Film Theory
Firstly there are different thicknesses of lubrication that can be applied. They are the 1) thick-film lubrication, 2) thin-film lubrication, 3) mixed-film lubrication, 4) boundary-film lubrication and 5) solid-film lubrication.
In thick-film regime, the lubricant completely separates the workpiece and the tool. As the film gets thinner, the influence of the bulk properties lessens and the surface-lubricant interaction begins to become more important. (i.e. the lubricant with its additives is able to form chemically-formed bonds that can also helps to lower the friction) Full film lubrication that means the whole workpiece to be FULLY covered with a layer of lubricant is hard to reach for most practical scenarios. Hence boundary or mixed-film lubrication with the help of additives is better.
Operating Parameters
The metalforming process is dependant on several factors:
1. Contact macro geometry
2. Load (contact force, tool to workpiece)
3. Speed
4. Environment (air, moisture etc)
5. Lubricant properties
6. Contact micro geometry (surface finish)
For hot-rolled steel, the type of surface is normal surface. It tends to hold lubricant that is applied to them. They do not need any special wetting or polarity agents to obtain sufficient lubricant.
If non-metallic surfaces are coated like vinyl paint, lacquer, or other organic coatings, lubricants used must be compatible and clean and not cause the surface coating to peel, blister, blush or stain. Lubricants that work well on coated surfaces are synthetic and chemical soluble and certain natural emulsions.
Types of Lubricants
There may be oil-based and water dilutable. Though both uses the same types of additives sulfurized fats or oils; chlorine (usually in the form of chlorinated paraffin wax commonly known as honey oil; and phosphorous. Fats are added to improve the wetting of the stock with the lubricant and also to increase slipperiness or oiliness.
Properties of lubricants can be formulated to suit a particular operation.
There are three basic ingredients in the makeup of a typical metalforming lubricant. The main component of most drawing compounds is their vehicle, which may be oil, solvent, water or a combination of several vehicles. The second ingredient is a wetting or polarity agent. Some of the commonly used agents are animals fats, fatty acids, long chain polymers, and emulsifiers. The third basic ingredient added to many stamping lubricants is an EP agent such as chlorine, sulphur or phosphorous. Numerous combinations of carriers, additives, and EP agents can be used in formulating lubricants for metal stamping operations.
Petroleum-based lubricants are especially useful when high load characteristics are present.
Liquids
Liquid components include:
Mineral oils
Natural oils (fatty oils)
Synthetic oils
Compounded oils
Extreme-pressure oils
Emulsions (soluble oils)
Solutions (mixtures of water with other fluids or additives)
Eutectic salts (liquids under process conditions)
Glasses (liquid under process conditions)
Mineral oils are viscous hence mainly used as coolants and heat-transfer media while they fuction poorly as boundary lubricants. However mineral oil are inexpensive, also there is a number of viscosity ranges, controlled compressibility and stability to molecular shear stress under high stress.
Natural oils are used in metalforming, either neat or with combinations of mineral oil. They produce better boundary lubrication as they contain free fatty acids thich can form boundary films. When added to mineral oils, these fatty acids are known as “oiliness”. Also they have a relatively low pressure coefficient of viscosity.
Synthetic oils usually shows better visciosity temperature characteristics, lower volatility and flammability, and better oxidative and thermal stability than mineral oils. It is best used at extreme temperature, as when they evaporate, they leave very little carbonaceous residue are available.
Compounded oils may be comprised of as much as 30% fatty acids by weight and 5% fatty acids. However they tend to stain metals if not completely removed before annealing or further processing. And they have fairly low critical temperature.
Extreme-pressure oils are mineral oils or synthetic fluids containing highly active chemical compounds in additives concentrations. Although elements like sulphur can be dissolved in mineral oil or acids, in order to be effective lubricants, they have to be loosely bonded which in turn might produce undesirable staining.
Emulsions’ main purpose in it’s cooling capacity.
Solutions are usually concentrated aqueous solutions of inorganic salts, water soluble detergents, and amines. Although water solutions are fire resistant, they have limited lubrication ability and they tens to build up inorganic deposits on the tooling and workpiece.
