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Rubber Mixers· 5 to 110 Liters· Vacuum Mixing· Computerized Controls

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TMP Inc., A Division of FrenchPiqua, Ohio U.S.A. · 937-773-3420

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Serving the WorldwideRubber Industries withTechnology, Quality & Value

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Optimizing mix quality and productivity By Quentin HartleyHF Mixing Group, Farrel Ltd.

The mixing process can be separatedinto two components, distribution anddispersion.

Distribution is the action of mixingfillers evenly throughout the rubber ma-trix so no area of the matrix is eitherrich or poor of filler. Essentially, this isachieved by moving materials aroundthe mixing chamber.

Dispersion is the breaking down of thefiller particles (often large agglomera-tions of particles stuck together by weakbonding) into their lowest possible par-ticulate state—achieving a state of ho-mogeneity throughout the rubber ma-trix. This is achieved by tearing, squeez-ing and rolling materials within the cham-ber, subjecting them to stress, strainand shear.

It is possible (and often the case) to

have well distributed material with poordispersion, or poorly distributed materi-al with areas of high dispersion, and it isimportant to recognize the significanceof achieving both for a good mix.

These two actions are performed indifferent ways with the different mixingtechnologies.

The tangential mixer achieves its dis-persive function in much the same wayas the rolling bank in a two-roll mill.The mixture is rolled in front of the ro-tor wing tip between the tip and thechamber wall, being squeezed and de-formed as it passes over the tip. Distrib-ution is carried out by the rotor wingspushing material down their length tothe opposite end of the chamber, and bytransferring the material from one rotorto the other in the “window of interac-tion” between the rotors.

The intermeshing mixer achieves itsdispersive function in a similar (but re-

duced) way between the rotor and thechamber wall, but more so by the “squeez-ing” action that occurs between the ro-tors. Distribution is achieved again bythe rotor wings (or “nogs”) pushing ma-terial down the length of the chamber,and by the physical pushing of materialsfrom one rotor to the other.

Although the same functions are per-formed, the relative amounts of each aregoverned not only by the rotor-to-sidewall interaction, but also by the rotor-to-rotor interaction. Because the method ofexecution is different, each type of mixerhas comparative advantages over the oth-er (depending on application).

In tangential mixers distribution oc-curs between the rotors and the sidewall and from rotor to rotor, and disper-sion occurs between the rotor wing tipsand the side wall. There is good materialintake because of the greater gap be-tween the rotors.

This also assists in a rapid dischargeof the batch. Because of the gap betweenthe rotors (a greater “rotor center” dis-tance) the comparative volume of thechamber is bigger, and so a higher batchweight per unit volume is achieved.

A tangential mixer compared to an in-

termesh mixer with the same batch sizewill be physically smaller. Because thedispersive action is carried out over thewing tips, and because there is less“physical” interaction between the ro-tors, the energy input per unit weight ofcompound is lower on the tangentialmachine.

In intermeshing mixers distribution isbetween the rotors and between the ro-tors and side wall, and dispersion is alsobetween the rotors and between the ro-

tors and side wall, and this gives im-proved dispersion because of the moreintense dispersive action between therotors.

There is a better temperature controlon these mixers because of the highersurface area to volume ratio in the mix-ing chamber. More metal to compoundcontact offers a greater area throughwhich to extract heat—assuming themetal surfaces can be adequately cooled.Because of these there is a greater ener-gy input per unit weight of compoundthan in the tangential machine.

Because of these differences it has tobe concluded that there is no such thingas the best mixer, but there is such athing as the best mixer for a particularcompound, or series of compounds. Eachstyle of mixer has its own particularniche in the market place, and each mix-er has been designed to fill that particu-lar niche. It is important to recognizethis fact, and to choose the type of mixerthat fits the needs of your business.

HF Mixing Group offers the widestrange of mixer types and rotor geome-tries of any mixer manufacturer: the Nseries tangential mixers, the E series in-termeshing mixers, and also the vari-

Executive summaryIn 1916 Fernley H. Banbury patented the first “Banbury” tangential mixer.

Based on the principles of the two-roll rubber mill it was intended to remove theinfluence of the operator and significantly reduce the time taken to mix a batchof rubber. Some 16 years later the first intermix intermeshing mixer waspatented, employing the same basic principles as the “Banbury.” So, what’s thedifference?

● Both types of mixer perform the same fundamental task of mixing rubbercompounds;

● Both employ the same basic principles to achieve this aim; and● Both styles of mixer are market leaders in the increasingly complex busi-

ness of mixing increasingly complex rubber formulations.It is the differences between the mixer types that provide specific advantages

to specific markets, and it is important to understand these differences tochoose the right style of mixer for the right application, which is detailed in thispaper.

