How Lubrication Strategies Affect Changeover Time
Excerpt from the article in the September 2012 issue of Stamping Journal by Troy Turnbull
To survive, a person’s body relies on getting fluids to the right places at the right time, and the same goes for the stamping process and die lubrication. When should the lube turn on, and for how long? Does the process require flooding, split-second lubrication, or minimal lubrication? The list of variables goes on, and if a technician overlooks any one of them, he may produce rejected parts, damage tools, and cause lost production time.
Stamping requires a system to distribute the die lubricant mixture accurately, thoroughly, and economically with the proper pressure and volume to supply fluid consistently to each spray nozzle. Stampers must ensure nozzles are positioned properly and the lubricant mixture mixed correctly, with proportions tailored for the application. Finally, stampers can find ways to optimize changeover to reduce setup time and, just as important, the possibility for press setup error.
Die Lubricant Selection
When running a takeover tool or a new product, shops should always discuss process requirements with their die lubricant supplier. In a job shop, every takeover tool might have specific customer requirements when it comes to lubricant application. Automotive companies, for instance, have specific, approved fluids for their tools and those are the only fluids operators can run for those jobs.
This becomes especially critical when stamping parts that will be painted or plated. Certain lubricants cause extreme problems during plating or painting, so in these cases, parts washing needs to be considered in the per-piece price. Certain equipment may not be able to spray or roll-coat in an area needed for the application or mix the lubricant to the proper dilution ratio. A die lubricant supplier should be well-versed on the metallurgical forming characteristics as well as the painting and plating of the required metal.
Multiple-metal lubricants can be a worthy investment for operations with frequent changeovers. In some cases, the same lubricant can be applied to hot-roll, cold-roll, and even high-strength steels, just at different dilution ratios. Of course, other job-specific lubricant variables—like fluid pressure, application method, and lubricating locations—still apply.
The Art of Dilution Ratios
All lubricants are not created equal, which is why stampers should never use generic or homegrown lubrication concentration charts. Each lubricant has a specific viscosity and level of solids. Even at identical concentrations, two different lubrication grades will produce different results on a refractometer. That’s why, to establish proper concentration levels, setup personnel must use the concentration chart supplied by the die lubricant supplier.
Some steels may run better than others with a certain lubrication product, which is why establishing the proper dilution ratio is so important. The supplier’s concentration chart is the right starting point, but some design of experiments (DOE) may be required, depending on the application.
Some lubricants may cause staining or a white chalk film, which might not be acceptable. Different material stock of the same thickness may have different forming, blanking, or fineblanking characteristics. Die age and wear can play a role here too. In many cases, determining the precise dilution ratio becomes more of an art than a science.
Basic QDC Considerations
Lean manufacturing has become a major focus of so many modern stamping businesses. Many companies must maintain process controls for short- to medium-volume production jobs. And even with fully documented process datasheets completed and available, stampers can overlook certain process variables. These can include many variables in quick die changeover (QDC), such as lubrication.
If the next job on a press requires a different die lubricant, a stamper should purge the nozzles’ spray lines with air, water, or a cleaner. This becomes especially critical when an operation has in-die lubrication points buried in the die steel. If technicians don’t purge spray lines adequately, this may cause clogs and, ultimately, several hours of downtime.
A shop can install simple equipment that will purge the lubrication system automatically before the die changeover. Installers can wire such equipment directly to the press controls, and time it so that it purges when the press is down for a certain period. Or, personnel can activate the system with a selector switch to clean the spray lines before they remove the die from the press.
Straight oil and water don’t mix. So what if an operation requires a change between a water-based and straight-oil lubricant? The oil will float to the top, and the leftover water-base lubricant remains at the bottom of the tank. As the stamping operation starts, the system draws the water-based lubricant from the bottom of the tank, and not the straight oil the application requires. This is why it’s ideal to have separate holding reservoirs for different die lubricants and mixture ratios (see Figure 1). This helps eliminate the possibility of incompatible die lubricants contaminating each other and plugging the spray system.
Once the die lubricant is changed over, technicians should check the lubricant concentration before each production run to ensure they have the proper dilution. They can use a refractometer or a moisture analyzer machine that determines the percentage of solids in a lubricant. And again, they must compare the reading with the chart supplied by their lubricant supplier.