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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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Select A Flexible And Transparent Supplier – Arul Shanmugavelu

Select a supplier who is flexible and transparent, says Arul Shanmugavelu, Chief Executive of L&T Kobelco Machinery Private Limited, in an exclusive interview with Rubber Machinery World. He further adds that in the project situations there are always changes that would crop up. Hence you need suppliers who are ready to listen to the tiny requirements of the customer and be ready to meet the same with open and transparent manner.

Well, Arul knows best being associated with the tire industry for over two decades now.

Soft-spoken and astute, Arul is a credible business leader amongst tire machinery manufacturers and I am happy to present before you his interview in this edition of “Know A Rubber Leader” series.

Know A Rubber Leader

In this crisp interview, you will find him share his thoughts on the challenges and customer frustrations and some plain advice that could lead to a win-win situation between rubber machinery manufacturers and users.

Here’s is Arul’s complete interview reproduced for you.

  1. Hello Arul. First of all thank you for accepting an interview with Rubber Machinery World and sharing your thoughts. You have been head of marketing at L&T and now head of L&T Kobelco, which is a JV. What has changed for you? How has your leadership style evolved over the years?

I am associated with the tire Industry for over 2 decades. As you mentioned, I was Head of Marketing in L&T Rubber Processing Machinery unit, before moving over to L&T Kobelco as the Chief Executive. In short almost, everything has changed for me, except the customer segment. Since I am associated with them for many years, I am able to understand them very fast and most of them know me well. This gives me and also the customer a comfort feeling. Now that, I have a full responsibility of the business, I have the opportunity to fulfill the requirement of the customer faster and better, this I enjoy a lot, as I always want to be close to the customers.

  1. What’s a challenge you spend a lot of time thinking about these days?

One of the challenges that we face is the varying choice of components by the buyers. Each one has their own choice of make (or brand) when deciding bought-out components, be it, RTDs or PLCs, switch gear items, etc. The varying choice of bought-out parts puts limitation on the Machinery manufacturers in terms their flexibility of price and delivery. If the users leave the choice of makes of the elements of the machine to the OEMs, machinery manufacturers will be in a position to provide better returns to end customers in terms of price, delivery and service. Any way the overall guarantee of the machine is given by the rubber machinery manufacturers and hence I feel the choice and responsibility should be left to the manufacturer.

  1. What is the biggest frustration today for buyers of rubber and tire machinery? How are L&T Kobelco/Kobelco Products and Services addressing this?

Tire making is a complex process and each of the tire companies’ are continuously improving their product through various product design and process improvements. Each time a green field project is conceived, the project engineers are required to validate all their assumptions of the previous project, as they are required to show improvements over the previous execution, in terms of better machines, better productivity, shorter project lead time, etc. This puts lot of pressure on the Project team.

We have often seen that the Project engineers are never able to repeat their specification of equipment between 2 consecutive green field projects, even if the projects are phased by only a years’ time.

Moreover due to the shorter lead-times on the project, they are required to concurrently do machine ordering, layout finalization, civil construction, etc. Sometimes this leads to changes during execution of the project, which becomes tough and expensive.

Know A Rubber Leader - Arul

Read PDF

  1. How does L&T Kobelco propose to change the rubber and tire industry in the years to come?

Currently L&T Kobelco’s focus is on the BB270, BB310, BB370 & BB430 Mixers and TSR330 & TSR450 Twin Screw Extruders. This product range covers almost the complete requirement of Tire Industry.  In this segment, where L&T Kobelco operates, the customers are generally used to one manufacturer and have not experienced any other technology.  The resistance to change comes out of the fact that the customers need to undergo the learning curve while using our technology to realize its benefit. Here, we assist our customers with the support of the process expert to stabilize the mixing process and also offer the facility of Trial mixing at Kobelco’s lab in Japan, if required.

  1. Is the rubber machinery industry leading or lagging behind the customer expectations when it comes to customer’s rubber processing expectations?

I think the Tire Machinery Industry is leading the customer expectations in several segments viz. Mixers, TBMs, automation, etc. We have seen manufacturers from these segments coming with new products which push the quality and the productivity of the customers.

  1. Can “superior-technology” and “low-cost” ever go hand-in-hand in rubber and tire machinery/equipment?

Certainly superior technology and low(est) cost cannot go together. To manufacture a quality product, we need to have a certain infrastructure i.e. good machines, trained manpower, good sub-suppliers, and good quality control. In addition for a product to sustain in the market there needs to be investment on R&D.  All of these cost money. At the same time, if we are able to couple appropriate strengths of different countries, we can come out with best value for money.

India with manufacturing expertise and availability of skilled manpower offers a best platform for making machine at competitive price. When this is coupled with the technology of Kobelco, Japan, I feel this is winning combination.

  1. Sustainability, Environment and Innovation are the key global themes today. How is Kobelco incorporating this in your business that has a positive impact to your customers?

Who can ignore this? No one.  Kobelco in Japan is working on these areas of Sustainability, Environment and Innovations. Though I cannot share what is being done in Japan, I can assure Kobelco takes these as very important and works on these relentlessly.

Kobelco Make Mixers

Kobelco Make Mixers

  1. What do you think is the awareness levels of customer on advances in machinery and its availability, superior technology and its adoption here as compared to the west and developed nations? Is there a disparity? How is L&T Kobelco working to expand your market on newer technologies here?

Today the world is a single market. In the Tire machinery market, the number of customers and number of machinery manufacturers are small. With the current information technology, I do not think any customer whether he is in India or Europe is lacking any information. In some occasions, I have seen that the customers in India are more knowledgeable in terms of availability of machines than some of the customers in the developed world. This may come out of the fact that they strive hard to beat the competition with the products made by them in India, compared to the products made by the developed countries.

