the maintenance of extrusion presses with specific reference to ...

handling systems; semi-automatic and automaticstretchers; and saws and stacking systems havereplaced many of the older, manually operatedequipment. In the intervening years, competitiveand economic pressures have resulted in the reengineeringof many features of the equipment, andthe resulting changes have not always been to thegood of its operation or its economic life.Manufacturing machinery is purchased on theassumption that it is “fit for purpose,” and that it willdeliver product at the required output and quality.The specification of any new (or used) piece ofequipment will always include the acceptancestandards that set out the teats to be met beforefinal payment is made. An analysis of competitivebids will also include a comparison of the weight ofproposed equipmentBefore any discussion of why tooling fails, it isappropriate to refer to John Ruskin’s (1829-1900)often-quoted description of “quality”:Quality: there is hardly anything in the worldthat some men cannot make a little worse,and sell a little cheaper; and the people whoconsider price only are this man’s lawfulprey. It’s unwise to pay too much, but it’sworse to pay too little. When you pay toomuch you lose a little money, that is all.When you pay too little you sometime loseeverything, because the thing you boughtwas incapable of doing the thing you boughtit to do. The common law of businessbalance prohibits paying a little and getting alot – it can’t be done. If you deal with thelowest bidder, it’s as well to add somethingfor the risk you take. And if you do that, youwill have enough to pay for somethingbetter.An examination of the failure of returned toolingand ancillary equipment reveals numerousexamples where the desire to reduce the up-frontcost of tooling has resulted in poor, and, in somecases, plain “wrong” engineering design.HOW TO MAINTAIN EXTRUSIONPRESSESThe press systems described in the introductionare a combination of many components operating inunison. Whereas earlier presses operated withseparate, discrete sub-systems including furnaces,run-out, handling, stretcher, and saws therebyallowing for some operation in the case ofbreakdown or adjustment, the overall reliability ofthe modern system is no better than the reliability ofthe weakest link.In recent years, extrusion press systems havebecome more reliable through the use ofprogrammable logic controllers and sophisticateddiagnostic software programs. Improvements to theengineering of components such as belts,couplings, valves, and micro-switches haveincreased the mean time between failures (MTBF)of these components. The development of noncontactposition sensing devices has removedsome components from the system. Understandingof the mechanisms of wear has resulted in thedevelopment and use of new lubricants, andincreasingly stringent requirements for the control ofcleanliness and temperature of hydraulic fluids.The combination of these developments hasimproved inherent reliability and has permitted theintroduction and development of automatedextrusion press systems. The downtime of asystem of log or billet racks, furnace, hot shear,press and run out, pullers, cooling table, stretcher,batching table and feed conveyors, saw table (withits feed conveyors), saw and stacker will, as waspointed out earlier, be dependent on the reliability ofeach of the component working parts.In planning to improve reliability, the design andmaintenance engineers must consider the inherentreliability of each part, the probable mean timebetween failure of each part, the number of workingparts, and the time taken to fix a problem. Aproblem that only takes a few seconds to correct isnot much of a problem if it occurs infrequently. But ifit recurs with every billet pushed (as with a billetloader that must be manually assisted, a buttdiscard that fails to separate from the shear, orspacers that do not feed consistently to the stacker)it will require the undivided attention of an operatorat some time during each working cycle. Again, thereliability of the system is a function of the reliabilityof the sub-systems.Press AlignmentThe alignment of press components must bekept within tight tolerances. Improper alignment isone of the causes of inconsistent die performance;it causes premature failure of fixed dummy blocks

