History of rubber mixing

• Early compounding took place either in single rotor machines such as Hancock's Pickle, or on two roll mills.



• The first use of the two-roll mill was in the 1830s in the USA.



• Hancock's Pickle was patented in 1837, although models had actually been in use from the early 1820s .



• The first reference to double rotor internal mixers would appear to be dated 1865, the Quartz mill of Nathaniel Goodwin, although there is some doubt whether this machine would have been suitable for rubber due to its apparent lack of strength.



• The first machine that appears suitable for rubbers was a twin rotor design patented by Paul Pfleiderer in 1878/1879.



• Today the compound preparation is accomplished today predominantly in internal mixers.



• Nowadays the mill is used mostly to sheet out and cool the rubber compound coming from the internal mixer. In some cases the final mixing process like the incorporation of sulphur and vulcanization accelerators as well as other minor components is done on the mill also.

Mills were first used for compounding in about 1835 Where internal mixers are available the mill has generally been retired to a simple sheeting use in the mill room, however for specific types of compound, particularly roller covering materials, the mill is still very often the preferred mixing unit. It is also widely used in developing parts of the world for normal compounding mainly used to prepare coloured, tacky of very hard compounds.

Primarily the two-roll mill is utilized in small factories or for small size compounds.

• Additionally mills have their importance as follow-up equipment after internal mixers of as breakdown and warm-up equipment in front of calenders of extruders.

Mixing and sheeting mill

The mixing mills are constructed from two horizontal parallel arranged rolls made from hard castings which are supported through strong bearings, e.g. friction bearings, in the mill frame made from steel casting.

• The rolls run at different surface speeds (friction ratio) against each other Conventionally mills have had a friction ratio between the rolls of between 1:1,05 and 1:1,2 to maintain the compound on one roll.


• For heating or cooling purposes the rolls are hollow where it its important that the wall thickness in uniform in order to obtain an even surface temperature. In contrast to plastics processing, the rolls are seldom heated and the mostly only for start-up of production. Cooling is of great importance because of the heat generated due to viscous flow that is generated continuously inside the elastomere in the roll nip. The cooling water enters and leaves the inside of the roll through a rotary union.



• In practice it is better to utilise rolls running at the same speed, with temperature control systems on each roll to force material onto the front roll. It is better still to have individual roll drive, with drilled rolls and temperature control systems, such that both temperature and friction ratio can be set to direct the rubber onto the correct roll. Modern mills are often fitted with hydraulic motors on each roll, dispensing with a gearbox, and hence become a very sophisticated tool in the mill room on which compound quality can be controlled.

Continuous Mixers

• The continuous mixing of rubber compounds is still very much in its infancy, even though the Du-Pont Delphi probe of 1971 predicted a significant change to powder type processing similar to the plastics industry by the mid 1980s.



• The earliest machines used in continuous processing of true curable materials were the EVK (Extruding, Venting, Kneading) machine made by Werner & Pfliederer and the MVX (Mixing, Venting, Extruding) made by Farrel Bridge



• The EVK and the MVX made some inroads into the compounding market, the EVK primarily into the EPDM extrusion compound area using powdered polymer and the MVX into cable compounding and into production of tyre compounds using granulated polymer.



• Recent work on continuous mixing of rubbers has centred on modified twin screw compounders that have been used for some considerable number of years in the plastic compounding industry.

The problem to overcome with all continuous compounding systems is to achieve good dispersion with adequate distribution of compound ingredients.

• All continuous mixers only mix a small quantity of compound at any one time. The requirement has therefore been that all ingredients used in a rubber compound mast also be present in a very small amount of that compound.



• Compared with batch mixing, where batches are both extensively and intensively mixed and small ingredients will be adequately distributed in the mixer of the following mills, continuous mixers achieve only the intensive mixing. The extensive mixing must be done outside of the machine. This can be done for instance by making preblends of all materials as have been used on the EVK and MVX.



