Correct design of the mixing room.
In the tire production industry, one of the first work phases takes place in the mixing room. The correct design of the flow of raw materials, the respect for the work environment, the reliability of the batching operation and their accuracy are all bases for obtaining an intermediate high quality product which will guarantee the success of the following work phases.
In the past, and also today, in countries in the process of industrialization, the automation degree of the plants upstream the mixer depends only on investment costs and the availability of cheap labor. The modem tendency, represented by the largest world producers, is to push automation to the highest levels and integrate the plants in the EDP-system of the company. This is possible if the mechanical part of the plants is highly reliable and appropriately designed.
In mixing rooms for tire production, the final compound quality is researched through the improvement of the uniformity. A better uniformity of the final compound can be translated into better feeding of the cold feed extruders in the semi-manufactured product department/room and better quality of the final product.
In order to improve the uniformity of the final compound, the following is necessary:
* an automatic control system of on-line batching of raw materials and of mixer cycle;
* an automatic weighing system for small ingredients (or manually guided system);
* blending of natural robber bales;
* mixers with a high cooling capacity, to control and always repeat the batch thermic profile;
* introduce into the mixer all the products/ingredients having always the same characteristics and temperature (mainly with the same viscosity);
* mixer tempering systems with interlock on mixer cycle;
* blending of the compounds;
* machines which allow for long processing/working of final compound at a low temperature.
In order to better understand and analyze the plants needs for a mixing room, paying special attention to the parts upstream from the mixers, we find it appropriate to examine the typical architecture of a mixing room. Reception, storage and batching of raw materials For the handling of polymers, carbon blacks, white fillers, oils and small ingredients, various solutions are possible, depending on local goods availability, which characterizes the way these products are packed when reaching the plant.
Natural or synthetic rubbers, in most cases, are packed in bales and conveyed on pallets. Warehouses with controlled humidity and temperature and with pre-heated rooms are necessary parts of the techniques used for optimizing the conservation and the use of polymers. In traditional plants and in plants where the quantity produced does not justify granulation (the limit is traditionally set to 10 tons/day), the polymers are used in their original form and their desired weight is obtained by manual cut.
As already mentioned, viscosity is a key factor for obtaining a better uniformity. The natural rubber (NR) is the product having a higher variation of viscosity than all other materials. This viscosity variation depends on type of rubber used, manufacturer (for the same rubber used) and production lot (for the same manufacturer).
To reduce the NR viscosity variation, it is possible to use natural rubber having controlled viscosity (e.g. SMR 5 CV 60), but more expensive than other natural rubbers and to blend the NR bales.
The methods used for the NR blending are:
* preparing and weighing of the single rubber load (mainly on-line with the mixer), using rubbers either coming from different lots of the same manufacturer or different manufacturers. In this case, the blending benefit is limited by the mixer size and the working formula/recipe (e.g. a 270 liter mixer is generally fed with maximum two or three bales of the same rubber). This system also requires enough space or room in front of the mixer for allowing storage of different rubber pallets.
* granulation of the rubber bales and then blending a large lot of rubber granules, to be used for the preparation of the single rubber batches.
The amount of the blending lot depends on the technical prescriptions/ideas of the final blend producer, but generally it is not less than the NR amount used for the daily production of the single formula/recipe).
The natural rubber granulation plant allows automatic handling of the granulated rubber, automatic blending of the granulated rubber (according to technical requirements), automatic weighing of the granulated rubber (according to technical tolerance requirements) and use of a cheaper robber type (e.g. SMR 20 RSS 3, etc.)
Rubber granulation plants characteristics Granulation, blending and automatic weighing are the main characteristics/performances of a complete rubber granulation plant. Advantages include:
* saving on buying costs, using a less expensive rubber quality;
* improving the uniformity by reducing the variation of the NR viscosity;
* clearing the space in front of the mixer (elimination of many pallets around the rubber bale scales);
* granulation of the rubber directly in the warehouse;
* reduction of one man working time for rubber pallet handling;
* possibility of feeding more than one mixer with the same plant;
* possibility of dividing total investment in different steps.
Disadvantages include the use of anti-adhesive for avoiding granules sticking and anti-adhesive consumption according to granulated robber type.
