Lubricants and Lubrication Part 11 pdf

In the straight strip drawing test, a strip of metal is drawn between two flat dies.The drawing force required is measured against the normal force to determine thecoefficient of frictio

557 15.1 Sheet Metal Working Lubricantsinteractions between solid matter and workpiece surface, on the load during theforming process and on type of solid matter used.

As cleaning residues may have an effect on the layer production during the ical surface treatment and thermochemical processes, even after profound rinsing,only especially developed cleaners are used in order to avoid failures and distur-bances In addition, the natural character of the basic metal which is to be treated(iron, aluminum, copper, brass, zinc etc.) has also to be taken into consideration

chem-According to the pH value of the alkaline cleaning solutions, a difference is madebetween:

. low-level alkaline cleaners (pH between 7.0 and 10.5),

. medium-level alkaline cleaners (pH between 10.5 and 11.5), and

. high-level alkaline cleaners (pH above 11.5)

Low-level alkaline cleaners are often called neutral cleaners’, as they are terized by a low tendency to saponify natural and synthetic esters A standard defini-tion of the term neutral cleaners’ does not exist

charac-Acid cleaners (pH below 7.0) are used for special applications Apart from thedegreasing effect, these cleaners show a caustic effect for the removal of rust, rustflake or other solidly-bonding layers which cannot be removed in alkaline media.Such caustic degreasers consist mainly of hydrochloric acid, sulfurous acid or phos-phorous acid added with acid-resistant tensides

In addition to the degreasing of lubricant films, acid degreasers based on phorous acid are also used to produce a thin iron phosphate layer (approximately0.1–0.3 lm), thus providing a key for a painting layer, e.g cataphoretic immersionpaintings and/or powder coatings

phos-Application Range for Aqueous Cleaners

The decisive criterion in the selection of cleaners is the applied cleaning method:

. immersing-bath cleaners (dip cleaners) which are basically emulsifying and. spray cleaners which are basically demulsifying

For special applications there are:

Subsequent to the immersion method, the parts are cleaned in a bath solution athigh temperatures for a relatively long treatment period Often, one stage will not be

558 15 Forming Lubricants

sufficient and multistage cleaning methods are applied in accordance with the taskrange

Immersion Cleaning Facilities

Standard immersion facilities can be constructed simply The easiest solution is aheatable container made of sheet iron, e.g the decoction bath which operates at boil-ing temperature and is nowadays applied in few cases only Here, the surging of theboiling solution serves as additional mechanical cleaning support Modern facilitiesavoid the disadvantages connected with the decoction’ cleaning: high heat losses,problematic steam swathes, splashes and boiling-over

The cleaning effect can be mechanically supported by circulating the cleaning lution or by flooding during which the solution is re-pumped and pressed into thebath through numerous valves positioned, staggered, opposite each other As analternative, the bath is re-filled and drained several times with a flood pump Byusing an additionally installed sprinkler system, creamed grease is rinsed off thebath surface into a spillway, thus preventing that the cleaned parts are re-oiled withdemulsified oil when taking them out of the bath In comparison with the decoc-tion’ bath, well circulated baths have the advantage that they ensure a perfect clean-ing result at a lower temperatures (60–90 C) and after a shorter period of treatment.Occasionally, the bath solution is circulated using pressurized air instead of thepumping method Here, cleaners with a particular low foaming tendency areneeded

so-Ultrasound Cleaning

Another immersion cleaning method is ultrasound cleaning In some industries,e.g in the manufacture of jewelry and silver products, cutlery, optic glasses anddevices, as well as in the production of high-precision fine-steel tubes, fittings andmany structural components, extremely high requirements are made on the cleanli-ness of the mostly polished surface areas Residues of the materials used and metalabrasion particles can only be removed completely with great effort because suchpigment contamination cling to metal and glass surface very strongly

Mostly, a treatment with aqueous cleaning solutions alone will not be sufficient if

it is not supported by additional mechanical actions In these cases, the use of sound will have more effect than the flooding of the cleaning solution or a spraytreatment Ultrasound cleaning is based on high-frequency sound vibration which isgenerated with special vibrator systems

ultra-The Effects of Ultrasound

In the cleaning solution ultrasound is transmitted in longitudinal direction as alongitudinal wave Due to strong pressure variations, the solution produces numer-ous small low-pressure bubbles which immediately collapse releasing energy Here,the micro-roughness of the surface areas and the accumulated particles act as cavita-tion nuclei This is particularly true for corners, drill holes, decorative embossingsand other areas where conventional cleaning methods will not lead to optimalresults

559 15.1 Sheet Metal Working Lubricants

In clear solutions one can observe after a few seconds, how the dirt particles areremoved from the surface areas This cleaning method is based on the pressure gen-erated by the collapsing bubbles, which can locally exceed 1000 bar

Sound frequencies of between 20 kHz and 40 kHz, as well as operating tures of between 50 C and 70 C will lead to the best results Generally, the mini-mum cleaning period does not exceed two minutes

tempera-The ultrasound generator of a cleaning system should be set so that a level of thesound energy of between 5 and 20 W l–1solution of the immersion bath is achieved

Spray Cleaning

This modern cleaning method is based on one or more coordinated stages Theintensive mechanical effect of the spray jets has the advantage that in comparisonwith the immersion bath the period of the treatment is considerably shorter and theoperating temperature often can be reduced to 40–70 C

Much lower quantities of cleaning solution are needed than in the immersionbath, thus reducing its concentration to 10–30 g l–1 This means less use of chemi-cals, less consumption of heating energy and lower costs for waste-water processingand disposal

noz-The number and length of the individual chambers of continuos spraying ities have to be adjusted to the respective pretreatment problem The type of nozzledepends on the function of the various chambers Flat-jet nozzles are used in clean-ing, degreasing and pickling chambers, whereas hollow cone nozzles are used inrinsing, passivating and neutralization chambers The number of nozzles and theirorder is predominantly determined according to the shape and size of the parts to

facil-be treated

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The most important prerequisite for the perfect operation of spray cleaning ities is the use of quickly emulsifying cleaners By skimming or filtering, it can beensured that a clean’ cleaning solution is available over a longer period of time.Products for Spray Cleaning

facil-Spray cleaning requires a different construction of the cleaners than in immersioncleaning The most important characteristics is the different foaming behavior:spray cleaners contain demulsifying tensides with low foaming tendency It must beobserved that the foaming behavior depends on the temperature, i.e minimumoperating temperature has to be reached in order to ensure the self-defoamingeffect This means that the water in the spray cleaners has to be heated while prepar-ing the cleaning solution, prior to adding the cleaner concentrate The temperaturehas to be kept on the same level also during down-times to avoid unwelcome foam-ing during the restarting process

Manual Cleaning, Cleaning with Steam Jet and High-pressure Machines using Aqueous CleanersThe Cleaning Table

In many workshops and smaller manufacturing companies, metal cleaning is stillcarried out manually using a brush Today, however, several developments havebeen introduced to improve and facilitate this manual work Instead of using thepetroleum tub with brush and cleaning cloth, a cleaning table with a hose-pipebrush for the cleaning solution is used In many cases, the petroleum can bereplaced by high-performance aqueous cleaners which are often better suited for thecleaning of repair parts

