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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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7 Must-Ask Questions When Buying Used Rubber Internal Mixer

An Internal Mixer, whether it is a Banbury mixer or Intermix mixer, is the heart of your rubber processing plant. The market for used rubber internal mixer is wide with sellers spread across. You can find an excellent quality pre-owned rubber mixer that can produce high-quality compound mix, without spending excessively.

Rebuild Farrel F270 Mixer From Pelmar Engineering Ltd

Rebuild Farrel Make F270 Mixer – Pelmar Engineering Ltd

Your purchase decision on used or pre-owned machinery has to be thoughtfully made. Because you will see that even in the secondary market for internal mixers, the costs are relatively high considering there could be additional rebuild costs (if not already refurbished by the rebuilder). In any case, you are making a significant investment from your affordability standards and hence you need to consider a variety of factors to help assess the mixer’s value to your rubber compounding requirements and, ultimately, to your bottom line.

Here are 7 Must-Ask Questions when buying used rubber internal mixer that will help you appraise the second-hand or rebuilt mixer value and usefulness to your rubber compounding operations.

  1. What are my mixer requirements?

You need to have a clear idea of what you wish to buy. This entails knowing capacity, the mixing process for your rubber compounding requirements, matching upstream and downstream machinery availability in your rubber mixing room and the remaining useful life of the equipment you are willing to live with.

(Read my post on the Internal Mixer Selection Questionnaire where you can also download a template. You can modify this template to clarify your needs and refine your decision process).

  1. What is my budget?

Your budget will be a crucial purchase factor including the brand, exact model and vintage of the mixer that you can buy. You should have clarity of breakup of costs associated with your batch mixer purchase.

This includes the cost of additional space required (in case you are expanding operations), cost of dismantling (if mixer is running at a particular location) and transporting the mixer to your factory, actual cost of mixer, its controls and accessories to be paid to the seller; plus various duties applicable, to name a few.

  1. Should I partner the right people – the pre-owned equipment sellers?

Given the global nature of rubber compounding business, there are internal mixers available across the key global markets. Hence, it is not possible for you to be informed about the best deals out there in terms of overall cost and mixer quality. This is where pre-owned equipment sellers or dealers come in to help you.

I think, a good dealer will be able to present you with multiple options and help you select the best used mixer for your requirement.

  1. Is the mixer I am considering to buy in Good Working Condition?

Whether buying used or rebuild mixer, you must always test them whenever feasible or you should ask for a start-up guarantee assurance. This is a precaution to be sure that the mixer is in good running order before your final purchase decision.

If you are buying a running mixer, you can easily test them on-site before dismantling. Or if you are buying from a warehouse, many of the reputed used rubber machinery dealers provide arrangements to allow you to test the mixer at least on a test-bed (if not on-site) to help you make a quick purchase decision.

Else, the last resort is a start-up guarantee assurance from the seller. Reputed used machinery dealers  will be transparent on the condition of the mixers they sell, but it is always smart that you check.

When buying with motor and controls, you should verify the operations and safety of electrical components and software licences along with its adaptability to your country of installation. (If not working properly of found unsuitable, you need to factor in the cost of its replacement into your purchase cost)

Another key aspect to check is whether the pre-owned mixer that you propose to buy comes with complete set of manuals, schematics and diagrams. (You may read my earlier post on mixer maintenance here.)

  1. Should I do Visual inspection?

Absolutely. Though the internet has made your communication easy and you can conduct a lot of your business communication online. You can even demand pictures and videos of your mixer in consideration through email. However, there is no alternative to physically inspecting the machinery you are going to purchase.

Used mixers are usually not warranted. This means you need to know the extent of rebuild or refurbishment, and get an idea of the actual state of the internal mixer.

You should insist on a test run of the mixer in your presence and keep your eyes and ears open for tell-tale signs of machine ill-health such as unusual vibrations or noise. Question the maintenance practices of the previous owner and keep your eyes open for worn out parts and leakages.

Additionally, your visual observation of the machinery empowers you to negotiate better with the seller.

