Dust stop systems for internal mixers--part 1.
At first, the finely dispersed ingredients are pasted with an oil film, called paste-bonding. Due to the high pressure inside the mixing chamber, the paste created is slowly squeezed outwards along the rotor shafts. To restrict the flow of the paste, a pair of rings seals the rotor shafts at each end.
The seal itself consists of two superimposed rings which are generally equipped with hard-coated surfaces in the sliding area. One ring is fixed on the rotor shaft; the counter-ring is mounted to the end frame (ref. 1).
The dust stop systems essentially differ in the way the surface pressure is applied. One differentiates self-sealing dust seals (SSA); spring-loaded dust seals (GA); and hydraulic dust seals (WYH).
Dust stop design
The sealing principle if the SSA seals is based on the use of the internal pressure in the mixing chamber. If the internal pressure in the mixing chamber rises, the pressure is transferred to the rotating sliding ring, whereby the locking force of the sealing is increased. For the application of a minimum surface pressure, disc spring assemblies are used to press the inner sliding ring against the outer fixed ring. Today, new internal mixers are equipped with the GA and WYH systems.
In the case of the spring-loaded design (GA), the sealing rings are located outside of the mixing chamber. Here, the inner ring is fixed to the end frame and the outer rotating ring is mounted on the rotor shaft. The surface pressure is applied by several disc spring assemblies arranged around the rotating ring. By preloading of the springs via a nut, the surface pressure can easily be adjusted individually for each dust stop. To paste the finely dispersed compound ingredients, a lubricating plasticizer is injected into the annular gap in front of the dust seal. For lubricating of the sliding surfaces, a further lubrication hole is located in the fixed sliding ring.
In the case of the hydraulic dust seals (WYH), the seal is positioned closer to the rotor at the inside of the end frame of the mixing chamber. The inner rotating sliding ring is fixed to the rotor shaft, whereas the outer fixed ring is pressed against the inner ring by a yoke. The contact force is generated by a hydraulic cylinder and a disc spring package. The force is then transmitted by the yoke to the pressure ring of the outer sliding ring. The lubrication situation is similar to the GA dust seal. Due to the fact that the sealing cross-section is shifted inwards to the mixing chamber, the pasting takes place in the axial gap between the rotor and the wear plate.
Comparing both systems (GA and WYH), it is noticeable that the position of the sealing is a differentiating factor. Due to the proximity to the rotor, there is only little dead space with the WYH sealing, in which material could get stuck. On the other side, free abrasive fillers can directly access the sliding surface very easily, which may unfavorably affect the wear. The GA sealing possesses a relatively long annular gap. Due to the long residence time in the annular gap, the free fillers can be bound better by the pasting oil. Therefore, the sliding surfaces are protected in a better way and the load of the sealings is minimized. However, at the same time, the danger rises that parts of the compound get stuck in the annular gap. Here the sealing oil is of great importance.
The lubrication system is divided into two separate parts, including the lubrication of the sliding rings and the paste-bonding.
The tasks of the lubrication of the sliding rings as used with conventional lubricated systems can be summarized as follows (ref. 5):
* Compensation of the speed difference of the surfaces by the shear in a liquid;
* evacuation of frictional heat;
* protection against adhesive failure;
* cooling of the components;
* evacuation of wear particles from the contact areas;
* admission and evacuation of foreign particles like compound ingredients; and * corrosion protection.
The tasks of the paste-bonding cannot be defined as clearly as for the lubrication of the sliding rings. The main task, as just defined by the name, is the pasting of the finely dispersed ingredients. The use of very high viscous process oils supports this pasting effect. This task is of great importance in the case of the hydraulic sealing. Due to the proximity of the sealing to the mixing chamber, abrasive fillers are more likely to directly access the sliding surfaces. Simultaneously, the free fillers can enter the surrounding atmosphere more easily. Moreover, the paste-bonding rinses the gap between the end frame and the rotor shaft so that no material can get stuck and start to vulcanize. This cleaning effect becomes very important in the case of the GA dust stops, because of the long annular gap. At the same time, the addition of process oil reduces the adhesion between the rubber, the rotor shaft and the end frame. The material transport along the rotor shafts toward the dust stops can be reduced in this way. For this reason, the addition of pasting oil is necessarily recommended.
