CARTER

Contact: Sales Manager

Mellor Street
Lancs, England OL12 6XQ
Europe

Phone: 01706 638301
Fax: 01706 526569

Website

E-mail

ENGINEERS TO THE WORLD'S
RUBBER & PLASTIC INDUSTRIES

Typical compound uses in manufacture :-


Natural & Synthetic Tyre Production

Polychloroprene hoses

P.V.C. Cabling

Thermoplastic polyolefine

Low smoke, Zero Halogen Cables

Polethylene & Carbon Black Masterbatch

ABS

Plus a wide range of specialist compounds


The name Carter stands for excellence, not only in engineering perfection and technical know-how - but in service. Service is something in which we at Carter pride ourselves. Whether your requirement is for something as simple as a radial bearing, an exchange mixer body, or the total capacity of our biggest mixer. We also provide full turnkey plant solutions.



Technical Services for Compounding Equipment.
Our company is accredited to BSEN ISO 9001:1994, for the design, manufacture, installation and commissioning of Rubber and Plastic Processing Machinery.
Our company has one of the most enviable Safety records in the industry, and has recently won a prestigious award from the British Safety Council, in recognition of this fact.
Our company is also accredited with the EEF, the ISM, the IEE and the IMECHE.
Any type of emergency repair can be carried out in our machine shop, and Stellite Hard Surfacing can be carried out by our Welding Department, complemented by our outfitters who will dismantle and re-assemble to your requirements on-site.

You'll get the complete service, efficiently, courteously and most
of all ... fast.

THE HISTORY AND DEVELOPMENT OF CARTER BROS LTD

Founded over 100 years ago, Carter's main product was originally transmissions and line shafting for textile machinery in the then thriving Lancashire cotton industry. With the gradual decline of the industry, it became apparent that diversification into other areas was essential for the company to continue, and the decision was made to enter the field of rubber and plastics processing machinery.

Over the years new two roll mills, stockblenders and three and four roll calenders were manufactured, together with the overhaul of a wide range of general rubber machinery. Naturally from this interest in the rubber industry and from the excellent reputation achieved, the question of repair and overhaul of mixing machinery soon developed.

Initially mixers were reconditioned for companies local to Rochdale, UK, so that the quality of the workmanship could easily be monitored by the company. Early machines were overhauled for the Dexine Rubber Company, Dunlop and the Firestone Tire and Rubber Company in the UK.

As the Bridge family gradually relinquished their interest in David Bridge and Company (which was one of the first companies to take licence to manufacture the tangential mixer), several of the family bought interests in Carter Bros Ltd. This brought even more knowledge and expertise into the company.

The introduction of the internal mixer saw Carter Bros Ltd choose the path of becoming specialist engineers to the rubber and plastics industry. Because of its involvement in manufacturing new machinery for the rubber industry and overhauling internal mixers, it was a logical move to progress to the design and manufacture of its own range of internal mixers, employing the latest technology.

The internal mixer is considered to be the "heart" of any mixing room. The Carter range of mixers has been developed with this in mind, also taking into account that end-users are continually looking to achieve shorter cycle times and improved dispersion.

Today, the Carter range of internal mixers are regarded world-wide as the most advanced and efficient mixing machines available on the market. A skilled and experienced workforce have drawn on a wealth of knowledge acquired over many years to produce mixers with optimum designs for efficiency and durability.

The best testimonial to the total quality of the Carter internal mixer is its use all over the world by rubber companies of all sizes. Apart from the quality, it is the philosophy of Carter Bros Ltd which has played a big role in this success. The supply of every mixer is considered not just a commercial transaction, but the supply of a machine built with pride and integrity by a workforce and management team totally committed to the satisfaction of its customers.

It was a matter of great pride and satisfaction for the management team when Carter Bros Ltd received in 1992 the ISO-9001 certification for the design, manufacture, installation and commissioning of rubber and plastics processing machinery.

Carter's policy has always been and still is, that ongoing research and development is essential both to meet the ever increasing demands of the industry, and maintain its established reputation worldwide.

The company's current mixer specification has emanated from internal research and development and feedback from actual end-users and is proving superior to competition in many ways.