Manual Sprayers

Often, a spraying process is also used, e.g in the cleaning of vehicle engines or ities and machines which can not be transported The cleaning solution is sprayedonto the surface and allowed to work in Higher contaminated surface areas aretreated with the undiluted cleaning concentrate

facil-Then the dirt particles are sprayed off with a strong water jet The surface areasare dried in the air The drying process is accelerated by blowing pressurized air onthe surface

Steam Jet or High-pressure Machines

Manual cleaning can also be carried out using steam jet or high-pressure cleaningmachines which will normally not work with solvent cleaners but with aqueouscleaning concentrates The cleaning concentrate is mixed with hot steam or water.This mixture is sprayed onto the respective surface under high pressure The tem-perature and jet pressure can be adjusted to the individual application

Cleaning with Organic Solvents

The solvent cleaners mainly consist of chloro- and chlorofluorohydrocarbons, andhydrocarbons and solvent cleaners

561 15.1 Sheet Metal Working Lubricants

Chloro- and Chlorofluorohydrocarbons

In recent years, the use of organic solvents, especially halogenated hydrocarbons,was made more difficult because of legal measures covering work place protection(Chapter 9) Nevertheless, higher emissions and the immediate exposure of the staffcan be avoided with the introduction of corresponding technical measures

The most important substances in this group are trichloroethylene ethene), perchloroethylene (tetrachloroethene) methylene chloride (dichloro-methane), 1,1,1-trichloroethane and, the chlorofluorohydrocarbons R 11 and R 113,which are also used as refrigerants The most important criteria for their use aretheir high ability to dissolve polar substances, the quick drying of the degreasedparts, the avoidance of fire hazard in contrast to pure hydrocarbons and the possibi-lity of regeneration by means of distillation Distillation methods make it possible toseparate foreign components However, it has to be taken into consideration that theboiling point of the compound will increase due to the dissolved lubricant compo-nents and that an accelerated decomposition process, accompanied by the formation

(trichloro-of hydrogen chloride, starts at about 120 C for trichloroethylene and at 150 C forperchloroethylene

The solvent decomposition accompanied by the formation of hydrogen chloride isone of the largest problems in the area of cleaning with chlorohydrocarbons Thisdecomposition can also start at temperatures which are clearly below the mentionedfigures and can be catalytically accelerated by foreign matter, e.g water Conven-tional solvents for metal cleaning contain effective stabilizers in order to avoid thisdecomposition which is accompanied by a corrosion process Therefore, it is impor-tant during distillation that the stabilizers mix with the distillate and do not remain

in the sump Normally, however, inhibitors are added for re-stabilizing purposes,thus neutralizing produced hydrogen chloride which has already been produced Aroutine test of the halogenated solvents to determine the acid index, the pH of theaqueous extract or the chloride content is useful in order to safely avoid corrosionproblems Test kits are often used for operational monitoring

Hydrocarbons and Solvent Cleaners

White spirit, petroleum and isoparaffins are the hydrocarbons most used in metalcleaning The comparably low prices and ease of disposal are advantageous, how-ever, when using these substances the fire hazard has also to be taken into account.Application Range of Solvent Cleaners

The degreasing of pigmented forming-lubricants using solvent cleaners generallyconstitutes a problem After dissolving away the lubricant film’s soluble organic sub-stances, the solid matter lubricant remains on the surface (see aqueous degreasing).Normally, solvents are used at room temperature or in closed systems at the boil-ing point of the solvent – the so-called vapor phase degreasing The vapor phase, inwhich the part to be cleaned is immersed, is produced above the boiling solventbath It then condenses highly pure solvent matter on the cold workpiece until thetemperatures are the same The condensed solvent rinses the oil film off the work-piece, which will either dry quickly or be immersed for short-term intermediate pre-

562 15 Forming Lubricants

servation in a solvent bath to which 1 to 5 % of a corrosion protection concentrate isadded

Solvent cleaners are a product group of their own They are predominantly used

in the cleaning of heavily contaminated or oiled repair parts, e.g engines, gearingsystems and major machines (locomotives) These products consist mainly of hydro-carbons with special additives They are inflammable

As a product group, solvent cleaners are related to solvents They consist of carbon mixtures or synthetic hydrocarbons and contain further additives in accor-dance with the respective application, e.g to improve the corrosion protection orwashability

hydro-Today, solvent cleaners which do not contain aromatic components have proventheir suitability in many applications

Solvent cleaners are used either in immersion baths or to spray the parts to becleaned According to the composition, they can be used as pure concentrate ordiluted with petroleum or, some of them, with water After it has been allowed toreact for a certain time, the solvent cleaner is carefully sprayed off with a strongwater jet When so-called separating solvent cleaners are used, the emulsion gen-erated during water spraying will split very quickly into an oil phase and a waterphase so that the oil components are safely retained in the oil separator

Emulsion Cleaners

The concentrates are to some extent analogous to water-miscible cutting fluids andconsist of mineral oil, synthetic hydrocarbons and other organic fluids together withemulsifying agents and anti-corrosive compounds They are mostly used in a con-centration of 2 % in water They do not lead to a cleaning level up to water wettabil-ity, which makes them suitable for the cleaning between two operations Suitablewater-mixed coolants with a low foaming behavior are often also used for this pur-pose The residual film protects iron materials against corrosion

Maintenance of Cleaning Baths

For reasons of cost minimization, the industry focuses more and more on the vice life of cleaning baths

ser-In solvent cleaners, the solvent can be retained and kept fresh through tion, as already mentioned More effort is made for the maintenance in case of aque-ous cleaning procedures

distilla-Apart from the development of, for example, special spray cleaners with a verygood demulsifying behavior, physical methods for the prolongation of the servicelife of cleaner solutions are also used These include oil separators, skimmer, filtra-tion systems or centrifuges for solid matter separation and microfiltration The use

of these methods not only ensures the separation of introduced contamination butwill also sometimes cause the cleaner’s components to be separated which thenhave to be re-dosed

The ultrafiltration method facilitates the separation of most organic componentsfrom the aqueous phase Inorganic salts reach the permeate Thus, this method isalso suitable for waste water treatment

563 15.1 Sheet Metal Working LubricantsWhen vacuum-distilling cleaner solutions (e.g in thin film vaporizers), only theaqueous phase is regained The non-distilled residue contains the components ofthe cleaner and the contamination, including the salts.

15.1.11

Testing Tribological Characteristics

Various methods are used to evaluate the deep drawing performance of lubricants Theprincipals are described in Section 15.1.3 The most common is the strip drawing testwhich is performed with and without deflection This method simulates the conditionsfound under the blank holder and during the flow of material into the die To examinewear, abrasion and the special behavior of metal coatings (powdering, cracking, smooth-ing effects) methods with strip deflection, e.g draw-bead test or Erichsen test are used

In the straight strip drawing test, a strip of metal is drawn between two flat dies.The drawing force required is measured against the normal force to determine thecoefficient of friction In addition, the time is also recorded The most importantmeasuring parameters are tool and material dimensions, drawing speed and themaximum drawing force applicable Apart from the coefficient of friction, the uni-formity of the force time-frame is an important criteria for evaluating the tribosys-tem Ideally, the material flow between the tool is uniform and free of stick–slip.Finally, the maximum normal force possible until the strip jams is a measure of alubricant’s ability to separate the sheet and the tool under pressure Evaluation ofsuch data allows lubricants to be developed which are ideal for metal forming opera-tions An example for a strip testing equipment is shown in Fig 15.18 The teststand used by the Institute for Production and Forming Technology of the Univer-sity of Darmstadt, Germany, enables tribological systems to be examined with largetool dimensions, variable drawing speeds and very high normal contact pressures.The various stations are de-coiling, cleaning and oiling Tools are equipped with sen-sors to measure forces Finally, the strip metal is coiled again [15.26]

Fig 15.18 Strip-drawing tester and tools [15.5, 15.6].

