EXTRUSION Extrusion is the technique of pre-forming unvulcanised rubber compounds by forcing material through fixed apertures or dies, to obtain definite shapes and sizes. An extruder is a machine designed to produce a continuous length of material of desired cross section by forcing the material through an orifice or die under controlled conditions of temperature, pressure, rate and homogeneity. In rubber extrusion, rubber compounds are forced under pressure through a die to get continuous profiles. In tyre industries the major part of the extruded articles are the tread and sidewall. While the history of the pioneering days of rubber industry can be traced to the early 1800s, extrusion as a process that bears comparison with our current understanding did not appear on the scene until the first forcer (extruder) was manufactured around 1880. It is commonly thought that the prolific inventor and patentee of many processes, Joseph Bramah, introduced the extrusion process to the world back in 1797.M/s Bewly and Brooman produced one of the earliest forms of extruder in 1845, from which various patents evolved and developments took place. The machines were essentially ram operated. In 1887, Willoughby Smith, an employ of the Gutta Percha company, turned the process into a continuous one by providing continuous feed from a hopper through hot feed rolls into a chamber fitted with a rotating gear pump, thus providing a positive pressure feed to the extruder. The concept of screw extruder as we know it today is generally linked to a patent filed by Mathew Grey in1879. John Royle of New Jersey built the first screw type extruder in 1880. Engineering standards and improvements in metallurgy, construction technology, developments in motors and gearboxes, etc. lead to considerable improvements in the reliability and efficiency of the machines. The next major step forward was the introduction of cold feed concept in the late 1950s. I. EXTRUSION PROCESS TECHNOLOGY 1. GENERAL Knowledge of the rheological properties (flow characteristics) of rubber is necessary for an understanding of extrusion. The major points are 1. Rubbers are strongly non-Newtonian; i.e. the rate of shear deformation is not proportional to the shear stress. The shear rate increases more than proportionality to stresses. Newtonian Non-Newtonian Shear Stress Shear Strain rate 2. At any given shear stress, the shear rate increases with temperature. 3. The rate of flow under a fixed stress is time dependent 4. Rubbers show more or less elastic recovery when the deforming stress is removed. Therefore it is important that these variables of shear rate, temperature, pressure flow and recovery must be controllable and repeatable in terms of the precision required in the finished product. 2. BEHAVIOR OF RUBBER INSIDE THE EXTRUDER Even though the non-Newtonian nature of rubber – that is, the fact that the relationships among the factors affecting flow characteristics are not linear in reaction to changed conditions complicate things, the general behavior patterns of rubbers within the extruder are fairly well established. a. There is little or no slippage of the rubber over the walls of the screw or barrel. b. The drag of the rubber over these surfaces accounts for the rapid transfer of heat and the effectiveness of the cooling on screw and barrel. c. Pressure varies linearly from feed hopper to head. d. Closed head pressure is proportional to screw speed. e. Screw speed controls out put. f. The extruder out put rate of any die can be predicted if the screw dimensions, speed and pressure are known. These basic postulates provide the foundations for consistency of extruder out put. These are varied if viscosity and temperature vary along the barrel by loss of control of the generated internal heat in the moving stock, so design is such that control is possible in all production conditions. To have an efficient and consistent extrusion process the following factors needs to be well taken care of. a. Stocks must be of consistent viscosity. b. Stocks must be delivered to the extruder with regular warm up. c. Feed must be even d. Feed should be sufficient in quantity to maintain a constant pressure column from hopper to head at the screw speed that balances the die throughput. e. Temperatures of barrel, head and die. f. Speed of screw g. Size, shape and finish of die. Thus, if the same stock is put through the same extruder under the same conditions, the result is predictable – i.e., the same extrudate as the previous run. 3. FLOW MECHANISM IN THE EXTRUDER There are four significant flow mechanisms taking place inside the extruder. a. Traverse Flow The rotating screw inside the barrel induces a rotational velocity to the stock in addition to the longitudinal movement of the stock. The main effect of this movement is the equalization of temperature through out the stock because of the rapid turnover and the effect of the wiping action of the stock on the walls of the barrel and the screw. b. Drag Flow The resistance to the forward movement induced by dragging against the walls. This is the basic phenomenon by which the material gets conveyed to the other end of the screw. c. Pressure Flow The