Graphite die lubricants had dominated forging applications for centuries. Changing industry needs and the drive toward safer, cleaner shops drove the development of synthetic lubricants. New polymeric lubes are now showing promising results in an assortment of applications.

In closed- and impression-die forging, the die typically amounts to 10-15% of the total forging cost. It makes sense then that any improvement in die life would improve productivity and profitability in the forge shop. 

An exclusive new lube formulation developed for use in the forging industry makes it easier to handle and eliminate many common defects and problems encountered with graphite die lubricants in a typical forging operation, such as dirty floors, broken electronics, plugged sprayers, and in-cavity and ex-cavity buildup.

A number of savings have been proven in the process. These include:

• No in-cavity buildup
• Little to no ex-cavity buildup
• Significant housekeeping savings
• Reduced cleaning of uniforms
• Reduced floor scrubbing and cleaning
• No agitation of concentrate or dilution
• Dilution control
• Polymeric can be used in a centralized dilution system, eliminating spray “carts”
• No plugged sprayers
• Reduced pump maintenance

History of Graphite Die Lubricants

The use of graphite as a lubricant easily predates the industrial revolution and has been used for millennia. Records of its use as a lubricant date back well over 400 years to the reign of Elizabeth I in the making of cannon balls that reportedly flew straighter and farther. Synthetically made graphite dates back to the time of Thomas Edison and the need to produce tungsten wire used in the incandescent light bulb. 

The use of water-based graphite is now the industry standard because the use of oil-based graphite continues to be a fire hazard at many forge shops. Nearly all graphite die lubricants used yesterday are very similar to those formulations currently in use. The same benefits of using graphite remain, while the same problems tend to occur wherever graphite is used.

Graphite Separation Issues 

Graphite forging lubricants are normally supplied as a concentrate that is diluted prior to use. Dilutions of the common water-based graphite lubricants vary from 1:1 to 1:10 on average. Graphite’s density is around 2 g/ml – more than twice as heavy as water. This significant difference in density results in graphite falling out of solution before use. 

This results in one of the biggest difficulties in using graphite – maintaining a consistent suspension of graphite in both the concentrate and dilution. Normally, the concentrate uses stabilizers to sustain a homogeneous mix of graphite throughout. When diluted, however, these stabilizers are at too low a level to assist in maintaining the mix’s homogeneity. 

This separation problem seen with dilution samples has a significant impact on the forging process. Fluid distribution lines or spray nozzles that are not constantly mixing and left stagnant will quickly clog up and become unusable. Some of these spray systems have an automatic water flushing system built in so that the spray nozzles can be flushed when not in use. 

Graphite Buildup

Graphite is well-suited to withstand the extreme temperatures of forging because of its high decomposition temperature of 3500°C (6332°F) versus the melting point of steel at 1370°C (2500°F). Graphite provides a solid film between the die steel and the forged part. The physical barrier explains why graphite provides lubrication, but this can cause buildup issues both inside and outside the die cavity. In-cavity buildup causes issues with the part dimensions and with identifying part-number lettering, which can result in part defects and customer returns. If the in-cavity buildup causes issues, the forging process must be stopped and the die cavity wire brushed to remove the graphite and get the parts back into the specification. 

Graphite is also an excellent electrical conductor and is typically used as an anode for lithium ion and alkaline batteries. Graphite’s superior electrical properties can wreak havoc in forge shops with electrical components such as computers and switches, causing shorts within the system. 

Ex-cavity buildup causes issues in the surrounding environment with black graphite getting into clothes, skin, hair and being tracked throughout the plant and into workers’ cars and to their destinations. Ex-cavity buildup may also lock in bolts and holder blocks, making them difficult to remove without drilling them out. 

Synthetic Lubricants

Decades ago, one solution to the graphite problem was the development of synthetic lube formulations. These helped meet the industry’s changing needs by eliminating graphite buildup and improving die life. The result was a cleaner and safer working environment. 

More recently, a new and distinct class of lubricants – polymeric lubricants – has been developed and is available for commercial use.

Polymeric Lubricants

The significant differences between polymeric lubricants and the older category of synthetic lubricants are as follows:

• Initial lab development focused on creating a barrier film without the solid-film theory of lubrication, unlike graphite or synthetics. Eliminating the use of any solid lubricants in the formulation should naturally prevent buildup, make the application of the die lubricant much easier and facilitate the use of automatic spray equipment.
• Polymeric lubes form a protective film, covering the die steel and providing release and lubrication at typical die temperatures of 250-700°F (121-371°C).
• Several lab studies demonstrated that polymeric lubricants have the ability to lower the surface tension of water at typical hot forging-die temperatures, unlike the synthetic or graphite lubricants. The relative heat-flux capability of a polymeric lubricant is 300% higher than graphite die lubricants, allowing not only significantly better film formation at higher temperatures but also controlled cooling of the die steel.
• The polymeric film has been shown in some studies to provide 400% more lubrication at 500°F (260°C) than graphite or synthetic using the ring compression test.
• Die life is a major concern among forgers. Maintaining a consistent die temperature is one key to long die life. Consistent die temperatures, typically in the range of 400-600°F (204-315°C), improve die life because this is where the die steel has its highest impact strength and hardness.