564 15 Forming Lubricants

Figure 15.19 shows the results obtained from uncoated cold-rolled steel The stripdrawing test without deflection is reported on at all times The lubricant is a mill-applied corrosion prevention oil The forces measured are recorded against time.According to the friction law the frictional force increases with higher loads From

5 N mm–2onwards stick–slip occurs

The coefficient of friction as a function of the load is outlined in Fig 15.20 wherethe drawing speed is varied The coefficient of friction falls with increasing speed.The begin of stick–slip is marked The curves end with the maximum load, whenthe strip jams and further drawing would cause the metal to tear

As a rule, lubricants with a higher viscosity have better lubricity To distinguishthe effect due to pure rheology from that caused by additives, some lubricants withidentical formula but with differing viscosities are tested The result is shown inFig 15.21 with a corrosion preventive oil as example The viscosity is chosenFig 15.19 Frictional force vs time with load as parameter, mill oil/uncoated sheet metal/50 mm s –1

Fig 15.20 Friction coefficient vs load with sliding velocity as parameter, mill oil/uncoated sheet metal.

565 15.1 Sheet Metal Working Lubricants

between 15 and 150 mm2s–1at 40 C to cover the most common applications fromblank washing to drawing oils applied by spraying

15.1.12

Sheet Metal Forming in Automobile Manufacturing

The pressing of car body parts is one of the most important sheet metal drawingprocesses The corrosion protection oil applied by the steel mills plays a part inevery sheet metal forming operation and also makes up the largest proportion oflubricants used The typical part transfer sequence from the sheet metal to the body-in-white is described in Fig 15.22 At every stage where oil is applied to the metalsurface a full compatibility with upstream and downstream processes is necessaryfor a high production reliability at minimum overall costs The single steps ofproduction are discussed in the special sections below [15.27, 15.28]

15.1.12.1 Prelubes

The idea to combine the corrosion protection properties of a corrosion preventive oilwith the lubricity of a drawing oil led to the development of the prelubes Prelubeshave existed on the American market for more than 20 years, they were introduced

in Europe in the early 90s Applied at the finishing lines of the steel mills, they serve

as the forming lubricant in the press shops As for the corrosion protection oils, aprerequisite for the suitability of a prelube oil is the absolute compatibility with allsingle processes from the coil to the body-in-white The use of prelubes in steelmills reduces the number and quantity of spot lubricants for additional press shopoiling dramatically But their true benefits can only be fully achieved if the compat-ibility principle is applied throughout the manufacturing chain Therefore, modernprelubes systems are modular, even different viscosities can be part of the same con-cept This results in a far-reaching multi-functionality for all applications

Fig 15.21 Friction coefficient vs load with oil viscosity as

parameter, mill oil/uncoated sheet metal/50 mm s –1

566 15 Forming Lubricants

Among the many specifications for mill oils, the comprehensive standard of theAssociation of German Automobile Manufacturers (VDA) [15.29, 15.30] describes allrelevant requirements of a mill-applied lubricant, defines all important propertiesand lists all test procedures The results – with the exception of lubricity – are com-pared with the standard corrosion preventive oil Anticorit RP 4107 S According tothese VDA requirements a prelube needs to be thixotropic for a reduced run-off and

is applicable by electrostatic spraying Of course it should protect the sheet metalagainst corrosion as effective as the standard oil Precipitation or other chemicalreactions must not occur when mixed with the standard oil Furthermore, the pre-Fig 15.22 Process line.

567 15.1 Sheet Metal Working Lubricantslube should be compatible with the cataphoretic paint and the adhesives used inthe assembly line Eventually, the prelube has to be easily removable with indus-trial cleaners in the preconditioning zone prior to phosphating and e-painting[15.15].

15.1.12.2 Skin Passing

To achieve the desired quality, the final surface finish of sheet steel is often rolled-onwet Water-based solutions are normally used for plain cold rolled steel but low-viscosityoils are used for zinc-coated sheet including electro-galvanized or hot dip galvanized As

a result of the increasing use of coated sheet steel in recent years, the compatibility ofskin-pass oils is becoming an increasingly important factor If the skin-pass and the sub-sequently applied corrosion protection oil are not compatible, the oil film can rapidlyseparate in spite of having been evenly applied The formation of dry islands is detri-mental to the tribological system and can cause tearings or oil dents

15.1.12.3 Coil Oiling

A prerequisite for the oil being used by steel mills is that it can be applied easily.The most common method these days is electrostatic spraying The optimum spray-ing temperature has been shown to be 50–60 C while the voltage, depending on theequipment itself, is between 80 and 120 kV Oils with a viscosity of 30–60 mm2s–1

at 40 C spray easily If thixotropic oils are used, a full solubility of the dispersedagents at the spray temperature is essential for an optimum spray pattern and a lowfilm thickness On the other hand, the oil may have to be very finely filtered toremove any impurities in return circuits

15.1.12.4 Transport and Storage of Sheet Metal

The principal objective of oiling coils and stacks in steel mills is to reliably protectagainst corrosion during transport and storage Fundamentally, temporary corrosionpreventives combine a barrier effect with specific inhibitors The specific adsorptionlayer is extremely thin The effect of inhibitors diminishes rapidly the further theyare away from the metal surface On the other hand, the barrier effect of the oildepends largely on the thickness of the film Thixotropic oils counter the loss ofbarrier protection by retarding run-off The stable film helps to maintain the originallevel of protection As this level of protection cannot be achieved with conventionalformulations, all of today’s prelubes are thixotropic The retarded run-off character-istics also prevent the soiling of press shop floors with oil as well as build-up of oil

in swage beads and cavities which may cause problems with adhesive bonding orpaint blisters during curing

15.1.12.5 Washing of Steel Strips and Blanks

Automobile outer body panels often have to be washed because of cleanlinessrequirements Advantages, however, are uniform oil film thickness, less die contam-ination and therefore low die wear As washing, particularly blank washing, does notgenerate high costs when compared to the overall manufacturing process, it is eco-nomically acceptable

568 15 Forming Lubricants

The steel strips or blanks coated with mill oil are washed with one of various able washing mediums, e.g water based emulsions, solvent diluted oils or low visc-osity oils When using emulsions, water related problems may occur such as corro-sion of stamped parts and tools, wear pick-up and growth of bacteria and fungi.Careful maintenance and special precautions are therefore necessary

avail-Washing machines do not fully remove the oil from the panels Depending on thewashing fluid, about half of the oil is washed off and is then squeezed off with fleecerollers The amount of oil remaining on the metal is influenced by the washingfluid and plant-specific parameters such as feed velocity and the number, diameter,material and pressure of the rollers The residual oil film increases as the fluid visc-osity rises Lower lubricity systems might require additional drawing oils [15.31].Washing fluids have to fulfil similar requirements to the mill-applied oils becausethey contribute to the total oil quantity entering the press and paint shops Apartfrom offering adequate pressing lubrication, they should be easy to remove, compat-ible with adhesives and paints and supplement the corrosion protection offered bythe underlying oil As all washing oils obviously settle on top of the mill applied oillayer and are thus relatively far from the metal surface, they tend to run off quickly.For these oils to have thixotropic properties is therefore a significant advantage.15.1.12.6 Additional Lubrication