and, in extreme cases, damage or breakage tostems, containers, and tie rods.Modern, self-contained extrusion presses areconstructed with a fabricated steel base extendingthe full length of the press. The rear platen is fixedto the fabricated base. The front platen, connectedto the rear platen by the tie rods, is keyed to thebase, but free to slide forward and backwardsduring each stroke of the press. Older presses,which are often constructed without the fabricatedbase, have separate foundations for rear and frontplatens.The base (and foundation) must be level and ingood condition, and the stem and container mustmove parallel to the base throughout the pressstroke. Optical (laser) instruments are used forthese measurements.Although it is often assumed that there hasbeen no change to the original symmetry of thecolumns with respect to the center-lines of theplatens, it is believed that alignment should startwith confirming the condition of the guide-ways,(including that of the front platen), the stem and diepressure plates; and the relative position of frontand rear platens, the stem and the container withthe ram in the forward and back positions.A complete alignment then starts with theplatens―with pre-stressing of the tie rods―andproceeds to the main ram, stem, container, dieslide, shear, and billet loader.Alignment can be continuously monitored, but isusually checked by press crew using the impressionof the fixed dummy block on card, which is placedbetween the block and the die face, by checkingbutt discards, or by the use of an alignment stack. [3]InspectionThere is no substitute for regular inspection.The press ways must be inspected for signs ofuneven wear. The stem, the fixed dummy block, thebutt discard, the container faces, and the shear areinspected for evidence of misalignment.As the press goes through its cycle, theinspector must look for uneven or hesitant motionsas they occur and note their possible origins (whichcould even be a misalignment of the shear blade,pressure plate, or inadequate control of die stackdimensions). Strain gauges that are fixed to thepress columns can monitor the loads on the pressand provide early warning of uneven loading. Inaddition, they are sensitive enough to indicatehesitation in the motion of the container, die slide,or shear.Attention to lubrication of press ways willminimize wear, but lubrication applied elsewhere isdetrimental to the extrusion process. Understandthat lubrication of the fixed dummy block, the shearblade, the face of the container, and the die stackshould be kept to an absolute minimum. Lubricatingthe face, or worse, the periphery of the fixeddummy block, risks introducing lubricants into thecontainer, with the attendant risk to product quality.Lubricants can be carried through to the longitudinalwelds of hollow aluminum sections. They will alsobe entrained in the transverse welds in solid andhollow sections causing welds to fail duringstretching or in service.Excessive lubrication is sometimes justified asa means of extending the life of the fixed dummyblock. The moving surfaces of the fixed dummyshould be lubricated with high-pressure lubricantbefore assembly. But, any lubrication, which isnecessary to enable the butt to separate from theface of the dummy block, is best applied to the billetbefore the billet heater or, in the case of a hot logshear, between the shear and the press.Excessive lubrication of the shear blade or theface of the container suggests that the bladeclearances are not correct; this in turn suggests thatthe length of the die stack is not precisely controlledor the die stack is not seated properly. “Show theshear blade the lubricant, and wipe it off.” [4]The development of lubricants that contain nographite or petroleum-based products has removedmany of the risks associated with the use ofhydrocarbons in high temperature, high-pressureenvironments. Furthermore, the use of benignlubricants, such as boron nitride, appears tominimize or eliminate problems that can occur whenhydrocarbons or their residues enter thelongitudinal welds in hollow sections. Boron nitride(BN) can be applied to the back-end face of theheated billet, to the internals of the fixed dummyblock, and to the butt shear.Extrusion DiesExtrusion dies, particularly hollow extrusiondies, will fail if they are not properly supported. The

platen ring, the sub-bolsters, and bolsters must beflat, round and the faces must be square. Few, ifany, first class extruders still use common diebackers due to their deleterious effects on theperformance of the extrusion die. But bolsters andsub-bolsters are usually common to a number ofhollow and solid dies. Support tooling―commonbackers, bolsters and sub-bolsters, and thepressure ring―will distort with time. The flatness ofthe working surfaces must be monitored; distortedand dished tooling must be replaced. On pressesequipped with in-line quench systems, particularcare must be taken to monitor the condition of thepressure ring due to the possibility of corrosionbetween the ring and the platen.Extrusion