Polymerics Designed for High-Volume Precision Forging

Results from the commercialized installations using polymeric lubricants support the fact that these lubes are well-designed and well-suited to high-volume precision forging. Use of a polymeric lube can potentially result in a net decrease in cycle times based on the advanced chemistry of these systems. With the ability to maintain a more consistent die temperature, customers should expect to increase die life as it relates to overheating. Other alloys tested with the polymeric include titanium, aluminum and stainless steel. Testing will continue to expand the scope of this new technology and offer better ideas as to which processes will be best-suited to polymeric lubricants.

The biggest difficulty encountered when testing polymeric products is the absence of any visible coating after application. There is no white or black coating after spraying. Customer concern about a visible coating on the die caused alarm for die wear. This was after evaluating the polymeric lube successfully for over six hours, producing quality parts (checked each 30 minutes), and confirming with the plant manager and tool shop that there was no excessive die wear. In fact, the tool shop suggested they have never seen a better looking die after that many shots.

The polymeric requires more attention to application in the hot areas of the die and does not work well at temperatures below the boiling point of water. A fair amount of polymeric application training is necessary for those accustomed to applying other lubes.

Conclusion

The forging industry’s shift away from graphite lubricants has resulted in the application of polymeric lubes to lubricate dies and enhance die life. Polymerics have proven suitable to high-volume precision forging processes and have improved die life.

Research and testing continue as developers gather as much die wear data as possible. It’s clear that numbers are difficult to obtain because failure of a die has many facets, from poor die steel or construction to improper setup in the machine. 
 

Sidebar 1

Case Study: Hot, Closed-Die Forging Company

Field testing was done at a Midwestern hot, closed-die forging company that supplies a variety of steel forgings to markets requiring carbon, alloy, micro-alloy and stainless steel forgings. The company has the highly coveted TS16949 quality designation. 

The manufacturer currently uses a synthetic-type product containing graphite. They had moderately good control over their spray process but still had difficulty keeping the graphite from clogging their auto sprayers. However, their preventive-maintenance system was very well-designed, and they cleaned their spray systems each night with a water flush. Their dilution tanks got a thorough cleaning once per week due to graphite precipitation. They were not checking their dilution ratio because they had no means to do this effectively in the plant.

A quick check using the polymeric lubricant and a hand spraying technique (not the automatic system they normally use) was initiated, and test results were good. A longer, one-day trial using our portable system on their auto-spray unit showed that the polymeric lube was at least equal in performance to the graphite. 

Based on the significant benefits of running a product without graphite, they decided to switch to our polymeric lubricant, and the results have been excellent. This has also provided an opportunity to clean up their spray system with new lines and pumps that are not clogged with graphite. The benefits realized are as follows:

• A significant improvement in part dimensional accuracy compared to the previous lube
• A significant reduction in tonnage requirements needed to produce the parts
• No hourly maintenance on the press to keep the auto sprayers working and no cleaning of the dilution tanks
• Consistent dilutions (±2% of target) using an automatic dilution system
• Clean-running dies
• No in-cavity buildup
• Clean plant and clothing
• Appears to be a net increase in die life
 

Sidebar 2

Case Study: Forged and Machined Axle Manufacturer

A Midwestern forger that supplies forged and machined axle shafts serving the commercial vehicle, off-highway and farm equipment industries also participated in a trial. The company uses an upsetter process and was using a synthetic die lubricant. They had difficulties with the lubricant contributing to in-cavity buildup, resulting in a difficult-to-remove, cement-like residue. In addition, release was difficult, and the synthetic required significant maintenance on their pump system and sprayers due to clogging. We used our portable equipment to plug into their automatic spray system and began testing. Release was far superior to the synthetic lube, and the polymeric lube provided a significant benefit to their process as follows:

• 300% less lubricant is used
• Consistent dilutions (±2% of target) using an automatic dilution system
• No plugged sprayers
• No cleaning of dies during a production run
• No supervisor needed on a daily basis to maintain spray system
• Reduced maintenance calls to fix broken pumps
• No in-cavity buildup seen with the polymeric lube, reducing rework and scrap (60% decrease)
• Reduced tool-room time for routine maintenance by eliminating white buildup on dies and holders
 

Author Tom Camel is the technical manager of Cross Technologies Inc., Westland, Mich., a producer of chemical specialties. Camel may be reached at 248-697-8419. Author Joe Star is the sales manager of Cross Technologies. Star can be reached at 248-891-7091. Cross Technologies provides solutions to industries around the world since 1958. Product lines include specific lubricants and releasants serving the metal, rubber, and urethane and plastics industries.