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Editor’s Pick: Mixing And Mix Design – Advances In Mixing Technology (Part 2)

Continuing with Part 1 of this article, by Dr.S.N.Chakravarty, President – Elastomer Technology Development Society and Ex-Chairman, Indian Rubber Institute (IRI).

Now, let us look  more deeply into PRINCIPLES OF MIXING.

PRINCIPLES OF MIXING

Vulcanizable polymers cannot be used without compounding. Various additives like curative system, protective system, reinforcing agents, cheapeners and other process aids have to be mixed to the polymer or polymer blend “to make a coherent homogenous mass of all these ingredients, which will process satisfactory and on Vulcanisation will give the product capable of giving the desired performance, all with the minimum  expenditure of machine time and energy.”

Due to the partly elastic nature and very high viscosity of rubber, power intensive sturdy machinery like mixing mills or internal mixers is necessary to achieve the mixing of additives into the polymer. The ingredients are in form of liquids, solid powders or solid agglomerates.

Phases during mixing of rubber

Phases during mixing of rubber

The mixing of solid ingredients into the solid polymer occurs in phases. During subdivision large lumps or agglomerates are broken down into smaller aggregates suitable for incorporation into the rubber.

For instance carbon black pellets which have dimension of the order of 250-2000 µm get broken down into aggregates with dimensions of the order of 100 µm. Then these aggregates are absorbed or incorporated into the rubber to form a coherent mass.

During mixing, shearing of the rubber generates shearing stress in rubber mass which imposes in turn shear stress on these aggregates and breaks these into their ultimate fine size which in case of carbon blacks is of the order of about 1µm. in size. This phase is also known as intensive mixing or homogenization in micromolecular level.

Distribution or homogenization in micromolecular level or extensive mixing is “the moving of the agglomerates / particles from one point to another, without changing the shape of the particle to increase the randomness of the mixture”. The ingredients incorporation is a very slow process.

Another method of reducing incorporation time is to use powdered rubbers. In a simple ribbon blender the powdered rubbers can be mixed with the other compounding  ingredients.

The powdery mass is compacted in another machine and then fed to the internal mixer. Because of the large surface area of the powdered rubbers, the incorporation into polymer is very fast and only a very short mixing cycle in the internal mixer is adequate to achieve the mixing.

Even after all ingredient is incorporated, dispersion/distribution of the ingredient is not complete. Good distribution is comparatively easy to achieve by paying proper attention to  cutting and folding operations  on a mixing mill or by just prolonging the mixing cycle in an internal mixer.

Rebuild Farrel F270 Mixer From Pelmar Engineering Ltd

Rebuild Farrel F270 Mixer From Pelmar Engineering Ltd

Dispersion however is dependent on the shear stresses generated within the polymer and hence good dispersion may not be achieved by prolonged mixing . Careful consideration is necessary not only as regards the time of the mixing cycle but also for the order of addition of ingredients to the rubber.

Viscosity break down occurs during mixing and is essential for smooth processing of the stock.

Degree of dispersion of carbon black has profound influence on the physical properties of the vulcanisate. Undispersed carbon black (normally taken as carbon black agglomerates bigger in size than 9µm) act as gritty particles. Under tension, cracks develop at these spots.

Failure properties like tensile strength, tear strength and consequently abrasion resistance come down as the degree of dispersion comes down.

CONDITIONS FOR GOOD DISPERSION OF CARBON BLACK

To achieve dispersion of the carbon black, the polymer mass itself has to exert considerable shear stress on the carbon black agglomerate incorporated inside the polymer. This is achieved by passing the polymer carbon black batch through a narrow nip either between two rolls of a mixing mill moving at frictional speed or that in between rotor tip and chamber wall of an internal mixer.

In internal mixer two additional conditions have to be fulfilled. Chamber loading must correct & Ram pressures must be adequate to hold the stock within the chamber.

CONDITIONS FOR GOOD DISPERSION IN INTERNAL  MIXER

  • Narrow Clearance between Rotor
  • Tip and Chamber wall (High Rate of Shear)
  • Correct Volume Loading
  • Adequate Ram Pressure
  • High Viscosity of Polymer
  • Low Polymer Temperature
  • (High Viscosity and More Prominent Elastic Characteristics of Raw polymer)
Kobelco Make Mixers

Kobelco Make Mixers

For higher shear stress generation inside the polymer mass, polymer should have high viscosity. The temperature should be low so that thermoplasticity does not lead to lowering of polymer viscosity.

Any sweeping of carbon black at the end of mixing cycle is to be avoided in regular production.

The Master Batch (MB) is aged. Cooled MB goes to the cracker. Mechanical working of the cooled MB improves the degree of dispersion further. Then the MB is worked on Cracker mill, warming mills, feed mill and then to the extruder.

It is possible to follow the mixing process in the internal mixer with the help of power / time curve (or amperage of drive motor / time curve). When carbon black is added the torque does not rise immediately. The carbon black added as palletised black is about 30% higher than the total chamber volume. As the carbon black is slowly absorbed into the rubber the torque increases. As more and more carbon black gets absorbed, stock volume becomes lower and the power curve comes down.

Based on the power curve data on experimental batches, criteria like constant time or constant temperature are selected as dumping criteria. With constant time or constant temperature as the dump criteria, there will be variation in quality of the compound produced.

