Three approaches to an old problem
June 14, 2005
The stamping environment has been host to numerous attempts at process improvements over the past few years—some very successful, others discarded as unappealing lessons learned.
Stand-alone oil reapply systems are designed for precise oil deposition.
Preparing blanks for today's high-volume, quality-driven stamping operations continues to be a key issue in the quest for quality. Blank preparation is a much more refined process now than the colloquial terms "blank washer" and "blank oiler" indicated in days past.
Blank preparation is an integral part of the production process, so much so that plant personnel frequently resolve die-related problems simply by adjusting the oil-reapply equipment. The adjustments are becoming subtler as stampers continue to reduce the amount of lubrication they use in forming. For example, over the past few years, automobile manufacturers have decreased their film levels from highs of 150 milligrams per square foot to, in some cases, less than 30 mg/ft.2So as attention to die maintenance and fluid reductions increases, so does the demand for more precise oiling systems.
Blank preparation systems have two basic functions in the stamping process:
From the inside, the top and bottom of spray zones can be clearly identified.
A variety of machines are available to stampers for completing these tasks, including combination machines that handle both steps, as well as individual machines that address each operation separately.
A blank typically enters the stamping area coated with mill oil, which is intended as a rust preventive. Because most of this oil is removed during the washing phase, and because most modern blank washers wring off nearly all of the residual washing fluid, little is left to provide lubricity to aid in forming of the panel in a die. Thus, the now clean and dry material has to be relubricated before entering the press.
In the past some stampers approached relubrication by laying down a heavy, uniform layer of oil or a water-based emulsion. Some even flooded the blank to ensure that there was enough lubricity.
The reoiling approach has changed dramatically. At typical prices of more than $13 per gallon, die lubricants represent a major cost component of a high-volume stamping operation. Add to this the cost of housekeeping resulting from excess oiling, and it's easy to see why controlled, precise application is a matter of considerable concern.
This application guide offers a general comparison of LVHP, LVLP, and electrostatic systems to compile your own selection criteria. Ultimately, choosing the right system involves gathering detailed specifications that define your process needs, interviewing suppliers, and visiting their current customers to evaluate the performance and rate of return on their equipment.
Over the years many approaches have been developed to deal with the goals of depositing the correct amount of a fluid in the most effective place—everything from spraying and rolling to the occasional manual application with a rag mop or paint roller. Of these approaches, three systems stand out as being the most popular: electrostatic, high-pressure fluid spraying, and air-assisted fluid spraying (see Figure 1).
In electrostatic reapply systems, the part acts as a grounded target to which positively charged microdroplets of oil adhere. These systems rely on high-voltage, low-current charging of the oil, which is then attracted to the grounded target (in this case, steel blanks) (see Figure 2). This is a sophisticated method that results in a very precise, controllable deposition. It also requires a good deal of floor space, and operators should receive adequate training before running these systems.
Low-volume, high-pressure (LVHP) fluid spray technology uses specialty spray nozzles and high-pressure fluid-pumping systems to spray small droplets of oil. The droplets produced by these systems are not as small as those produced by the electrostatic and air-assisted systems, so users need to take precautions against overspray.
Each reapply system requires routine preventive maintenance to run at peak efficiency. This matrix recommends a general task schedule at regular intervals. The majority of the tasks can be accomplished quickly with proper training and the right tools.
Low-volume, low-pressure (LVLP) fluid spray, also called air-assisted fluid spraying systems, feed low-pressurized oil into a nozzle that introduces compressed air, resulting in a small droplet size that can be controlled to produce a uniform film layer. Some misting occurs, although it has decreased dramatically compared to previous air-assisted systems.
Each of these approaches can be found throughout stamping operations worldwide, and in some cases, more than one approach is used in the same stamping operation on different press lines, depending on the requirements of the application.
While all three systems have benefits, each requires some understanding of how to apply them optimally in a manufacturing environment and what should be expected in terms of operation and maintenance. The following matrixes give some guidelines on proper application and maintenance (see Figure 3 and Figure 4).
A Big Three automaker integrates its reoiler systems into the blank washers to save floor space. The access door to the reoiler is open, showing the washer wringer rolls and spray nozzles.
Having implemented all three methods, a Big Three automaker decided on an air-assisted spray system. For its applications, it found that the air-assisted approach can be more economical in design, maintenance, and operation as compared to the other approaches. Over a five-year period, this OEM deployed electrostatic spray, air-assisted, and high-pressure fluid spray systems (see Figure 5).
All engineering efforts for this OEM's production equipment in the North American stamping plants are coordinated through a centralized company resource. In 2003 the OEM adopted an approach across all the plants to wash the blanks with a water-based emulsion, then reoil them with straight oil. This approach resulted in substantial improvements, such as a reduction in the consumption of fluids, improved part quality, a cleaner working environment, and scrap reductions.