The up-to-date technology built into a new Carter mixer is also available for introducing into replacement new and service exchange body parts on older mixers.

The economic climate over the last five to six years has limited customers' capital available for new machinery but they have, however, been able to benefit greatly by the introduction of Carter's latest up-to-date technology into a replacement or service exchange body part, rather than simply a repeat of the out-of-date design body part.

The 'flexibility' of Carter Bros Ltd as a company means also that customers' own special requirements can be incorporated into an internal mixer.

Over the years Carter internal mixers have been supplied to customers worldwide, including most countries in the Pacific Rim.

Tangential Mixers

At the heart of a carter mixer is the unique heavy duty carter rotor assembly which has been developed specifically to give excellent dispersion and increased output, whilst eliminating deadspots. A much higher torque value is attained through the use of alloy steel rotors and heavy duty rotor blades tips ensure a constant mixing over an extended life period.

The carter internal mixer range has gained and outstanding reputation for high performance and consistency in a veriety of main stream and associated industries.
Many carter machines are shipped overseas for use in the production of high technology rubber and plastic compounds so it is essential that high quality control stnadards are constantly maintainedto meet rigorous production schedules in the most demanding environments.

Complete manufacture, machining, super hard welding, inspection , assembly and testing are undertaken in house in order that customers have total reliability and efficiency of production machines.

Interswirl Mixer
the new 'interswirl' rotor has been designed to maximise mixing and blending of the compound whilst minimising temperature rise. This is achieved with a varying width main nog which semi-mixed compound along it's length, whilst blended compound moves more readily through the shear zones of the machine.

in addition a reversal dam has been incorporated , which gives a degree of tangential mixing, as well as reducing dust stop pressure.

The Single-Rotor Continuous Mixer
(Carter Continuous Compounder)

Introduction

Continuous mixing holds out the promise of efficient and consistent rubber processing This has been recognised for many years, with the last major development activity occurring in the 1970's [1]. Then, the lack of a reliable supply of technologically and economically viable particulate rubber, coupled with a relatively undeveloped mixing technology, caused the movement to founder, despite the best efforts of enthusiasts. Now, continuous mixing is back on the agenda, with major materials suppliers offering a range of elastomers in particulate form. There is also a much better understanding of mixing to draw upon for process design [2,3,4].

In this paper a new continuous mixer is described, followed by presentation of results from a prototype system.


Design of the Single Rotor Continuous Mixer (SRM)

From analysis of existing continuous mixers [3,4] it is clear that some separation of incorporation, distribution and filler dispersion functions is desirable for an efficient design. Similarly, separation of conveying and mixing is necessary for versatility, to bring residence or mixing time under operational control. A layout of the SRM system, equipped with a roller die output device, is shown in Fig 1. Alternatively, a screw extruder can be substituted for the roller die. A schematic cross-section of the prototype, which has a simple fixed die, is shown in Fig 2.

A powder mixer delivers a particulate blend of the rubber compound ingredients to the feed unit. This device, which is essentially a screw extruder, compresses the particulate blend and causes the rubber to flow, effectively encapsulating and incorporating the filler. It also meters the rubber compound to the mixing unit, enabling residence time in the mixing unit to be controlled by adjustment of the feed unit screw speed.

The mixing unit has two zones, distributive and dispersive. The distributive zone is designed to remove residual variations of feedstock composition due either to the powder blending operation or to segregation in transit from the powder blender to the feed unit. Distributive mixing does not require high stresses or energy levels. Progressive subdivision and recombination of flows at modest strain rates is used to restrict the heat generation and rubber compound temperature rise in this zone. Following the distributive zone, the dispersive zone only has to accomplish the micro-scale re-distribution of fractured filler agglomerates. It is designed to subject the rubber compound to a series of high stress events, with a minimum of "wasted" flow between them. A multi-blade rotor is used for this purpose, to accomplish filler dispersion in a very compact unit.

Figure 1 Schematic layout of the continuous mixing system

The single rotor design gives simplicity of manufacture coupled with high rigidity, enabling a light construction to be used. It is more analogous to an extruder than to a traditional rubber mixer. A twin rotor or screw machine is only essential when the feedstocks are substantially segregated, as is the case with a bale fed batch internal mixer or with separate metered feeds to a continuous mixer. When the feedstocks are pre-blended the distributive mixing requirement is less severe and can be accomplished with a single rotor design. The RAPRA cavity transfer mixer is a good example of a proven single rotor design [5].

