Internal wear of the batch mixer--part 3.
With increased demands being placed on the performance of rubber and plastic products, new formulations, polymers, fillers, reactive and chemical additives have been developed. Many of these new materials and formulations are extremely abrasive to mix, and some contain potentially corrosive chemicals and emit corrosive gases during the mixing process. The compounding of these new products requires the mixer to be manufactured using specific materials of construction based on the application.
Besides the importance of selecting materials of construction that minimize mechanical wear and optimize corrosive resistance, the effect of these materials on the stick/ slip phenomenon required for efficient mixing and for a clean discharge from the batch mixer must also be considered (ref. 1).
The compound must flow properly in the mixer to as sure efficient mixing, and must release from the internal surfaces of the mixer when the mix is complete. Improper selection of materials of construction can have a significant adverse effect on the mixing process. It is important that, at the time of purchase of a new or rebuilt mixer, the application be carefully reviewed so the most appropriate design and materials of construction are specified.
Although not covered within the text of this article, the design of the dust and fume collection system is also important and can have a dramatic effect on the life of the mixer. It is important that generated corrosive gases be quickly and safely removed from the mixing machinery. It is equally important that the dust and fume collection system be able to capture and handle the effluents from the mixing process, but not be so aggressive that critical components of the mix, such as light fillers or minor concentrations of critical powdered chemicals, are removed from the mixing chamber.
In last month's Process Machinery column (March 2006), the batch mixer and the mixing process were examined, along with fume and dust removal.
Wear observed in the mixer
It is through field inspection of mixers and the standard practice of rebuilding machinery where the returned machine is cleaned, disassembled and inspected that we have been able to document wear and the effect of mixing on various mixer designs and materials of construction. Such innovations as replaceable throat and wear plates, rotor wear rings, as well as the use of specialty hard surface coatings for particular compounds, have been developed.
Figure 13, displaying different components of a Banbury mixer, shows the effect of wear seen in different applications on specific materials of construction of the mixer. The categorizing of the type of wear is interpretive and arbitrary. In fact, the wear observed in each case is most likely a combination of various wear mechanisms imposed by the mixing process.
Abrasive and corrosive wear has been seen in the body bore. Chemical wear has been seen on the mating parts of the sides, end frames, rotor end plates and the hopper assembly.
Abrasive wear is observed primarily on the leading and trailing angle of the rotor wing (rounding of the rotor wing tip) and at the start of the rotor wings at the rotor end plate. In cases where a porous hard surfacing has been applied for wear, corrosive wear has been seen as the corrodent penetrated the coating. Also, corrosive wear has been seen when a protective chemically resistant coating has been worn through and the base metal is exposed.
Rotor end plate/end frame
Abrasive wear has been seen on the face of the rotor end plate that is exposed to the mixing chamber. Corrosive-chemical wear has been seen between the mating parts between the rotor end plate and the sides/end frame of the mixer.
The door top has seen both mechanical and chemical wear on the working face of the door top. Impact wear is seen on the contacting face where it seals against the sides of the mixer.
History has shown that there is very little abrasive or impact wear on the weight. Chemical wear attacking the entire surface of the weight seems to be of most concern. There is a number of applications where the weight has to be constructed out of stainless steel to prevent corrosion. Chromed weights have been supplied for the ease of cleaning. The base metal of these weights is cast steel. It now becomes an issue if a steel weight is used within a steel hopper rather than a cast iron weight within a steel hopper.
Mixer hopper and throat and wear plates Replaceable throat and wear plates were designed because of severe wear observed in the lower part of the single piece hopper.
The following questions must be asked when addressing the wear effects of compounding in the batch mixer: What potentially corrosive ingredients will be mixed and in what concentration? What (if any) corrosive by-products or gaseous vapors will be generated during the mixing process, and how much will be generated (potential concentration)? What potentially abrasive ingredients will be mixed and in what concentration? Do I have product release problems which can be caused by the use of specific hard coatings or metals for the internal working surfaces of the mixer? Do I have a product that requires maximum heat transfer, dictating the use of a hard surface coating with good heat transfer characteristics?
It is through communication with the machine manufacturer as to the application of the mixer that a mixer can be specified which will offer the longest service life for the proposed application. Currently, we have over 31 surface coatings and eight types of base metal options available to address issues of mechanical and chemical wear in selected parts of the batch mixer. Further, there are multiple materials such as specialty cast irons, various grades of carbon, tool and stainless steel available for use in the manufacture of the mixer.
The first mixer bodies were produced primarily out of cast iron. Shortly thereafter, rubber hang-up issues in the mixing chamber resulted in the application of chrome onto the surface of the rotors, side and drop door for release.
The inspection and eventual disassembly of mixers showing signs of wear identified areas of the machine requiring different materials of construction, as well as the need for specialized hard coating in specific areas. As the rubber industry matured, higher speeds, shorter cycles, high torque requirements and the mixing of higher viscosity compounds forced a review of all mixer major components leading to the use of cast and wrought steel plate for many of the highly stressed mixer components.