Many drawing operations require additional spot lubrication which is applied in thepress shop These higher viscosity drawing lubricants lose out when prelubes areapplied Ultimately, they will become unnecessary if the lubricity of the mill appliedoil on the panels is adequate In fact, some enormous savings are reported on by carmakers Long-term plant trials which were exactly documented were evaluated[15.32] The savings resulting from the elimination of the drawing lubricant itselfwere only a part of the overall savings potential The bottom line is improved byadditional effects such as lower disposal costs and more stable manufacturing pa-rameters If in spite of the prelube, difficult deep drawing still requires the use ofadditional spot lubricants at critical points, these must be process-compatible Assuch spot lubricants are again applied on top of the other oil layers, a thixotropicformulation is also advisable to avoid run-off

15.1.12.7 Pressing

The central manufacturing process is that which occurs during pressing The tremely high investment and running costs of advanced, multistage presses make itessential that the pressing operation is consistent and reliable Rejects caused bytears, dents or poor dimensional conformity, but also die cleaning, grinding or set-ting time can soon generate high costs The large performance reserves offered byprelubes and the prelube-compatible washing oils used for outer skin panels canprovide decisive benefits Figure 15.23 illustrates the advantage of a prelube-type oil

ex-in comparison to a corrosion preventive mill oil on uncoated steel strip ably lower friction combined with a smooth flow of material up to higher loads isachieved in the strip drawing test without deflection when the prelube is applied

Consider-569 15.1 Sheet Metal Working Lubricants

Because cold rolled steel strip for car bodies is to an increasing extent zinc-coated,the prelube needs to be as versatile as possible with regard to various metal coatingssuch as electro-galvanized, hot-dip galvanized, galvannealed or pre-phosphatedsheet metal surface Different lubricants do not necessarily display similar behavior

on all coatings An example is given in Fig 15.24 where two prelubes are compared

on different substrates While the performance of both lubricants on uncoated sheetmetal is quite similar, the coefficients of friction differ dramatically on zinc-platedsteel [15.149, 15.150, 15.151]

Lubricants with optimum run-off characteristics are solid films known as dry filmlubricants However, the application of such products in the finishing lines of therolling mills as well as problems in removing or corrosion protection have turnedout to be the major obstacles for their widespread use up to now

Fig 15.23 Friction coefficient vs load with type of oil as parameter, uncoated sheet metal/ 50 mm s –1

Fig 15.24 Friction coefficient vs load with type of prelube and

surface coating as parameter, 50 mm s –1

570 15 Forming Lubricants

15.1.12.8 Transport and Storage of Pressed Parts

Pressed panels do not usually enter the manufacturing process immediately but arestored for a period of time In-house storage does not pose any particular demands

on the corrosion protection oil used However, if the pressed panels have to be ported to other plants, overseas, through or into tropical regions, particularly highperformance concepts have to be used A high-performance, thixotropic prelube canmake special protection oils unnecessary if it is combined with an intelligent pack-ing concept and vapor phase inhibitors (VCI) [15.19] The use of run-off-retardingproducts throughout the manufacturing process is especially worthwhile in thepost-pressing stages because oil no longer runs off during transport and storage.This feature is emphasized by the lower film thicknesses of prelubes and prelube-type low-viscosity washing oils

trans-15.1.12.9 Welding and Bonding [15.33]

Welding is not influenced by today’s corrosion protection oils and pressing cants and therefore poses no particular requirements However, this does not apply

lubri-to the bonding processes used in body building because of the substitution of anincreasing number of spot welds by adhesives and the increasing demands on thedurability of such bonded joints Of importance is the strength of the joint – up to itsbreaking point – but also its corrosion protection and its sealing properties As oiledpanels are bonded, the type and quantity of the oil, physical surface characteristics

of the metal and naturally, the type and thickness of the adhesive effect the bondingresult As the automobile manufacturers use a number of different adhesives intheir plants for different purposes and all combinations of metals, adhesives andapplication methods have to be compatible, the corresponding testing proceduresare enormous The formulation of a mill-applied corrosion preventive oil or prelubewhich has displayed optimum compatibility cannot be constantly adjusted to suitnew adhesives, not least because it may be used by a large number of companies Asdescribed, non-thixotropic or over-applied oils can run-off pressed panels and collect

in beads and folds In spite of originally positive compatibility tests, this can lead topoor bonding if the oil bead is too thick Here again, the specification of run-off-retarding mill oils but also thixotropic washing and drawing oils can eliminate suchproblems

15.1.12.10 Cleaning and Phosphating

Oil which is trapped in cavities is difficult to remove with the normal treatment cesses Usually, the cleaner spray does not reach these points Dip cleaners offerbetter performance with regard to removing oil from these inaccessible points butthey foam excessively when sprayed In spite of the use of spray and sprayed dipcleaners, components must be degreasable by simple dipping to remove the oiltrapped in cavities The oil must be easily removed from all surface qualities evenafter long-term storage or temperature induced aging during the pre-curing of bodyadhesives

pro-571 15.1 Sheet Metal Working Lubricants

15.1.12.11 Cataphoretic Painting

Even though mill-applied oils can generally be excellently removed, small quantitiesbeing carried into the electrophoretic paint bath is unavoidable The potential harmcaused by such contamination can be lessened to a certain extent by special additives

in the paint tank The danger of serious, i.e visible markings in top coats is bestminimized by the use of the most compatible oils A wide variety of contaminationtests are performed to establish an oil’s compatibility with electro-deposited paints.Usually small quantities of oil are emulsified in a paint bath After storing, the sizeand number of craters in the cured paint film are evaluated Excessive quantities ofoil in beads, swages and other cavities resulting from run-off can also be expelledwhen trapped water and cleaners are boiled off in paint drying ovens If the oil–water mixture is spattered onto wet and sensitive paint, serious paint irregularitiescan occur In the laboratory, this effect is simulated by the oil spray or blow-out test

As opposed to the contamination tests, the results however cannot be quantifiedwith figures but only by a relative evaluation

15.1.12.12 Savings Potential using Prelubes

The use of prelubes in steel mills reduces the number and quantity of spot cants required for pressing operations The savings potential of prelubes is not justthe result of one less lubricant required The true benefits are only available if thecompatibility principle is applied throughout the manufacturing chain, beginning

lubri-in the steel mill and up to the palubri-int shop It is therefore hardly worthwhile uslubri-ing ahighly specialized product for every application at the cost of subsequent manufac-turing stages It is much more important to optimize all the manufacturing stagestogether A suitable prelube offers the potential, but the biggest savings result from

a more rational overall manufacturing concept This would result in a far-reachingmultifunctionality for the corrosion preventives and lubricants used for different ap-plications, i.e specific characteristics under the umbrella of overall compatibility.Today’s prelube systems are therefore modular so that even different viscosities can

be part of the same concept

Since their introduction, thixotropic, run-off retarding oils have outperformed ventional oils It remains to be seen if dry film lubricants offer additional advantages.15.1.12.13 Dry-film Lubricants

con-There is still a growing demand for even higher-performing lubricants as high andhighest-strength steels are increasingly used for sheet-metal forming Complex partdesign in high-strength steel is a challenge for press shop managers making use ofconventional mill oils and drawing lubricants