Problems of dust stops
Apart from the wear of the sliding rings, the oil needed for the paste-bonding and the lubrication of the sliding rings may also lead to substantial problems. Large quantities of oil are needed to fulfill the tasks described, causing high costs for the oil purchase. Furthermore, the oil, which contains compounding ingredients like polymers, fillers, etc., and exits the mixing chamber via the dust stops, needs to be disposed of, which is expensive. It is well known that up to 80% of the used oil flows backwards into the mixing chamber. These portions of oil can cause significant problems in quality, as already described (ref. 6). If too little an amount of lubricants is used for the paste-bonding, parts of the compound could get stuck in the annular gap and may start to vulcanize, possibly causing contamination of the following batches. Secondly, the use of too little an amount of lubricants for the lubrication of the sliding rings will lead to rapid wear of the sealing rings.
As further determinant factors for the wear of the sealing rings, the kind and the hardness of fillers, the rotor geometry, the installation position and the adjustment of the surface pressure can be mentioned. Considering the wear and the development of the disposal as a function of the surface pressure, two opposed tendencies can be observed. If one regards the influencing factors individually, it can be determined that the application of a low contact force will lead to low wear rates. Alternatively, an increasing contact force will lead to rapid wear of the rings.
The interactions between the disposal and the wear also have to be taken into account. An increasing amount of disposal also contains an increasing amount of abrasive fillers passing the sliding area, which again leads to rapid wear. Therefore, the difficulty is to determine the right dust stop adjustment with adapted operational parameters.
Analysis of principle procedures in the annular gapof dust stop seals
The following study investigates the effect of the material transport from the mixing chamber towards the sealing rings, with special consideration of the influence of pasting oil.
The mixing experiments were done in a laboratory mixer with tangential rotor geometry and a chamber volume of seven liters. For this purpose, the sealing rings of two sealing positions were removed. As test material, natural rubber (RSS1, 100 phr) was selected both in the unlubricated operation and with addition of pasting oil (BP Enerpar 16) in the outer area of the mixing chamber.
The target figure of these studies was the rubber balance for the non-sealed dust stop positions. The rubber balance is to be defined as the ratio of the initial weight [m.sub.0] to the leaked rubber mass [M.sub.exit]. For this purpose, the amounts of leaked rubber were collected separately for each position. In the case of the addition of pasting oil, the collected rubber was separated from the leaked process oil.
After addition of the polymer, the mixing time was 15 minutes for all trials. As specific ram force, a setting of 50 N/[cm.sup.2] was selected.
For the rotor speeds of 30 and 50 rpm, the fill factor was varied from 50% to 80%, or from 40% to 70%, respectively. After the dump of the polymer, the batch temperature was measured by hand.
Both rubber balance curves showed a continuous increase of the leaked rubber mass. The development of the disposal can be divided into two parts. First, a slow rise of the disposal amount can be detected. After reaching a category temperature of 130[degrees]C, the rubber balance jumps up for both curves with nearly the same gradient. Due to the increase of the shear velocity, the increase of the rotor speed up to 50 rpm results in a doubling of the leaked rubber masses. This fact, together with the fact that the rubber mass leakage also appears in the case of the under-filled mixing chamber, leads to the assumption that the material transport is caused by the Weibenberg effect. At high fill factors, this effect is superposed by the rising pressure inside the mixing chamber. Considering the real mixing process, one consequence is a strong load of the dust stops additionally caused by the material transport, particularly for natural rubber-based recipes at high temperatures.
A rising tendency to wall slippage is connected to a decrease of the shear. Based on the Weibenberg effect, a reduction of the adhesion leads to a reduction of the shear, and therefore it should lead to a decrease of the leaked rubber mass. This reduction up to the prevention of the material transport can be obtained by the addition of pasting oil.
In comparing the development of the rubber balance for natural rubber for the unlubricated and lubricated cases, the oil volume was varied on two levels. The specified total volumes consist of the oil portions of both unsealed dust stop positions. By the wetting of the rotor shafts, the emission of rubber can be totally prevented up to a certain point. In the case of the highest flow rate of 1,016 ml/h., no material exits the mixing chamber in the investigated fill factor range. The reduction of the flow rate to 440 ml/h. leads to a maximum rubber balance of 0.7%. The emission of rubber starts when a fill factor of 65% is exceeded. With an increasing temperature, the oil absorbing ability of the natural rubber rises. The sliding friction again passes into adhesion. This way, a new or further meaning is attached to the addition of pasting oil. First, the dispersive fillers have to be bound. At rising temperature and homogeneity of the mixture, the material transport towards the dust stops can be reduced by lowering the adhesion between the compound and the rotor shafts. For this reason, the addition of pasting oil is strongly recommended.
This article will be continued in next month's Rubber World (December 2005), where process parameters will be analyzed and lubricant-free dust stop systems will be examined.
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|Title Annotation:||Process Machinery|
|Date:||Nov 1, 2005|
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