Problems with conventional approaches to forging-die lubrication can be eliminated with a system that uses PLC-controlled positive-displacement pumping systems.
Excerpt from the November 2007 Forging Magazine article by the Forging staff
In the forging process, friction between die and part, consistency of friction from part to part, and consistency of die temperature all affect not only the part quality and part tolerances, but also die life, and hence operating efficiency and operating costs.
Indeed, die life is affected not only by thermal cycling but also by friction between the die and the part being formed, variations of which greatly affect the severity of thermal cycling itself. Die designers base their designs on assumed, given friction. Presses also are designed based on loads caused by formation of parts, and these loads are affected significantly by friction, in addition to temperature and material being formed. Even billet weight is based on estimated material flow which, again, is greatly impacted by temperature and friction.
Friction can be controlled to a great degree by lubrication. While any lubricant's inherent lubricating ability is significant its ability to adhere to the formed parts and tools obviously also plays a critical role in a successful process, for what good does any lubricant do if it is absent where it is needed.
Finally, even if the lubricant used is the best available, but it is not applied effectively, covering the whole surface area of the dies, or is applied excessively, or is not applied at the right time, the process results still compromise part quality, efficiency, and economy.
It is easy to see from the foregoing how choosing the best lubricant, application method, and equipment all are integral parts of the most economical and successful forging process.
In most modern forging facilities, billet weight, billet furnaces, and presses are controlled with high-speed computers, and tool designers use 3D software with simulation capabilities to design dies. In over 95% of forging shops, however, press operators typically adjust and re-adjust their spray application systems, and hence friction, from hour to hour and from day to day, to keep a press going, or to get it going after a shutdown. This activity is thought to keep the press running well, but under closer scrutiny, is found to be ineffective, resulting in loss of production, poor yield, and significantly reduced operating profits. When one asks, why are the presses down more than running , the normal answer is, "We are adjusting for over-fill or for under-fill, fixing the lubricating system, grinding dies up, or changing worn-out dies."
To prepare the lubricant, many forgers simply guess the quantity of raw lubricant to dump into the mixing tank when it is running low and likewise guess the amount of water to add on top. If the lubricant is not mixed for an adequate time period, the heavy, raw lubricant from the bottom of the tank is used first, resulting in excessive consumption of lubricant solids with the applications of the batch and inadequate solids during the balance of the batch.
A lubricant's ability to adhere is dependent externally on die/part temperature and inherently on lubricant formulation and lubricant dilution ratio or solids level. The less the lubricant is diluted, the better it will adhere. If a lubricant with too high solids (low dilution ratio) is applied, and especially when dies are cold, the lubricant builds up on the dies, resulting in under-fill. If a lubricant with a too-high dilution ratio is applied, there are not enough solids and the lubricant will not adhere and dies overheat.
Any variations in lubricant mixing or in dilution accuracy will cause changes in lubricant's ability to adhere and, as a result, also in friction.
Typical application systems today consist of a pressure-pot or diaphragm-pump system used to deliver lubricant into a lubricant manifold that feeds several lubricant lines with spray nozzles or flooding pipes. Each line has solenoid valves installed on the manifold, controlled by timers used to adjust the spray or flooding time. The same approach is used on the air side; the source of air is divided into multiple lines controlled by solenoid valves with timers.
With these types of pressure-operated systems, the application quantity, or nozzle output of lubricant, is affected by any variations in the following: lubricant viscosity (see dilution ratio, above), plant air pressure feeding the pressure pot or diaphragm pump, set pressure of the pressure pot or pressure pump, lubricant line and nozzle conditions, and application system settings.
In addition, if air-atomizing nozzles are used with pressure-operated systems, the air and lubricant mix together in the valve body or nozzle body and atomizing air pressure will greatly affect the lubricant output through the nozzles. If the atomizing air pressure is too high, the resulting back pressure will fight against the lubricant pressure, resulting in an uncontrollable and unknown quantity of lubricant being applied.
Typically when more lubricant is required, operators adjust spray time. Alternately, they may adjust pump pressure and lube flow through needle valves, and then manually adjust atomizing air to match the lubricant flow rate. If a timer is used to close one line/nozzle earlier than others using the same pump or pressure pot, there will be a proportionate increase in the lubricant flow-rate through the other nozzles, making the lack of control even worse.
Additionally, if operators are restricting the lubricant flow to one of the dies using needle valves, the reduction in flow to that die will be transferred to other dies. This change is often routinely ignored.
Finally, increasing the lubricant amount applied by extending the spray time affects press cycle time negatively.