high pressure at the head side and the relatively low pressure at the feed end of the screw induce the screw to attempt a back-flow against the drag flow. This results in the back outs while running the extruder. d. Leakage The backward flow through the clearance between the screw and wall as a result of the increased pressure gradient from the hopper to head. These forces unite in the passage of the rubber through the screw and mix or agitate it violently. This agitation combined with the massive wiping action against the screw and barrel walls levels and tends to equalize all temperature differences within the stock and between the stock and extruder. Recalling the rheological property of rubber that the shear rate increases with temperature, if the stock has an elevated temperature as fed, i.e., the 190° - 200°F from a well-controlled feed mill, the internal heat generation is comparatively small. The cold walls and screw actually tend to cool down this stock temperature, thus increasing the shear stress, raising the pressure at the die, and increasing the out put of the extruder. The rate of flow of the rubber inside the extruder is directly affected by pressure. So equal extrusion rate across the face of die can only be achieved by delivering the stock to the die with the pressure as near equal as possible, then modifying the die to compensate for the pressure differences. It is obvious that the pressure must be maintained constant to insure a steady flow rate. Since the pressure varies linearly from hopper to head, only by maintaining the head and screw full can this constant pressure be maintained. Insufficient feed reduces this full die-to-hopper pressure column, which magnifies all the variations I rubber behavior, flow mechanisms, and stock properties making the process highly uncontrollable. 4. EFFECTS OF STOCK PHYSICAL VARIATIONS ON EXTRUSION The control of stock and process during all extruder operations is the only basis for constant dimensions from run to run in any extruded product. Some of the common variables and their causes follow: - 1. Viscosity For a stock that has received consistent mixing, the viscosity varies with the amount of warm-up and working the stock receives prior to extrusion. Stock that has been on the warm up mill for 20min.extrudes differently than stock on and off the mill in five minutes. Rework material extrudes easier, swells less, and shrinks less than fresh stock. Additions of such rework materials to the warm-up mill must be consistent or there will be variations in the size of the extrudate. 2. Temperature Viscosity varies with stock temperature. This may be internally generated heat or heat from a heat mill or extruder. Consistent machine temperatures maintained by adequate controls and consistent stock warm-up provides even stock temperature. 3. Scorch Stocks with excessive heat history extrudes slowly, may have a rough surface, and swells excessively and erratically. 4. Mixing variations Under-mixing, over-mixing, and variations in mixing cycles may produce viscosity variations which show greatly in the extruder. Similarly, viscosity and extrusion characteristics may be varied greatly by inaccuracies in weighing up the components of the batch or by variations in polymer lot. This type of variation is troublesome and difficult to find. 5. Porosity Some porosity is found commonly in extrusions, and if limited in scope and consistent in amount, can be compensated for in dies. This porosity will be spread evenly throughout the body of the extrusion. Above 3- 5 percent, the individual bubbles tend to be large and concentrated towards the center of the extrusion. This concentration at times results in a continuous open passage at the cortex. This allows water to enter the interior of the stock and results almost certainly in a blow in the cured product. Porosity is generally conceded to result from gas generated from some volatile material within the stock. Control of porosity depends first upon proper selection of rubber and compounds to assure that excess volatiles are not introduced with the stock. Second and probably more important are proper processing temperatures to prevent porosity developing in normal stock. Porosity is developed within the extruder itself in most cases. Failure of cooling of the screw will result rapidly in porosity. By far the greatest single cause of porosity is starvation. Under feeding or starvation almost immediately develops excess heat in the most critical area within the extruder and porosity follows almost invariably. 6. Swell Stock swell at the die is varied greatly by variation within the stock. Generally, swell is decreased by: - a. Lower viscosity b. Higher heat of stock or equipment c. More work performed on stock - Higher warm up d. Addition of rework e. Under-mixed stock f. Low head pressure Swell is increased by: - a. High viscosity b. Low temperatures of stock or equipment c. High head pressure d. Any scorch in stock Adequate controls of all these variables are built into compounding formulations, processing specifications and equipment design. Careful observance of all these are feasible and will result in both quality and quantity of production. 