by Josh Schroeder
When selecting a heat source for your heated platen press application, you should first consider your process requirements followed by the total cost of ownership. In your evaluation there are advantages and disadvantages you should consider for each method of heat transfer.
Heat Transfer (Fluid) Systems
The advantages of a Hot Oil or Heat Transfer System are that it produces the most accurate and uniform temperature control across the surface of the platens. Accuracy is typically from 1 to 5 degrees Fahrenheit depending on the platen design. Process temperature ramping can also be controlled consistently with this type of system and large heat loads are possible. The maximum operating temperature is around 700 degrees Fahrenheit.
Another advantage is that cooling of the platen can also be achieved utilizing the same circulating system. This will allow product to be removed from the press at a safe operating temperature for personnel. This can also be used to provide an end of shift cool down for equipment.
Maintenance of these systems can be considered both an advantage and disadvantage. The heat system is external to the press and therefore routine maintenance and electrical connections can be serviced without disrupting the press. The down side is that the oil is a flammable medium and can be a fire hazard if proper industrial hygiene and preventative maintenance are neglected.
These systems are very accurate and provide uniform temperatures even with large heat loads. The cost for this level of accuracy is much higher than metal sheath style heaters and maintenance of these systems can be demanding but if your process requires tight tolerances these systems are the best choice.
Metal Sheath Insert Heaters
The advantages of metal sheath insert heaters are that they can produce fast heat up and recovery times. They provide efficient energy usage with the heaters inside the platens and can achieve high temperatures up to 1,200 Fahrenheit. The initial cost is also much lower than the hot oil systems and maintenance cost would be considerably less. The risk of fire is greatly reduced as well.
The disadvantages are that temperature uniformity across platen surfaces is not as accurate. The delta range is typically from 15 to 30 degrees. Also the physical dimensions of the platen may limit the amount of KW that can be installed.
Maintenance on these systems is relatively low in comparison to hot oil however should a heater failure occur often the heater must be drilled out of the platen. This may lead to extended downtime.
Metal Sheath Insert Heaters provide efficient energy usage and fast heat up and recovery times coupled with higher temperature capabilities but they do not provide the same level of accuracy or uniformity. If you are looking for a lower cost option and your process does not demand the higher accuracy or uniformity of the hot oil systems then insert style heaters would be the best choice.
In an attempt to decrease margins and increased competition, today’s manufacturers are driven to consolidate processes, minimize operator error and reduce material scrap. In order to thrive in these conditions, manufacturers must push for a greater emphasis on technology. The hydraulic press market has grown with these challenging demands. One of the most popular advances is the utilization of Active Leveling Control (ALC). This technology counteracts the negative effects of off-center loading, significant breakthrough shock with high strength steels and reverse shock associated with the use of nitrogen in dies.
Active Leveling Control (ALC) is a technologically advanced system that utilizes closed loop control for each corner of the press ram, keeping the press ram in parallel and reducing any existing shock. Presses equipped with ALC utilize a high-speed motion controller, linear transducers and proportional valves to synchronize the press actuators. The 32-bit multi-axis high-speed motion controller provides less than 1 msec control loop time.
Active Leveling Control for bed-to-ram parallelism is offered in either 2 or 4 axis depending on the application. Two-axis control provides either left-to-right or front-to-rear parallelism. For users requiring both left-to-right and front-to-rear parallelism, four-axis control is recommended.
Off-Center loading can be caused by many issues and can cause unwanted symptoms to arise within the press, dies and overall application. Poor part quality & inconsistency, premature tool wear, and detrimental stress on the press structure are all likely to occur if the press has not been properly designed to handle an unbalanced load.
The most common cause of off-center loading is due to the geometry of the part formed not being conducive to an even load. Many applications do not form evenly over the press bed and can wreak havoc on a press or tooling. Other difficult Off-Center Loading processes many include progressive dies and the desire to consolidate multiple operations (and multiple dies) into a single machine. In addition, many presses can be improperly loaded due to operator error, creating additional alignment problems. This can especially be a problem for presses with big beds and/or small tooling.
A properly engineered Active Leveling Control system will provide many capabilities to counteract the negative effects of off-center loading. This improvement supplies protection for the press structure and tooling while decreasing maintenance costs. Many processes that historically could not be unified in one press are now being efficiently combined and properly executed with the integration of the ALC system. This feature enables companies to displace multiple presses and save valuable floor space, which in turn reduces labor costs and creates room for more capacity. The ALC technology is paralleled by servo mechanical presses but offers more flexibility at a significantly reduced price.
As hydraulic design technology advances, manufacturers will be presented with new capabilities to increase the efficiency of their production processes. Active Leveling Control can be integrated into a new hydraulic press to increase part quality, reduced die wear, and reduce complex setup time. This system provides the ideal solution for manufacturers who need to counteract the effects off-center loading, significant breakthrough shock, and reverse shock loading situations.
by Ryan Pendleton
The use of nitrogen springs in die sets is certainly a practical and proven asset in designing modern metal forming tool sets. However, their use often comes at the expense of the machine tool running them.
Nitrogen assists with softening breakthrough shock in punching applications, assisting material flow in forming applications and compensation in off-center loaded applications. When used properly, they assist with removing shock and detrimental forces in presses during the down stroke. Unfortunately, they can have the opposite effect on the up or reverse stroke.
In evenly loaded hydraulic presses, decompression circuits can be incorporated to minimize the negative effects of nitrogen laden dies. Once the press has compressed the tool set and nitrogen, the ram can be idled and allow for the nitrogen springs to push the ram up and begin the retract portion of the stroke. Position feedback or simple timers can be utilized to engage the press hydraulics for high speed retraction at the proper time.
In off-center loaded applications, Active Leveling Control can be utilized. ALC systems use sophisticated proportional hydraulics and a motion controller to monitor the bed-to-ram parallelism throughout the cycle. The motion controller adjusts the hydraulics to compensate for varying forces throughout the bed area to maintain parallelism. ALC systems can be programmed to provide a controlled retraction to maintain bed-to-ram parallelism on both the down and up stroke.
Through programmable decompression or more advanced Active Leveling Control systems, issues related to nitrogen decompression shock can be effectively countered by experienced hydraulic press manufacturers.