  1. Should I Buy a Standard Model of a Brand Name?

When it comes to buying pre-owned mixers, brand plays an important role. Buying standard models of branded and reputed manufacturers of used mixers can assure you about its quality. In addition to this, you will find it very easy to get spares and servicing for a standard internal mixer models in case of future repairs.

On the other hand, if you go for non-branded or non-standard rubber internal mixer, buying and maintaining the spares can prove a difficult task.

Kobelco Make Mixers

Kobelco Make Mixers – Image From Web

  1. Should I get everything on paper?

I think this is a very important step whether you are buying new or used rubber machinery.

You should get everything on record, from the first formal quotation, the details of the rubber machinery, the accompanying accessories, delivery terms, mode of payment, extent of buyer liability, seller liability, etc. It could be an exhaustive document or a simple set of key clauses basis your comfort – either way they are critical to your purchase. (You may wish to explore the rubber machinery purchase and sale agreement here. Preview – Rubber Machinery Purchase and Sale Agreement Template  To buy the full agreement kindly email me directly.)

Once you have answered these questions satisfactorily and determined which factors are most important in your current purchase decision, you can confidently negotiate and purchase a pre-owned mixer that will meet your rubber compounding requirements.

Summarizing, when buying used rubber internal mixer, you need to conduct a proactive due-diligence; identify and partner the apt seller for your needs, and have proper documentation in place. When you make an informed used or pre-owned internal mixer purchase, you avoid buyer’s remorse. 

Happy Buying!


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Rubber Dispersion Kneader – The Other “Internal Mixer”

In the world of rubber mixing, when “purists” refer to “internal mixer”, they either mean a Tangential Rotor Type (aka Banbury Mixer) or a Intermeshing  Rotor Type (aka Intermix).

While “practitioners” have a third category viz. Rubber Dispersion Kneader.

When you discuss with the practitioners (mostly in Asia) it is very common to hear the terms “Kneader Banbury”, “Kneader Mixer”, “Kneader Intermix”, “Intensive Mixer Kneader”, “Internal Mixer Kneader”, “Dispersion Mixer”, “Tilt Mixer” etc being used in the same context as an “internal mixer”.

So, what is this dispersion kneader and how is this different from an internal mixer?

Dispersion Kneader

Moriyama Make Dispersion Kneader

To start with, a kneader means a machine that specializes in kneading substances, something to a dough form. I suspect it originates from Germany when Heinz List, a pioneer of modern industrial processing technology, first developed a kneader reactor to process high viscosity materials.

The respective rotor, throat, chamber and floating weight designs are different in a dispersion kneader and batch mixer (or internal mixer) . While a mixer discharges the batch through a bottom drop-door, the kneader tilts 125-140 degrees to discharge the batch. Available in more customized capacities than batch mixer, this machine can be positioned on the ground level. While a mezzanine floor is required for mixers with drop door. For similar capacities, dispersion kneaders use lower power than mixer. The mixing time is higher than an internal mixer and hence production volumes are lower in kneader.

Though developed initially for mixing thermoplastics, dispersion kneaders have a unique place in the elastomer mixing industry. Users love the ease of cleanliness on this machine especially when they have to change the colour of their compounds frequently. These machines are also easy to operate and their varied applications include

  • Oil seal, Body seal, Gasket, Belt, Hose, Tube for Automobiles
  • Rubber for Electric wires
  • Conveyor Belts & Power transmission belts
  • Rubberized Rolls
  • Rubber based products like Plug, Cap, Glove, Dental for Medical line
  • Sporting goods like rubber ball, etc

Moriyama Japan (now merged with Nihon Spindle Manufacturing Co Ltd in 2014) enjoys a leadership status in Dispersion Kneader and continues to innovate regularly. Many regional players co-exist in India, China, and Taiwan catering to different categories of customers in rubber mixing industry.

Over a period of time, these kneader manufacturers have developed designs, features and automation for high quality and optimum mixing performance to position themselves closer to the internal mixer. Hence, the confusing terminologies (that I spoke of in the beginning) needs to be viewed in this context. The only caution being when trying to articulate, discuss and solve rubber compounding issues, it is very important to know exactly which rubber machinery is being used – is it dispersion kneader or internal mixer?