As discussed in Section 15.1.12.1, the idea of combining the mill-applied sion-protection properties of a corrosion-preventive oil with the lubricity of a draw-ing oil led to the development of the prelube oils Nevertheless, the film stability ofthixotropic oils is not perfect and may be further improved if the lubricant isenhanced by incorporation of an additive with a semi-solid or waxy consistency Oil leak-ing out of coils and running off pressed parts does not occur with firm films Theremust, however, be no deterioration of other beneficial properties of the coating

corro-572 15 Forming Lubricants

Automotive press shops have had much longer experience with drylube-coatedaluminum strip than with steel strip Water-based and hot-melt drylubes have beenused on aluminum for some years For reasons of compatibility, however, the mem-bers of the Association of German Automobile Manufacturers (VDA) prefer thesame drylubes for both steel and aluminum; this precludes use of water-borne sys-tems, because of their lack of corrosion protection For optimum versatility, the dry-lube should be suitable for all sheet-metal qualities – uncoated steel or aluminum,hot-dip or electro-galvanized steel, pre-phosphated steel and composite material, forexample polymer-coated sheet metal and double-bonded strip The major require-ments for the automotive sheet metal forming process are summarized in the VDAstandard VDA 230-202 [15.141]

All drylubes have in common the solid state, or at least a waxy consistency, atambient temperature The lubricant must have a fairly low viscosity for ease of appli-cation to the metal surface, however Solids or waxes are difficult to apply and areusually thinned before application [15.148]

For environmental and health reasons and the danger of fire, hydrocarbon vents are no longer appropriate Aqueous dispersions of wax or other solid organiccompounds, for example polymers, have the desired liquidity When the water hasevaporated a wax or polymer film is formed To accelerate this process, the freshlycoated metal strip must be heated for curing; this is costly and sometimes impossi-ble, because of limited space for the necessary heaters Emulsifiers are also needed

sol-to disperse the product in water This makes the finished film susceptible sol-to tration or removal by water; this can cause severe corrosion of steel substrates Waxdispersions are, therefore, predominantly used for lubrication of aluminum or stain-less steel for flat forming or tube hydroforming

pene-Apart from dilution, if a solid lubricant melts at a reasonably low temperature itmay be thinned by heating, to enable spraying This procedure is used when specialwaxes are used as bases for hot-melt drylube The hotmelt is commonly applied byelectrostatic spraying at the finishing lines of the rolling mills Existing equipmentcan be used but must be adjusted to higher spraying temperatures The hotmeltcan, however, serve just as well as a drawing lubricant in press shops using speciallydesigned spraying equipment for spot-wise application Squeegee rollers can also beused if an homogeneous film is wanted In addition to their major technical merits,ease of application is certainly a reason for the popularity of hot-melt drylubes in thesheet-metal-forming industry

The most obvious requirement for a lubricant is that it be present as a neous layer of adequate thickness in the zone of deformation A thinly and evenlyapplied drylube meets this prerequisite almost ideally The improvement in lubricitycan be verified by strip-drawing tests Figure 15.25 shows the graphs obtained whenelectro-galvanized (EG) sheet metal is drawn with 90 deflection The lubricity of thewaxy drylube film is a significant improvement even over that of the latest thixotro-pic prelube-type oils; this often enables cost-saving because of reduction of filmthickness

homoge-573 15.2 Lubricants for Wire, Tube, and Profile Drawing

prelube oil Anticorit PL 3802-39 S

hot-melt drylube Anticorit PL 39 SX

hot-melt drylube Anticorit PL 39 SX

Fig 15.25 Results from strip drawing The lubricant was

1.5 g m –2 hot-melt drylube Anticorit PL 39 SX, the substrate

was: EG, the die temperature was 40
C, and the speed

10 mm s –1 , with deflection.

15.2

Lubricants for Wire, Tube, and Profile Drawing

Theo Mang and Wolfgang Buss

Tab 15.5 Areas of application.

Low-carbon steels Wires, wire netting, nails, rivets, pins, screws

High-carbon steels Wires and rods for the massive forming and free-cutting

machining, wire ropes Alloyed steels Springs, welding rods, special components, steel cord

for tires Copper and copper alloys Wires, pipes and components for the electronic industry,

wire netting, screws and moldings Aluminum and aluminum alloys electric transmission lines, screws, moldings

574 15 Forming Lubricants

Solid or hollow bodies are generally drawn through a stationary drawing tool ing die, drawing nozzle, drawing ring) to reduce the cross section or change thecross section geometrically

(draw-The drawing hole is the opening in these drawing tools

The drawing of wire, in theory, is comparable with the drawing of round rod Whendrawing both, rod with non-circular cross section and pipe, the basic knowledge of fric-tion, lubrication and wear is employed for better defined round cross sections

In the case of wire and profile drawing the sliding friction between the surface ofthe workpiece and the drawing hole surface is reduced by lubrication, but whendrawing pipe with an inner tool the inner wall and inner tool also have to be lubri-cated

Wire drawing in the case of the slip-type drawing process has outstanding nomic significance Typical lubricants and their application are also used for draw-ing pipe and profile and as a result are to be studied more closely in the following.15.2.1.2 Friction and Lubrication, Machines and Tools when Wire Drawing

eco-In no other forming process the lubrication, the machine and tool technology are soclosely associated with each other as in the case of wire drawing We differentiatebetween dry and wet drawing, immersion and extrusion drawing machines anddrawing dies for dry and wet drawing; three different types of lubrication technologyare applied:

. Dry drawing: (in wire drawing this term does not mean the absence of cant but the use of solid, not liquid lubricants) with dry drawing soaps in thedrawing box These lubricants only lubricate the drawing die and hardly everhave a cooling function

lubri-. Lubricated drawing with pasty or high viscosity lubricants: which means drawinggreases, drawing oils or drawing emulsions

. Wet drawing: Wet drawing is generally done with oil-in-water emulsions, dom with non-water miscible oils, however, quite recently, also with special,highly viscous, degradable ester oils

sel-In wet drawing the task of the lubricant is to lubricate and cool the drawing toolsand keep the machines clean A further criteria for lubricant selection is the thick-ness of the wire

Classification of Wire Drawing by Wire Thickness

National or international standards such as ISO DIS 22034-2 or ADDMA 8911 areapplied for wire dimensions Trivial classifications are also still in use Where steelwire is concerned we differentiate between the following dimensions, for example

in Table 15.6

575 15.2 Lubricants for Wire, Tube, and Profile Drawing Tab 15.6 Dimensions of steel wire.

