Metal sheets are made by feeding the metal through rolling mills that squeeze or press it in order to reduce thickness. The rolling process is often repeated, with the thickness decreasing at every step until the desired measurement is achieved. Rolling usually begins with cast metal slabs, which can range from several centimeters to decimeters thick. It can end with foils of 0.03 millimeters or less or gold plating that is measured in microns.

Mills can operate at hot temperatures (typically 50 to 100 degrees above the metals crystalline temperature) or cold (below the crystalline temperature). Because it makes the metal more malleable, hot rolling has the advantage of potentially higher reduction rates. But cold rolling typically achieves a better surface finish and can also produce thinner sheets. Hot rolling can make sheets that are centimeters or millimeters thick, Baumgaertel said, while cold rolling is capable of final thicknesses in millimeters or microns.

The rollers that squeeze the metal can be configured in any of several arrangements. The simplest – duo – has one roller atop of the other and may be used for cold rolling of aluminum or to apply surface skins to stainless steel. A quarto arrangement is a stack of four with the metal feeding between the second and third rollers. It is used for cold rolling aluminum or copper, or sometimes stainless steel. Multi-roller arrangements have twin groups of several rollers arranged horizontally that sandwich the metal sheet. Operators use these for steel, stainless steel and hard copper alloys.

Any of these arrangements or processes requires a rolling oil to lubricate the metal as it passes between the rollers, although the makeup of the oil can vary significantly depending on the process.

The roll oil has to be designed as an integral part of the whole process of making metal sheets, strips and foil, since there are no standard rolling rigs [or] standard rolling conditions,

Baumgaertel said.

Cold rolling usually uses neat oils, but fluids made of straight mineral oil would create a fire risk at the temperatures involved with hot rolling. Therefore, Baumgaertel said, hot rolling uses emulsifications of oil and water.

Rolling oils serve a number of functions. The first, according to Baumgaertel, is to reduce the friction in the roll bite in a defined manner. That means lowering friction at the point of contact between the sheet and the rollers but maintaining a precise amount of friction to help keep a consistent surface finish and hardness for the finished sheet. The fluid should also cool the sheet and rollers, and it should flush out metal particles generated at the point of contact between roller and sheet. Finally, the fluid should prevent excessive transfer of metal from the sheet to the rollers.

One of the keys to ensuring the proper level of friction is the viscosity of the fluid. In general, Baumgaertel said, operations with higher reduction rates and higher feed speeds require thicker fluids.

Like most lubricating fluids, rolling oils consist of base stocks and chemical additives. Baumgaertel said formulators need to pay close attention to a number of base stock properties because of their influence on the process and final product. The viscosity of the base stock is the chief determinant of the rolling oils viscosity, he said. Formulators need to choose oils with flash points that are safe for their applications and that will avoid evaporating at mill operating temperatures. Thermal stability is likewise important, as is an ability to resist foaming. The proper base stock will also behave well during the annealing process. Finally, base stocks used in emulsifications for hot rolling should not separate too readily from water.

Reduction rate, friction and surface quality are highly influenced by additives, Baumgaertel continued. There are three groups of additives that are key for roll oils: long-chain organic acids, alcohols and esters. But within these groups there are huge performance differences. Chemicals in these categories can vary widely in:

base oil solubility;

tendency to form brown stains on metal sheets;

tendency to form soaps, which are undesirable;

and emulsibility.

In some cases formulators must perform a balancing act, Baumgaertel said, offering an example. Long chain molecules work better as friction modifiers, he said, But they can also lead to annealing problems.

Emulsion stability is one of the most important properties in hot roll oils, and it depends on the combination of the base oil and emulsifier, Baumgaertel said. Emulsifications that are very stable will show no separation and appear milky white. A typical rolling emulsion will have a certain volume of cream or oil on top of the emulsion, and these normally perform satisfactorily, he said. But a very unstable fluid separates into an oil or cream and a nearly translucent water phase. At that point the necessary performance properties will no longer be evenly distributed.

Baumgaertel added that it is important to note that emulsification stability can change during use due to emulsifier depletion, ingress of tramp oils, change in water hardness or coagulation.

Rolling oil filtration is an important consideration for any rolling mill, Baugaertel said. The industry uses several types, including horizontal plate filters, candle filters and supamic filters. The primary components remove solids from the rolling oil, but filters sometimes include aids such as active earthchemicals so that they also remove polar compounds that have dissolved into the oil.

But active earth filter aids also remove additives such as fatty acids, alcohols and esters, Baumgaertel said. Therefore, the additive content must be observed carefully several times per day. He added that chemical filtration can also reduce the conductivity of the rolling oil, leading to a static electricity buildup that can create a fire risk. As a result, some formulators add conductivity improvers.

The last step in a rolling mill can have an inordinate impact on the appearance of the final product.

Immediately after rolling, some metal sheets and foils undergo an annealing process that heats and cools them in a controlled way in order to strengthen them and make them less brittle. Staining may occur during this process for several reasons, Baumgaertel said, including the formation of soaps, contamination by tramp oil used to lubricate the rolling machinery, or insufficient annealing. To avoid this problem, some mills, especially those producing aluminum, use low-staining gear lubricants and hydraulic oils to minimize the effects of tramp oils.

Lube Report Asia occasionally includes articles originally published in sister publications of LNG Publishing Co. This article originally appeared in the May 2012 issue of LubesnGreases Europe – Middle East – Africa, under the title Easing the Squeeze.