Page 41 - Metalorming Magazine January/February 2023

Conventional Soft Contact Pulsating
20 rpm 5 rpm 20 rpm and one pulse 10 mm off bottom
Al6005/1.0 mm
Fig. 5—Forming results on 1.0-mm-thick Al 6005
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
-0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5
Fig. 6—Slide acceleration/vibration at two different contact speeds
Slide Acceleration [G]
Impact at 5 rpm
Impact at 20 rpm
38.0 40.0 42.0 44.0 46.0 48.0 50.0 52.0 Distance off Bottom [mm]
start of the forming stroke was set to 180 kN and a total of 3 g/m2 of lubricant was applied consistently on both sides of each blank. Draw depth: 40 mm for dual-phase steel and 50 mm for the Gen 3 steel and Al 6005. As a base line, they set forming speed to 20 rpm— approximately 270 mm/sec. or 245 mm/sec. at contact, depending on the draw depth. Later, they used motion profiles such as soft contact, pulsating, dwell and constant speed for compar- ison (Fig. 3).
The first parameter of interest: form- ing speed, or whether a drawing oper- ation should be conducted quickly or slowly. While the common opinion among stampers is to reduce forming speed to improve drawability, results of the tests using the servo press were counterintuitive. For all materials test- ed, researchers consistently observed better results at 20 rpm (245 to 270
mm/sec.) rather than at 5 rpm (61 to 67 mm/sec.). A lower force measured when drawing at 20 rpm supports these results (Fig. 4). Nevertheless, only adjusting speed proved useful only when forming the Gen 3-steel cups 50 mm deep; while drawability increased in all cases, they still observed fractures at 20 rpm when forming DP980 40 mm deep, and Al 6005 50 mm deep.
What About a Pulsed Press Profile?
As an alternative, researchers inves- tigated the use of a pulsating force pro- file (Fig. 3), designed with one pulse at 80 percent of the draw depth. This alternative method proved useful when drawing the Al 6005 cups (Fig. 5) but not DP980. Rather, to successfully form the DP980 cups, a dwell profile worked best, using a 0.5-sec. dwell at 50 percent of the draw depth. Finally, to improve
the drawing process on the Gen 3 steel, a constant- speed profile proved ideal, along with the addition of a soft contact to reduce vibration on the tools and the press (Fig. 6).
While the fundamental principles behind the results obtained remain under investigation, one clear con- clusion appears: Servo- motion profiles can help improve drawability. Never- theless, a tradeoff must be considered. That is: Any of the proposed motion profiles require additional process time, leading to lower pro- duction rates in exchange for
improved material formability and reduced press and die maintenance. Process engineers should compare the costs related to scrap rate, press down- time and tool maintenance versus the costs of a lower production rate.
To keep the production rate as high as possible using servo-motion profiles, it is important to use a servo press with low inertia and capable of quick accel- erations and decelerations. In addition, some of the proposed motion profiles require a press with a special lubrica- tion system that allows for dwell time under certain loads.
Finally, an optimized software is necessary to enable design of the motion profiles with the highest pro- duction rate while minimizing energy consumption. Here, be sure to work closely with your press manufacturer regarding press selection and motion- profile design. MF
38 MetalForming/January/February 2023
www.metalformingmagazine.com