5. BEHAVIOR OF THE STOCK AFTER EXTRUSION a. THERMAL SHRINKAGE The non-Newtonian nature of rubber polymers and ever changing formulations used in the tyre industry make the establishment of shrinkage equations very difficult. This means that each compound behaves differently with respect to shrinkage. Natural rubber compounds tend to have higher shrinkage. Variations in mixing and warm up produce much greater variations in natural rubber extrusions than in the synthetic rubber extrusions. The variability in dimensions will be the minimum when the extrudate has the minimum entrapped stress. These stresses are caused by the basic polymer rheological reasons, or by temperature. Giving a loop of moderate size immediately after the die facilitates relieving these stresses. A large loop will introduce stress. Also, a forced shrinkage will facilitate relieving of stresses. b. SHAPE STABILITY Even though low viscosity increases productivity and quality of the extrudate, they sag, collapse, and also will get badly deformed in subsequent stages of operation such as stitching. E.g.: Tread stitching in Tyre building Rolling of tread in Tread Rubber making c. TACK Generally higher extrusion temperature facilitates higher tack. However after a threshold temperature, reversal of soluble into insoluble Sulphur takes place, and may affect tackiness. All stocks tend to be stickier if run at higher temperatures; natural rubber stocks vary more than synthetic. Smooth surfaces are stickier than rough surfaces. Soft stocks are also tackier. Surface tack may be affected by cooling; very rapid cooling speeds up bloom and lowers tack. Air cooling and slow water- cooling maintains tackiness. Stocks wrapped hot into liners may loose tack because of the rapid migration of the processing oil and resins to the surface. 6. EXTRUDED COMPONENTS The major tyre components made using extruders are the tread and sidewall. These components are made of either single or dual compounds. The dimensions, weight and profile of these are of great significance in the subsequent processes as well as in the ultimate performance of the product. Typical cross sections are given below. CROSS SECTIONAL VIEW OF GREEN TREAD WITH CAP & BASE CAP COMPOUND BASE COMPOUND BASE COMPOUND CUSHION CROSS SECTIONAL VIEW OF SIDEWALL WITH RIMSTRIP SIDE WALL RIM STRIP CROSS SECTIONAL VIEW OF TREAD WITH WING TIP CAP WING TIP CUSHION II. EQUIPMENT Depending on the mechanism, by which the rubber compound is forced through the orifice, extruders can be classified into 1. Ram extruders 2. Screw extruders th Ram extruders are the machines which were developed early in 19 century to coat telegraphic wires with gutta percha. As this is not a continuous process, large amounts of compounds could not be handles. This lead to the development of screw extruders. In a screw extruder, a screw with a thread configuration rotates within a close fitting barrel and produces the pressure to force rubber through the die. Comparisons of various aspects of ram and screw extruders are given below. SCREW EXTRUDER RAM EXTRUDER ADVANTAGES 1. Higher output 1. Short rapid runs 2. Continuous operation 2. Scorchy compounds can be extruded 3. Less air entrapment 3. Easy to clean 4. Plasticising action inside the barrel 5. Lower operating cost DISADVANTAGES 1. Higher heat development 1. Intermittent operation 2. Scorchy compounds cannot be extruded 2. Size variation 3. Difficult to clean 3. Air entrapment 4. No Plasticising action inside the barrel 5. Lower output Depending upon the temperature of feed, extruders can be classified as 1. Hot feed extruders 2. Cold feed extruders Comparisons of various aspects of hot and cold feed extruders are given below. HOT FEED EXTRUDER COLD FEED EXTRUDER DESIGN PARAMETERS 1. Short barrel 1. Long barrel 2. Lower L/D ratio 4:1 to 5:1 2. Higher L/D ratio 10:1 to 15:1 3. Compression ratio approx.1 3. Compression ratio >1 th th th 4. Depth of the flight is 1/6 of Dia. 4. Depth of flight is 1/8 to 1/10 of Dia. 5. Constant or variable pitch 5. Variable pitch HOT FEED EXTRUDER COLD FEED EXTRUDER 6. Constant flight depth 6. Variable flight depth PROCESING FACTORS 1. Compound pre warmed 1. Cold compound 2. Tendency to scorch 2. Less scorching 3. Feed varies in temperature & viscosity 3. Constant temperature & viscosity 4. Difficult to maintain consistency 4. Better dimensional control of extrudate 5. Production equilibrium attained 5. Takes longer time for extrusion to stabilize within a short time ECONOMIC FACTORS 1. Higher investment 1. Lower investment 2. Higher labour cost 2. Lower labour cost 3. Requires large floor area 3. Less space required 4. Lower frequency of replacement 4. Frequent replacement of screw and liner of screw and liner 5. Higher output 5. Lower output 1. SIZING The size of an extruder is expressed in terms of the diameter and length-to-diameter ratio (l/d) of the screw. The diameters can be expressed in ‘inch’ (mostly American manufacturers) or in ‘mm’ (mostly European manufacturers). Thus a 6”/150mm extruder means an extruder with a diameter of 6”/150mm. The L/D ration depends on whether the extruder is hot or cold fed. For the same feed the L/D ratio changes significantly depending on