The Prototype SRM System

The prototype system shown in Fig 2 has a 90 mm diameter feed unit screw and a 200 mm diameter mixing unit rotor. At maximum speeds of 30 rev/min for the former and 100 rev/min for the latter the theoretical output is 200 kg/hr, based on conventional one-dimensional non-Newtonian, non-isothermal modelling. At this output, complete mixing of a simple SBR compound with 40 phr N330 carbon black is predicted from earlier work on the micro-mechanics of dispersive mixing [6,7], coupled with an output rubber compound temperature in the region of 105C. The latter figure assumes a cooling efficiency similar to that attained in an intermeshing rotor internal mixer.

Figure 2 Cross-section of the prototype Continuous Mixer

Mixing Trials and Results

Two dissimilar compounds were selected for initial evaluation of the SRM. The purpose of the SBR compound shown in Table 1 is to explore the dispersive mixing behaviour of the SRM while that of the EPDM compound in Table 2 is to determine its ability to deal with high filler and oil loadings.

Table 1 The SBR Compound Table 2 The EPDM Compound

The Union Carbide Elastoflo EPDM is produced directly from polymerisation with a spherical granule in the size range 0.5 to 1 mm, while the SBR is granulated bale material with an irregular particle in the size range 4 to 7 mm. No difficulties were experienced with the feeding of the SRM system. In the results which follow rubber compound viscosity is used as a measure of filler dispersion [6,7].

Table 3 Single Rotor Continuous Mixer Results for the SBR Compound

Table 4 Francis Shaw K1 Intermix Results for the SBR Compound

A great deal has been learned from the mixing trials to date. The screw and rotor speeds of 8 and 40 rev/min for the SBR compound and 20 and 40 rev/min for the EPDM compound are simply the first found to give a visually mixed rubber compound. Initially, the output rate from the system was substantially below expectations, particularly for the EPDM compound. The cause was traced to wall slip in the feed unit, a factor not taken in account during process design and was remedied by reducing the flow resistance of the transfer channel between the feed unit and the mixing unit.

The results in Tables 3 and 5 confirm that the mixing treatment can be varied over a wide range by independent adjustment of feed unit and mixing unit speeds. In addition, the outlet impedance (die resistance) of the mixing unit has been found to be a powerful and useful variable. A reduction of impedance reduces the effective "fill factor" in the mixing unit. The second result in Table 3, marked by the *, was obtained with a reduced impedance. Unit work is reduced, accompanied by a reduction in rubber temperature but an unacceptable deterioration in dispersion, indicated by the high viscosity. In contrast, Table 5 shows that reducing the outlet impedance for the EPDM compound gives an overall improvement, reducing unit work and rubber temperature while maintaining a good dispersion. Outlet impedance clearly has a strong influence on the intensity of mixing; and a much lower intensity is needed for dispersive mixing of N550 carbon black than for N330. It is also interesting to note that output is controlled by the feed unit screw speed and is unaffected by changes of output impedance, for both the SBR and EPDM compounds.

Table 5 Single Rotor Continuous Mixer Results for the EPDM Compound

Table 6 Francis Shaw K1 Intermix Results for the EPDM Compound

The unsatisfactory filler dispersion of both the SBR compound and the EPDM compound at maximum speed is attributed to the high rubber temperature causing a substantial drop in the stress available for dispersion. There is substantial scope for improving cooling. The prototype SRM was designed for simplicity and ease of modification, which precluded the incorporation of efficient cooling. The heat transfer rate is approximately half that of the flood cooled Francis Shaw K1 Intermix used for performance comparisons in this study. Use of drilled cooling channels and turbulent water flows in future machines will give a three or four fold improvement. In both Tables 3 and 5 the last row gives the best result that can be achieved with the current prototype - maximum output with satisfactory filler dispersion. This will be increased when the cooling efficiency is improved.