The better criterion is the constant energy criterion. This is very versatile, and will automatically take care of any minor variation in operating conditions as well as of even major ones to give a consistent quality output. It can also be kept constant even when rotor rpm is changed or ram pressure is increased, while the time or temperature criteria will have to be re-established after a series of experiments.

BLENDING OF POLYMERS

In compounds, sometimes polymer blends are used in order to cover deficiencies of one polymer by partial use of the other. However a homogenous dispersion of two different polymers on molecular scale is not possible.

Most important condition for achieving good blending of polymers is that both polymers should have as near viscosities as possible during blending.

Rubber Mixing Room

HF Mixing Room Image

FUTURE DEVELOPMENT

In order to mix uniform, high quality, low-cost rubber in an environmentally clean area, the mixing systems in future must provide the following:-

  • Accurate, automatic, clean and flexible weighing of all materials used in the mixed compound.
  • Mixers that use :
    • Either tangential or intermeshing 4-wing variable speed rotors depending on the product.
    • Variable ram pressure and position during the mix cycle.
  • Mix time based on feedback from instrumentation sensors that monitor and control in “Real Time “ temperature, viscosity, dispersion and energy.
  • Greatly improved dust stops, rotor and chamber metal surfacing as well as mechanical and electrical components that will increase up-time and reduce overall maintenance cost.
  • The down-stream equipment will be similar to what is used today but automation will either eliminate or minimise a need for the operator at the mill, former or batch-off unit.
  • Online automatic sampling and testing of each individual batch will be performed after the mill or forming machine and this data will be used to make minor adjustments to the formula of the remaining batches as well as further processing down-stream.
  • Controls will be more sophisticated with feedback loops to make sure each batch and formula will be compounded properly. They will automatically record and control the conditions of the mixer to provide a more consistent uniform mix.

ZONE ANALYSIS OF UPSIDE DOWN POWER PROFILES :

Power Curve Of Typical Banbury Mix

Power Curve Of Typical Banbury Mix

ZONE – I

Loading + wetting stage – Formation of a Single Mass of filler and rubber – penetration  of Polymer in to filler voids – As the C-black is slowly absorbed into the rubber the torque increases. When the volume of rubber + C-black becomes equal to the chamber Vol., the raw comes to the lowest  position, the raw hydraulic pressure on the stock disappears. The power shows first peak. More and more C-black gets absorbed, Stock volume becomes lower & the power curve comes down.

ZONE – II

Most of the real dispersion work takes place. The filler agglomerates are gradually distributed through the polymer and then  broken down tto their  ultimate size. The power curve also starts rising till the whole stock with oil & C-black has consolidated. At this juncture the second power peak occur .

ZONE – III

Plasticization takes place.

The power curve decrease beyond the second power peak has been found to obey first-order kinetic law,

Log [(Po – Pt)/(Pt  –  Px)] = Kt

The mixing should continue till dispersion half time. (i.e. (Po-Pt)/(Pt-Px) = 0.5)  after 2nd power Peak.

TOTAL MIXING TIME = Black incorporation time + Dispersion half-time.

To handle variety of rubber compounds on the same mill required that mill to have.

  • Independent speed control on both rolls
  • Widely variable speed on both rolls
  • Independent temperature control on speed, friction ratio and temp. to be adapted to each individual .
  • Hydraulically operated nip adjustment
Two-Roll Mixing Mill

Two-Roll Mixing Mill

 

RECENT DEVELOPMENT FOR IMPROVING MIXING EFFICIENCY :

  1. Increased rotor speed
  2. Higher Ram Pressure
  3. Improved Rotor Design
  4. Improved Cooling System
  5. Continuous Mixing Process

 

MAJOR CHANGES IN RUBBER & PLASTICS MIXING

OLD

INTERMEDIATE

NEW

i)     2 Speed Rotor (20 – 40 RPM) Variable (0 – 90 RPM)
ii)    Low Power (e.g. 11- max. 800 HP) High Power (e.g 11 Banbury Max 1500 (HP)
iii)   Low Pressure Ram (40 Bar) High Pressure Ram (80 Bar)
iv)   Tap water cooling Refrigerated water cooling Tempered water cooling
v)    Spray side cooling Cored (channel) Sides cooling Drilled sides  cooling
vi)   Spring Drop Door Drop Door
vii)  Spring Tension Seals Hydraulic Seals
viii) Chrome internal Surface Alloy internal surface
ix)   2- Wing Rotor 4 – Wing Rotor
x)   Mix unit till it sounds Right Mix by Power Consumption Computerized control of all variables

 

INCREASED ROTOR SPEED (Size 11 Banbury)

Rotor Speed(r.p.m) Mix Time( % ) Out Put rating( % )
30 133 80
40 100 100
60 64 140
80 48 160

 

At high pressures the average HP required was found to be inversely proportional to the rotor speed to the 0.97 power.

P1 = P2*(V1/V2)   where, P = Horse Power , V = Rotor Velocity

 

INCREASED RAM PRESSURE (Size No. 11 Banbury, 40 rpm)

Type Pressure Ram Pressure (Psi) Effective Pressure (Psi) Mix Time (%) Output Rating
Normal 90 25 100 100
Intermediate 135 35 84 120
High 280 70 70 143

 

IMPROVED COOLING SYSTEM

Tempered water / controlled water temperatures are selected relative to the coefficient of friction of the Sp. Polymer being mixed. Lowest possible Temp. at which the polymer gripping the metal surface enabling shear and turbulent flow of the polymer to take place rather than slippage.