the feed. 2. BASIC PRINCIPLE Extruders are generally driven by variable speed motors running through reduction gears to obtain the somewhat slower speeds that are normally employed. The interaction between the compound, the screw and the barrel surface is central to the extrusion process. In a screw, rotary motion is translated to part rotary and part linear motion. Thus, if a right hand threaded screw rotates in an anti clockwise direction, it will try to unscrew, but as it is restrained by the thrust bearing, it cannot move, but propels the material in the screw to the head and die against the frictional resistance of the screw, barrel, head and die. The resistance to forward movement of the material causes work to be done in the form of compression, shear, and mixing, and some of this work is turned into heat. The rotational velocity of the screw surface can be resolved into a longitudinal component, parallel to screw flight, and a transverse component, perpendicular to screw flight. The longitudinal component, ‘drag flow’, results in material being conveyed towards the die. The transverse flow, although not contributing directly to the extruder output, results in a circulatory flow which contribute to improved heat transfer and distributive mixing, leading to better physical and thermal homogeneity of the stock when it reaches the die. The pressure gradient resulting from compression due to the flow restrictions caused by the die and the design of the screw cause pressure flow, which opposes the drag flow. Also, this pressure gradient results in leakage flow through the clearance between the screw flute and the barrel. Hence, the volumetric output through the die equals the quantity conveyed by the screw, minus the back flow (pressure flow) down the screw and the leakage flow. Q = Qdrag – Qpressure – Qleakage The value of Qdrag depends only on the geometry and rotational speed of the screw. The value of Qpressure depends on the screw and barrel geometry, the developed pressure and the viscosity of the compound. The value of Qleakage depends basically on the clearance between the screw and barrel. The capacity of an extruder is rated in terms of Kilograms of compound extruded per hour. The design and selection of an extruder are governed by factors such as capacity or output rate, speed, temperature of extrusion desired and the type of extrudate. The variation in speed of the screw of extruders gives a variable output for the same size of extruder. The speed range is limited for a given extruder. The composition and flow behavior of the compound has a significant effect on the output and temperature of the extrudate. In the extrusion process, heat is generated as the compound progresses along the screw. The screw and barrel are therefore water cooled to remove the heat generated. However in order to reduce the flow resistance at the die and to get a smooth extrudate, it is required to maintain the die at a slightly higher temperature than the compound. This is achieved by using tempered water. The extruder, its cooling/heating system and die must be properly designed to produce a uniform extrudate at a safe temperature. 3. PARTS OF AN EXTRUDER The extruder can be divided into six basic parts for ease of description. a. The barrel b. Screw c. Feed hopper d. Head e. Die. f. Drive and g. Temperature control FEED HOPPER BARREL SCREW WATER CIRCULATION PATH FOR HEATING/COOLING HEAD DIE a. BARREL The barrel encloses the screw and is made of hardened steel. Two types of barrel constructions are found - the stressed casting and tie rod type. The inner lining of barrel is made with a very hard alloy. Walls must be heavy to resist the radial pressures developed inside. Even greater are the longitudinal pressures as the screw generates enormous separating forces in pushing the stock through the restricted die. These forces are taken up in the rear by thrust bearings and the massiveness of these bearings reveal the magnitude of the pressures developed. The cast outer barrel has a thimble or bushing insert with ribs running diagonally around it in the pressure area. When the thimble is inserted in the barrel, the spaces between the ribs form the channels through which the cooling water circulates. The thimble in turn contains an extremely hard metal removable liner, which provides a wear resistant surface surrounding the screw. This can be replaced rapidly and economically when worn. Usually the wear of metal liner and screw takes place due to the abrasiveness of the compound being extruded. When excessive clearance develops between the two, stock bleeds over the flutes of the screw (i.e. excessive leakage flow) reducing the extruder output, and increasing the residence time of a portion of material giving a tendency to scorch. Therefore the screw-barrel clearance should be in the specified limits. Replacing the liner is one way for maintaining this clearance. The rubber compound is sheared between the walls of the barrel and flights of the screw by the friction between rubber and barrel .For this reason, the barrel should not be cooled too low, as condensation of water on the wall will reduce the friction and cause inefficient extrusion. Also the barrel must not be warm too high, as this might cause scorching. The output of an extruder is dependent on the relative grip between the screw and barrel, the higher the grip of barrel, higher the output. b. THE SCREW The rotating spiral screw is the heart of the extruder. It is a threaded shaft, which lies co- axially and horizontally inside the barrel. It has, as a rule, right hand threads. It is connected with a motor and rotates anti-clock-wise. F= flight depth P= pitch W= flight width F=flight depth Basic screw parameters illustrated above are defined as follows: - 1. The length-to-diameter (L/D) ratio: - The ratio produced by dividing the effective length of the screw by its diameter. 