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.
After all adjustments are made, the press can be operated; however, if lubricant viscosity or solids change, or if any dirt gets into the lubricant lines, needle valves, or nozzles, or if lubricant starts drying in the nozzle tips, lubricant flow rates will change accordingly.
Depending on shop quality-assurance practices, several hundreds of substandard parts are sometimes produced due to a partly or fully plugged nozzle not being noticed immediately by the operator. In order to correct or compensate for changed flow, the operator must adjust settings or shut the press down to clean the lines and/or nozzles. While this is being done, the dies cool down. Then, the operator must run the press until the dies heat up back to normal operating temperatures to find out if the setting corrections were appropriate. If not, the adjusting starts all over again.
Lack of tools for operators
With typical application systems, operators have no tools to work with die cooling and lubrication. They adjust flow rates and timers to get production running. However, they cannot record their adjustments or know the effect of their adjustments in actual flow rates per actual die. When the next operator starts his shift, he starts with existing settings. When conditions change in lube solids, back pressures build up in the lines, dirt gets into the application systems, etc, the new operator starts to learn what he started with.
Any changes in lubricant solids, die temperatures, or lubricant application quantity result in changes in friction, part tolerances, part quality, and die life, yet operators are not provided tools to control them.
Typical solution — Due to the lack of controls in pressure-operated application systems, and due to the inability to apply high-solids lubricant in small, controlled quantities to make the lubricant adhere in higher die temperatures, forgers simply flood the dies with highly diluted lubricants. Since highly diluted lubricants do not adhere at high temperatures, operators resort to applying large quantities of liquid in hopes of fixing the problem, resulting in additional related problems of low die temperatures, high thermal cycling of dies, and, consequently, reduced die life. Also under the above conditions, the forger faces high levels of pollution and even waste of lubricant.
Normal end result — Due to flooding practices in some shops, the lubricant delivery truck arrives once a week, and so does the pump truck to remove from the pit the 80-90% of the total lubricant previously purchased, while at the same time maintenance people continue to clean the shop and machinery, and unplug and re-build lube systems. Billet weight often is increased to compensate for variations in material flow caused by variation in friction, increasing billet and machining costs.
Some shops reuse the lubricant from the pit. This lubricant contains abrasive scale, plus different types of hydraulic oils and unknown levels of solids and additives, making it not only inconsistent in performance, but in some cases the cause of high die wear due to the presence of iron oxide in scale.
In most forging processes, it takes only about 5-10 grams of dry, high-quality graphite-based lubricant per square meter of die surface area to provide an adequate lubricant film on typical dies (0.02 to 0.04 oz/ft2). When the dies change color from shiny to gray, approximately 5 grams/m2 of graphite solids. When converted to liquid lubricant by diluting 18% starting solids by a ratio of 8 to 1, the application quantity of liquid is 250 grams to 500 grams/m2, or 0.9-1.8 oz/ft2.
Yet, most forgers apply many times this amount, resulting in 80 to 99% waste of lubricant.
(It is easy to check how much lubricant one is wasting each month: calculate in square feet the average surface area of dies used to make a part; multiply this figure by parts produced per month; multiply this figure by 0.04 oz of graphite solids per square feet; and multiply this figure by 100/18 if lubricant is supplied with 18% solids. This gives total consumption of die lubricant required to produce the parts in optimal conditions, still with 50% waste.) *
In order to lower the lubrication cost, some forge shops are testing different types of lubricants. However, to differentiate between the different types of lubricants, it is important that the lubricant solids and mixing are controlled properly during testing. In addition, when comparing the performance between different lubricants, it is important to measure the application quantity of each lubricant. Switching to a higher-performance lubricant will help specifically because it will be more forgiving in terms of inaccurate diluting and mixing.
However, if one cannot control the lubricant solids, lubricant mixing, lubricant flow rate to each individual die, and lubricant atomization, the optimal benefits cannot be realized solely through the use of high-performance lubricants.
The cost of a lubricant and its effectiveness is affected directly by the graphite-particle size and additives used. As the graphite-particle size decreases, more of the lubricant can be diluted and the performance of the lubricant improves, but the production cost of the graphite also is higher. Higher-purity graphite also provides better lubricity and lower pollution, but it costs more, too.
There are thousands of additives that are available to a chemist. Among them are high-performance additives that are environmentally safer, but more expensive. There is no cheap, good lubricant sold in any industry regardless if they are graphite-, oil-, or synthetic-based.
When forge shops push lubricant prices down to save money or due to inconsistent results in lube tests, the lubricant producers must push down the cost of raw materials to stay in business. This results in negative benefits to both parties, particularly to forge shops. See Table 1.
The most important factors affecting operating profits are efficiency, quality, yield, and cost management. Having these factors under control should enable forgers to keep current customers and acquire new ones. As shown in Table 1, die-temperature control and control of friction through lubrication greatly affect these all important goals.
