Differences Between Tryout and Production Conditions

October 1, 2019


Along with many other parameters, a stamper specifies maximum die weight, shut height and specific quick-die-change provisions to ensure that the tool fits within the press-line infrastructure. But even this communication may not result in quality parts as the product moves from build to production.

A good practice is to conduct a dimensional-layout study during pre-production runs prior to tool buyoff at the construction site, and use a statistically significant number of randomly selected sample parts to validate that the product meets print specifications and tolerances. Ensure that the tool-construction source produces these parts while the process runs in a continuous mode for a minimum specified time or rate, perhaps 30 strokes/min. for at least 10 min. Do not accept data on a part produced from a single stroke.

Despite diligence on the part of a stamper during tool development and buyoff, numerous factors as described below can influence success when producing parts at production rates.

Material: Temperature Effects

Depending on the sheet metal and forming system, the 10-min. run-at rate performed during buyoff may not be long enough to display the effects of heat buildup. Higher-strength sheet metal grades tend to become noticeably hotter with increasing contact over critical radii. Lubricant additives designed to improve metal flow may not be active at these elevated temperatures, thus worsening the problem. Differences in local metal flow change the strain distribution, leading to increased springback. Direct contact with the die surface may result in scoring as well as permanent tool deformation, as the hardness of some advanced high-strength steels approaches that of tooling.

Material: Simulation

Perform forming simulations using realistic inputs covering the full range of properties that you might receive based on the specific sheet metal grade ordered. Ensure appropriate distribution of binder and ram tonnage at the four press corners during simulation, tryout and production, and avoid overloading at any one corner of the press. Evaluate the impact of allowable thickness tolerance on clearances and forming. Software allows for simulation of the manufacturing process as well, providing the ability to check for interference at production speeds.

Process: Blank Edge

If a developed blank is needed for a part running with line dies, the blank die may be the last to be completed. Laser cut blanks, often used when blanking-die construction is not complete, allows for adjusting the blank contour between runs to accommodate lessons learned in tooling tryout. But blank-edge quality differs between sheared and laser-cut edges. Cut consistency also differs, with sheared edges affected by knife clearance, alignment and sharpness. Be aware of methods used to create the blanks in tryout and production, and the associated characteristics of the blank edges.

Process: Lubricant

Lubricant type, application method, amount and distribution all influence metal flow. Differences can be expected when comparing hand-applied shop lubricant during tryout with application methods used in production.

Ensure that the die-tryout shop uses production-intent lubricant, even if they cannot use a production-intent application method. Guide the tryout shop on acceptable and non-acceptable methods. For example, when comparing use of a rust-preventive oil in a spray bottle against wiping a vanishing compound on each blank, one approach may better represent production plans. Note that neither approach lends itself to generating run-at-rate panels.

Process: Destacking and Transfer Between Stations

Hand-loading blanks during tryout may be easier in some cases than the automatic destacking from a lift of blanks and the subsequent transfer between stations in production. When running in automatic mode, lubricant tackiness may increase the risk of lifting multiple blanks simultaneously. Remember that not every sheet metal is magnetic.

Resting softer metals on conveyor belts increases the risk of underside damage and print-through, due to foreign materials on the belts coming in contact with the blank or formed part. Transfer bars or fingers may be required for transfer between stations. Ensure that all trim scrap can fall free of each die at production rates with no risk of interference or scrap backup.

Press Equipment

Because the industry overall has sufficient knowledge of the differences between hydraulic and mechanical presses, those differences will not be covered here.

Be aware of differences in derated tonnage (tonnage available at a specific distance off bottom) between the tryout and production presses, and consider press-energy characteristics when working with high-strength material.

Stamping higher-strength sheet metal increases reverse tonnage (snapthrough) severity. Tryout presses may have greater capacity to accommodate a wide variety of projects. The production press and process must have features to minimize the impact of greater snapthrough tonnage. Over time, unless steps such as placing shear on punch faces and staggering engagement heights are taken, press alignment and durability can be affected

If you have questions or would like to see specific topics covered in future columns? Let me know at ScienceOfForming@EQSgroup.com. MF

Industry-Related Terms: Blank, Corner, Die, Forming, LASER, Ram, Scrap, Shut Height, Surface, Thickness, Tolerance, Transfer
View Glossary of Metalforming Terms

 

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Training

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