  1. What do you envision for L&T-Kobelco in the next 10 years?

I feel that L&T Kobelco would have emerged as a major Mixer and allied machinery manufacturer from India catering to both Indian and export markets. L&T Kobelco will play a major role in the overall strategy of Kobelco as a leader in Mixer and allied machinery in the world.

  1. Great! And one last question, what would you advice on machinery selection to buyers and users of rubber and tire equipment?

My input to the buyers regarding machinery selection would be to evaluate the features that are required by you for your processes. Please do not beef up the product with an intention of having all the features whether required or not. Because every penny counts in the project stage. Whatever money you put in, requires a return to be paid to the owners. We have seen customers who use the bare minimum features for their machines and keep their investment on machine to the minimum and yet be a leading player in the tire business.

Hence making of the functional specification is the most important. Please leave the detailing of the machine to the machinery manufacturers and demand the performance parameters. In some cases the project team starts to specify the design parameters of the machine, which is best left the Machine manufacturer.

Select a supplier who is flexible and transparent. In the project situations there are always changes that would crop up. Hence you need suppliers who are ready to listen to the tiny requirements of the customer and be ready to meet the same with open and transparent manner.

xxxxxxx

Download the full interview in PDF here.

That’s some profound practical approach from Arul Shanmugavelu, I think. What do you think?

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Internal Rubber Mixer: It’s Not as Difficult as You Think

Having trouble comprehending the two types of Internal Rubber Mixers? Well, it’s not as difficult as you think.

Here’s an info sheet that gives you all the critical aspects that you need to differentiate between Intermeshing Type Mixer and Tangential Type Mixer. The Rotors, Configurations, Fill Factor, Position inside the Mixing Chamber, Mixing Nature, Rotor Cooling and Mix Quality all explained in a single page for your ready reference.

Banbury Vs Intermix Info Chart

A Quick Reference Chart on Internal Mixer from Rubber Machinery World.

I hope you will have this handy when you plan discussions for a new or used internal mixer or while addressing purchase selection questionnaire.

Let me know if you found it informative.

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Here’s Why TCU Is An Irresistible Ancillary For Your Rubber Machinery And Worth Your Investment.

TCU or Temperature Control Units are ancillary equipment to your rubber machinery that control process temperatures during rubber processing. During rubber processing, you continually seek optimal temperatures for the best product quality.

A TCU achieves temperature control by circulating water or an oil based fluid through the process application (in a closed loop). This results in heating of the application and temperature control of the process.

Temperature control units service a single process at any one time. In rubber industry, water is most popular circulating media.

For example, you have Three-Zone Water TCU’s for your internal mixer where One Zone supplies water to the Rotors, Second Zone to Drilled Sides (or Mixing Chamber) and Third Zone to Discharge Drop Door (and Floating Weight, RE Plates as appropriate to the mixer model). Usage of TCU in an intensive internal mixer is found to improve carbon black dispersion, eliminate first-batch-effect, maintain batch-to-batch consistency of your rubber mix compound, increase productivity, and reduce machine stress (which in turn increases the life of your internal mixer).

Similarly, you have 4 or 5 Zone TCU’s for your Extruders, where each of the zones supply water to screw, forward and rear barrels, die head, feed roll, etc as is required for your rubber extrusion process. Or you may have Two-Zone TCU for open two-roll mixing mills where each zone supplies water to your peripherally drilled or cored rolls. Most Rubber Calenders also have TCU supplying water to the rolls to control the calendering process temperature.

TCU for a 3-Roll Calender

3-Zone TCU for a 3-Roll Vertical Calender – Image from Bainite Machines Website

A properly installed, operated, and maintained TCU system gives you years of reliable operation.

Each TCU is a self-contained system consisting of a centrifugal pump, electric heater, cool/vent solenoid valve, electrical control, including a PID microprocessor controller and thermocouple along with standard safety devices like a mechanical over temperature safety thermostat, a pressure relief valve, motor overload protection, a low pressure cut out switch, etc.

Temperature Control Units come both in portable and robust sizes (depending on the application) with a variety of different control instruments, heater sizes, and cooling vales.

A Compact TCU

Compact TCU – Image from AEC Website

And you are right when you imagine that the portable TCU’s are visually appealing and popular because of their compact size.

At your installation site, you need to provide adequate water (from portable or central chillers or towers) to these TCUs from your plant water supply source. Check your manufacturer’s operations manual to verify the recommended water pressure requirement. This is important because lower than recommended water pressure can cause pump cavitations while excess pressure can damage internal components.

The pump in your TCU is used for rapid circulation of a relatively small amount of water. This in turn ensures close and uniform temperature differences between the delivery and the return lines.

You use temperature control units, when you want to preheat a process to the desired operating temperature. The heater and  cooling valve works together to give fast and accurate response to bring the water up to desired temperature or to change the settings when needed. This controls your circulating water temperature.

Here is an animation, that I found on the website of Sentra Temperature Controllers. It is quite informative and I hope you too like it. (Click on the picture to start animation)

Advantage Make TCU

Upon reaching your desired operating temperature the TCU can continue to add heat or convert into a cooling device by exchanging a small amount of circulated water with cooling water from an external source. Modern TCU have the controls wherein cooling water is precisely metered into the system by the cooling valve.

Most TCU’s in rubber industry for your machinery has temperature control from 30° to 300°F and designed for easy maintenance.

Summarizing, rubber processing under optimal temperature conditions is a must for your final product quality. And the features of TCU makes it irresistible as an efficiency enhancing ancillary equipment, which control process temperatures on your rubber machinery. Thus worth your investment.

Do you agree?

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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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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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