Internal wear of the batch mixer--part 1.
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 assure 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 he 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.
Tribology is an interdisciplinary study that deals with the design, friction, wear and lubrication of interacting surfaces in relative motion (ref. 2). There is a huge body of information available with numerous definitions of wear and the effects of wear on machinery. Tribology involves the technology disciplines of mechanics, lubrication, materials engineering, metallurgy, ceramics, surface chemistry, polymer science, contact mechanics, nano-scale surface physics, chemical engineering and surface processing. As one can imagine, there is a multitude of different approaches to defining what one may consider the types of wear that exist. Reading through the literature on tribology, one soon will come to the conclusion that there is a number of opinions as to what the types of wear are and how they can be applied to describe the degradation of machinery components. A general literature search will result in a description of the following types of wear that I believe can be categorized into either a mechanical or chemical wear category, as can be applied to batch mixing (table 1).
Abrasive wear is material removal by hard particles that have embedded themselves into a soft surface and plow grooves into the opposing softer surface. Abrasive wear is sometimes subdivided into two-body and three-body abrasive wear. In three-body wear, hard particles are free to roll and slide between the two, perhaps dissimilar, sliding surfaces (figure 1).
[FIGURE 1 OMITTED]
The most obvious abrasive wear occurs in the high shear areas of the mixing chamber, specifically where there is an obvious loss of metal on the leading edge of the rotor wing for a tangential mixer and on the minor and major diameters of the rotors of the intermeshing designs. Abrasive mixing can also be seen on the wing and hog ends of the mixers and in the body bore. The potential for abrasive wear occurs anywhere in the mixer where there is a direct contact between the mixing materials and moving parts of the machinery.
Adhesive wear is material transfer between two surfaces, or the loss from either surface, due to localized bonding between the contacting asperities (tiny projections and irregularities) (figure 2).
[FIGURE 2 OMITTED]
Adhesive wear occurs in the same areas of the mixer that see abrasive wear; small amounts of metal have been detected within the structure of very hard fillers, leading to metal contamination of the product. Also, adhesive wear has been seen in the dust stop assemblies of both types of mixers where hard formulation filler has been imbedded into the metal of the dust stop gland rings.
Erosive wear refers to the type of surface loss where hard particles carried in a gas or liquid (polymer) stream strike a surface, causing loss of the surface (figure 3).
[FIGURE 3 OMITTED]
Impact wear is structure deformation based on the cyclic application of stress where no loss of material is noted (figure 4). The repeated movement of the ram in the enclosed hopper of the mixer and the contacting face of the door top and body in the discharge area is where there is repeated metal to metal impact, causing the degradation of the contacting surfaces. Tramp metal within the mixing chamber can cause impact wear to virtually all working surfaces of the mixer, and in some cases has led to catastrophic failure, requring the replacement of major components of the mixer.
[FIGURE 4 OMITTED]
Corrosive-chemical wear can be considered the loss of surface due to a corrodent or the gradual destruction of a metal or alloy due to a chemical process such as oxidation or the action of a chemical agent (ref. 6). A dramatic example of corrosive-chemical wear is shown on specific components of a continuous mixer. Figure 5 shows the upper half of the working surface of a two-piece mixing chamber. The figure clearly shows the chemical attack on different components of the mixing chamber made of tool steel, 410 stainless and 300 (low carbon) stainless steel.
[FIGURE 5 OMITTED]
The corrosive-chemical wear occurring within the batch mixer involves the reaction between metals, oxygen, water and/or acids. Due to the complexity of chemical wear, a detailed explanation of the chemistry will not be discussed. However, it is safe to say that practically all corrosion is electro-chemical; that is, anodic and cathodic regions on the metal surface are involved. As examples of corrosion, the oxidation process (figure 6) can be accelerated by the presence of heat and, in situations where direct corrosion is occurring, the acidity of the contacting solution can accelerate the corrosion process (ref. 3).
Corrosive-chemical wear can occur both within the mixing chamber in the high and low shear area of the mixer, as well as up in the hopper and mating surfaces of the chamber assemby, where corrosive gaseous/vapors generated by the mixing process can attack the mixer materials of construction.
It is my belief that wear of the internal surfaces of the mixer that occurs due to the mixing action of rubber and plastic compounds is primarily due to corrosive (oxidation), chemical attack and the abrasive, adhesive and erosive wear mechanism. However, there are sections of the machinery where there is repeated metal to metal contact, where impact wear takes place. It occurs in the hopper and discharge area of the mixer. Although associated with mixing, this type of wear and that caused by tramp metal are not associated with the abrasive or corrosive nature of the components of a product mix.
This article will be continued in next month's Rubber World (March 2006), where the batch mixer and the mixing process will be examined, including fume and dust removal and typical corrosive applications, along with wear observed in the mixer.
(1.) "Effect offull-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 by A. G Guy Addison, Wesley Publishing.
Frank J. Borzenski,
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|Title Annotation:||Process Machinery|
|Author:||Borzenski, Frank J.|
|Date:||Feb 1, 2006|
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