Polymer Tempered Water Temp. (max.) (°C)
Highly Cryst. EPDM 60 – 70
Natural Rubber 40 – 60
SBR 50 – 60
Low Cryst. EPDM 30 – 35
Hypalon (CSPE) 30 – 35
NBR (Nitrile) 20 – 25
IIR (Butyl ) 20
CR (Choloroprene) 15

 

  1. Total Power consumption is reduced because More time is spent mixing rather than flopping around.
  2. Greater fill factor is obtained because the mix is hugging the metal during the whole time it is in the mixer.
  3. Batch to Batch consistency is improved  because the temperature of the metal fluctuates  in a very narrow range and each batch is exposed to essentially the same metal conditions  at each step of the loading and mixing cycle.
  4. Improved dispersion due to absence of unbroken down polymer lumps.

Automated Mixing Line

*******

Dr. Chakravarty can be reached on kpspltd@gmail.com


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Editor’s Pick: Mixing And Mix Design – Advances In Mixing Technology (Part 1)

An earlier post on injection moulding of Dr.S.N.Chakravarty(President – Elastomer Technology Development Society and Ex-Chairman, Indian Rubber Institute (IRI)), on this site was refreshing to many of the readers who wrote back to me because rubber machinery and rubber processing go hand in hand. One cannot be alienated from the other nor viewed in isolation.

On this note, here is another informative paper sent to me by Dr. Chakravarty – MIXING AND MIX DESIGN: ADVANCES IN MIXING TECHNOLOGY.

Here is Part 1 of this two-part series.


INTRODUCTION

Rubber compounding is one of, if not the most difficult and complex subjects to master in the field of Rubber Technology. Compounding is not really a science. It is art, part science. In compounding one must cope with literally hundreds of variables in material and machine. There is no simple mathematical formulation to help the compounder. That is why compounding is so difficult a task

OBJECTIVES OF COMPOUNDING

  1. to secure properties in the finished product to satisfy service.
  2. to attain processing characteristics for efficient utilisation of available equipment.
  3. to achieve the desirable properties and processibility at the lowest possible cost.

REQUIREMENTS FOR THE SUCCESS IN COMPOUNDING:

  1. The properties and functions of hundreds of elastomers and rubber chemicals are to be understood.
  2. Knowledge of the equipment used for mixing, extrusion, calendaring, moulding and vulcanisation are required.

THE INGREDIENTS AND FORMULATION OF A MIX:

Today, a technical vulcanisate is made up of the following constituents :

  1. Base polymer or blend of polymers
  2. Crosslinking agents
  3. Accelerators
  4. Accelerator modifiers (Activators / retarders )
  5. Antidegradents
  6. Reinforcing fillers
  7. Processing aids
  8. Diluents
  9. Colouring materials
  10. Special Additives

 

In addition to the above, reclaimed rubber or vulcanised rubber crumb may be included and according to the manner of their use, function under groups 1, 7 or 8.

SELECTION OF POLYMER AND COMPOUNDING INGREDIENTS:

POLYMER

Rubbers are viscoelastic materials of low rigidity exhibiting large strain elasticity. The deformation imposed on rubber components are far larger than those encountered for most other materials, and the stress-strain relationships are correspondingly more complex.

The ability of rubber to store elastic energy depends largely on the type of polymer used. In general, polymers having relatively high glass transition temperatures exhibit the highest energy losses during deformation. These energy losses are exploited in components  intended to damp motion, but generally the higher the damping obtained the more sensitive are the modules and damping to frequency and temperature.

The tensile strength of a rubber is low compared with other materials but the energy storage capacity at break can be greater than that of an equivalent mass of steel. Failure of rubber components rarely occurs by simple tensile failure: tearing or fatigue crack growth is more likely.

A major factor determining the strength of rubber is an ability to crystallise under the influence of an applied strain. Rubbers possessing this ability (e.g., NR and CR) are intrinsically strong while those that do not crystallise rely on the incorporation of reinforcing fillers to impart adequate strength.

A limitation on the use of rubbers in some applications is the effect of certain fluids. The extent of swelling or property change in a given fluid is critically dependent on both the rubber and the fluid. Selection of a rubber for a given application should take into account its resistance to any fluid it is likely to be in contact with in service. A similar consideration applies to the effects of temperature and the climate in which a product is to be used.

Relative ratings of different polymer vulcanisates are as shown below:

TEMPERATURE RANGE OF MOST COMMON ELASTOMERS

Elastomer Base      Temp Range (°C)
CR                         40 – 100
IIR                         40 – 120
NBR                       40 – 100
NR                         55 – 90
SBR                        50 – 100
CSM                        20 – 120
EPDM                      50 – 150
FPM                        20 – 200
VMQ                       60 – 200

Next on the list would be the vulcanising agent. This would be between sulphur, sulphur donors, metallic oxides, urethane crosslinkers, or resin cures. This decision is somewhat easier since it depends largely on the type of polymer.