In Tables 4 and 6 the results for satisfactory carbon black dispersion in a Francis Shaw K1 Intermix (intermeshing rotor internal mixer of 5.5.litres chamber volume) are given, together with the batch viscosity which provides a measure of this state-of-mix. Comparison with the unit work results in Tables 3 and 5 shows the high energy efficiency of the continuous mixer. For the SBR compound the energy required is approximately 1/3 that of the batch internal mixer and 1/2 for the EPDM compound. In addition to the obvious energy saving, this improvement enables the size of the whole mixing system to be reduced. With the simplicity and stiffness of the cylindrical feed and mixing unit barrels, the mass of the continuous mixer can also be much lower than that of an equivalent batch mixer.

Energy Balance

A simple process energy balance has been undertaken with the SBR compound to determine the distribution of drive energy between the zones of the system and to assess the transfer of heat to and from the system. The drive energies are calculated by multiplying power by residence time in each section, while the heat transfer to and from the system is obtained from the difference between the measured rubber temperature rise in each zone and that calculated from the drive energy input (adiabatic). The very small differences between inlet and outlet water temperatures in each zone made it impossible to obtain accurate direct measurements. The heat transfer will therefore include both the energy transfer to and from the circulating water and from losses. Table 7 summarises the results.

Table 7 System energy balance for the SBR compound at 8/40 rev/min

As expected, there is a energy input to the feeder compactor from the circulating water (set at 70C) and cooling thereafter. The estimated cooling efficiency of the distributive zone is high and may reflect the difficulty of obtaining an accurate measured rubber temperature at the boundary between the distributive and dispersive zones. Hence the figure for latter zone will be correspondingly low. The overall heat transfer efficiency is as expected for a simple flood cooling system in which the water channels are separated from the rubber by substantial thickness' of metal. As noted previously, substantial further improvement can be gained for production systems by using drilled cooling channels in the mixing unit barrel and by changes to the internal water ways in the rotors.

Development of State-of-Mix

"Dead stop" experiments have been used to track the progress of mixing as the rubber passes through the SRM. After running at equilibrium conditions, the system was stopped while full of rubber and then opened up. Rubber samples were taken from accessible locations and filler dispersion checked by viscosity measurement.

The general shapes of the mixing curves in Fig 3 are similar to those for viscosity vs time in an internal mixer [6]. Viscosity rises to a maximum as incorporation progresses and the air in the filler agglomerates is replaced by rubber, rendering the compound essentially incompressible. As it reaches this state dispersive mixing starts. It becomes possible to apply high stresses, which fracture the agglomerates to release the immobilised rubber and cause a progressive decrease of viscosity. In the early stages of dispersive mixing, the agglomerates are large and the stresses needed for fracture are low, so it is not surprising that dispersion can occur in the feed unit head and in the distributive zone [8]. As the agglomerate size is reduced, the stress intensity needed for further fracture increases; and it is the purpose of the dispersive zone to supply high stresses.

Figure 3 Tracking the progress of mixing through the Single Rotor Continuous Mixer

The results for the EPDM compound in Fig 3 were obtained with a high outlet impedance. They show that the transition between incorporation and dispersion occurs in the head of the feed unit. Comparison with results in Tables 5 and 6 reveals that the amount of dispersive mixing that remains to be done after the compound leaves the distributive zone is small and explains why it is possible to reduce impedance without loss of dispersion quality.

In contrast to the EPDM compound, incorporation of the SBR compound is not completed until after it has entered the distributive zone and, from the shape of the curve in this zone, it is clear that the rate of dispersion drops rapidly towards the exit. This can be attributed to the available stresses becoming inadequate for further dispersion8. The further decrease of viscosity in the dispersive zone shows high stresses are developed. This result is interesting, since the energy balance shows that the dispersive zone power demand is low and pressure measurements near the outlet indicate gross slippage. It suggests that improved exit pressure control, with a roller die or extruder output device, will give a substantial improvement in mixing performance.