With the actual practical knowledge, it is possible to granulate all types of natural robber and some types of synthetic rubber (rubbers having a high Sticking degree, for example butyl rubber are excluded).
The blending of synthetic rubbers is generally not provided, but it is possible to foresee an appropriate blending system, if required, keeping in mind the different sticking degrees of the rubbers.
The generally used blending systems for natural rubbers are two bin stations with rotating distributor (no. of bins/stations according to blending lot amount required) or two rotary drums (capacity of each drum according to blending lot amount required). For synthetic rubbers the blending sysrams are two bin stations (as above) or a conveyor belt with reversible translation for stratification of granulated rubber (capacity/length of conveyor belt according to required blending lot amount). In order to avoid unwanted humidity problems of the rubber granules, appropriate granulators which require dry lubricant of the knives should be used.
In this case, it will only be possible to introduce the antiadhesive powder into the granulator, as machine knives lubricant or anti-adhesive agent for the granules. Vice-versa, if the granulator requires a liquid lubricant, an expensive drying unit of the granules must be installed.
Typical data of a complete rubber granulation plant are: NR feeding - from six rubber bale pallets from different lots of the same manufacturer or from different manufacturers; the granulator equipped with a bale feeding device for reducing absorption peaks and granulator screen hole diameter 50 80 - 100 mm; the NR blending capacity is 6 t; and the granules storage time is < 24 h for natural rubber and <4h for synthetic rubber.
Carbon black is considered, and not wrongly, the "black sheep" in these plants. It is characterized by its high volatility and fineness and therefore it has a large covering capacity. The percentage of fine parts contained in carbon black, even if pelletized together with the fines generated during handling, represents a potential danger for the environment for the state of being of the operators and therefore for the quality.
Studies carried out on workers normally in contact with powders because of their work have shown psychological aspects more depressive for carbon blacks than for white products, even in cases when the white products have a higher contamination level. The above mentioned reasons in addition to the technological importance of the product have led to great efforts in optimizing the reception, the handling and the storage of carbon black.
The classical packing methods are bags; big-bags; mobile metal containers (flow-bin, tote-bin) and road tankers with discharge by gravity. The ideal solution in all cases is to concentrate the unloading of the containers to one single area for the introduction of the carbon black into the system, physically separated from the rest of the plant with systems preventing contamination of the area and guaranteeing a clean and healthy environment.
Solutions which imply unloading in several places directly in the plant are not recommended. Each "open" place in the carbon black cycle is a potential source of contamination. In addition, the handling of any containers within the production area will always be a considerable source of dirtying because of the unavoidable losses from the container.
Where it is possible, the carbon black supplied in tankers is the best solution regarding the reduction of environmental contamination. The tankers allow for automatic unloading of the black loads in a closed and controlled way. Where this is not possible, one should provide for an automatic centralized unloading system of bags and/or big-bags and/or metal containers. The next phase will provide for a sufficient storage capacity and batching system.
Carbon black pneumatic conveying
Before examining the different lay-out possibilities of the plant for carbon black distribution, we would like to give some information regarding the handling systems. It is well known that in the past, plants with mechanical handling of blacks have been realized (screw-conveyors, bucket elevators, conveyor belts).
The attempts to use pneumatic conveying, tightly considered very attractive for a substance a dirtying as carbon black, have given disastrous results. But today, because of the development of low speed conveying systems, appropriately made for conveying of carbon blacks in pellets, it is completely outdated to still consider using mechanical systems in new plants.
The pneumatic conveying systems are absolutely fight, the conveying air is conditioned in order to avoid transfer of humidity to the product and the plant. They occupy just a small floor space, the runs can easily be modified (a pipeline passes everywhere), the maintenance required is moderate and localized in easily accessible places.
The carbon black used in the rubber industry is carbon black in pellets. Only by keeping the integrity of the pellets to a maximum degree, the reliability of the technology and of the plant is guaranteed. The pelletizing must guarantee a grain hardness which enables it to surpass stresses from conveying and batching, but must allow for compounding and easy dispersion in the batch. In fact, these are two contradictory requirements. During the years, many compromises have been found in order to obtain the easiest conveyable products, with a minimum percentage of fine powders, but with characteristics which allow for easy compounding and dispersion. The tire producers' specifications require that the percentage of fine powders does not exceed precise values (from 10% to 15%) when reaching the mixer. The definition of "fine" is any value below 125 gr. (microns).