Happy Mixing!


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Single-Stage or Two-Stage Mixing?

Have you encountered the often dilemma, “Should I do single-stage or two-stage mixing for my compound; which machinery to use?” – do not be surprised! You are not alone.

Single-stage mixing is considered for productivity reasons (and cost-effectiveness). While Two-stage mixing gives a better dispersion of the finer size blacks. And interestingly, for some compounds with high levels of blacks, even three or more mixing-passes may be necessary.

Rubber mixing as a subject would have been quite simpler, if we could answer this topic effortlessly. Unfortunately, it is not!

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

Single-stage mixing in an internal mixer is a cost-effective solution but difficult for all compounds. If the compounds have high filler loadings, you may be forced to mix in two-stages due to the high amount of shear and heat generated in the mixing cycle. If you use peroxide cures or are mixing expensive FKM, then you must be even more worried of the batch temperature.

Most experts feel two-stage mixing, with short time spans for each of the mixing stages, is helpful.

One school of thought advocate an open two-roll mill for second-stage mixing because the dispersion of the batch and the mastication is higher (than an internal mixer). Open mills, though slower, are safe for short scorch compounds.

A traditional mixing line comprises of an internal mixer above a dump mill then one (or two) mill before the batch off cooling line.

Traditional Mixing Line with Two-Roll Mill Set-up

Reference Image Courtesy: Bainite Machines

Internal mixers are high-capacity rubber compounding machinery. Hence they need to be supported by open mills with advanced features to keep pace with production. The rotors of these mixers operate at high-speed to maximise dispersion of the bulk ingredients and dump the batch at high temperatures. Curatives, blowing agents, etc are added on the open mill and final homogenization happens on the last mill before batch off. Also, adding the cure system on downstream mill eliminates the batch contamination problem from “leftover’s” trapped (between the rotor end plates and ends of the rotors) in the internal mixer. These open mills are recommended to have peripherally drilled rolls to take out heat of the compound before adding heat sensitive curatives.

Open mill mixing is operator dependent and hence quality of compound varies from beginning of shift to end of shift. (Read about Stock Blender). As compared to rubber mixing in a closed environment, the probability of “fly loss” is high in open mills. Hence, an alternate school of thought propagates second-stage mixing also performed in an internal mixer. This can be at a lower speed, energy and dump-temperature configuration setting on mixer.

Single-stage mixing in an internal mixer is possible, when you mix and drop the batch within 120⁰C. The present range of internal mixers have advanced designs to effectively control batch temperature. With many designs and rotor geometries for faster mixing, accompanied by quicker cooling features, mixers like tandem mixers allow traditional two pass to be reduced to single pass cycle. (I will cover newer mixing lines with Twin Screw Sheeter, Dump Extruder, etc in different posts). As a side note, if you opt for single-stage mixing with internal mixer; the Intermeshing Type Mixer has the best quality and efficiency.

Single-stage mixing is not always cheaper and two-stage mixing is not always better. The best way to decide is to make a cost-benefit analysis between the two processes for the different polymers that you work with. Quantify how much of your product defects are linked to poor dispersion. Analyzing them, you have your customized solution to mix effectively.

Summarizing, there is no one best way for all compounds. Your mixing process has to be designed to the polymer; depends on the viscosity of the elastomers used, the quantity of filler, mixing temperature, machinery employed, time at every stage of mixing and desired physical properties for the end use product. If you get your “desired” characteristics in a single-stage mixing, adopt it or wisely opt for two-stage mixing.


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Peripherally Drilled Rolls or Centrally Cored Rolls?

Do you use centrally cored rolls or peripherally drilled rolls in your two-roll mixing mills? Or a combination of both?

Roll selection for a mixing mill is of decisive importance for the quality of many high-tech products manufactured by the rubber industry. Open two-roll mills in rubber processing are recommended when quick cooling for the batch being mixed is sought, say for example in final mix compounds.