Purging Characteristics

Minimum wastage of rubber compound and time during a compound change-over are natural objectives of continuous mixer design. Experience with the prototype SRM has shown that the feed unit screw and the dispersive zone are completely self purging but physical removal of rubber from the feed unit head and from the distributive zone is necessary. Design modifications to improve the self-emptying of the distributive zone are being investigated. In addition, a trial to explore the extent to which a direct change-over is possible, without emptying the machine of rubber has been carried out. The SBR and EPDM compounds, with and without curatives, were

Figure 4 Purging Behaviour of the Single Rotor Continuous Mixer

used to track the progress of purging. The feed hopper was run empty of the particulate blend with curatives and followed immediately with a blend which omitted the curatives. Samples of mixed compound were then taken at intervals for cure testing. The results shown in Fig 4 were obtained at an output rate of 35 kg/hr for the SBR and 44 kg/hr for the EPDM.

The reduction in the amount of material present with curatives is tracked by the change in crosslink density (Tmax - Tmin), until it is eliminated at 1 minutes for the SBR and 2 minutes for the EPDM. After each of these trials the mixer was opened and samples of compound removed from the head of the feed unit for further cure tests, to check for residual compound with curatives. These samples showed zero crosslink density and confirmed that the head is free of regions of re-circulatory flow or stagnation.

Scale-up

Scaling rules have been developed for the SRM, using non-isothermal, non-Newtonian flow analyses to extrapolate from the prototype results. They are conservative in that they do not assume any increase in dispersive mixing efficiency resulting from the planned improvements in heat transfer efficiency and output pressure control. The predictions in Tables 8 and 9 are for single-pass mixing with all the curatives included in the blended particulate feedstock. Consequently, the maximum output rate is dictated by rubber temperature and a target outlet value of 100C has been used. The specific heat transfer rates have been set at levels expected for drilled cooling channels, except for the prototype characteristics in the top row, which are experimentally determined. For comparison, the experimentally determined heat transfer coefficient of the flood cooled Francis Shaw K1 Intermix used in the work reported here is 770 W/m2/C. Net power does not included allowances for drive and friction losses.

Table 9 Scale-up predictions for the SBR compound

Table 10 Scale-up predictions for the EPDM compound

Conclusions and Further Development

Preliminary results from a new design of continuous rubber mixer have been presented which provide proof of principle and show the way forward for improvement and for the design and build of production systems. In particular, the SRM:

· is compact, simple and robust

· has simple and versatile operational control to mix a wide range of compounds

· feeding is insensitive to rubber particle size and shape within the range investigated

· is energy efficient

· can achieve complete mixing in a single pass

· has good purging behaviour for rubber compound changes

Substantial improvements to the heat transfer efficiency of the mixing unit are possible, to combine high output with low rubber temperature. This will also improve the dispersive mixing capability. The prototype unit is relatively inefficient due to simplicity of construction and design for adaptability.

Acknowledgements

The work reported here has been carried out in co-operation with Carter Bros (Rochdale) Ltd and funded by the Engineering and Physical Sciences Research Council of Great Britain. The support of Avon Rubber plc, Cooper-Avon Tyres Ltd and Union Carbide Inc is also gratefully acknowledged.

References

1. C. W. Evans, Powdered and Particulate Rubber Technology, Applied Science Publishers, London, (1978).
2. J. L. White, Rubber Processing, Hanser/Gardner, Cincinnati, (1995).
3. I. Manas-Zloczower and Z. Tadmor (Eds), Mixing and Compounding of Polymers, Hanser/Gardner, Cincinnati (1994).
4. H. Ellwood, 'A Tale of Continuous Developments', Eur. Rubb. J., 169 (3), 26-32 (1987).
5. R. S. Hindmarch and G. M. Gale, 'Application of the Cavity Transfer Mixer to Rubber Extrusion', Elastomerics, 114, 8, 20-25 (1982).
6. J. Clarke and P. K. Freakley, 'Reduction in Viscosity of an SBR Compound by Mastication and Dis-agglomeration During Mixing', Rubb. Chem. Technol., 67, 4, 700-715 (1994).
7. P. K. Freakley and J. Clarke, 'Comparisons of the Mixing of Rubber with Carbon Black in an Internal Mixer and in a Biconical Rotor Rheometer', J. Appl. Polym. Sci., 53, 121-132 (1994).
8. J. Clarke and P. K. Freakley, 'Modes of Dispersive Mixing and Filler Agglomerate Size Distribution in Rubber Compounds', Plast. Rubb. and Comps., Proc. and Appl., 24, 5, 261-266 (1995)