The traditional pneumatic conveying systems, even though allowing for product conveying, if they are slightly altered, cause an increase in the percentage of fines, which might reach 80%, depending on the characteristics of the run.
The pneumatic low speed conveying system "Pulsairpack" allows for transfer of carbon black in pellets at such a low speed as to consent an increase of fines, always within the preset limits. The "Pulsair-pack" system is a high dense phase pneumatic conveying system with extemal by-pass and pressure control along the conveying line.
The pneumatic low speed conveying allows for plant design where reliability, grain size and environmental cleanliness contribute to the realization of the target of the desired quality.
Carbon black batching
Weighing by gravity When possible, in order to guarantee the maximum premises for obtaining the best quality, the best solution is to provide for daily storage silos installed above the internal mixers. This arrangement allows for weighing of carbon black, and then for discharge by gravity into the mixer, allowing for very short cycles. It guarantees optimal emptying of the downstream part of the weighing hopper and does also minimize the contamination problem among the various black loads (cross-contamination).
This solution can be applied when storage silos are required, which implies conveying from silos to daily silos, or when storage is done, directly in bags or big-bags, etc. In this case, the conveying takes place from the unloading system to the daily silos whose functions are to guarantee a minimum operative stock as well, The advantages include a high reliability plant; cross-contamination reduced to a minimum; possibility of keeping a stock of carbon black above the mixer, allowing for production if the conveying system is temporarily out of operation; possibility of carrying out very short cycles; and high weighing and transfer accuracy into the mixer.
Some of the disadvantages are higher investment in building; higher investment in plant; difficulties in adapting existing structures to this type of installation; and a high number of different powders complicates the lay-out considerably.
When the lay-out of the plant does not allow for installation of daily silos above the mixers (existing building, height limits), a good compromise consists in using the so-called "remote weighing" systems. This implies the installation of the weighing systems directly beneath the storage silos, which may have variable capacity depending on supply system and consumption. They can be installed at "zero" level in the factory area where sufficient space is available.
In this case, the carbon black is being weighed and then conveyed to the mixer or mixers. The problem is to guarantee complete transfer of the required lot, without degradation, as fast as possible. It is anyhow clear, that even when the most sophisticated technical measures are applied, this system is not quite as reliable and precise as the system described before.
Anyhow, this solution is very often used because of the reduced investment costs and the possibility of using already existing buildings without important modifications. Some of the advantages are that it can be adapted to nearly all existing situations or requires only small modifications to existing structures; lower investment in building; lower investment in plant; allows for future expansion; and is easily adaptable to a large number of raw materials. Some of the disadvantages are plant complications for conveying of weighed product; and to guarantee precision of weighing and transfer, the plant generally requires more time to carry out a cycle - the line is immediately completely out of service if the transfer system of the weighed product does not work.
Characteristics of equipment
used for handling and
batching of carbon black
The plants in the tire industry using carbon black are characterized by their use of very special components, appropriately studied, which impede the carbon black tendency of adhering to any surface it gets in contact with. But carbon black in pellets is normally in most cases characterized by its free-flowing qualities. But the fines, which are invariably contained in the product may, as time passes, create deposits and incrustation everywhere in the plant. Years of experience have led us to choose materials and solutions in order to avoid these inconveniences.
* Conveying pipelines - high density polyethylene in a special formulation of high carbon black content allows for conveying of any types of carbon black without causing incrustation.
* Silos unloading - the fluidization with dehumidized air has proven to be the best system for guaranteeing unloading of silos and hoppers in any weather conditions.
* Horizontal conveying systems - use of fluidized airslides instead of screw conveyors allows for slightly sloped conveying of large quantities of the product with air-tight equipment without degradation of the pellet size.
* Batching - use of fluidized air-slides and specially designed weighing heads allows for fast, clean and precise batching. The weighing hoppers are in rubberized fabric (neoprene) or with steel frame with inflatable rubber lining in order to guarantee fast and total unloading of the batched products. The chutes for the loading of the mixers are also in rubberized fabric or fluidized.