2. Lead (L): - The horizontal distance the flight progresses in one revolution of the screw. 3. Pitch (P): - The axial distance between two adjacent flights. The pitch equals the lead if the screw is single start, or in the more conventional designs of rubber processing screw, the pitch equals half the lead. 4. Starts: - The number of flights in one revolution of the screw. 5. Flight width (W): - The width of a flute. Also called land. 6. Root diameter: - Diameter of the screw without flight. 7. Flight depth (F): - Half of the difference between the screw diameter and the root diameter. Most rubber screws are produced from high molybdenum steel, turned and ground to shape, flame hardened, then chrome plated. This improves the life by reducing the wear rate against the liner. There are two types of screws: torpedo type and full flighted, with flat or pointed end. The torpedo type screw is so named because of the torpedo like extension at the extrusion end, and is mainly used in plastics. Rubber industry uses the full flighted screw, in which the flight runs clear to the end. The screws can also be classified based on the pitch sequence and the number of starts. The larger the pitch of the screw, more material is moved per rotation, but the pressure is less. The smaller the pitch, less material is moved per rotation, but the pressure is more. So a decreasing pitch screw is more common. However, this leads to a pulsating output. This can be taken care of by a double start decreasing pitch screw. The major disadvantage of this design is that the feeding is extremely difficult. The most commonly used screw in Tyre industry is the single to double thread screw. This screw offers the advantages of superior feeding by offering a single thread at the hopper area, then as the stock approaches the die, bringing in a second thread. This provides a high die pressure & even extrusion, and a greatly reduced temperature. The bull nose prevents stock from sticking and scorching at the screw tip. As the stock spirals from the screw, it eddies around and leaves a center area of little movement, which tends to cure – the bull nose scavenges clean. The primary purpose of the screw is to convey material along the barrel. The rubber compound is conveyed from the feed end to the die end by the reciprocating movement of the screw and builds up sufficient pressure so as to force the rubber through the die. The barrel has to exert a grip on the rubber to prevent the rubber from rotating at the same rate as the screw. Only then will the rubber progress down the barrel. A first estimate of the potential output of an extruder per minute can be calculated from: - Output = A (L/2) R d Where, A = Cross sectional area of the flow channel L = Lead length R = Revolutions per minute D = density of the rubber A simplistic but realistic approach would be to accept that a well designed and efficient screw profile gives between 45 to 52% of the calculated theoretical output based on the volumetric displacement of the screw per revolution. Screw has got a definite L/D ratio. The L/D ratio is a major factor in the selection of an extruder to match the process requirements and again one that presents problem in quantifying a clearly defined general principle. The L/D ration controls to a large extent the plastication of the compound inside the extruder. The most common L/D ratios used today are 4:1 to 5:1 for hot feed extruders and 10:1 to 12:1 for cold feed extruders. As the rubber gets worked inside the extruder, a lot of heat is generated. A part of this heat is taken out by cooling the screw by circulating water inside the screw. The screw is drilled internally to permit water to be circulated on its inside. The wear on screw occurs most rapidly on the tips of the lead, which contact the barrel wall. Original screw to wall clearance on a new screw and barrel should be double the screw diameter in thousandths; i.e. clearance on a 10” extruder should be about 0.020”. When a feeler gauge inserted between screw and barrel, one diameter length, indicates a total clearance of 6-8 times the screw diameter in thousandths the screw should be changed. Normally, the liner must be changed only on the third or fourth screw change. Design of a screw depends on the extrusion rate, nature of die, stock material, etc. As the pressure of compound at the discharge end is to be maintained to have a uniform extrudate and also to maintain output, screws should have lower volume in flights at the discharge