Evaluating film lubricants for stamping

Deep-drawing test makes the most sense

STAMPING JOURNAL® JUNE 2007

June 12, 2007

By:

The ERC/NSM conducted extensive deep-drawing tests to evaluate various dry film lubricants for several companies. Compared with other commonly used friction tests such as draw bead and twist compression tests, deep drawing emulates realistic friction conditions that exist in stamping operations in terms of the pressure and forming speed interface .

Deep drawing diagram figure 1

Figure 1

In today's stamping plants, dry film lubricant (DFL) is gaining acceptance for its uniform application on the material surface and the possible elimination of washing operations after forming.

Center for Precision Forming (CPF) of The Ohio State University conducted extensive deep-drawing tests to evaluate various dry film lubricants for several companies. Compared with other commonly used friction tests, such as draw bead and twist compression tests, deep drawing emulates realistic friction conditions that exist in stamping operations in terms of the pressure and forming speed interface.

Several systematic evaluation criteria were developed for the deep-drawing tests conducted at the CPF:

  • The maximum drawing load attained (the lower the load, the better the lubricant)
  • Measurement of draw-in length, Ld, or perimeter at the drawn flange (the larger the Ld, the better the lubricant) (see Figure 1)
  • The maximum applicable blank holder force (BHF) without any fracture in the cup wall (the higher the maximum BHF, the better the lubricant)
  • Evaluation of surface topography under a microscope

The dimensions of the tooling used in the CPF study were as follows:

  • Punch diameter = 6 inches
  • Die diameter = 14 in.
  • Punch corner radius = 0.787 in.
  • Die corner radius = 0.591 in.
  • Initial blank diameter = 12 in.
Test Lubricants
Lube Name Lube A Lube B Lube C Lube D Lube E
Lubricant Type Dry Film Dry Film Dry Film Dry Film Wet
Physical State Solid Solid Solid Solid Liquid
Coating
222~318 mg/ft.2
N/A

Figure 2

The BHF was provided by CNC hydraulic cushion pins. The press ram moved down the blank holder and the die while the punch was stationary.

Sheet blanks used in the tests were coated with selected dry film lubricants. No additional lubricant was used during deep drawing. For comparison purposes, a general-purpose, wet-type stamping lubricant (Lube E) was sprayed on some of the nonlubricated samples. Detailed specifications of the tested lubricants are shown in Figure 2.

Evaluation of Lubricants Based on the Maximum Punch Force

Most lubricants were tested using two different ram speeds (0.56 and 2.56 inches per second) at three different BHFs (30, 50, and 80 tons). The load-stroke curves for various lubricants tested at a BHF of 30 tons are compared in Figure 3. Lube A gave the lowest punch force, while Lube E gave the highest.

To determine how various lubricants perform under more severe, higher-pressure conditions, the BHF was increased to 80 tons. As the BHF increased from 30 to 50 tons, sheet specimens coated with Lube E and Lube D fractured during deep drawing at both test speeds. Figure 4illustrates a comparison of load-stroke curves between fractured versus formed samples. Samples of successfully formed and fractured cups are shown in Figure 5.

In Figure 6, the measured maximum punch forces are compared for the different lubricants tested. The maximum punch force increased slightly with increasing BHF, because the normal pressure between the sheet flange and blank holder increased, thus slightly increasing the friction.

Lubricant test results figure 3

Figure 3 Most lubricants were tested using two different ram speeds (0.56 and 2.56 inches per second) at three different BHFs (30, 50, and 80 tons).

Samples with Lube D and Lube E showed fractures when the BHF was 50 tons. Lube A performed best, regardless of the magnitude of BHF. About half of the tested samples coated with Lube B and Lube C showed fracture at a BHF of 80 tons with a high test speed (see Figure 6), although both lubricants performed well at a BHF of 80 tons with a slow test speed. The lubricants were ranked based on their ability to allow fracture-free drawing with the largest BHF.

Evaluation of Lubricants Based on Draw-in Length

Several geometric parameters can be used as a friction indicator. In this study, the draw-in length and the perimeter of flange area were used to differentiate the performance of lubricants.

As shown in Figure 7, regardless of the test speed, Lube A showed the longest draw-in length, which means it provided better lubrication. The ranking of lubricants coincided with the ranking determined using the maximum punch force as a criterion.

Determination of Friction Coefficient for Lubricants Tested

Using commercial finite element method (FEM) software, researchers calculated friction coefficients for tested lubricants by matching the draw-in length and maximum punch force obtained from simulation and experiments (see Figure 8). The material properties of A1011 DS Type-B steel were determined by the viscous pressure bulge (VPB) test at the ERC/NSM and used in FE simulations.

  COF
(at BHF 30 tons)
COF
(at BHF 30 tons)
COF
(at BHF 30 tons)
Lube A
0.0
0.0
0.05
Lube B
0.07
0.07
0.07
Lube C
0.07
0.07
0.07
Lube D
0.15
>0.1
Lube E
0.15
>0.1

Figure 8

This study illustrated that the deep-drawing test can evaluate the performance of stamping lubricants under near-production conditions.1The friction coefficients also can be calculated with the aid of FE analysis.

Hyunok Kim is graduate research associate, Ji Hyun Sung is graduate research associate, and Taylan Altan is professor and director of the Center for Precision Forming, 339 Baker Systems, 1971 Neil Ave., Columbus, OH 43210-1271, 614-292-9267, www.cpforming.org. The authors would like to acknowledge the technical support provided by Randy Kirkland (rkirkland@hannasteel.com, 205-783-8214) and Tim Nichols (tnichols@hannasteel.com, 205-783-8367) at Hanna Steel Corporation, Fairfield, Ala., throughout the project.

Note

  1. H. Kim, J. Sung, R. Sivakuymar, and T. Altan, "Evaluation of Dry Film Lubricants Using the Deep Drawing Test," Report No. ERC/NSM-05-R-16, The Ohio State University, Columbus, Ohio, 2005.
Formed cups figure 5

Figure 5



Hyunok Kim

Graduate Research Assistant
Engineering Research Center for Net Shape Manufacturing
339 Baker Systems
1971 Neil Ave.
Columbus, OH 43210
USA
Phone: 614-292-9267

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