June 27, 2014
What makes a hydraulic press equipped for hot stamping? Access for auxiliaries and process equipment, a rapid deceleration rate, correct tonnage, and off-center load handling are all considerations.
Fuel prices are at an all-time high and the current U.S. administration has implemented tough new targets for corporate average fuel economy (CAFE) standards equivalent to 35.5 MPG for the 2016 model year and an astonishing 54.5 MPG by the 2025 model year. Producing lightweight passenger vehicles that can still meet the industry’s stringent safety standards is now more important than ever to automakers’ success.
Though many advances have been made in North America to reduce vehicle weight by manufacturing them with lightweight materials, additional safety equipment often offsets the weight loss. In addition, an ever-increasing array of creature comforts, such as powered actuators and electronics, have added weight as well.
The use of hot-stamping technology, also referred to as press hardening, is one key approach manufacturers are using to produce both very strong and lightweight structural vehicle components. Hot stamping literally is stamping very hot boron steel. Heating the steel makes it pliable for forming and eliminates some of the problems, such as cracking, associated with forming hard metals at room temperature. It is the properties of boron steel that make it extremely hard when it is heated to ultrahigh temperatures and then quickly cooled.
The hot-stamping process involves a number of steps. The first step is destacking a blank and loading it into an oven or furnace to be heated above 900 degrees C. Next, the blank is transferred to a hot-stamping press, where it is formed and then cooled in a rapid quenching step. Last, it is automatically unloaded from the press.
Automatic loading and unloading of the material into the furnace and press is required for safe handling of the material as it undergoes these high temperatures and rapid cooling (see Figure 1). The automation can be either by robot or high-speed feeder.
Examples of automotive components currently manufactured in a hydraulic hot-stamping process are:
A hydraulic stamping press is the heart of a typical hot-stamping line. Some key variables that should be considered when specifying a hydraulic hot-stamping press are frame style, guiding configuration, ram closing speed, tonnage, off-center loading, quick die change, and operator safety.
The two press frame styles most commonly used in hydraulic hot-stamping presses are prestressed housing and monolithic type.
A prestressed housing-style press frame comprises separate structural components including the bed, crosshead, housings, four tie rods, and regular and prestressing nuts. This press frame style is required when the frame is too large to be shipped in a single piece, and so must be disassembled for shipping purposes and reassembled on-site.
A monolithic-style press frame is a single unitized weldment that is thermally stress-relieved before it is finish-machined. Because it is a single piece, this frame style offers simplified installation. It is a viable option when the frame size is small enough to be shipped as a single unit. It typically is less expensive than a prestressed housing of the same size.
Regardless of which frame style is used, all frame structures should be designed with 3-D modeling software and optimized by finite element analysis (FEA) to ensure that they can handle deflections and stresses properly. This is particularly important for hydraulic hot-stamping presses that may have off-center loading conditions during the stamping cycle.
In most hot-stamping lines, the process runs through the front and out the back of the press, requiring that the die change occurs through one or both sides of the press. It is important that the press frame be engineered with side openings large enough to allow the dies and any peripherals attached to them to be removed from one side or both sides.
The optimal guiding configuration for a hydraulic hot-stamping press is an adjustable 8-point gib type. A centralized automatic lubrication system minimizes maintenance and ensures continuous lubrication of guiding system components.
The slide should be designed with a high depth-to-width ratio.
When combined with 8-point guiding, this provides a high degree of mechanical guiding during the forming cycle. Replaceable, precision-ground hardened liners should be installed on the guiding surfaces of the press frame. Guideway covers are an option to further protect the guiding system.
Because the heated blank begins to cool rapidly immediately after being removed from the furnace, it is critical that the press close and generate tonnage to form the part very quickly. Automated part loading typically requires that the press be open a considerable amount to allow ample clearance. This large clearance makes it even more critical that the press be able to open and close very quickly. Typically, closing speeds of 500 to 1,000 mm per second are required.
An optimal hot-stamping press should incorporate three main cylinders whereby the flow of all hydraulic system pumps can be directed to the center cylinder during the prepressing portion of the press cycle. This allows for gas springs to be compressed in the tool or the blank to be preformed at lower tonnages and high speeds. The remaining cylinders should be prefilled so they will be ready to press under full tonnage immediately when required.