end. There are four ways to achieve this: - 1. A reduction in the pitch of the screw 2. A reduction in depth of the base of the screw 3. A reduction in overall diameter of screw and barrel and 4. An increase in number of starts in the screw. c. HOPPER In all extruders, the screw is provided with a feed hopper, which may be of rectangular or circular opening, which receives compound in a strip form continuously and guides it down into the ‘feed flights’ of the screw. The shape and location of hopper section are important factors in the control and output of the extruder. These must be such that the feed strip will be presented to the screw bite with the least interference and in a consistent and efficient position. The hopper is the largest compatible with the size, is offset toward the bite side of the screw as it revolves, and has gently tapered smooth sidewalls to prevent the feed strip from bridging as it falls. The barrel is undercut on the floor of the hopper beneath the screw – that is, it is cut away so that a clearance of a half-inch or more between the tip of the screw and the floor before it is pinched off. d. HEAD The head of an extruder is the means of consolidating the rolling column of rubber emerging from the barrel and screw into a homogeneous mass; of distributing this mass to the die as evenly as possible; and of mounting the die and accessories. It holds the compound as a reservoir, which ensures even pressure at the emerging end of the extruder in spite of variation in feed rate. Ideally, the pressure and velocity of the rubber presented to the die opening should be equal vertically and laterally, throughout the area. The size and shape of the cavity controls to a great extent the change from the circular mass which spirals out of the screw to the long narrow extrusion emerging from the die. Briefly, the most successful transistorizes change from the circular head end to the rectangular die opening in a length between 2 to 3 times the circle diameter, with the same cross section area being retained at each section of the cavity. A choke or frog splits the mass at the head stock entrance. Heat transfer from the head is very important. Much of the generated heat has been developed when the stock leaves the screw, so temperature controls, which provide and maintain consistent conditions while the stock is in the head, are necessary. This means that the head is heated prior to the run so that the stock at the start of the run does not lose its heat in warming the head. After start up, a temperature controller will maintain optimum extrusion conditions. Each head is cored or chambered for steam and water circulation. Usually both provisions are given to heat the head initially and then to cool. Alternatively, temperature control units are used. The design of the head for duplex or triplex extruders is more complicated. Even though the basic design principle remains same, the head has a wedge inside to prevent merging of individual compounds prior to exit from the head in the case of face to face duplex extruders and some piggy-back extruders. In the case of duplex and triplex piggyback extruders, wide ranges of designs have been developed for clamping the head sections together. The most usual versions are clamping heads and hammer heads. All movements of the piggyback head, including clamping, are hydraulic in the more common designs. The dual extruder (face to face) head is a massive Y-shaped receiver with openings on opposite sides for matching the tuber barrel outlets. The central cavity is partially filled and sealed at this open by a V shaped wedge placed centrally which deflects the two stocks entering the head horizontally and separately guides it vertically down through the pre former and die. The head and the wedge on the cap side direct the stock from the 10” tuber and hold it in the middle; in the base area they spread and thin the stock from the 8.5” tuber nearly across the full width of head. The wedge seals into the head and is held by heavy bolts tightly against a lead lip around the inside edge of the tread. A TCU unit controls the temperature of the head. e. PREFORM DIE Preform dies are used with multiplex extruders only. As the two stocks emerge from the head through their individual openings they are merged into one strip in the preform die. It is machined of die steel to fit precisely into cavity at the bottom of the head. Its exterior dimensions are extremely critical – it must fit tightly on its upper surface so that no undesirable cross flow occurs, yet it must fit on its tapered sides so that it cannot move sidewise as this changes the dimension of the extruded component. Its function is to control the amount of cap & base compound in the tread and also to equalize flow through the final die to prevent uneven shrinkage of the extrudate. The preform has an opening through it, which opens at the top into the two openings on the bottom of the head. The two stocks join in the preform for the first time, and the size and shape of this opening in part determine the shape of the tread through the final die. It is held in position by air cylinder retractable fingers which move and forth horizontally to allow