Extruded aluminum parts are some of the most valuable materials produced by heavy industry today. From simple extrusions for door and window frames to complex components using specialty aluminum for automotive and aerospace applications, the aluminum extrusion press has been, and still is, a powerful and productive manufacturing machine.
Today’s extrusion presses are, in most cases, hydraulically powered. Many of them have been running for decades. However, press owners and operators are often reluctant to invest in improving the hydraulics on their systems despite the significant — and constant — wear and tear on the machine. As a result, decades of extrusion parts production can take its toll on critical hydraulic components such as pumps, manifolds and control valves.
Pump servo valves are a classic example: there are many current presses with valves that are 30-40 years old. These devices are critical, controlling the displacement of the press pumps which directly affects the press performance and part quality. Some presses have one or two, but many can have up to seven control valves controlling seven pumps. Many press operators are recognizing that these control valves are becoming too difficult and problematic to continue repairing them, so modernization is needed.
Bosch Rexroth has developed a smart, simple solution to this challenge: a family of Rexroth proportional control valves to replace legacy servo valve technology. Engineered to easily fit into existing extrusion press hydraulic systems, these proportional valves are rugged and tolerant of contamination and provide equal or better dynamic hydraulic flow control.
Limitations of Legacy Control Valves
Legacy servo control valves commonly used in extrusion presses have certain features that, when compared to the current generation of proportional control valves, make them substandard for today’s extrusion press requirements. Older valves have delicate internal parts that can ultimately wear out after years of service; those parts can be difficult to obtain if replacements are needed. They are also more susceptible to damage and clogging, causing poor performance in many extrusion processes.
In addition, the feedback sensors for pump displacement are very limited, using analog contact-style potentiometers subject to wear and needing repeated adjustment, or amplified-style analog devices requiring additional electronic signal conditioning devices and more complicated wiring.
When these sensors start to fail, unlike current digital feedback devices, it can be difficult to tell that the device is malfunctioning; instead, the pump that the control valve is regulating may get stuck at a different stroke than it should be operating at for that extrusion cycle, and the press operator won’t know there is an issue.
Worn-out feedback sensors and control valve parts can cause major disruptions downstream in the press hydraulic circuit. It can lead to problems with breakthrough pressure at the start of an extrusion run, as well as velocity control throughout the push. The result is often poor part quality, with the problem of chattering marks on the surface of the extrusions.
So why not replace these older valves? Many press operators face concerns that replacing the valves may idle a press for multiple shifts or days — and a press that isn’t producing is losing money. Some press operators have found workarounds for poorly functioning control valves, such as sending valves to third-party repair shops. In some cases, these rebuilt valves (because they are much older products) can be made operational, but the valves may not be set up or balanced correctly, raising the risk of further problems.
Press operators may also be concerned that they have to upgrade all their legacy valves, as well as legacy pumps and other decades-old equipment, all at one time to gain the full measure of parts quality and press productivity moving to current technology offers.
Installing Proportional Valves: A Good First Step
Fortunately, proportional control valves can eliminate many of the persistent operational and product quality issues associated with legacy servo valves. Upgrading to the current generation of smarter, more reliable and more precise proportional valves can be a good, cost-effective “first step” for aluminum press operators to migrate their legacy hydraulics to state-of-the-art technology.
Proportional control valves feature dynamics that are equal to or better than the dynamics supplied by legacy servo valves. Their reliability is built, in part, on having fewer moving parts to wear out over a lifetime of operations. This makes them more rugged and tolerant of contamination; plus, with fewer moving parts, they are less expensive to repair or replace.
Proportional pump controls such as the Rexroth HS5 and HS5E feature a mechanical valve spool that has extremely high tolerances, which enables it to provide the hydraulic performance of a servo valve with only one moving part. In addition, the Rexroth proportional valves feature digital electronics that provide much faster and more precise response times so that the valve dynamics, and thus the response of the pumps, are much more efficient.
The Rexroth HS5 and HS5E support closed-loop swashplate control, pressure control and power control, enabling press operators to improve how they manage extrusion performance. The HS5 utilizes the Rexroth VT-HPC controller, located separately in a control cabinet, whereas the HS5E has the controller built into the valve, making installation much easier.
These digital controllers make integration and start-up easier for press operators, with a user-friendly software setup “wizard” as well as Ethernet connectivity so they can be connected to the overall press controller platform. They also provide a range of built-in diagnostic features to alert press operators if there is a performance issue or operational thresholds are being exceeded.
These convenient features let press operators avoid common problems with legacy equipment, such as malfunctioning feedback devices or improperly tuned servo valves. The diagnostics also provide a platform for extrusion press operators to gradually implement Industry 4.0 data capture and analytics steps to help improve overall efficiency and also the quality of the parts they produce.
Smart Extrusion Control Platform
One of the key reasons the Rexroth HS5 and HS5E pump control upgrade platforms are a “good first step” is that they can be done on an incremental basis. Many extrusion presses could have anywhere from two to seven hydraulic pumps, but it is not necessary to upgrade all the valves at once to begin improving pump control and part quality.
For operations that are seeking to significantly improve hydraulic performance, Bosch Rexroth also offers an Extrusion Controller Upgrade solution. The digital add-on controller can control from one to seven valve and pump configurations, as well as controlling ram velocity and pressure.
It features a touchscreen HMI (human machine interface) for configuring multiple pump operations, as well as for displaying diagnostic testing and hydraulic system trending reports. This controller platform provides closed-loop control of ram velocity so press operators can create more complex parts, improve the surface finish of the parts they produce and reduce scrap rates during production runs.
The platform also includes Bosch Rexroth’s proprietary “bumpless transfer” technology that cleanly manages the transition from pressure to speed control. This is particularly useful when pressure peaks during extrusion breakthrough and then needs to quickly switch to speed control once breakthrough has occurred.
Upgrade to Connected Hydraulics
For more and more aluminum extrusion press operators, the time is fast approaching where wringing out a couple more years of operation of problematic, decades-old servo control valves no longer makes economic or operational sense.
Bosch Rexroth has extensive experience working closely with press operators to provide sensible hydraulics upgrade solutions. These include the Rexroth HS5 and HS5E pump controls which can be done on a valve-by-valve basis, as well as the more advanced controller upgrade platform. In addition, Bosch Rexroth offers modular, drop-in hydraulic pump and manifold replacement programs that can be easily tailored for the specific needs of individual extruders who may also still be using legacy pumps.
All of these solutions incorporate advanced digital controls and connectivity to provide smart, sophisticated, easy-to-implement hydraulics products and solutions which benefit performance, functionality and lifetime operation.
This article was written and contributed by Mike Kramer, Senior Applications Engineer – Presses, and Rick Garcia, Application Engineering & Simulation, Bosch Rexroth Corporation