For the final pressing or holding and hardening phase of the press cycle, pressure across all main cylinders is equalized so that full tonnage is available.
Controlled decompression and opening to complete the cycle is necessary to prevent counterpressure from the tooling’s gas springs, which may cause uncontrolled slide return. Once that step is done, rapid return speeds are necessary to further shorten the overall cycle time. The rapid return (opening) speeds typically are similar to the fast advance (closing) speeds.
The hydraulics and control systems for a hydraulic hot-stamping press should be capable of fully programmable and repeatable tonnage control. This optimizes the process and reduces energy consumption.
The press should be able to produce enough tonnage to form the part and hold/harden it, but excessive tonnage should be avoided. Tonnage applied beyond what is required could cause excess energy consumption and tooling wear.
After optimal parameters are established and saved for each part, the press controls must produce a precise and repeatable cycle to ensure consistent part quality and high production rates.
The press loading conditions should be fully balanced when possible. Off-center loads should always be minimized to the greatest extent possible. However, circumstances may arise that result in limited off-center loads, including the use of several dies at the same time or multipart dies designed to produce multiple parts simultaneously (see Figure 2).
Limited off-center loads can be accommodated by heavy press frames and guiding systems and also by using a deeper slide. Another option is to incorporate a servo-controlled slide parallelism system. Both options can effectively reduce the impact of off-center loads but increase the cost of the press.
It is advisable to discuss potential off-center loading with the press manufacturer in advance to ensure that the press is engineered to handle it.
Many quick die change options exist for hydraulic hot-stamping presses. The most common are T-type die tables, die carts, and rolling bolsters (see Figure 3).
T-type die tables, which can be installed on either side of the press, use a push/pull arrangement to shuttle the die out the press’s side. A separate lifting apparatus then transfers the die onto a separate axis, for example, in a front-to-back direction, where it can then be indexed out of the way to allow a second prestaged die to transfer into the press.
Double-die carts are used the same way as a T-type die table and also can travel away from the press, but on fixed rails. The fixed rails allow access to the side of the press for maintenance and other purposes that otherwise would be unavailable with a fixed-place device such as a die table.
Single or double rolling bolsters can be used; however, a single rolling bolster does not allow for prestaging of the next die to be transferred into the press. Double rolling bolsters require a considerable amount of floor space on either side of the press, because the bolster must exit on one side of the press and the second rolling bolster with the next die must enter from the other side.
If floor space limitations cannot accommodate this, a T-type rolling bolster arrangement can be used that allows both bolsters to exit from the same side of the press in a T configuration. This is the most expensive solution, however, because additional lift, drive, and fixed rail arrangements are required.
The slide should include automatic traveling clamps fully integrated with the main press control system to facilitate quick die changes. Manual clamps are usually sufficient for clamping of the lower die half because they can be removed at a slower pace during prestaging, after the die change has occurred.
A multicoupling-style quick-disconnect system enables rapid disconnection and reconnection of lines required for the process, including die cooling and auxiliaries. These must be disconnected before transferring the die out of the press.
Whichever die transfer option is chosen for the operation, it is important that it incorporate prestaging, which significantly minimizes downtime between die changes.
The last and most important consideration is operator safety. The production and operational details should be discussed to determine the best solution for the application. The approach must meet specific plant safety requirements and all applicable safety codes and practices.
Safety fencing and gates provided with the hot-stamping line must be integrated with the press to ensure that the press does not operate if personnel are in a hazardous position.
The press should feature a slide lock device to secure the slide in the fully open position for die change and maintenance. Die blocks interlocked with the press stop safety relay should be used whenever personnel will be working inside the press for any reason.
Side rollup doors can be used to further restrict access to the press while it is operating. If desired, optional light curtains can also be used on the press sides to protect personnel from press movement while the doors are open for die change and maintenance.
An access platform and enclosure for the press-mounted hydraulic unit should be provided to protect personnel when they are on top of the press for maintenance purposes. This arrangement should include stairs or a ladder to access the platform. A spring-loaded gate at the top prevents personnel from accidentally falling through the opening.
The key variables discussed here are not a complete list, but should cover most considerations. A press builder that has hot-stamping press experience and the ability to customize the design to the stamper’s specific requirements should be able to fill in any remaining blanks.
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