The third to choose is for the most appropriate filler and the amount. This of course does not occur with purge gum compounds. The colour desired, the hardness if specified, the service environment will be some of the factors in that decision.

HARDNESS IMPARTING RATIO OF CARBON BLACKS FOR EVERY 1 POINT RISE IN HARDNESS ADD
SAF                 0.80 SAF                 1.6
ISAF               0.90 ISAF                1.8
HAF                1.00 HAF                 2.0
FEF                 1.05 FEF                  2.1
GPF                 1.10 GPF                  2.2
SRF                 1.20 SRF                   2.4
  • For every 1 point rise in hardness add 2.0 phr of carbon black (HAF)
  • For every 2.0 phr of process oil added, hardness drops by 1 point.
  • For every 2.0 phr of oil added, hardness drops by 1 point.

Then the selection of accelerators and activators are made. Their choice depends primarily on the vulcanising agents chosen, next the polymer and then curing and service conditions.

Plasticisers and/or softeners have to be compatible with the elastomer, effective with the type of filler, and should not cause problems of their own.

The last fundamental question to be resolved is the age resistor package. Two conditions have to be satisfied here, providing suitable protection against the environment and not choosing agents inimical to the softeners or curative system.

Antioxidant Class  Natural ageing  Heat ageing   Flexing  Resistance to Staining Copper & manganese
Acetone/Diphenylamine  condensates 2-4 4-6 1-5 1-2 2
Acetone/Aniline condensates 2-4 4 1-2 1-2 1
Phenyl-betanapthyl amines 4 4 4 1 2
Para-Phenylene diamines 4 4 4-6 1 4-5
Substituted Phonols 2-3 1-3 1-2 5-6 1

Obviously for some compounds further choices have to be made: fire retardance, electrical conductance, particular colour etc.

COMPOUND FOR VULCANISATE PROPERTIES

Hardness : Hardness, as measured by an indentation test is a semi empirical measure of modulus .

Tensile Strength : Tensile strength is one of the most widely determined properties of rubber. Tensile strength varies widely with rubber type and is sensitive to compounding, passing through a maximum as cross-link density and hardness are raised. Elongation at break usually decreases with increasing stiffness.

Tear resistance : Tear is a localised strength failure in a material whose bulk strain is below its breaking point. Resistance can be improved by reinforcing fillers.

Fatigue resistance : Like other materials rubber can undergo fatigue failure as a result of crack growth and , as in the case of tear, this can occur at tensile deformations well below the breaking strain.

Fatigue life increases rapidly as the maximum strain imposed is reduced. Atmospheric oxygen can reduce the fatigue limit and increase the rate of crack growth. At strains below the mechanico-oxidative fatigue limit as slow crack growth many occur as a result of ozone attack in unsaturated rubbers. IN strain crystallising rubbers, fatigue life is enhanced in components that do not return to zero strain during cycling.

Compression set : The extent of resultant deformation when rubber is subjected to a distorting load after release of load is known as compression set.

  1. Optimum resistance to compression set is developed as cure continues beyond the      level normally considered adequate to obtain a good general level of properties.
  2. Thiurmas in conventional systems give lower compression set than do thiazoles and sulphonamides alone.
  3. Partial replacement of sulphonamide with thiuram gives compression set resistance approaching that of thiuram used alone. These systems give good compromise between cost and technical performance.
  4. Semi EV system are superior to conventional system.

 

Tension Set : The tensile analogue of compression set is termed tension set and is defined a residual tensile strain in a rubber after it has been stretched either to a given tensile strain and released.

Resistance to liquids : By proper selection of the rubber type and other compounding ingredients, products can be designed for satisfactory use with wide  range of liquids. Contact with an aggressive liquid can have two effects on rubber. The more obvious is change of dimension due to swelling which may be positive because of absorption of liquid or negative because of extraction of soluble compounding ingredients, e.g. Ester plasticizers by fuels. Swelling is diffusion controlled process. The rate of penetration depends more on the viscosity of the fluid rather than its exact chemical nature.

Ageing resistance : It is known that many factors such as oxygen, ozone, sunlight, metal irons, heat and mechanical conditions may markedly contribute to the deterioration of rubber with the passage  of time. Although complete prevention of degradation is impossible inhibitions can be done by use of antioxidants to minimise oxidation and carbon black to reduce sunlight effects.

Heat resistance : In general, resistance to high temperature is a function of polymer structure and Crosslinking systems.

Low temperature resistance : As the temperature falls there is an increase in stiffness the rubber passing through an intermediate transition  state until it becomes brittle solid  (glass hardening). The use of plasticizer may improve the low temperature flexibility. An additional factor in low temperature behaviour is crystallisation which may occur with certain rubbers particularly with NR and CR.

In Part 2 of this article, you will read more deeply into PRINCIPLES OF MIXING.

Dr. Chakravarty can be reached on kpspltd@gmail.com


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Why We Love Twin Screw Sheeter (And You Should, Too!)

Twin Screw Sheeter replaces the dump-mill and sheeting mill combination in a traditional rubber mixing line (an image you had seen in my earlier post – Single-Stage or Two-Stage Mixing?). This means you could visualize Twin Screw (Extruder) Sheeter, as a rubber machinery that accepts mix compound directly discharged from an internal mixer into its hopper chute and converts it into a continuous, seamless rubber sheet that is then fed into a Batch-Off Cooling Line.