Coarse wire (special dimensions) 20–12

Coarse wire (rolled wire) 12/8–3.5/1.5

15.2.1.3 Drawing Force and Tension

The cross section is reduced in the drawing hole (Fig 15.26)

If, for example, a wire 1 mm2thick is drawn through a hole 0.8 mm2then e is 0.2(or 20 %) If a section of wire is stretched from 1 mm to 1.2 mm, then k is 0.2 (or

20 %) At the same time this reducing operation means a considerable increase inthe surface area Accordingly, the sliding friction in the drawing hole takes placeunder these difficult lubricating conditions

In the wire drawing machine the drawing force FZis applied by a drawing disc(best known as cone) which causes the reduction in the drawing hole and overcomesthe counter pull on the wire infeed side Frequently the proportion of the drawingforce related to the reduction in cross section is stated namely the drawing tension

rZ It can be well understood that the drawing tension must not be greater than theyield stress or the tensile strength of the wire so that the wire does not tear To beable to work reliably at high drawing speeds the drawing tension must be clearlylower than the yield stress so that fluctuations in material properties and the coeffi-cient of friction can be compensated The drawing tension relationship has beendefined to indicate the necessary safety allowance [15.35]

The Siebel formula is to be mentioned as a further theoretical issue, according towhich the drawing tension is broken down into one part for forming without loss,one part for friction and one part for displacement (or shearing) [15.36, 15.37] The

576 15 Forming Lubricants

Siebel equation is given by ignoring the back pull If the amount of friction is sidered then this increases with the coefficient of friction l and the reduction incross section e and declines with the drawing angle a Since the drawing tension forproportion of displacement increases with the drawing angle, it can be seen thatthere must be an optimal angle a for the drawing tension Not only the significance

con-of the drawing hole geometry can be seen from this but also that a minimum indrawing tension over the drawing angle does not indicate a minimum of friction.The course of the drawing tension over the drawing angle is given by taking a brassmaterial (Fig 15.27) with the assumed prerequisites whereby the minimum isapprox 20, i.e the hydrodynamics are favored at this angle

In the area of wire drawing, empirical values have had a greater influence on theoptimization of the drawing hole geometry than theoretical issues On the otherhand in practice it is not the drawing force but the surface finish and especially thedrawing die wear that is the prime consideration

Fig 15.26 Friction and drawing tension when drawing wire m 1 = initial speed;

m 2 = final speed; F Z = drawing force;

F N = normal force; r Z = drawing tension; F R = friction force; k f = average deformation resistance; e = reduction of cross section; a = half reduction angle (drawing angle); l = average friction coefficient.

Fig 15.27 Dependence of drawing tension on drawing angle.

577 15.2 Lubricants for Wire, Tube, and Profile Drawing

15.2.1.4 Drawing Tool and Wear

Drawing tools (drawing dies and drawing rings) are made of steel [ISO 1684], hardmetal [ISO 1973], diamond single crystals or polycrystalline diamonds (PCD) [DIN1546] PCD layers consist of randomly oriented diamond particles sintered together

in composite materials backed with cemented tungsten carbide

Steel drawing dies are used especially for drawing steel rod and pipe drawing.Hard metal drawing dies are used mainly in the steel wire drawing sector Smallerdiameter wire is still almost only drawn with drawing dies made of sintered hardmetal Diamonds are used in the medium wire sector but above all for drawing fineand finest wires in copper, steel, molybdenum and tungsten Pressure sintered hardmetal alloys have proved particularly successful for wet drawing in the hard metaldrawing die sector

The porosity of normal-sintered drawing dies, as usually used in dry drawing, isdetrimental to drawing and leads to unfavorable wear behavior Pressure-sintereddies are a further advantage because they can be polished better

Of decisive significance for the wear of natural diamonds, alongside the degree ofinhomogeneity, is the crystallographic orientation [15.38] Polycrystalline sinter dia-monds [15.39] have gained significance for the profile drawing, for the mediumwire drawing sector, and to some extent, also for drawing fine wire Compared withsingle crystals these have the advantage of orientation-independent wear properties,are more resistant to impact and have higher heat conductivity

Fig 15.28 Drawing hole geometries.

578 15 Forming Lubricants

When designing the drawing hole geometry, the theoretical and empirical values

of lubrication film development have to be considered as well as the specific criteria

of different wire materials The different drawing hole geometries of hard metaldrawing dies (Fig 15.28a) and diamond drawing dies (Fig 15.28b) are generallyused In the case of hard metal dies a longer infeed entrance favors the hydro-dynamic lubrication when using dry drawing soaps, even in the case of low drawingspeeds (Fig 15.29) Limit analysis of flow-through conical converging dies is known

in detail [15.142] General drawing die geometry details are found in DIN 1547

In the case of thin wire, drawing die wear is used to assess the drawing lubricant

as well as the number of wire cracks The causes of drawing die wear are very plex in nature and depend on the influence of the machine engineering and on for-eign substances which are carried in the lubricant or on the surface of the wireFig 15.30) Different forms of drawing die wear, which correspond with the differ-ent causes, are known, the extent of which also depends on the nature of the drawnmaterial

com-The possibility of the lubricant reducing wear is especially given in the area ofboundary friction and the prevention of cavitation

The service life of drawing dies is a significant factor for economic wire drawing.Giving the wire weight as a reference variable has become established practice inthe coarse and medium wire sector despite comparable details only being availablewith exactly the same wire thicknesses Typical figures for this are, for example, 70tons copper wire with a final diameter of 2.50 mm and a drawing speed of 17 m s–1,and the figure is 50 tons in the case of final diameter of 1.60 mm and 30 m s–1drawing speed The expansion of the final drawing die is in this case max 0.04 mm

Fig 15.29 Drawing hole, shape of diamond dies (a) shape used for hard wires (high carbon steel, stainless steel, tungsten); (b) shape used for copper wire; (c) shape used for soft wires (gold, silver, aluminum).

Fig 15.30 Modes of die wear (a) annular wear, especially from

soft materials; (b) grooving caused by hard particles in the

drawn material or in the lubricant, cross section; (c) expansion

of the die, roughened; in the case of diamond, single crystals

change of shape, cross section.

579 15.2 Lubricants for Wire, Tube, and Profile Drawing

A clearly better service life is achieved through the use of polycrystalline sinter monds, with re-polishing being necessary in each case after drawing 50 tons withoutwidening occurring

dia-A more expedient reference value, namely the wire run-through length (that is tosay the wear path) for the drawing die service life, is very seldom used With goodbasic wire material, well set-up machines and good lubricating conditions, drawingdie control and change, can be determined independent of diameter, for example,

12 000 km [15.40] With a final diameter of 0.50 mm this means a drawing die city of 21.6 ton wire but only 1.92 ton wire with 0.15 mm dia

capa-15.2.1.5 Wire Cracks

To assess the suitability of lubricants in the case of wet drawing of fine and finestwire, besides the wear phenomenon on the drawing dies and the quality of the wiresurface, the frequency of wire cracks is also applied There is a large number ofcauses of wire breaking of which only a few can be associated directly with the lubri-cant or the lubrication system such as, for example, deficient lubrication (applyingtoo little lubricant or inappropriate lubricant performance) or the development ofresidue (inadequate rinsing and washing effect of the lubricant) in the drawing cone

of the tool On the other hand overrolling (rolling faults) or wire inclusions rities from the melts) cannot be influenced by lubrication

(impu-15.2.1.6 Hydrodynamic Drawing

There has been no lack of studies on the influence of hydrodynamic conditions onwire drawing It can be assumed that the hydrodynamic friction in the drawing holeplays an essential role at high drawing speeds (Fig 15.31) Nevertheless, the effect

of polar and wear protection additives is of greater significance for the boundarylubrication conditions and to avoid stick–slip [15.41]

