Choosing a lubricant for deep drawing
One of the most common metalworking methods is drawing, which involves forming flat sheet metal into "cup-shaped" parts. If the depth of the formed cup is equal to or greater than the radius of the cup, the process is called deep drawing.
Deep drawing involves placing a sheet metal blank over a shaped die and pressing the metal into the die with a punch (see Figure 1). The piece produced may be cylindrical or box-shaped with straight or tapered sides or with a combination of straight, tapered, or curved sides.
The punch must provide enough force so that the metal is drawn over the edge of the die opening and allowed to flow into the die. The sheet metal blank must be strong and ductile enough to avoid breaking in areas where the metal flows from the punch face to the sides of the punch.
Characteristic of deep drawing is the high pressure—on the order of 100,000 pounds per square inch (PSI)—involved in the operation. To deal with such force, the choice of lubricant is critical to the success of the operation. Under such pressure, the drawing lubricant should:
- Cool the die and the workpiece.
- Provide boundary lubrication between the die and the workpiece.
- Prevent metal-to-metal adhesion or welding.
- Cushion the die during the drawing operation.
In deep drawing, a sheet metal blank is secured and then pressed into a shaped die to create a "cup-shaped" part.
Drawing compounds used in deep drawing are known as boundary lubricants. The tooling and sheet metal surfaces are pressed so tightly together that the liquid is squeezed out and only a very thin adsorbed film remains. This film forms a "boundary," preventing direct metal-to-metal contact under conditions of high pressure and temperature produced during the deep drawing operation. Proper lubrication prevents friction, which causes heating and can ultimately lead to the breakdown of a lubricant.
Different types of drawing lubricants are used, depending on the depth of a particular draw. Generally, the effectiveness of a deep drawing lubricant depends on its ability to form an adsorbed film of sufficient strength and oiliness on the metal surface being drawn.
Three types of drawing lubricants are used:
- Drawing oils. Drawing oils form an adsorbed film, and they take the form of light or soluble oils such as straight mineral oil or emulsions of soluble oil and soap, or of heavy oils, fats, and greases such as tallow or lard oil.
- Emulsions. Aqueous solutions of nonoily lubricants containing some suspended solids are called emulsions. These lubricants are not widely used in deep drawing because they contain little or no oil.
- Lubricants containing both oil and solid substances. Used in applications involving severe drawing, these lubricants contain oily components that reduce friction and heat. The combination of the oil and the solids together produces enough lubrication for severe drawing applications such as deep drawing.
A typical lubricant used for drawing is a soap/fat paste-type material that has a formulation of 5 percent soap, 25 percent oil, 25 percent water, and 45 percent solids.
The soap and oil are responsible for forming the adsorbed film between the metal surface and the die, and the solids include materials such as graphite, chalk, zinc oxide, carbonates, and borates. that further aid in the forming of the boundary film between the metal surface and die. The combination of oil, water, and soap helps to form the emulsion, preventing separation of the product. In addition to these major components, lubricant formulation can include additives such as defoamers and biocides.
A lubricant compound can be used as a paste or as a liquid after being diluted with water, depending on the required concentration and the severity of the drawing operation. Methods for applying lubricant to sheet metal include dips, swabs, brushes, wipers, rollers, or recirculation. Of these, the three most common are:
- Manually wiping lubricant onto a surface with a rag.
- Roll coating, during which metal blanks pass through rollers that apply the compound.
- Flooding, during which tooling and metal sheets are drenched with lubricant, and the excess liquid is recovered via a filtration and recirculation system.
Animal fats and petroleum oils have been traditionally used as paste-type products in deep drawing. Although animal fats are inexpensive components of such compounds, they can be difficult to remove, even when drawn and formed parts proceed immediately to a cleaning system. They can also load up and "kill" a cleaner tank quickly, and difficult-to-dissipate soap foam can form rapidly when highly alkaline cleaners are used with the compounds.
Removing lubricant from a formed part after the deep drawing operation is important because any lubricant left behind can interfere with subsequent steps in the manufacturing of the part. Mineral oils, animal fat, and vegetable oils can be removed with an organic solvent by emulsification or saponification, or with an aqueous alkaline cleaner. Greases can also be removed from sheet metal with an organic solvent or an alkaline cleaner. However, an alkaline cleaner may be slower in removing the drawing lubricant.
Solids are more difficult to remove because they are not readily soluble. The presence of solids often requires that additional cleaning methods be used.
Petroleum oils can raise special issues from removal through disposal. These oils require the use of alkaline cleaners for removal, which can then contaminate cleaner tanks with oil, leading to potential disposal challenges.
Vegetable oils can be removed with hot water if the parts are cleaned immediately and with a mildly to moderately alkaline cleaner if the parts are cleaned after they have been left standing for a few days.
A manufacturer should consider the severity of the drawing operation, the choices of lubricant available, and how easily the substance can be cleaned from the metal after drawing to decide which lubricant best suits a particular application.
One manufacturer, for example, produces sinks and bathtubs and has extensive transfer and coil-fed press lines that function, on an average of 18 hours per day. Much of the work is deep drawing that is usually conducted in one hit. That is followed by an operation in which the drains and overflows are formed and peripheral cropping is completed in one pierce-and-crop hit.
The tubs are formed from 54- by 74-inch sand-blasted carbon steel blanks on a 700-ton press. Final draw dimensions are 13 inches deep, 22 inches wide, and 51 inches long.
In the past, problems arose during subsequent operations, however, because the lubricant the company used was difficult to remove. The company experienced a 45 percent rejection rate during the porcelainizing and painting phases of the manufacturing process because of "popping through" that occurred during curing, resulting in costly rework.
The manufacturer decided to substitute a synthetic lubricant that is a blend of vegetable oils, alkalis, secondary polymeric lubricants, and additives that include a defoamer, biocide, inorganic buffering agent, and a nonferrous metal passivator.
The vegetable oils used are blends of assorted C18 to C24 hydrocarbons with varying degrees of saturation. The secondary lubricant included is a polymeric-type material. It enhances the lubricity characteristics of the product.
The new lubricant was initially applied with roll coaters at 100 percent concentration, but subsequent rippling during the deep draw operation indicated that too much lubricity was present. The lubricant was then continually diluted until it reached a 30 percent concentration by volume, at which time the rippling stopped, and acceptable tubs were produced.
The tubs are put through a three-stage washer system in which the lubricant is removed completely. During subsequent porcelainizing, the reject rate has fallen to zero.
A second manufacturer of bathtubs and sinks was also experiencing a high rejection rate caused primarily by difficulty in removing the deep drawing lubricant it was using, which made the products troublesome to properly porcelain or weld.
Its tubs are formed in a single hit, which simultaneously produces the bowl of the tub and a front bend—a difficult undertaking because three sides are straight and the fourth has a 10-degree angle. The tubs' curved sides further increase the difficulty of the operation.
The tubs are made from 65- by 52-inch carbon steel blanks that are 0.056 inch thick. The sinks are produced from 32- by 38-inch blanks of the same thickness and require a 9.5-inch draw.
The manufacturer experimented with the synthetic lubricant, applying it by roll coater at decreasing concentrations until it reached 60 percent. At that level, the lubricant provided the proper level of lubricity and had sufficient viscosity to hold onto the roll coaters. After the switch in lubricants, the company saw a decrease in material buckling during forming and a lower rejection rate.
Mary Linda Schumann and Doris Simoes are senior chemists with Oakite Products Inc., 50 Valley Road, Berkeley Heights, NJ 07922, 800-526-4474 ext. 2107. Oakite Products is a manufacturer and distributor of specialty chemicals and support equipment.
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.