For those who have been following my blog, you have already viewed a video of this equipment in action in my earlier post Rubber Mixing Room.

When you explore this equipment for purchase, you should not be surprised with different OEM’s calling it in similar sounding names. For example, you will get a Conical Twin Extruder (CTE) with Roller Head from Colmec SpATwin Screw Roller Head Extruder (TSR) from KobelcoTwin Screw Discharge Extruder (Convex™) from HF Mixing Group or simply Twin Screw Sheeter (TSS) from other rubber machinery manufacturers like Bainite Machines.

In construction, they all appear similar as shown below.

Kobelco Make TSR

Kobelco Make TSR with description

For reading simplicity, let me address this machinery simply as “TSS” for the rest of this article.

You will find the TSS to be ideal for conventional and diverse applications including tire manufacturing, custom compounding, hose & belt manufacturing and technical rubber goods production.

So, here’s why we love Twin Screw Extruder Sheeter (And, I feel, You Should, Too!).

  • Energy Saving: Rubber compounding is a energy-intensive process. So, any technological advancement that has the potential to reduce energy consumption receives my first preference (and I hope you will agree with me here). Let me help you with a quick back of the envelope calculation. If you are using a 270 Liter Tangential Internal Mixer, you are engaging at least two units of 26″x84″ two-roll mills in the downstream section. Each 26″x84″ two-roll mill, requires around 180 kW (minimum) motor power – totaling to 360 kW (=180 x 2) only for the mills. For a similar capacity mixer, a TSS downstream will not seek more than 300 kW power (again, there is energy-efficient models available here). So, this rough calculation, when a TSS replaces the traditional dump-mill with sheeting mill set up, straightaway gives you 16.7% savings in energy (60 kW less).
  • Labor: The second aspect is the reduction is labor cost. Unlike two-roll open mills (with or without ), where you will need two separate operators, a TSS can be set up to perform sheeting function of rubber sheet without an hands-on operator at its vicinity. Even if not fully automated, you do not need an operator once the discharge of rubber sheet from TSS is fed into a Batch-off.
  • Reduced Contamination: In open two-roll mixing mills, your rubber mixing is exposed to the environment and it is difficult to control any dirt or moisture absorption by the compound during milling process. In a TSS, this is eliminated. Your rubber and its recipe constituents are mixed and sheeted-out in a closed environment under temperature controlled conditions right from the time you feed it into your internal mixer. Hence, with reduced contamination, you get a guaranteed higher quality of your mix compound.
  • Self-Cleaning Feature: The Screw and Barrel of the TSS is at a downward inclination (15º) angle from the feed chute section to exit of the roller die head. This incline ensures that compound flow to the exit of the barrel is reinforced and no material remains inside the TSS – hence, the self-cleaning feature.
  • High Mixing Line Efficiency and Productivity: When you install a TSS , your compound batch from the internal mixer is converted into a continuous sheet and the working of TSS can be automatically synchronized with rubber mixing line speed. This in turn, improves the mixing line performance making it more efficient. The continuous sheeting without operator involvement increases your mixing line throughput and overall productivity. Original Equipment Manufacturers (OEM) can offer you customized TSS models beneath internal mixers with throughput capabilities from 500 Kg per hour to 21000 Kg per hour (….and that’s a vast range by all means).
  • Effective Temperature Control: Your rubber compound discharge temperatures from TSS is reduced while sheeting out the material because no additional work (hence no additional heat) is introduced into your compound. Additionally, there is circulation of tempered (or chilled) water inside the conical twin screws, barrel and the peripherally drilled rolls of the roller die. This flowing water facilitates an effective heat exchange to take away the heat from the rubber mix and reduces the compound temperature at the discharge sheeting section.
HF Twin Screw Extruder

HF Make Twin Screw Extruder

  • Compact Layout: Most manufacturers offer various drive options, making the design of the TSS very compact yet sturdy. This means that a TSS can be accommodated under most internal mixers starting from the lowest production range of 16-25 Liter capacities based on the OEM standards.
  • Easy Maintenance: Further, the screw tips of the energy-efficient conical twin-screws do not touch each other and hence there is minimized wear of the screws. A rapid action hydraulic cylinder arrangement for clamping and moving the roller-die calender on rails facilitates the cleaning of the screw tips and insides of the barrel tip during your scheduled maintenance. Also, the TSS does not require external pushers, as in case of single-screw dump extruders. These features make a TSS maintenance easy for you.
  • Additional Features: With increasing trend of Silica usage in rubber compounding, you need to be cautious of the metallurgy and surface treatment characteristics of any rubber compounding machinery you buy. Hence, explaining the major ingredients of your recipe to your OEM is of paramount importance. For example, in TSS you can seek rollers that has hard-surfaced rolls if you are processing silica compounds. This will minimize the compound sticking to the TSS roll and increase its life.

Lastly, this physically very sturdy and robust, rubber machinery is designed for intrinsically safe mixing line operation.

Summarizing, with its capabilities for conventional and diverse applications, a TSS is emerging as the standard downstream equipment in the rubber compounding process for masterbatch and final mixing lines. And that is why we love Twin Screw Sheeter.

How about you?