Eutectic salts that forms high-visciosity liquids at high temperatures act similarly to conventional mineral oils at lower temperatures; that is, boundary effects are minimal and lubrication is predominantly a physical process that depends on the merit of the viscosity of the salt.
Additives
Additives used to influence the chemical properties of the lubricant are:
Antioxidants
Corrosion inhibitors
Oiliness, antiwear, and extreme-pressure agents
Metal deactivators or passivators.
Detergent dispersants
Additives used to improve physical properties of lubricants are
Viscosity-index improvers
Pour-point depressants
Antifoam agents
Emulsifiers
Antimicrobial agents
Thickness and tackifiers
Solid lubricants
Odor-masking agents and dyes
Lubrication Application Methods
Correct lubrication application can attain:
Reduced lubricant usage
Increased press speed
Longer die life
Cleaner operation
Reduced shop maintenance (cleanup)
Reduced scrap
Reduced lubricant carryoff
Simplified waste disposal
There are five basic methods for applying lubricants. They are manual, drip, roller, spraying and flooding.
Manual Application
Advantage: the tools of application, eg. Brushes or swabs are readily available, inexpensive and require no maintenance. Useful in short run work for applying lubricant at a selected place on the workpiece.
Disadvantage: this method is wasteful. It may also require the operator to reach in to the die area that is dangerous.
Drip Application
Advantage: relatively inexpensive and are readily available. Regulating valve can be used to adjust the correct flow of lubricant so that it can applied consistently. Also the drip can be installed where lubrication is most needed.
Disadvantage: the lubricant must be of stable compounds that do not separate into its constituents. For a large stock area, a drip applicator is not suitable because of relative small reservoir capacity.
Roller Coating
There are basically three types of roller coaters: 1) unpowered plain, unpowered with recirculating system and powered. Preferred position of the roller should be in between the fabricating equipment and the feeding mechanism. Pumps have to be selected based on the viscosity of the lubricant selected. Roller surface can also be altered to retain lubricant with special grooves or surface texturizing. A soft roller can be used to prevent scratching the workpiece’s surface.
Unpowered plain roller
Advantage: it is easy to maintain and provides some degree of control over the amount and uniformity of lubricant.
Disadvantage: like the drip, it also needs a very stable luricant compound. Many a time, roll tension is also not verified, which means that control is uncertain. It might cause slippage if the feeding mechanism is also a roller.
Unpowered roller with recirculation
Advantage: the excess lubricant is squeezed off by wiper rollers which is then returned to the reservoir, where it is filtered and available for reapplcation. Some designs can be installed after the feed hence reducing slippage problem.
Disadvantage: high cost for initial setup and purchase.
Powered roller with recirculation
Advantage: ability to coat wide widths evenly.
Disadvantage: cannot be used to lubricate formed or partially formed work.
Spraying
It is the most versatile and there are 3 types of spray: air spray, airless spray and electrostatic spray.
Air spray system
Advantage: readily available and relatively inexpensive. It can be used for single or multistation equipment. Lubricant usage can be controlled precisely and special nozzles are available for pattern spray. They can be connected to press control circuit and operate only when and where it is required.
Disadvantage: it may create fog or mist. High-viscosity drawing lubricants cannot be used.
Airless spray system
Pressure is applied by means of an intensifier and is conveyed through a pressure resistant hose to an orifice in the nozzle where the lubricant is expelled as a fine spray.
Advantage: no atomizing effect, no mist or fog. Can handle high-viscosity lubricant. There is no bounce or overspray. Heavy films can be applied to form works.
Disadvantage: system is complex and initial cost is high.
Electostatic spray
The lubricant will be attracted to the work and wraps around it. Therefore there is no overspray or bounces. This system can be very effective, however there are restriction on the types of lubricant that can be used.
Flooding
Advantage: positive lubrication can be applied at all stations. Ability to prolong die life. As there is large supply of lubricants to critical points, the cooling effect can allow the press speed to be faster. As the lubricants are recirculated and filtered, there is little waste.
Disadvantage: installation of this system is very difficult if it is not with the original press. Sensing system has to be waterproofed and dies have to be vented.Electrical system also needs to be rerouted. Initial cost is high.