All these devices are applied in order to guarantee the repeatability of all batching operations independently of climatic and environmental conditions. The repeatability and the precision are the bases of any qualitative process.
Regarding the supply systems, bags, big-bags and road tankers, what has already been mentioned for carbon black products can be applied. The exception in this case is that the road tankers are of a self-unloading pressurized type. This kind of tanker is being pressurized and discharges the product through pipelines without requiring appropriate unloading systems, as is the case for carbon blacks. Also for systems such as weighing by gravity and remote weighing, the same considerations are valid. It should be noted that even though the white fillers do not have the grain size problems of carbon black, they do anyhow have considerable sticking qualities. This is why these products also require appropriate devices, in many cases similar to the ones required for the carbon blacks.
They can be supplied in road tankers or in drums. When supplied in tankers, they require storage tanks and daily tanks. When supplied in drums, only daily tanks are required.
In many cases, the oils must be heated in order to be pumped; the best quality choice is to use hot water where the exchange surface is a contact surface, and steam or electric resistances for radiation. In this way, one avoids the formation of excessively hot spots which might crack the oils. The pumping and distribution of the oils take place by using hollow disc or gear pumps.
The typical layout in a tire provides for storage in tanks with adequate dimensions regarding the tankers arriving and consumption. It also provides for transfer to dally tanks and from these to the batching systems, one for each mixer, and to the eventual feeding tanks of the mixers dust stop rings.
There must always be the possibility of feeding the daily tanks directly from drums for emergency or for special oils. In these cases, drum pre-heating systems are provided.
The safest, most reliable and rational way is to weigh the oils in a gravimetric way and then to pump the weighed oil to the mixer injectors using high capacity gear pumps. Cleaning systems of the oil injection line with compressed air might be used in order to further guarantee the precision of the operation. The oil circuits and the weighing system are heated. Small ingredients
This is the section most frequently neglected in the mixing room. It is not uncommon to see ultramodern systems for black and white fillers and oils combined with outdated systems for the preparation of the small ingredients with unreliable and empiric control systems. But still, it is well-known that weighing errors of hundreds of grams on the fillers are less important than errors of just a few grams on the additives.
There are basically three ways of solving these problems weighing guided by the control system, but manually carried out outside the line; completely automatic weighing online: or completely automatic weighing outside the line.
These solutions are the base of many plant architectural layouts, and two of them are fixed raw material storage mobile scale and mobile raw material storage - fixed scale.
Regarding the solutions described above, several alternatives are possible, but the philosophical concept is always the same: the operator receives instructions from the control system and carries out required operations. The control system does not allow for the carrying out of step number two if step number one has not been carried out with the required modalities. This method consents to record all carried out operations. The batched product is manually packed in bags with a low melting point.
Automatic weighing on line/off-line
This consists of providing automatic weighing, withdrawing the various products from hoppers separately fed with bags or big-bags. In this case, because of the severe requirements against cross-contamination, centralized systems for unloading and conveying of raw materials from containers are not possible.
Regarding in-line weighing, the products weighed in one or several weighing hoppers are sent directly to the mixer together with the other raw materials. Regarding off-line weighing, the products weighed in one or several weighing hoppers are packed in bags of a low melting point material or with automatic machines.
In all cases of batching, manually or automatic outside the line, marking systems are provided enabling the identification of the packed bags. Other configurations are possible, for example using mobile scales combined with automatic machines for positioning of the empty bags and automatic welding and marking machines or with loss-in-weight scales, one for each ingredient. This last solution, even being the most expensive, is the one which guarantees less cross-contamination among the products. In fact, there does not exist common parts among the various products to be batched.
Introduction into the mixer
The different ingredients can be introduced into the mixer separately, one from the other. These ingredients are polymers, carbon black, white fillers, oils and small ingredients.
When granulation is not provided for, the polymers manually weighed on a belt, according to the control system instructions, are loaded through the mixer loading door in one or several steps depending on the process. In order to carry out this operation, a loading belt is used. This belt may also be used as an accumulation belt enabling the preparation of one or several polymer loads beforehand.