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Top 15 Skills Required For An Internal Mixer (or Kneader) Operator

Rubber Mixing is a capital and energy intensive operation. And mixing machinery are the mother equipment. This could be an Internal Mixer (Banbury or Intermix) or Rubber Dispersion Kneader depending on the size of your organization and/or products manufactured.

Hence, the cost of errors or omissions are very high when compounding a batch in a mixer. You need a skilled operator. Ever pondered on the skills that make a mixer operator successful?

Internal Mixer Operator

Here is the list of top 15 skills for an successful batch mixer (or kneader) operator. (Updated on 23rd Dec 2015: Flip through this post in our digital edition and download here)

  1. Control of Operations – Your mixer operator should be able to adjust ram pressure, control the mixing process, set parameters and ensure its completion as per SOP (temperature or time or energy as programmed/specified).
  2. Monitoring Operations – The most important skill of your operator should be to have a keen eye for watching gauges, dials, or other indicators in the control panel or HMI to make sure the mixer is working properly. He has to ensure that the mixer is kept clean, safety features are functional,  upstream and downstream equipment along with all accessories (like cooling water, hydraulic/pneumatic system, temperature control unit (TCU), lubrication system, etc) are ready
  3. Active Listening – Your operator should be a skilled listener. He should actively listen to the sounds of the mixer and its motor during a mixing cycle; pay full attention to what his supervisor (or you) or his colleagues on the mixing room safety are saying, take time to understand the points being made, and ask relevant questions.
  4. Speaking – Your operator should be able to talk to you (or his supervisor) to convey information effectively be it to report data/problems/incidents as applicable in a timely manner
  5. Reading Comprehension – An operating and maintenance manual is normally supplied together with the rubber mixer. This is a crucial document. Again your compounding process may involve specific work related instructions or SOP. Or there could be a training manual in some instances. Your operator should be able to understand written sentences and paragraphs in these documents. Hence, reading skills is very important for a successful operator. It is not necessary (while it is preferred) that they read English, because you could translate these documents to your operator’s local language for ease of reading.
  6. Troubleshooting, Judgment and Decision Making – Your operator is the first point of contact with your mixer in operation. Hence, he should have the experience or knowledge on mixers to determine/read the causes of any operating errors when they occur, judge the gravity of the error and also decide what to do about it – whether to reset the mixer, or escalate to supervisor or raise a service visit request of the manufacturer’s engineer.
  7. Critical Thinking – Operating a rubber mixer requires critical thinking skills because your operator should use logic and reasoning to identify alternative solutions, conclusions or approaches to problems he faces while mixer is in operation.
  8. Quality Control Analysis – Your operator should have basic skills on quality control with an outlook to meet your set mix quality parameters in every batch. This may involve need for appropriate fine tuning like helping you fix the batch weight, or sending the sample of specified compound/ batch in specified form to lab for testing.
  9. Social Perceptiveness – Emotions could run high in the rubber mixing room. Your operator should display “awareness” of others’ reactions and understanding of why they react as they do in a particular circumstance.
  10. Repairing – Your mixer operator should be able to use the required tools to both repair and assist repair of mixers when needed in the most urgent manner.
  11. Time Management – Your operator should manage his own time and display sensitivity to the time of other co-workers involved in the mixing room.
  12. Mixer Maintenance – Performing routine maintenance on mixer and determining when and what kind of maintenance is needed is an important skill that your operator should posses.
  13. Active Learning – Your operator should display active learning skills. This is because, mixers get upgraded, automation and new controls might get introduced or new methods of mixing could be introduced all of which he might have to learn or get trained in.
  14. Writing – If you could get an operator who could communicate effectively in writing to you (or his supervisor) or to other departments, then I would say you have a great asset.
  15. Complex Problem Solving – Your operator should develop skills to identify and solve complex problems when they occur at site and support maintenance department effectively over a period of time. This reduces the downtime of your mixer.

Do you agree the above listed 15 skills, required for an Internal Mixer or Kneader Operator, are comprehensive? Let us know.


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7 Quick Tips About Batch Weight Calculation For An Internal Mixer

Internal mixer is a standard rubber machinery for volume mixing in both tire industry and non-tire rubber industry.

When you use one, your most elementary requirement is to calculate the batch weight for your respective mixer model. Because when mixing rubber compounds, you should understand that different compounds based on the same polymer might require different batch weights. And different polymers will almost certainly require different batch weights.

Bainite Make Intermeshing Mixer

Image Courtesy: Bainite Machines Pvt Ltd

Here’s 7 quick tips for you to fix the batch weight for your rubber mixing. (Updated on 23rd Dec 2015: Flip through this post in our digital edition and download here)

1) Theoretical Equation

The thumb rule is the theoretical equation

W= NV x SG x FF

where W – Batch Wt [kg]; NV – Net Mixer Volume [dm³]; SG – Specific Gravity (density) of the mixed batch [kg/dm³]; and FF= Fill Factor.

Generally, most mixer manufacturers share this calculation with you. But remember, what they give you is only a theoretical number. This is only a starting or reference point and you need to arrive at your own mixing batch weight for your compound recipes, following some of the other tips stated below.

2) Net Mixer Volume (NV)

Since Internal mixer has a fixed volume mixing chamber, knowledge of the net volume (in liters) is required. This can be obtained from the manufacturer directly or in some cases from their literature for their various models.