• Because of the numerous compound ingredients used in tire industry and their varying physical form, an economic and sufficiently accurate proportioning of the compounds in a continuous mixer is barely possible. In contrast, batches, for example, one consisting of elastomer and filler and another one containing the chemicals, can be well mixed when special mixing screws are used.



• Continuous mixing lines are applied up to now preferably only for partial operations, e.g. the mixing of some few main compound constituents, i.e. the preparation of batches or the pre-heating for the calender or the final mixing and extrusion of compacted powder compounds in one step.
  

Internal Mixers

• In large rubber factories, especially tire factories, the internal mixer has practically replaced the two roll mill for the preparation of compounds.



• The mixing process in the internal mixer is accomplished inside a closed chamber by rotating kneading rotors.



• The movement of the rotors achieves the mixing process either between the rotors or between the rotor and the chamber wall. In contrast to the mill, more uniform compounds and large batches can generally be prepared by the internal mixer.



• In modern, large-scale production facilities the material flow is extensively automated using microprocessor equipment which controls and monitors the whole mixing room There are two basic internal mixers available: the Banbury (tangential rotor type) mixer and the Intermix (intermeshing rotor type)
  
  

Banbury Mixer

The Banbury mixer was initially developed by Fernley H. Banbury from 1916 onwards The Banbury mixer was developed in response to problems encountered with the existing range of mixing machinery in the USA during the early part of 1900 century. As the first truly successful batch mixer, this machine has given its name as the generic term for all tangential internal mixers.

• The mixing principle of Banbury mixer relies on a tapering nip between the rotor and the sidewall of the mixer to give mix dispersion. It also relies on the transfer of material around the mixing chamber and from one rotor to the other to give mix distribution. Mainly the mixing effect is achieved between rotorwings and chamber wall.



• The distinguishing characteristic of this machine is that the paths of the blade tips only just fail to touch. In other words rotors do not interlock each other. This means that in machines of this type the rotors can turn at different rotor speeds. (1:1,1 as a rule).



• Various rotor shapes are available for different mixing jobs. It is likely that at least 50 % of all rubber mixed in the world is still mixed on 2-wing tangential rotors.



• A significant proportion of the tyre industry has changed over to 4-wing rotors, on the basis of both productivity and quality improvements when larger mixers are used.



• What 2-wing rotors do not have is high productivity, hence the development of the 4-wing rotor. The 4-wing rotor was initially developed for larger mixers, but it has gradually introduced for all sizes of machine when the process requires the faster mixing which this type of rotor provides.

The Intermix

• The concept for the Intermix was developed in the U.K. during the early 1930s by an unknown engineer of the ITS Rubber Co Construction and detail design of the Intermix was contracted to Francis Shaw and Company of Manchester, who eventually acquired and patented the design.



• Intermix gave a different approach to the problems of rubber mixing, the emphasis was given to the transfer of material along its length and in the opposite direction to the other rotor. Transfer from rotor to rotor occurs due to the interlocking nature of the rotors.



• Intermeshing rotors are rolling with same speed and the mixing effect is achieved as in tangential system but with this system mixing takes place also in the nip between the rotors.



• Compared to tangential system intermeshing system has more effective temperature control, drive power is around 10-20 % higher but optimum fill levels are about 5 % lower because of the narrow intermeshing zone.



• The use of intermeshing rotors has increased considerably during last years especially in technical rubber industry.
  

The VIC (Variable Internal Clearance) Mixer

The VIC mixer has been developed in 1987 in Italy to address what some regard as problems in intermeshing rotor mixer designs.

• On this machine a device allows axial movement of the rotors to increase or decrease the gap between them during mixing. This is said to allow easier feed of large batches of rubber at the start of the cycle, and also to allow adjustment of the rotors to an ideal mixing position for each type of rubber compound. This in analogous to the requirement for different nip settings during mill mixing.



• How valuable the rubber industry will find this development is not yet known, but initial reports, primarily from the manufacturers indicate a promising start.