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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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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 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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