When the mixer is used regularly (or if you have procured a used-mixer) the effective volume increases due to wear on the rotors and mixing chamber. If not compensated for this inside wear, your batch volume will be effectively too small leading to insufficient ram pressure on the compound, poor dispersion and longer mixing times. Annual measurements of chamber are recommended to update your batch weight correctly.

Excessively worn out mixers will have to be rebuilt or reconditioned (Read our posts on mixer rebuilding – Top 25 Things You Should Know to Discuss with Mixer Rebuilder and 17 Essential Questions to Select the Right Rebuilder for your Internal Mixer)

3) Guesstimate the Fill Factor (FF)

If you have a Tangential Mixer (aka Banbury) , then your FF can range between 0.70 and 0.85. And for a Intermeshing Mixer (aka Intermix), your FF can range between 0.62 and 0.70.

Knowledge of the fill factor is necessary because an under-filled mixing chamber results in the ram bottoming out too soon. This reduces the pressure on the rubber stock and increases the mixing time. An over-filled chamber leads to unmixed ingredients staying in the mixer throat. This creates a mess under the mixer when the batch is dumped.

For example, NR-rich compounds in an intermeshing mixer has a fill factor of around 0.65 while for the same compound in a two-wing tangential mixer, it is about 0.75. This compound will have an increased FF of about 0.78 for a tangential mixer with four-wing rotors. Each polymer also has its ideal fill factor and that varies again with Mooney viscosity and filler system.

Fill factor of a mixer depends on the age of the machine, wear and tear of the rotors and chamber, the rotor type, rotor speed, rotor friction ratio, nature of elastomer, ratio of elastomers/ fillers, mixing sequence, kind of polymers, fillers and individual SG of the ingredients in your recipe, viscosity of ingredients, etc. Generally, the lower the compound viscosity, the fill factor is higher.

Hence, we initially guesstimate the FF before stabilizing on the figure later on through actual trials.

4) Estimate the Specific Gravity (SG) of your Compound

You can estimate the density of your compound by multiplying the quantity of each ingredient with its individual density (you can get this figure in any compounding handbook or ingredient supplier literature). Sum up your individual results and then divide this number by the total sum (usually phr). The result will give you the estimated density of the compound. (Mathematically, this is the weighted average calculation).

For example, lets consider a sample recipe (I got this recipe from a web search) as below:

Recipe Ingredients  Volume Density Volume x Density
      (L) kg/L kg (or PHR)
SMR 10 106.4 0.94 100.0
Zinc Oxide 1.8 5.55 10.0
Stearic Acid 2.2 0.92 2.0
N550 Carbon Black 27.8 1.8 50.0
Oil 10.9 0.92 10.0
Antioxidant TMQ 1.9 1.08 2.1
Antiozonant DPPD 1.6 1.22 2.0
Sulphur 0.1 2.07 0.2
TBBS 1.6 1.29 2.1
TMTD 0.7 1.35 0.9
Total 155   179.3
Compound SG  (179.3/155) 1.16

Calculating, the SG of this Compound mix is arrived at 1.16 (=179.3/155).

5) Know your Internal Mixer

Knowing your internal mixer – its capabilities, design features like rotor (tangential or intermeshing), ram (pneumatic with dedicated air supply at the plant or hydraulic), variable speed capabilities of the motor, SCADA, PLC, automation and control features, etc.

Rotor speeds are critical because you can use higher speeds at the initial mix and then reduce the rotor speed to allow the batch to “knead” well.  This will allow you to get both your dispersion and distribution tasks of mixing right. Hence, when selecting a mixer explore variable speed drives since it give you advantage in your mixing process.

(If you are planning a new purchase, read and download our Questionnaire for Internal Mixer Selection)

Similarly, think of ram pressure.  If your ram pressure is too high you will cause excessive heat build up and poor flow of ingredients across the rotor tips. In intermeshing mixers, this will also cause internal pressure within the mixing chamber and might cause mixer failure. If ram pressure is too low, then you will not get the ingredients down into the rotors and this will result in poor mixing. (Read more about Hydraulic Ram here)

Banbury Mixer

Image of HF Mixer

6) Watch the Ram Action

After the above reference calculations are done and mixing initiated; watch the ram action during the mix. The ram should start high, move up and down about an inch or two and bottom out when mixing is complete. Good mixing practice dictates that when the ram bottoms out about 30 – 45 seconds before the batch is dumped, you can be assured that the chamber is properly filled and mixed compounds will be of high quality.

You need to observe the position of the ram by watching the tell-tail rod attached to the top of the ram. Hence, this requires more of practice and experience than theoretical knowledge.

If you have a good mixing system with controls and feedback features, you can correlate the position of the ram with the current and rise in temperature – these are important to get an optimized batch size and high quality of mix.

7) Optimize Your Mix Batch Size (…Do Not Maximize)

The key to successful mixing is optimizing your mix batch size, and not maximizing. And good mixing is a form of art.

Most mixer users want to get the most out of their internal mixer (quite natural!) and they test its capabilities to the full. Finally, when they get poor mixing, they wonder if they have done the right investment! 

If you try to take your batch size to the upper limits of the mixer’s “capacity” as specified in the manufacturer’s manual (that is usually a peak magical figure) and you have raw material variations such as particle size or bulk density changes in your fillers, this can lead to poor mixing (dispersion and distribution of ingredients).