A number of changes has been made directly on the drawing tool or to the cating system to completely separate the wire from the drawing hole wall However,

lubri-as is also the clubri-ase with other forming processes, inadequate or no contact betweenthe workpiece surface and tool leads to a mat or even rough surface finish As aresult, the use of hydrodynamic drawing by sliding action is restricted to prelimin-ary or interim drawing

In the case of wet drawing the pressure of the lubricant can be applied in ple by pumps [15.42], which, in turn, results in considerable expense In the case ofliquid lubricants special wire infeed devices can be used while for dry drawing mul-tiple pressure drawing dies can be used Well-known infeed processes are the Chris-topherson method for wet drawing and Bisra for dry drawing soaps [15.43] Fig-ure 15.31 shows a pressure drawing die with infeed pipe for drawing under hydro-dynamic conditions

princi-15.2.1.7 Wire Friction on Cone

In single drawing machines the wire is drawn from the drawing drum through thedrawing hole, but only sliding friction occurs in this case As far as multiple drawingmachines are concerned there is a cone between two drawing dies which, due to the

580 15 Forming Lubricants

stretching of the wire has to turn faster than the cone in the proceeding drawingstage The wire stretch can alter as a result of drawing die wear This means that thespeed of the cone has to be adjusted while drawing, or the changed drawing speedhas to be taken up on the cone as collection wire

As an alternative, the changing length of wire can be compensated by wire slip onthe cone on so-called slip-type drawing machines The slip occurs with the slidingfriction between the wire and cone (Fig 15.32) As a general rule, the greatest slip is

on the first cone (up to 30 %) whereas slip no longer occurs on the last disc

In the case of wet drawing the lubricant lubricates, cools and keeps the cone clean.The friction between wire and cone is taken up by the cone torque and as a result isconcerted into drawing tension The force available for pulling through the drawinghole is reduced by this, which is why the deformation per drawing stage by slip-typewire drawing is less then when compared with the non-slip method

The transfer of the force to the cone is in accordance with the rope friction law(Fig 15.33) The coefficient of friction must be adequate in size to be able to trans-mit force No force can be transferred in the case of l = 0, i.e F1= F2 The smaller l isthe larger is a However, the number of wrappings is limited to ensure trouble-freerunning With smooth coarse wires the number of windings can be as high as 7 Inthe case of super finest drawing, the numbers of coils can even be less than a fullwrapping and the wrapping angle a must be set by special jigs, for example, to300

Fig 15.32 Contact between wire and cone – sliding friction on slip-type drawing machines.

Fig 15.31 Triple stage pressure drawing die Drawing under hydrodynamic conditions.

581 15.2 Lubricants for Wire, Tube, and Profile Drawing

Cone wear can quickly lead to disc failure if the same zones are continuouslystressed To avoid wear grooves the wire can be moved on the cone by pushing thedrawing die to and fro This results in a more even wear and a considerably longerservice life for the cone

The running surfaces of the cones, the so-called drawing rings (also called ing cones) can be made of different materials, depending on the application Metaldrawing rings, for example, made of flame plated materials are used frequentlywhen drawing copper wire on coarse wire machines Generally, ceramic drawingcones are used for medium, fine and finest wire Even compound constructionsmade of ceramic and steel play a role, especially with thicker medium wire In thecase of ceramic materials these are often aluminum oxide or zircon oxide with lowaverage roughness (Rabetween 0.3 and 0.1 lm)

draw-Slip-type wire drawing machine can be different in design and are not so cated in construction Non-slip type wire drawing machines can be operated withoutsliding friction on the cones but have to have a sensitive speed control system forthe cones

compli-As a general rule, in the case of tandem arrangements the individual discs aremounted next to each other In the case of cones of uniform size the speeds (or theangular speeds, x) are stepped according to the wire length With machines work-ing on the cone principle these are very compact in construction On a third type,cones of the same size are mounted on a shaft but run via gears at different speeds.Compared with the cone system this type of machine has large cones which offeradvantages for the surface of the wire, especially in the coarse wire sector because ofthe lower bending stress of the wire

Modern wire drawing machines on the tandem or cone principle are designed asmulti-wire machines, especially in the fine wire sector for copper drawing where up

to 40 wires can be drawn in parallel Thus a 24 wire drawing machine with 31 dieseach wire holds 744 dies at the same time In this case the application of the lubri-cants calls for the lubricant to be directed accurately to the required spot [15.44] To

be also considered are the increasing demands put on lubricants (for example, bility to aging, abrade carrying capacity, filterability, etc.) Moreover, an adequatelylarge volume of lubricant must be available and provisions must be made for cooling

sta-Fig 15.33 Chained forces before (F 1 ) and after (F 2 ) cone The equation

of rope friction is applied: F 2 = F 1 e la , where l is the coefficient of friction and a the wrapping angle.

582 15 Forming Lubricants

where necessary In the case of copper wire drawing, for example, the working ume of emulsion can fluctuate between 0.5 and 2.0 m3for each wire

vol-15.2.1.8 Lubricant Feed in Wet Drawing

Depending on the type of machine, a choice (Fig 15.34) is given between two basicprinciples, the immersion machines and the spraying machines, which are usedmore frequently today

There are no consistent rules for lubricant delivery The users proceed in manyvaried ways Either the lubricant is supplied to a circulating central lubrication sys-tem or a separate circulation system is used for each individual machine

Immersion machines provide advantages for thicker wire drawing or for larly difficult to form materials with high heat development Hollow draft can occur

particu-at higher drawing speeds over approx 20 m s–1 This can cause inadequate tion, especially when using lubricants which contain finest dispersed or dissolvedair

lubrica-The cones are only partially immersed on special types of immersion machines(Fig 15.34b) Only the drawing dies are flooded in other applications (for examplewith super-finest wire) not the cone (Fig 15.34e)

15.2.1.9 Dry Drawing

Dry drawing is applied most frequently for drawing steel wire Dry drawing cants (also called drawing soaps, drawing stearates or drawing powder) are applied

lubri-Fig 15.34 Lubrication feed in wet drawing – lubrication,

cooling, and cleaning of drawing die and cone (a) immersion–

immersion, (b) immersion–immersion, (c) flooding–spraying,

(d) spraying–spraying, (e) flooding–dry.

583 15.2 Lubricants for Wire, Tube, and Profile Drawing

as a general rule through drawing boxes (Fig 15.35), which are located immediately

in front of the drawing die The applied lubricants must have the necessary powderfineness for the respective wire, which means it must be neither too coarse nor toofine in order to enable sufficient quantities to get into the drawing hole

The granulation must be selected so that there is no tunnel forming The drawingsoap must neither melt during the drawing operation nor be baked but must allow easylayering and remain in motion and an optimal heaping angle must set in (Fig 15.35).Frequently, it is advisable for the drawing die, which is not cooled by the dry drawinglubricant, to be cooled by special devices, for example, by water cooling (Fig 15.35)

15.2.1.10 Applying Lubricant as Pastes or High-viscosity Products

The application of lubricants as pastes or as very viscous oils or emulsions follows,

in circulating systems in so far as this is ensured by flowability and suitability forpumping The drawing machine also has to be appropriately designed for this Fre-quently, a drawing box is used for the lubricated drawing if no other devices areavailable, which causes sealing problems, in particular at the wire intake of thedrawing box

15.2.2

Drawing Copper Wire

In the wire drawing of non-ferrous metals, copper wire drawing still plays an tant role, especially because of its outstanding significance in electrical engineer-ing – although copper wire has lost some of its significance in the wake of the intro-duction of glass fiber cable and has been replaced by aluminum wire in other sec-tors

impor-Copper wires are produced in final thicknesses between 20 mm and 10 lm It isnot possible to work with the same cross section reduction e: coarse wire e
25 %,super finest wire e
9 % This corresponds to wire lengthening k
33 % for coarsewire and k
10 % for super finest wire

Dry lubricant

Cooling water

Drawing die

Wire Drawing direction

Fig 15.35 Dry drawing using a drawing box with die cooling.