In cases of robber granulation introduction of the automatically weighed product is provided for. The introduction is carried out using one of the side openings of the mixer. Also in this case, the introduction can be done in several steps, operating in loss-in-weight on the polymer scale.
Carbon blacks, white fillers
These are introduced together or separately in one of the mixer side openings. They can be introduced in several steps, as for granulated polymers, using the scales with the loss-inweight functions. In the step division, the required precision is not extremely high and this allows for control of the unloaded flow only with the discharge valve of the scale.
In some cases, a metal-detector is provided. When activated, this works on the mixer isolation valve, enabling the unloading of the product outside the line.
As already mentioned, the oils are directly pumped to the injectors under pressure. The eventual steps are carried out operating by time on the volumetric gear pump.
In cases of batching in line, the small powders are loaded in the mixer using one of the side openings. The eventual steps are carried out in cycle by using intermediate hoppers. When the small powders, packed in bags, reach the mixer, they are mainly introduced from the main door of the mixer. The introduction is only seldom done from the side openings.
The bags can be loaded together with the polymers on the same belt or in a more sophisticated way being placed on loading belts, perpendicularly to the axis of the mixer loading belt. The number of loading belts is similar to the maximum amount of steps one wishes to use. It is the mixer which, according to the process, withdraws the bags for the various steps.
Control and automation
Everything examined until now is the base for design and construction of a reliable plant regarding conservation of the raw material characteristics, of the batching accuracy and the introduction and the constancy of the operation's repeatability independently from climatic and human factors. In order to ensure the quality, it is also necessary to adapt a series of controls which allows for the process repeatability and operations recording.
Raw material arrival
The products reaching the plant in road tankers must be accompanied by a self-certification form from the producer with bar-code. The products arriving in bags, big-bags, in metal containers or drums must have an identification with bar code. The supplier is responsible for the correct certification. When this is not possible, it is necessary to establish a procedure which involves the laboratory, supplying a test sample and emitting an acceptance form of the product with identification and bar code. The reading of the bar code enables the recognition of the product and facilitates the plant searching for the special product (diverter positioning, door releases, etc.). The same procedure must be carried out for the polymers and the small powders which must have a certification from the supplier or laboratory.
In a plant for feeding of a mixer producing batches for tires, the scales to be controlled are in the widest configuration: polymers scale; carbon black scale; white fillers scale; oils scale; small powders batching scale (on-line or outside line); and a small powders control scale (when weighing outside line). The scales for the small powders can be more than one, especially for automatic batching the tendency is to use, in a dedicated way, scales depending on the quantities to be weighed in order to ensure maximum accuracy.
The weight revealing system is an analogue type system with high precision load cell. Also mechanical lever scales are used with the load cell on the end lever for the signal transmission. This last solution has the advantage of being less sensitive to vibrations and possible bending of the support structures. It also enables mechanical zeroing of the tare and will therefore give a signal only for net weight.
The analogue revealing is transformed in a digital signal and as such, it is compared and elaborated. For the control of all elements needed for the management of formulas and batching, PCs combined with PLCs are used. Only the software is customized and dedicated, the hardware uses equipment easily found on the market.
This configuration enables the use of always up-to-date equipment which uses the latest inventions in the technological field and which will not require worldwide after-sale services. Use of computers which have been specially developed is not justified today. Moreover, in this way, it is possible to connect the equipment in line with the rest of the EDP structure of the company.
The scope of a software program managing the batching plant in the tire industry is not only for ensuring that the weights always are accurate and constant. It must also enable other functions, such as:
* Raw material file: it is the list of all ingredients which may be part of the formulation, also if not present in stock. It is possible to do a print-out, in any moment, of the complete raw material file. It is also possible to know the total consumption of each raw material.
* Stored product file, with association between raw materials and storage stations. Stock availability.
* Recipe file, contains all formulations regarding the production. Each recipe is identified by a code and name. Contains all ingredients requested, carbon blacks, white fillers, oils, polymers and small ingredients.
For each ingredient, the following is set: identification code; weighing station where the hatching is to take place; the required batch value; the pre-set value, which implies the passage from fast weighing to slow weighing; the fly value, which keeps account of the dropping product; the required tolerance; and the number of steps required for unloading the ingredient in the mixer.