TECHNICAL NOTEBOOKEdited by Harold Herzlichh

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Many rubber, plastic and silicone based products can be dramatically improved with fibers. Adding treated or untreated cotton, polyester, pan carbon, aramids and/or nylon boosts performance in a multitude of ways. Characteristics such as coefficient of friction, abrasion resistance, compression set, tensile modulus, temperature control, green strength, cut and tear resistance, noise reduction and many others can be positively altered.

Contact the fiber professionals at Finite Fiber. We will provide feasibility studies and work with you through implementation to completion.

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able intermeshing clearance mixers, orVIC.

Rotor designWithin the mixer ranges we also offer

a variety of rotor designs, tailored tomeet specific applications. The function-ing of these rotor designs deserves someexplanation.

The requirement of the rotor is to dis-tribute material around the chamber ef-fectively while dispersing the ingredi-ents appropriately. To do this it is im-portant that the materials do not stickto the rotor surfaces (sticking results instagnant areas, or dead spots within thechamber), and do not reach excessivetemperatures (causing premature cur-ing or thermal degradation).

With these considerations it is hardlysurprising that the majority of rotor de-velopment over the years has been fo-cused on cooling efficiency within themixer (Fig. 1):

● Core pipe cooling. The body of therotor is flood cooled, like the originaltwo-roll mill.

● Core spray cooling. Again, a copy ofmill cooling, but generally more even in

heat distribution than simple flood cool-ing.

● Wing tip cooling. Here the coolingwater pathway is directed, giving a bet-ter heat transfer where dispersive workat the wing tips is required; and

● Spiral tip and body cooling design(super cooling, or SC.) The ultimate incooling efficiency across the entire rotoris achieved.

Hand in hand with these improve-ments in “internal” design has been theexternal design of the rotor. Again, it isimportant to understand the functioningof the rotor wings (or nogs) within thechamber and how these affect distribu-tion and dispersion (Fig. 2).

A distributing wing is angled suchthat it pushes material across the mix-ing chamber (obviously, there will besome flow of material over the wing tipand this can be controlled by the widthand clearance of the wing tip). A dis-persing wing is angled such that materi-al will move over the rotor wing tip—anarea where it will be subjected to highshear—resulting in increased levels ofdispersion but reduced distribution.

The more dispersion is carried out thehigher the temperature rise in the mate-rial. Obviously, it is important to bal-ance these two functions to achieve theoptimum conditions for the particularcompound being mixed, and, obviously,

Fig. 1. Rotor cooling.

Fig. 2. Rotor wing configuration.

Fig. 3. Application advantages for rotor designs.

these conditions will vary for each indi-vidual compound. The different rotor de-signs therefore result in different per-formances and can be roughly dividedinto these applications (Fig. 3):

● PES rotor for intermeshing mixersgiving a high energy input per unitweight, low temperature developmentand high dispersion—good for technicalrubber goods and silica compounding;

● NST rotors—a tangential “all round”rotor designed to give high fill factor(batch weight), good overall uniformity,good Mooney reduction and relativelylow temperature development. General-ly good for high filler loading and silicamaster batching;

● MDSC rotors (maximum dispersionsuper cooled)—a tangential rotor de-

signed to give a high energy input perunit weight, high throughput, high dis-persion and high temperature develop-ment. Good for high filler (carbon black)loading; and

● ZZ2 rotors—a tangential rotor de-signed for low temperature develop-ment, low energy input per unit weightand therefore the highest possible revo-lutions during a cycle, making it the ide-al rotor for silica master batching and fi-nal stage mixing.

Machine and process designNow that we understand a little about

the differences in the functionality ofthe mixer types and rotor designs wecan start to look at the ways we can fur-ther influence the behavior of the mate-

Continued from page 23

Mixing

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Fig. 4. Energy cost of drive systems. Figures are taken from “Advancements & Fu-ture Trends in Elastomer Mixing,” by Dr. Eng. Andreas Limper at IMS 3.0 in Topeka,Kan., September 2011.

Fig. 5. Energy savings through conversion from pneumatic to hydraulic ram. Fig-ures are taken from “Hydraulic Hopper Retrofit,” presentation by I. Wilson of FarrelLtd., given in 2011.

rials within the mixing cycle. We willconsider these in the following sections:

1) the influence of friction ratios andsynchronous technology with the tan-gential mixers;

2) the effect of variable speed;3) the effect of ram pressure; and3) the effect of temperature control

units, or TCUs.Obviously there is a degree of com-

monality of these factors across the mix-er types, but now that we understandthe basic mixing principles we can startto appreciate their relevance.