dies can break and fail to run properlyif they are not heated to the correct operatingtemperature. The die should be heated as quicklyas possible, without overheating the surface, andheld at temperature no longer than is necessary toaccommodate the production schedule. Properlydesigned and constructed single cell die ovensensure that extrusion dies reach the correct,uniform, temperature in minimum time. Visualmanagement systems will ensure that the correctdie is delivered to the extrusion press just in time.Figure 2. The heated die slideThe electrical heating systems must be properlycontrolled and sustained. Maintenance of thecorrect operating temperature for all die stackcomponents ensures that dies run properly from thefirst billet and that productivity and recovery ismaximized.Press ContainerThe press container is the most expensive itemof press tooling, and its design has evolved withtime. However, it is evident that some designs,presumably to reduce the cost of manufacture,have introduced features that weaken themechanical structure and promote prematurefailure.Figure 1. Die oven development, showing aninstrumented die being monitored to ensure thatoverheating cannot occur in serviceThe merits of heating support tooling are wellunderstood, but, although some extruders pre-heatthe bolster, few extruders use bolsters with integralheating systems. In most applications, preheatingthe bolster will improve the performance of theextrusion die. However, electrically heated bolsters,die slides, and ram-noses (for indirect extrusion)are increasingly being used to maintain the correctthermal gradients during extrusion operations.Figure 3. Introduction of a keyway, as opposed tothe use of an external bolt-on key, introduces astress raiser and, in effect, makes the outer surfaceof the container a redundant structural elementThe basic principles of designing thick-walledcylinders subjected to internal pressure are set outin many texts. For example, Extrusion [5] , by Laue

and Stenger, explains how using multiple layersconsisting of a liner, sub-liner, and mantle canstrengthen the container structure. It also explainshow the resulting assembly is weakened by theimproper placement of heating elements.The press container has to be preheated priorto extrusion. However, poorly controlled wraparoundheating systems are known to be the causeof premature failure since they can soften the outermantle.The wrap-around heaters are being replaced bymulti-functional heating elements inserted in holesin the container. In addition, can heaters, which areinserted into the bore of the container, are used forpreheating and maintaining the temperaturegradients.Containers and their thermal control systemsmust be inspected to ensure that the liner remainsin good condition, that the mantle is strong enoughto support the liner under pressure, and that theheaters and thermocouples are working asdesigned.fixed dummy block, so that the maximum unitpressure on the stem is higher than that on the faceof the dummy block. The stem is designed to beuniformly loaded. Eccentric loading, which canoccur for a number of reasons, may result in thestem bending or breaking. Improper pressalignment (the result of inadequate inspection andmaintenance procedures), a hanging buttpreventing proper sealing of the container, or, forexample, poorly cut billet can result in eccentricloading.Because it is highly stressed, the surface of thestem must be kept free of surface defects. Improperhandling of the stem, accidental mechanicaldamage or rapid heating or cooling of the surfacefrom direct flame impingement or stray water mustbe avoided. The surface of the stem must be free ofstress-raisers; the stem should periodically bestress-relieved.Table 1. Recommended Practices for StressRelieving StemsThe Fixed Dummy BlockFigure 4. A container that has been overheatedThe Press StemThe press stem (ram) must be properly installedon an inspected and maintained pressure ring. Thestem should be heated prior to use by advancing itinto the hot container.The alignment should be periodically checkedto ensure that the stem is concentric to thecontainer bore throughout its stroke. The diameterof the stem is smaller than the container bore, andthe face of the stem is usually designed to carry aFixed dummy blocks must be properly sized forthe press and aluminum alloy being extruded, andthey should expand and contract a fixed amount oneach pressure stroke. Single cell ovens should beused to preheat the fixed dummy block, and, whilein service, the dummy block can be kept attemperature by advancing the press ram to bringthe dummy block into the container whenever thepress is stopped.The fixed dummy block should be inspecteddaily. It should be visually checked for aluminumbuild up on the face and the land. The land shouldalso be checked for evidence of explosions. Themandrel should be free and forward from the face ofthe dummy block.Once each week, the dummy block should beremoved from the press and cleaned in caustic. Itshould be visually inspected for signs of wear, and

