data can be entered and displayed, and the DCS which controls the machine.
Embedded in this is the software tool Intelligent Program Generator®, which ensures rapid and simple creation of programs. In addition, the IPG offers the unique optimisation function that carries out the calculation of adjustments needed on the basis of the results of the inspection of the first parts pressed. In this way, it also takes into consideration the parameters unknown and the setting process to take account of the current powder parameters is enormously speeded up.
Zero level in the press, as reference value for all calculations, is defined by the stationary die. This again means a considerable simplification of programming of the pressing cycle, because no arbitrary definition is required for this. Selection of an unfavourable reference level may have a negative effect on the execution of the program.
The second computer is part of the electronic system of the machine, and it is responsible for the closed-loop control of all servo hydraulic axes of the automatic press.
Visualising control
Here the necessary volume flows and pressures, required for an optimum pressing cycle, are calculated. The interaction of these two computers is ensured by Ethernet connection, so that all data in both systems are practically available at the same time and can be processed immediately. This architecture allows closed-loop control at high precision combined with extremely fast processing.
This “brain of the press” is supported by measuring systems operating with a precision in the range of several μm within normal operating temperatures, irrespective of ambient temperature influences. This means an accurate conversion of the control commands into machine movements, and consequently extremely high precision of the compacts. It is this property which allows the control system to compensate the compression of the tool system resulting from the pressing force, without any intervention required by the operator. This increases precision and repeatability of the machine, because even small differences in powder filling between one cycle and the next, will result in differences in density of the pressed part.
The compensation of compression can prevent this effect to a large extent.
The stationary die opens up the additional option of side compacting. Side compacting requires a movement transverse to the direction of main pressing, and it means a spectacular step forward in technology from uni-axial pressing to multi-axial pressing.
Undercuts can now be formed in the pressed part by up to six independent pressing punches which are freely adjustable and closed-loop controlled, operating in a direction transverse to the main pressing direction.
This allows new processes for parts, which hitherto could not be pressed, and it results in considerable reduction of costly subsequent machining.
This technology has been applied in the production of saw blades in large quantity lots for several years now. The saw blades are pressed from hardmetal powder and have a chip deflecting recess below the cutting edge, which can only be pressed as an undercut. The system consists of six identical punches mounted in the die with individual closed-loop control, a pressing force of 30 kN and a punch travel of 4 mm.
In the hardmetal industry, DORST Technologies introduced the pressing of cross holes successfully several years ago with the TPA 15 HS, where it is used in daily production.
Lateral penetration
Now it is also available for higher pressing forces and allows considerable cost reduction for many product groups. The principle of pressing parts with cross holes is simple.
Two punches penetrate the powder laterally and the powder columns above and below are compacted against these punches. It is in the demoulding of the green compact that this process becomes a challenge, because even small differences in density will result in different spring-back behaviour of the upper and lower powder column, so that the sensitive pins could be sheared off. At this point the intelligent pressing technology of the TPA-HS presses steps in. Spring-back compensation can be carried out very sensitively with very small forces of only a few Newtons. Forces up to about 10 per cent of the pressing force are subjected to hydraulic closed-loop control by the respective tool levels. If they are still too high for the compact, the optionally pneumatic hold-down can be engaged, by which the force can be reduced to about 1 per cent of the pressing force. This allows safe, reproducible demoulding, even for most sensitive green compacts. In this range, the control system reacts with a repeating accuracy of ± 0.05 per cent of the maximum pressing force.
This article is drawn from New Generation of Hydraulic Presses Type TPA HS, by Lutz Lackner of Dorst Technologies and given at EuroPM2007 in Toulouse.