preform removal. A machined groove on the bottom of the preform holds the final die. f. DIE Die designs and manufacture is a major part of extruder operation. The function of the die is to give the required profile to the extrudate. Although a set of basic rules governs dies, so much of the final die dimension depends upon the particular extruder, stock properties and profile with which it is used, that familiarity with these particulars is of more value than general calculations. When preparing the extrusion die, it has to be considered that the emerging profile does not have the same dimensions as the die opening but that a certain extrudate swell occurs. When passing through the die, the compound shrinks along their length and swell across the cross section (Die Swell). This swell occurs both in the vertical as well as the lateral direction. Die swell depends on: 1. The shape of the head and the extrudate. 2. Pressure in the head. 3. Head and compound temperature. 4. Compound’s Rheological characteristic Die swell has to be accounted, i.e. the die has to be made for a particular compound and head. Thinner regions in the extrudate have to be made thicker and vice versa while designing the die. Rubber flow towards larger openings in the die should be restricted either by means of baffles or by reduced or nil tapering. As a thumb rule, the cross sectional area of the die should not be less than 5% or more than 35% of screw cross sectional area. Too small a die will lead to high-pressure areas and ‘dead spots’ in the head that causes stagnation and subsequent scorching of the compound. Bleeder holes may be drilled in the die to relieve high-pressure areas or eliminate ‘dead spots’ in the flow. Too large an opening will lead to insufficient pressure development and subsequent under dimension of the extrudate. Also, it can cause cavitations and subsequent porosity in the extrudate. The die types could be fish tail, multiple manifold, teardrop, or coat hanger shaped. Die surfaces are polished and also beveled to facilitate easy flow. For precision articles, dies are even chrome plated. For a particular die, the extrudate profile depends to a great extent on the viscosity of the stock, the amount of stock breakdown, the volume of the feed strip, the stock temperature, the extruder temperatures and the screw speed. It becomes obvious that this die will produce the same extrudate profile only when the conditions under which it was developed are repeated. Dies are heated prior to fitment on to the head. g. DRIVE Extruder drives must provide adequate torque to turn the screw over wide range – 0 to 200 rpm – with precise speed control. The rotational speed at the extruder end is normally achieved by coupling the extruder shaft through a gear train with the motor. The gear reduction system is used to reduce speed and multiply torque. Until recently, D.C motors have supplied power. The D.C drive provides infinite speed variation with fairly precise control when properly sized. However, it has certain disadvantages. 1. Slight variation in speed as the load increases, which magnifies variations in stock breakdown and viscosity, leading to an increase in the range of deviation from extrudate specifications. 2. Requires a D.C source of current 3. Low in efficiency 4. Higher maintenance cost Alternatively modern extruders use AC motors with variable frequency drives (VFD). h. TEMPERATURE CONTROLS Temperature is to be controlled in the head, barrel, screw and die of the extruder to produce quality good cross sections and to maintain consistency. A temperature change of 10°C in the stock will change the shear rate by approximately 1.3%. This will affect the rate of flow through the die as well as the die swell characteristics. The consistency of temperature is more important than the actual temperature, which may be set depending on the process and compound characteristics. For precise control of extrusion, multi –zone barrel, head, screw and Die/ Die clamp block temperatures are separately set and controlled. Two methods of temperature control are in use: 1. Direct where the cooling medium, say water is directly admitted into the extruder zones as required – normally for high cross section extrusions >150 sq.mm 2. Indirect system, where a circulating fluid is passed through a heat exchanger which removes the heat from the circulating medium A rule of thumb is that the heat exchanger should be able to take away at least 60% of the power input into the process. The water circulation system is occasionally treated with a sludge treatment solution to remove sludge and prevent scaling. Usage of steam heating for a start up etc. may lead to the following problems. 1. Condensation of steam on cold metal accelerates formation of scales. 2. Temperature control using steam is somewhat difficult. 3. Sudden thermal loads may crack the body parts. The dies also are to be heated outside the extruder before fitting into the extruder. An insufficiently heated die gives a faulty extrudate (poor finish, cold and therefore dimensional variations, torn edges etc.) and results in considerable loss of productive time. In such extruders, where the die change is likely to take time, the die will be cold by the time the extruder is ready for a re-start, and therefore systems of heating the die, die clamp etc while running is usually practiced. 