measured accurately across the face. Thedimension should be recorded and compared to thediameter of the block when it was new, since thedummy block will eventually grow to take up apermanent set.from the extrusion die face. Hard alloys are typicallysheared using a blunt or block shear.As the diameter of the dummy block increases,blisters will form on the extrusion. Machining theland to the original diameter can prevent theseblisters and extend the dummy block’s life.Figure 6. The butt shearPress MaintenanceFigure 5. Fixed dummy block in good condition ona 2500 ton pressAll the components mentioned above must bemechanically and thermally aligned at all times.The Butt ShearThe butt contains the surface skin of the billetthat has flowed inwards and across the face of anadvancing dummy block. At the end of each pressstroke the butt must be discarded. With thecontainer open, and the stem and dummy blockinside the container, the butt, which is still attachedto the back of the die, is exposed, ready forremoval.Modern extrusion presses are equipped toposition the butt so there is about a 0.020-inch(0.51mm) gap between the die face and the path ofthe shear blade. Older presses must be maintainedso that the relative position of die face and shearblade retain this clearance.The design of the shear blade varies with thesize of the extrusion press and the alloy beingsheared. Common alloys, with smooth as-castsurfaces and little shell zone, can be extruded downto a very thin butt. This butt must literally be peeledThe press system must be maintained to meetits performance capability. The billet furnace mustdeliver the billet at the specified temperatures. Thepress must be correctly aligned. Pressures, speeds,and clearances must be maintained to eliminate thecreation of mushrooms and flashes, and to ensurethat the butt shear cuts cleanly, and the fixeddummy block operates properly. The equipmentmust be available for operation at all times.Making equipment available for use requires anunderstanding of why it breaks down.Understanding why machinery does not workcomes from taking appropriate measurements andan intimate knowledge of machine design.SUMMARYExtrusion dies–hollow and solid–break whenthe load, and hence the stress in the H13 tool steel,exceeds its ultimate strength. Although thousandsof extrusion dies are put into use every workingday, die breakage is extremely rare. Breakages areusually limited to details that are difficult to support.However, H13 is a hot-work tool steel and must bepreheated prior to its use. Improper preheating intraditional chest ovens, where continual loading andunloading of dies disrupts the intended heatingcycle resulting in cold dies being loaded into theextrusion press, has been shown to be one of thecauses of premature failure. Whereas too low atemperature can result in breakage, dietemperature that is too high can result in prematurefailure because of excessive wear.

The press stem, although not in the true senseof the term structural column, has to withstand highcompressive stresses. Although on many extrusionpresses, the unit pressure will not exceed 80,000pounds per square inch, modern high-performancepresses consistently operate with a unit pressure inexcess of 110,000 pounds per square inch. Thefixed dummy block is designed to ensure that theloads are not concentrated.Because of the high unit pressure, the staticalignment of the press stem, container, and the diestack must be maintained within close toleranceduring the operation of the extrusion press. Thisensures that the load on the stem remains axial tothe stem.REFERENCES1. Goldratt, Eliyahu M., The Goal: A processof Ongoing Improvement, Revised Edition,North River Press, Croton-on-Hudson,1986.2. Fielding, Roger A. P., "The Maintenance ofExtrusion plant: Operating ExtrusionPresses for Maximum Efficiency," SixthInternational Extrusion TechnologySeminar, ET’96, Chicago 1996, Vol. I, 275-280.3. Robbins, Paul, “Dummy Blocks, Clean OutBlocks, Lubrication and Film Coatings, andAlignment (The Enemy),” AluminumExtruders Council Preventive MaintenanceWorkshop, April 24-26.4. Hains, R. W., Unpublished internal report1989.5. Laue, K., and H. Stenger, Extrusion,originally published in German asStrangpressen, Aluminium-Verlag GmbH,Duseldorf, 1976, American Society forMetals, Metals Park, Ohio, 1981.