Friction ratio vs.synchronous technology

The intermeshing mixers obviously donot run at a friction ratio. Although theyare even speed, a friction ratio does existbetween the rotor surfaces by nature ofthe differing diameters at any point inthe “juncture” between the rotors. Tan-gential mixers traditionally had a fric-tion ratio across the rotors mimickingthat of the two-roll mill they were de-signed to replace.

It was recognized, however, that thisfriction ratio did not always give the op-timum results, and a mixer’s perform-ance could be enhanced by keeping therotors fixed at a certain orientation toeach other.

With the NST rotor this orientationwas found to be best at 0° to 90° orienta-tion. At this orientation the area be-tween the rotors remains consistent, re-sulting in good material intake and agood current stability. Compared to oth-er orientations and to a friction ratio theeffect of the mixer “gulping” periodicallyis greatly reduced. These results havebeen seen across the range of tangentialrotors.

The use of synchronous technologyhas led to a series of developments with-in the mixing chamber to assist in thetransfer of material from one rotor tothe other—issues that were not appar-ent with friction ratios.

In intermeshing mixers, the contourof the drop door follows the radius of themixing chamber wall, but this has beenaltered in the tangential mixer to assistin material flow from one rotor to theother.

Similar modifications have beenshown to be effective around the rambottom profile, resulting in the “keel bot-tom” design.

Synchronous technology has providedsavings of up to 5 percent in energy.These modifications have resulted notonly in an increase in chamber volume—allowing for larger batch weights—butalso in improved flow around the cham-ber with the associated reduction in cy-cle times ~ 5 percent (to be confirmed)

Throughput increases in the order of5-13 percent have been reported usingthe new design of weight.

Variable speed mixingOld technology typically offered two

mixing speeds, high and low. High speed gave the associated high

shear rate and low speed the associatedlower shear rate. The rest was deter-mined by rotor geometry. Now that weunderstand the dynamics within themixing chamber we can see why vari-able mixer speed offers such a consider-able advantage over this two-speed mod-el.

Variable speed allows for the requiredamount of mixing to be done while con-trolling the temperature rise within thecompound, and this has resulted in awide range of compounds now beingmixed in a single stage where once a two-stage process was required.

There is an obvious advantage here interms of energy savings (the batch onlyneeds to be heated up once), and also avastly simplified process with reducedmaterial handling and improved produc-tivity.

Drive technology has advanced signif-

icantly over the past years (and contin-ues to improve). The example given hereshows how the use of an AC VariableFrequency Drive (1,400 kw) can offer anannual savings over a DC drive systemof about $96,000 for a 270-liter mixer(Fig. 4). Added to these savings is the

further advantage of an AC drive need-ing no maintenance.

Experience has shown us that movingfrom a fixed drive to a variable drive hasrealized a throughput increase of some20 percent before energy savings throughuse of a modern system have been deter-mined.

While we recognize that new drivessystems are not always an option, a re-view of your current systems will alwaysbe worthwhile.

Ram pressure Over the last 30 years there has been

a move away from pneumatically oper-ated ram assemblies to hydraulic sys-tems. The economic argument for themove has been based around the effi-ciencies of the hydraulic system over thepneumatic systems, where substantialsavings can be realized. On a 270-litermixer this can be as much as $256,000(Fig. 5).

Added to these savings are additionalprocessing improvements where rammovement can be adjusted to suit theparticular part of the mixing cycle, withrapid ram movement when needed anda slower movement when bulk fillers areused.

Despite a slower movement, reducedcycle times can be realized by reducingcleaning times where fillers had previ-ously been blown onto the ram top.IRAM (intelligent ram control) allowsthe speed and position of the ram to becontrolled dependant on operator set-tings for individual recipes and stepswithin the recipe.

Temperature control unitsIt has long been considered that be-

cause the mixing action produces a tem-perature rise in the compound that fullcooing is required to extract the maxi-mum amount of this energy into thecooling water.

This simplistic approach is, however,often detrimental to mix quality andadds greatly to the energy consumptionin the process.

Returning to first principles, it is im-See Mixing, page 26

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portant for the materials to move withinthe mixing chamber without stickingunduly to the rotor surface. There is anoptimum level of “slip-stick” for eachcompound that facilitates both the dis-tributive and dispersive aspects of mix-ing.

The interaction between the machine

surface and the material is paramountin energy efficient mixing.