The right batch size will be smaller, but your internal mixer throughput is increased by shorter mixing time and thus more batches in the same period. Thus, optimizing your batch weight will allow you to get consistent batch quality and repeatability that are of paramount importance to your (or your customers’) downstream processes.

The key factors that will influence your mixing optimization are compound formulation, ram pressure, mix procedure, mixing speed and rotor design.

Each mixer is different and it would be very difficult to determine the optimized fill factor without actually conducting several mixing trials. Experience is a key to good mixing.

Summarizing, when mixing rubber compounds, different compounds require different batch weights. These 7 tips will help you calculate the optimized batch weight for your compounding recipes on an internal mixer quicker.


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9 Things About Tandem Technology Your Boss Wants To Know In Rubber Mixing

Dr Julius Peter, then Chief Technical Officer at Continental AG patented his idea of Tandem Mixing Technology in 1989. His colleague, G. Weckerle, manager at Continental Technical Rubber promoted this technology at his factory in Northeim, Germany on K2A, K4, K5 and K7 type of mixers.

Francis Shaw & Co had sole world rights for supply of the intermeshing type tandem mixers. Today, HF Mixing Group (Harburg-Freudenberger Maschinenbau GmbH) are owners of tandem mixing technology by virtue of their acquisitions in the rubber machinery world.

(Updated on 23rd Dec 2015: Flip through this post in our digital edition and download here)

Here are 9 key things about tandem technology in rubber mixing you should know to impress your boss.

  1. Tandem technology separates the two main tasks in your rubber mixing process viz. dispersion and distribution. Dispersion means breaking down of your solid materials such as the fillers. Distribution involves achieving homogeneity within your rubber mix compound with its different chemicals added. The temperature profile which is absolutely essential for inducing chemical reactions during your rubber mixing process can be better controlled when these two stages are separated.
  2. In Tandem technology you interconnect two “mixers” in series, a ram type mixer on top of aramless mixer. Each machinery is optimised to perform one rubber mixing task. Ram type mixer does dispersion well whileramless tandem mixer does the task of distribution.

    HF Tandem Mixer

    HF Tandem Mixer

  3. Your masterbatch produced in the primary ram mixer is transferred without intermediate storage to the ramless tandem mixer below. Here your batch is cooled and finals mixed. At the same time a new masterbatch is prepared in ram mixer above. The upper mixer with ram is preferably (but not necessarily) intermeshing type. As your masterbatch mixing does not involve the addition of curatives or accelerators and is essentially a heating operation, the mixing cycle may be carried out rapidly without any need to cool your mixer before the next mixing cycle.
  4. Between the two mixers is a discharge flap and chute which would be closed at all times except when the lower tandem mixer receives the masterbatch dump from above.
  5. The mixer below must be intermeshing type to enable self-feed without pressure and work without a top ram. The finals rubber mixing function is usually a shorter process than the masterbatch stage. This means that the tandem mixer has an idle time after the discharge and before receiving the next hot masterbatch. This idle period with the discharge door open allows the tandem mixer to cool.
  6. The final mix compound is then dumped into a two-roll mill or a dump extruder and processed in the normal way.
  7. When the two tasks of dispersion and distribution are separated, your compound weight is relatively smaller in the larger lower machine. Hence, you can operate this ramless mixer at a higher speed. This improves the quality of your mix because your compound is moved around the mixing chamber more number of times.
  8. Excellent cooling water circulation to the mixers is a must in tandem mixing technology.
  9. HF Mixing Group expert, Dr Harald Keuter, emphasize that a Tandem mixer improves your throughput rate by up to 25 per cent when compounding with carbon black compounds and can rise up to 100 per cent with silica compounds. Hence you can cut costs and increase output with this technology. Depending on your choice of mixing line, say for a mixing room with five tandem mixing lines and production of approx. 100,000 tonnes of rubber compound annually, he says you save up to one million euros per year. (….And that’s lot of money!!)

The population of tandem mixers is higher in the tire industry while its economy of operations is tempting the non-tire rubber industry as well.

Do you plan to reduce the mixing stages for your rubber compound (Read on Single-Stage or Two-Stage Mixing here) using tandem technology? Let us know.


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Rubber Mixing Room

Think rubber mixing room, and the image that conjures up in your mind is that of a “black room“!

Factories that have open two roll mill mixing have carbon black particles all over the place, lending credibility and authenticity to “black mixing rooms”. Dust collection system reduces this to a large extent.

When an internal Banbury mixer or Intermix mixers with semi/automatic carbon feeding, filler feeding, oil dosing and effective dust collection systems are deployed, they present a safer and cleaner environment. Maintenance is easier and mixing rooms need not be “black” any longer.

A representative image of a mixing room is as below.

ThyssenKrupp Mixing Room Image

ThyssenKrupp Mixing Room Image from the web

Depending on the technology and layout adopted, the set-up could vary. And if you are curious to visualize how the complete system works, here is a video of a rubber mixing line.

Automated Mixing Line From Bainite Machines

Automated Mixing Line From Bainite Machines

Newer and most state-of-the-art rubber mixing and compounding factories invest in automated rubber mixing rooms to reduce reliance on labour, increase efficiency and production.


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