In the European Union, a rising number of forging shops and their customers are demanding total process control for increased quality assurance and operating efficiency, and are installing high-accuracy, fully automated lubricant application systems connected to robotics and controlled by PLCs and computers.
The cost of a lubricant, when properly analyzed, and when applied only in sufficient controlled quantities required by the process, is a relatively insignificant element in the total cost of forging operations.
The same can be said about the cost of application equipment. Operators that lack the "tools" are adjusting the most important variables blindly from day to day, losing production time, causing premature die failures and changes in part tolerances, and decreasing overall quality.
The payback from using high-performance lubricants together with high-accuracy PLC-controlled application equipment is realized through a 50-80% reduction in lubricant consumption and a 10-15% improvement in productivity and die life, together with a significant reduction in rejects and resulting re-runs. Considerably reduced pollution contributes additional savings. See Table 2.
The CMT solution
To overcome these problems, CMT started a development program five years ago to provide new technology for the forging industry to reduce pollution and lubrication cost while improving operating efficiency, operating cost, and quality. Several lab tests were done with different types of lubricants, pumping systems, and nozzles. Some field tests proved negative, but in the end a new conceptual design was created.
The main thrust was to design a system that overcomes the above-known shortcomings of pressure-operated systems while keeping the cost and practicality of the system in control.
In the past CMT systems were built in Europe, but today to reduce the costs significantly, the manufacturing is done in the US using mostly commercially available parts.
The new system design:
a) Lubricant is automatically mixed and diluted properly to selectable solids levels.
b) Lubricant is filtered and re-circulated to prevent system problems.
c) Nozzles and nozzle pipes are washed after each spray cycle to prevent lubricant build up and nozzle failures.
d) Nozzles are pre-pressurized for instant spray fan and instant set flow rate.
e) Instead of using expensive lubricants to cool the dies, an adjustable quantity of well-atomized water is delivered by individual PLC-controlled positive displacement pumps.
f) Patented, PLC-controlled positive-displacement pumps, specifically designed for abrasive materials such as graphite, are used to deliver to each individual die a controlled and adjustable quantity of lubricant. The flow rates of the positive displacement pumps remain consistent regardless of lubricant viscosity, plant air pressure, nozzle or line back pressure, or atomizing air pressure.
g) Lubricant solids and die-lubricant quantity, along with die-cooling water quantity, remain consistent for each die at all times. Instead of using delicate airless nozzles or other expensive nozzle arrangements with wear and other issues, the new system can be connected to existing nozzle pipes or inexpensive, disposable nozzles without fear of nozzles plugging up.
h) A monitoring system provides the following alarms: lubricant "tote" empty, low lubricant level in holding tank, pressure in any lube line exceeding preset limits, general alarms as specified by the customer, and, in case of power or signal failure, shutting the press down to prevent damage to the dies, and production of bad parts.
i) All adjustments are done from the monitor screen or directly from the press PLC, eliminating all manual adjustments and guess work.
j) Due to total monitoring and use of PLC-controlled, electrically driven positive-displacement pumps, the established optimized setup can be recorded and recalled when returning to specific parts, reducing change-over time.
CMT uses patented PLC-controlled, electrically driven positive-displacement pumps designed specifically for use with abrasive materials, such as graphite-based lubricants. Standard gear pumps or typical positive-displacement pumps cannot be used due to severe wear caused by graphite particles.
In order to test the CMT system, to make it operator friendly and practical for operations, and to adapt the system to short-cycle-time presses with high output of different demanding part shapes, CMT worked closely with the well-established, high-quality, forge shop Impact Forge Group Inc. Pooling their staff and expertise, CMT and Impact developed the optimal system design that uses a part of the existing air and water valve arrangements.
The first CMT system was installed in June 2006, and the second and third in July, with the fourth commissioned in September.
As a result, the lubricant consumption and expense, as well as pollution, was reduced by approximately 80%. The total control and consistency in application conditions of water and lubricant for each die eliminated all operator guesswork and the need to continuously adjust and re-adjust the application system.
Consistency in die cooling and application of lubricant provided stability in die temperatures and friction between the part and die. That resulted in improved quality, tolerances, and die life, thus improving the overall operating profits.
Clean, controlled application of lubricant, together with controlled die cooling, allowed them to use of the best performing and environmentally safest lubricants to improve the overall operations.
Lubrication expense and pollution can now be reduced significantly, while frictional conditions can be controlled to provide consistent quality, part tolerances, and operations.
Operators and/or supervisors now have an application system with total control without manual guesswork, and all optimized application settings can be stored and later re-called for added day-to-day operating consistency.
|Type of graphite
|Particle size distribution
|Table 1: Factors affecting the cost and performance of lubricants.
Raimo Peltoniemi is president of Coating Management Technologies, Charlotte, NC. CMT is grateful to Impact Forge Group Inc. and Terry McInerney for the support and expertise they provided in making this new principle and system available to the forging industry. For more information, contact [email protected], or call 704-895-3796.