To this end the use of temperaturecontrol units, or TCUs, to warm the in-dividual elements of the mixer are nowstandard across the rubber industry.

The mixer typically is separated intothree zones: the chamber, the rotors andthe drop door (the ram and rotor endplates are connected to one of thesezones depending on the style of mixer).Correct temperature settings can signif-icantly reduce the kwh/kg figures while

maintaining (and often improving) com-pound quality and throughput.

In the standard TCU system thepumps are run at a constant (maximum)speed, and the cooling water tempera-ture is maintained at set point by eitherusing full heating or full cooling. Newenergy-saving TCU systems recognizethat often the cooling and heating capac-ities of the TCU system exceed the re-quired need.

These systems utilize a frequency con-trolled pump motor. The pump speed iscontrolled depending on the differencebetween the inlet and outlet water tem-peratures.

The heating and cooling elements ofthe system are regulated to maintainthe required temperature setting. Inpractical tests this has been shown toreduce the energy consumption of theTCU by up to 50 percent.

Looking at the energy balance withina rubber mixing cycle when thermalequilibrium has been achieved we cansee some impressively large figure—upto 38 percent of the energy input into abatch is removed in the cooling water,up to 51 percent of the energy input intoa batch goes into increasing the com-pound temperature.

This doesn’t leave much energy left todo what we might consider to be themost important part of mixing: Actuallymixing.

Mixing philosophy“The mixer is the heart of the mill

room”The mixer is the single most impor-

tant part of the mill room because themixer is the piece of equipment that pro-duces the quality materials that yourproduction relies on. Without it youhave no product.

But what does this really mean? Allthe material handling equipment up-stream should be designed around sup-plying the mixer in a timely fashionsuch that the mixer is not “waiting” formaterials to be weighed or prepared. Ifit is, then it isn’t running to its full ca-pacity.

All the materials need to get into themixer as quickly as possible, or the mix-er isn’t running at its full capacity.Looking at power traces very quicklyhighlights where the mixer isn’t doingthe job you bought it for.

All the materials need to be removedfrom the mixer as quickly as possibleonce they are mixed so that the next cy-cle can start. All the downstream equip-ment needs to be able to keep up withthe mixing cycle so that the mixer isn’tdelayed.

Pretty simple really, but it needs to belooked at more closely (Fig. 6).

In this example we have an “average”cycle time of 155 seconds. This is dividedup into sections as follows: 25 seconds

initial feed, 30 seconds mixing, 30 sec-onds loading, 25 seconds mixing, a 10second sweep, 25 seconds mixing andaround 10 seconds to discharge.

The mixing cycle is triggered on tem-perature, and so the times are “aver-ages” rather than precise.

It would, of course, be naive to thinkthat this cycle could be reduced to the 80seconds that the mixer is actuallypulling power. It does, however, high-light where we should be focusing ourattention.

The “quality” driver is the amount ofmixing time, and improvements in effi-ciency of any other part of the cycleshould have little effect on this. There-fore, the obvious place to start looking forefficiency savings would be inconsisten-cies in total cycle time, the long (and split)load times, and the low batch weight(which is evident by the lack of ram move-ment and rapid power reduction).

We have already seen how individualaspects of the mixer design and the con-trol of particular components can carvethe cycle times and energy consumptiondown, but in this example almost half ofthe cycle time is not attributable to“mixing.” More than one third of the cy-cle is attributable to feeding materials,and the variations in cycle times makeup < 10 percent of the cycle time.

With the use of modern mixer controlmethodology it should be possible to re-duce sections of the cycle, or at worsthighlight those aspects outside the mix-er control that need attention.

The control systems within the mixingroom need to be able to monitor the com-plete process from material supply tothe end of the batch-off if comprehensivecycle time reductions are to successfullybe implemented.

Control systemsWe need to be able to monitor and

control all aspects of the mill room. Thecontrol system needs to look at weighingand feeding equipment, the mixer, themixer control system, the drive trainand the downstream equipment. All ofthese must be understood if comprehen-sive cycle time reductions and properenergy management are to be success-fully implemented.

HF Mixing Group recognizes the im-portance of this type of control and mon-itoring system throughout the millroom, and with our Advise system areable to offer all aspects of this controlmethodology.

Coupled with our upstream and down-stream equipment knowledge (from rub-ber bale cutters, strip-feeders, oil injec-tion systems etc. to convex twin screwdump extruders, mills and batch-off sys-tems) we are capable of creating a “uni-fied” mill room, utilizing the “best of thebest” approach across all aspects of mix-ing high quality materials.

Fig. 6. Example of mixing cycle.

MixingContinued from page 25

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