Generally, these rolls are made of Chilled Cast Iron (CI) through a process of vertical casting. Chilled CI has greater resistance to deflection and uniform heat transfer characteristics. Depending on the presence or absence of alloy, the hardness of the outer working (chill) zone could be in the range of 460-650 HV with a thickness 12-20 mm.

Basis the application, manufacturers take extreme care on the properties of the rolls that include breaking strength from journals and core material, thermal conductivity, surface quality and wear resistance of the roll, overall machining and surface quality.

As these rolls operate at high speeds, precise concentricity with proper balancing of rolls is a prerequisite for efficient utilization of material and energy. The surface quality of the rolls is crucial for the products to be produced. The smoother and more precise the rolls, better the product.

Viscous deformation of the rubber compounds occurs between the rolls of mills during mixing and mastication. This generates heat that needs to be removed through effective cooling. Hence, water circulation passages for cooling are an essential feature of the roll design in rubber mills. These passages allow a pre-defined circulation of the cooling agent (mainly water) and ensure that the temperature can be kept within a prescribed tolerance over the entire face length of the rolls.

Two designs are normally available – centrally cored rolls and peripherally drilled rolls. Peripherally drilled rolls are possible for diameter greater than 150mm (or 6 inches). The cross-section of a centrally cored roll is easy to visualize. But, ever wondered how the insides of a peripherally drilled roll looks like?

Well here is with a sectional view with water flow.

Peripherally Drilled Roll

The water entering into the roll is cooler (blue colour) and as the heat transfer occurs, the water temperature rises gradually (red colour at exit).

The efficiency of heat transfer is relatively higher in the case of peripherally drilled rolls than in centrally cored rolls due to close proximity of the water channels to the roll surface. In peripherally drilled rolls, the passages for heat exchange are provided approximately 25mm under the roll surface and can vary nominally between manufacturers. Reputed roll manufacturers like Walzen Irle, Leonhard Breitenbach and Karl Buch, in their decades of existence, have built their own standards. Roll manufacturers are also available in China, Taiwan and India for various sizes.

The manufacturing processes and costs involved in producing a peripherally drilled rolls is relatively high, hence they are priced higher than cored rolls. Your choice of peripherally drilled or cored rolls depends on the quality of rubber processing required in mixing mills and the marketability of your rubber products for a price that profitably covers your investment.

Any state-of-the-art Calender in rubber processing also use peripherally drilled rolls for its stated advantages.


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Why Hydraulic Ram in an Internal Mixer?

In internal mixers, hydraulic ram assembly is an apt replacement (over pneumatic ram) because they improve rubber compounding quality.

In pneumatic rams, pressure may vary while the hydraulic version ensures constant, reproducible ram pressure. When you cut compressed air, operating costs reduce because you remove the central compressed air requirement in your plant. You also remove the maintenance issues associated with these air systems in the process. This leads to energy savings in your operations.

A constant pressure from hydraulically actuated rams eliminates variations in ram pressure caused by varying compressed air-supply to the mixers’ pneumatic cylinders. Hence, these are more efficient than air compressors for application of sustained batch-mixing pressure. This in turn, provides consistent mixing process conditions that enhances product uniformity – all of which leads to improved mix quality.

The hydraulic rams can be fine controlled with the hydraulic power. The result is shorter mixing cycles, higher productivity and improved batch-to-batch consistency. Further, the hydraulic ram reduces the noise level within the vicinity of the mixer. So, apart from improving the work environment they use less energy and are quieter in operation.

Representative Image of A Rebuild Internal Mixer

Representative Image of A Rebuild Internal Mixer

In intermeshing type of mixers, cooling water passed through ram provides extra cooling. This control feature is critical when processing heat-sensitive compounds.

Please remember, this is a technology upgrade. Hence, when you choose your tangential and intermeshing mixer, you need to ask for this option. Else, you may continue to receive manufacturer’s standard pneumatic ram only.

Existing pneumatic ram on your internal mixers can be converted to hydraulic ram. Internal mixer manufacturers like HF Mixing Group, Kobelco, Bainite Machines, etc can guide you with energy-saving data for your mixer and offer conversion at an optimized investment.

Let us know if you found this post useful to shortlist energy-efficient features for your next internal mixer purchase.


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