A closed loop system tends to reduce the deviation from zero to a minimum within the tolerance range, automatically modifying the fly value depending on the results of the previous weight operations.
Other data being part of the formulation include the operative mode of the mixer and the ingredients loading modality in the mixer. The recipes contained in the memory can be printed equipped with the main data,
A production plan may be set, which is thereafter taken over by the automatic management system. Each formulation may be given a lot number. The system verities the stock, calculates the total weight to be produced, based on required cycles. The system is equipped with video graphics which enable the operator to follow the work taking place.
History, file, traceability
The actual values of the weighing of the ingredients in each produced batch are kept, together with corresponding theoretical set values, in an appropriate memory area, identified by recipe code and by production lot number and arranged chronologically with applied date and time of production.
It is possible to recall on screen or send to the printer the file data relating to a certain time period, either in a synthetic way (code and name of recipe, lot number and quantity produced, date and time of production) or extended (for each lot, in addition to the main values already listed, also all data contained in the memory regarding weighing of the ingredients are supplied).
A "memory soon full" indicator regarding available memory is provided; if the operator does not free the memory by copying the data to floppy discs, the new data will cancel older data if the memory is full. The file data might later be combined with laboratory tests done on the batch. In this way, all information is available in order to "identify" each batch, and supplied with laboratory test results.
The system allows for configuration of the plant through the definition of some parameters and values. These include: zero tolerance; maximum batching time; maximum unloading time; weight full scale value; and verification tolerance of the control scales
In the case of manual off-line batching of small powders and in-line batching of polymers, the system controls the "guided" operations, communicating information of the formulation to the operator and comparing the obtained results with the set values. Each following operation is authorized only if the previous one has been carded out according to order and tolerance.
The information communicated to the operators does not only regard the guided batching systems, but also, in various parts of the plant, the type of formulation being produced, the batch number and the products to be loaded in storage silos.
Control of mixer operations
PC and PLC, hardware components available on the market, are used for this system as well. The appropriately studied software is applied in order to guarantee repeatability of the process. Depending on the constant quality of the raw materials and accurate batching, the batch quality can be pursued independently of production site, external conditions and human intervention. This enables one to set the production cycle in one unit and to repeat the same production in other units, obtaining the same results.
A standard control program normally consists of the following controls: opening/closing of loading door; temperature control of bearings and wear rings; temperature control of cusps and side frames; position control of the ram; pressure control of the ram; control of hydraulic unit; control of grease lubrication; control of dust stop rings; control of main gear reducer and lubrication; control of mechanical gear speed change and lubrication (in cases where a D.C. motor is not provided); and tempering control (temperature and distribution pressure switches).
The control of the cycle is carried out by dividing the cycle in at least 20 phases. A macro instruction corresponds to each phase, and contains all micro instructions in order to let the requested operation take place. Each phase can start by setting time, energy or temperature. These parameters can be individually used or linked together in an "and" or "or" logic function.
The function "and" means that the passage to the next phase takes place when both conditions are satisfied. The "or" function means that the next phase takes place only when one of the two conditions is satisfied.
A third possibility exists: "if'. In this case, to the temperature parameter, a second parameter is linked, which "must" be reached before the temperature, to let this phase end and to pass to the next one. Example: [120 degrees C] (temperature) if 350 kWh (energy) According to the above mentioned conditions, if the energy set is reached first, the system will wait for the temperature to be reached before turning to the next phase. If the temperature set value is reached before the energy set, the system provides for lifting the ram, opening the door, reducing the number of revolutions when possible, in order to avoid temperature rise, until the set energy is reached. When the two parameters are reached, the system turns to the next phase.
The cycle can also be led by setting a temperature curve to be followed by time. The program provides also for introduction of raw materials previously batched, even by successive steps. Even the open mixers downstream of the internal mixer can be controlled, when requested, by an automatic system, which combines all the mixing parameters.
The correct design of the mixing room and adequate work methods are extremely important in order to guarantee a very high level of the batched products while respecting the work environment. This is absolutely necessary if a finished high quality product is required.
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|Title Annotation:||tire production plants|
|Date:||Feb 1, 1994|
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