584 15 Forming Lubricants

The production of copper wire with different tensile strengths is only possiblethrough the forming process It is not possible to quench and temper later as is thecase with steel This is why working with very different yield stresses is necessary inthe case of copper, which means very different pressures in the lubrication gap Thetensile strength of hard or spring hard wire is > 370 N mm–2 and 200 to

250 N mm–2with soft copper

The basic wire material and its production have a great effect on friction, wearand lubrication when producing wire An old, widespread method of producingrolled wire is based on cast billets These are welded together and subsequently hotrolled In the case of the Southwire method the molten copper solidifies in thegroove of a casting wheel The resulting copper rod is hot rolled directly afterwards(conti-process) In the case of the dip forming process a peeled copper wire is drawnthrough a copper belt and hot rolled afterwards, whereby the immersion and therolling process is carried out in an inert gas atmosphere Produced are wires with anoxygen content of less than 20 mg kg–1

Hot rolling leads to scaling of the wire surface with copper oxides, except in thecase of dip formed wire This oxide layer must be removed by pickling before thefollowing drawing operations Mixed acid is used for pickling Wire with a mat sur-face is obtained, with a characteristic pickled profile and typical porosity Smoothsurfaces free of pores can only be reduced by wire peeling Dip-formed wire on theother hand has a clean, smooth surface almost without pores right from the outset

It is used in an unpickled and unpeeled state for wire drawing and is suitable cially for further processing to produce fine and finest wire

espe-Because of the different basic materials there are also various application ogy consequences concerning the lubrication of the drawn wire As a result, theworking requirements for the lubricant and lubricating system depend not least onthe main qualities of the wire Dip-forming wire, for example, demands a higherquality lubricant as a rule, or at least a higher application concentration

technol-15.2.2.1 Lubricants

Water-mixed lubricants are used exclusively Just in the first draft of coarse wire, aspecial lubricant of high viscosity is frequently used in the lubricated drawing or,alternatively, the concentrate used in a following wet drawing is used in a pre-draw-ing block to ensure more favorable lubricating conditions at the given low drawingspeed The lubricant used for this application must be compatible with the lubricantused afterwards

Emulsions are the most important of all the lubricants in wet drawing Apartfrom these, tenside solutions, free of mineral oil and fatty oil, are significant To asmall extent other synthetic solutions are also used The three groups of lubricants(see Table 15.8) can be characterized on the basis of the most important ingredients

585 15.2 Lubricants for Wire, Tube, and Profile Drawing Tab 15.8 Lubricants for wet drawing copper wire.

Type of lubricant Ingredients

Emulsions Hydrocarbons, mineral oil

Natural fatty oils Synthetic esters Nonionic surfactants Anionic surfactants Stabilizers, inhibitors Antifoam agents Other additives Tenside solutions Alkali soaps

Alkali salt of sulfated fatty oils Nonionic surfactants Other additives Other synthetic solutions Polymers

Organic salts Inorganic salts Other additives

In practice these differ mainly through their degree of dispersion or their ity properties in water

solubil-The emulsions can vary from relatively coarsely dispersed emulsions with an age drop size of approx 5 lm to the finely dispersed emulsions of an average dropsize under 1 lm These are colloidal solutions where soap and tenside solutions areconcerned The other synthetic solutions can be real solutions or partial colloidalsolutions [15.45, 15.46]

aver-The planned use of the drawn wire has an influence on the choice of lubricant,depending on wire thickness and material quality The use of higher quality lubri-cants was a parallel development to higher drawing speeds and the greater demandfor surface quality In addition, greatest importance is being attached to cleanlinessand enamel adhesion, particularly in the enameled wire sector

Some 100 drawing stages, which have to be lubricated, have to be run through toproduce 10 lm super finest wire from 8 mm thick coarse wire, whereby drawing speeds

up to 50 m s–1are achieved The mean drawing speed comes to 20–25 m s–1

Working with a standard lubricant for all stages is just as impossible as using adifferent lubricant for each stage As a result, present day drawing plants only gen-erally differentiate between two groups of wet drawing lubricants: lubricants for thecoarse and upper medium wire sector and those for the medium and fine wire sec-tor Occasionally special lubricants are also used for finest and super finest wire

15.2.2.2 Lubricant Concentration

Despite the many different types of lubricant, specific concentrations are appliedaccording to the wire sector (Table 15.9) These concentrations are differentiated bytheir fat content in the conventional way

Super fine drawing 0.5–2

Very often one has to compromise when using central lubrication circulation tems with neighboring drawing thickness sectors, in order to keep the costs withinlimits At least one tries to incorporate those drawing machines to a central supplyunit on which certain dimensions are to be drawn despite different quality wire(peeled, pickled and dip-formed wire)

sys-15.2.2.3 Solubility of Copper Reaction Products

Monovalent and bivalent ions of copper are developed during the drawing operationwhich react with specific substances from the lubricant Numbering amongst theseare especially the oxidation products of hydrocarbon materials, fat substances andother ingredients It is mainly copper soaps that are formed Apart from this anionicsurfactants react and copper reaction products are formed, likewise mainly soaps.The study of the emulsions used has revealed that the main quantity of copper reac-tion products are given in undissolved state and react hydrophobically and, as aresult, are found in the oil phase of the emulsion That is why the oil phase fre-quently is green in color Reaction products also form in the water phase, which aregenerally blue in color and come under the copper amine complex salts In all thecopper reaction products only contribute to a very low extent to the conductivity ofthe drawing emulsion

15.2.2.4 Water Quality and Electrolyte Stability

Very frequently copper drawing emulsions are mixed with water of low hardness toavoid initial foaming problems with the fresh emulsion Demineralized (deionized)water is used only in machines or central systems which generate very little foam inoperation A key value to assess the salt content and the degree to which salt can beadded to the drawing emulsion is the electrical conductivity generally given in

lS cm–1(micro-Siemens) or mS cm–1(milli-Siemens)

By making up the evaporation losses with the soft water the conductivity will notincreases as quick as it does with hard water, while diluting the used emulsion withdemineralized (fully deionized) water only a very slow increase of the conductivity isfound When the so-called limit conductivity is achieved the emulsion starts tobecome unstable This value varies from lubricant to lubricant and depends, on theone hand, on the type and volume of electrolytes and, on the other hand, on thecomposition and the ingredients contained in the lubricant Experience has shownthat bivalent cations (mainly earth alkalis) destabilize more strongly than the mono-