The mixing of highly abrasive materials, such as floor the formulations, silica and Ti[O.sub.2], resulted in the rapid wear of the internal surfaces of the mixer. The use of iron, cobalt and nickel based hard surface coatings was introduced. Design changes were made which would allow relatively quick replacement of wearing components. Concurrently, it was discovered that in many applications corrosive effluent gases were being generated within the mixer during compounding. These gases penetrated the porous chrome surfaces of the internal components of the mixer, causing a loss of the chrome, as well as the rapid loss of the base metal of the mixer. The corrosive gases/vapors also penetrated the mating area between metal parts, creating crevice corrosion issues. Special (crack free) coatings have been and are being developed and tested to protect against specific corrosive agents. Due to wear, the design of the mixer has been changed and the materials of construction are now specific to the compounding application. Typical materials of construction are presented in table 6. The base metal used for specific applications depends on the tensile and yield strengths, corrosion resistance and manufacturing requirements for specific components.
The internal working surfaces of the mixer in most applications need some type of surface coating or treatment to protect the mixer against mechanical wear. Also, the surfaces of some of the structural components of the mixer which come in contact with high concentrations of the corrosive gases and vapors generated during the mixing process need protection as well. There is a number of options available that can be used to protect the base metal and extend the usable life of the mixer. In all cases, the internal surfaces of the mixer contacting the mix must support the mixing mechanism of the batch mixer and must allow for a clean release of the product from the mixing chamber. Table 7 lists the categories of wear protection options that are currently available.
Through exhaustive studies and years of data collection, it has become possible to establish specifications for the materials of construction for the batch mixer for specific applications. Figure 23 and table 8 present selected data of comparison for the materials of construction for an internal mixer for general purpose applications, an extreme mechanical wear application and for an extremely corrosive application.
[FIGURE 23 OMITTED]
With the changing demands of the rubber and plastics industries, it is increasingly important that the compounder not only be aware of the type of mixer that is best for his application, he must also know the abrasive and corrosive nature of the products he is expected to mix. Knowledge of the effects of available base metal materials of construction, as well as that of the available surface treatments, can greatly extend the usable life of the mixer. It is through working with a qualified vendor of machinery that the proper selection of materials of construction can be made that will optimize the service life of machinery in application.
1. "Effect of full-slip condition along rotor on the mixing efficiency of internal mixers', " Antec 2003, University of Louvain & Michelin Clermont-Ferrand, France.
2. Merriam Webster On Line.
3. Elements of Physical Metallurgy, A.G. Guy Addison, Wesley Publishing.
4. Friction and Wear of Engineering Materials, I.M. Hutchings, 1992.
5. Handbook of Chemistry and Physics, The Chemical Rubber Co., Cleveland, OH.
6. Dictionary of Scientific and Technical Terms, McGrawHill
Table 6--typical materials of construction Description Type Comment Cast iron, i.e., Multiple -- Low cost and high strength -- Grey grades and -- Ductile properties -- Difficult to weld and hard -- Malleable surface -- High tensile -- Varying composition based on application and use Low and high alloy Multiple -- Medium cost to high cost steels, i.e., grades and properties -- Wide variety of strength, -- Cast steels chemical resistance, -- Wrought steels abrasion resistance -- Tool steels -- Stainless steels Table 7--base metal protection options Surface treatment - Quenching and tempering - Shot peening - Nitriding Surface coating - Chrome and iron, nickel and cobalt based matrix materials with different compositions of boron, carbon, chrome, copper, magnesium, molybdenum, silicon, tungsten and/or vanadium Specialty plate - Iron, cobalt and nickel based alloys and and castings composites Table 8--Banbury mixer materials of construction Mixer Standard Mixer Highly abrasive Highly component applications corrosive applications Hopper sides, Cast iron and Cast iron and Stainless steel charging door wrought steel wrought steel Base metal and back door offers No special Chromed surface protection for protective for improved corrosive coatings mechanical applications required wear resistance Floating Cast iron Cast steel Stainless steel weight No special FA # 27 (iron Base metal protective based alloy) offers coatings on bottom for protection for required improved corrosive mechanical wear applications resistance Replaceable Wrought steel Wrought steel Stainless steel throat and wear plates No special Chromed surface Base metal protective for improved offers coatings mechanical protection for required wear resistance corrosive applications Rotors Cast steel Cast steel Cast steel - Rotor tips - Rotor body FA #12S (cobat FA #27 (iron FA #22 (cobalt based alloy) based alloy) based alloy) - standard FA #28A - FA #22 (cobalt thickness (nickel based based alloy) chrome alloy) (coatings for (coatings for (coating for extreme mechanical wear extreme corrosive wear resistance) mechanical wear resistance) resistance) Rotor end Wrought steel Wrought steel Stainless steel plates Standard chrome FA #26A (nickel FA #26B (nickel on working face based alloy) based alloy) (coating on (coating on internal face internal face for mechanical for mechanical wear and corrosive resistance) wear resistance) Drilled side Wrought steel Wrought steel Wrought steel - Frame - Liner (bore) Dual metallic Dual metallic Tri-metallic steel liner steel liner and liner (steel, and FA #13 FA #19 (iron stainless steel (iron based based alloy) and FA #13B alloy) iron based (coating for alloy) (coating for extreme mechanical wear mechanical wear (special resistance) resistance) construction for mechanical and corrosion wear resistance) Drop door Cast steel Wrought steel Wrought steel FA #27A (iron FA #26A (high Chromed surface based alloy) nickel based for product alloy) release and (coating for mechanical wear extreme (coatings for resistance mechanical wear extreme resistance) corrosive wear resistance)