Controlling motion in a press that is 3 stories high, exerts forces of 36,000 kN (8,000,000 lbf) and that completes a cycle in 19 seconds is a significant challenge for any metal forming company. To make it even more complex, this machine is used for producing some of the latest plastics materials such as car doors, panels and even whole cars that require exacting quality right off the press. Dieffenbacher and Moog have worked together to realize these technical requirements and also provide additional benefits such as energy savings, remote diagnostics and support.
This article focuses on the latest generation plastic press from Dieffenbacher, a leading manufacturer in the field of SMC/GMT/LFT technology based in Germany. SMC/GMT/LFT technology is a process for fiber-reinforced plastics used for manufacturing complex lightweight components. Crash-resistant lightweight structural components with a cosmetic surface are manufactured on this machine from advanced plastic compounds producing exceptional finish quality.
Moog has been developing and supplying hydraulic systems including integrated hydraulic manifold systems and advanced valves for high-performance metal forming presses for over 10 years. The machine called the COMPRESS PLUS from Dieffenbacher is their response to the needs of their customers to be more competitive, productive and cost effective.
Customer Benefits of COMPRESS PLUS
Plastic presses are for direct processing of fiber-reinforced thermoplastic and thermosetting plastics. This is a challenging high production application that is often used in automotive manufacturing of large components such as doors and panels.
- Energy: In response to steadily increasing energy prices, Dieffenbacher and Moog, developed a machine that reduces energy consumption up to 50% by means of a new closing device and an optimized hydraulic concept.
- Quality: The new generation press enables the end customer to produce parts of highest quality due to an innovative high performance motion control solution. The increased stiffness of the hydraulic system and the use of high response servovalves with digital electronics guarantees high precision and repeatability of the control axis.
- Reliability: The control concept is based on fieldbus technology. Servovalves and sensors are equipped with EtherCat Interface that enable diagnostics and support remotely via the Internet.
- Cost: In order to address the increasing cost pressure of the customers, the new generation press combines high output, high reliability, low energy consumption and an attractive price-performance ratio in one machine.
Technical Specifications for Speed and Force
One of the challenges of this application is speed where complex motion must occur at extremely rapid rates.
Figure 1 - Shows an exemplary press cycle for an 36,000 kN (8,000,000 lbf) press closing time 4 sec., pressing time and cooling 6 sec., opening time 4 sec., loading/ unloading time 5 sec.
The weight of the mold is approximately 60 ton and the closing movement is driven by the gravity force and no energy is required. During 90% of the pressing cycle, only 10% of the maximum force is required. Figure 2 shows the force- stroke characteristics.
This means that the maximum pressing force is needed for a short interval of the process cycle. Typically vertical hydraulic presses are equipped with a fast stroke auxiliary cylinder (also known as a kicker cylinder) to control the speed and position of the upper die in combination with the main press cylinder precharged with the use of prefill valves and a tank that is on top of the press. The maximum speed of the upper die cylinder is limited by the size of the prefill valves.
New Press Design
An analysis of the process parameters like force and speed was conducted, resulting in a new optimised press design concept. Figure 3 illustrates the principle of the new design of the hydraulic system.
In its new design, Dieffenbacher overcame the conventional limitation with a new upper die concept, where a mechanical locking device is transmitting the press force from a short stroke cylinder. This eliminates the need for prefill valves and the prefill tank, thereby reducing costs and increasing the maximum closing and opening speed.
- New Press Design COMPRESS PLUS, Dieffenbacher, with short stroke press cylinder and mechanically locking device
- Technical data of a 36,000 kN (8,000,000 lbf) press COMPRESS PLUS:
|Speeds: fast closing, opening||1,200 mm/s
|Speed Pressing||1- 80 mm/s
(.04 -3.5 in/s)
L/min (872 gpm)
|Press force at centric load||36000 kN
|Press force at maximum parallel levelling force||32000 kN
Main Advantages of the New Concept
Due to the short stroke press cylinder the oil volume is reduced significantly. This leads to increased stiffness and higher natural frequency of the hydraulic system.
The use of a servo-proportional valve with integrated failsafe functionality in combination with a position-monitored active cartridge fulfils the press safety requirements. The main stage of the servo-proportional valve is spring centered, when the failsafe directional valve is switched off by the machine control. The electronics detects the safe position within a safety window and generates a logical signal (> 8,5 V), that is available on the main valve connector.
Due to the active cartridge design a closing time of less than 150 ms to switch off the accumulator circuit is achieved thereby meeting Machine Safety Standards. This concept allows a compact manifold design with less hydraulic components and intersections. Low pressure drop in the hydraulic circuit improves the efficiency of the hydraulic control circuit.
Main Press Cylinder Manifold
In addition, the adaptation of system pressure with a low pressure and high pressure accumulator reduces significantly the pressure losses during the pressing mode. Depending on the force requirements the high-pressure accumulator is activated for the final pressing stroke only.
Moog Industrial Group and Dieffenbacher collaborated on an innovative new solution for a latest generation plastic press that significantly reduced the energy consumption and met tough technical challenges to increase the productivity and reliability of the machine. The combination of a new mechanical design of the closing device with a customized hydraulic control concept meets the future requirements of customers in one of the most demanding and advanced applications involving high forces and speeds as well as exacting finished part quality.
Volker Treffler is the Engineering Manager of Moog Luxembourg based in Bettembourg. Employed since June 1995, he is responsible for developing manifold systems solutions and cartridges valves. He holds a Master of Science degree in Mechanical Engineering from the University of Krefeld (Germany) and has over 25 years of experience in hydraulic systems for presses and injection moulding machines.