4. ACCESSORIES a. CRACKER MILLS The objectives of using cracker mills are to reduce the compound viscosity, homogenization, better filler dispersion, and for warming feed stocks. Cracker mills are similar to ordinary roll mills but with a serrated back roll for getting a better shearing action. The mills are cooled using chilled water. b. FEED MILS From the warmer mill, the stock is conveyed to a feed mill. The feed mills are provided with a stock blender for better homogenization of the feed, and also, this transfers the fresh stock from one end of the mill to other. At the feed end a pair of knives cut a strip of specified width and is fed to the extruder continuously. The feed mills have both rolls smooth and circulating chilled water along the centrally drilled hole cools rolls. Cracker and feed mills are required only in the case of hot feed extruders. In the case of cold feed extruders the compound stripped to specified dimensions at the banbury is directly fed into the extruder. Compound warm up will taken place inside the barrel due to the high L/D ratio. The feed rate in an extruder is very critical to get smooth continuous product. Too low feed rate will cause starvation in the extrudate, whereas too high feed rate can block the extruder. Therefore the feed rate is specified for each die, at a specified rpm of screw. The thickness and width of the feedstock is set for each die and variations in them affect the feed rate. c. CUSHION FEED MILL This mill is used to warm the cushion compound and feed it to the cushion calendar. The cushion compound is applied as a layer down the base of the tread to have good adhesion to ply compounds during building and in service. d. FEED CONVEYOR Cap, base and cushion compounds are transferred from the cracker mills to the respective feed mill and then to the respective feed zones by independent conveyers. e. TAKE AWAY CONVEYER The extrudate coming out of the die is taken away using a take away conveyer. The speed of this conveyor is controlled so as to closely approximate the rate at which the rubber leaves the die. It is a belt made of heat resistant rubber-coated fabric. A length marker is provided to mark points on the tread at 500mm apart to get an idea regarding the shrinkage properties. f. CUSHION CALENDER Cushion calendar is used to calendar rubber compound into a very thin sheet. This sheet is the applied under the extrudate. Two- roll calendars with roll diameters of 10” or 14” and varying lengths are generally used. The basic rules of set up and operation of calendars apply to these as well. However, the following points may be taken care: - 1. Feed strips must be variable and the conveyor carefully designed to insure adequate feed. 2. Temperature of the rolls must be controllable. 3. Speeds must be synchronized with line speed, usually 0.5 to 1.0% faster. g. CONSOLIDATING ROLLERS They are used to consolidate the cushion compound from the cushion calendar to the bottom of tread without air entrapment. The consolidating or stitching roll consists of a large number of steel discs stacked together on a bar. The diameter of the center hole of the disc is bigger than the rod diameter. This arrangement makes the roll capable of taking the shape of any tread profile and consolidating it with the cushion compound. h. SHRINKAGE CONVEYER These are conveyers with independent motors. The speeds of these conveyers are adjusted in order to give either a stretch or shrinkage to the extrudate. This is to accommodate elastic recovery of rubber after extrusion as well as to take care of the variations in extrudate. i. CODE PRINTING/LINE MARKING SYSTEM Identification lines are marked on the tread using line markers. The code-printing unit is used to print the product code and date & shift of production on the extrudate. j. RUNNING WEIGHT SCALE This is a knife-edge suspended balance scale, which weighs a constant length of the extrudate as it travels over the roll-equipped table. The specified running weight is set on the balance and read as zero if the running extrudate is of correct weight. The read out is in plus or minus units. The weight of the running extrudate is obtained from this. It gives an idea about the final booking weight of the extrudate. This helps the operator to adjust the process variables early to reduce scrap. k. COOLING SYSTEM The extrudate coming out of the die is at a very high temperature, normally above 100°C. This needs to be brought down closer to the ambient temperature, (1) To prevent dimensional distortions of the extrudate and (2) To reduce the possibility of scorch, especially the inside of thicker extrudate. Cooling is accomplished by spraying the top or both top and bottom of the extrudate with chilled water or water of the lowest temperature available. This combines the rapid cooling through evaporation with the flood cooling for maximum efficiency. The evaporation cooling is improved by finer atomization of water and application to the hot treads as soon as possible. Flood cooling requires close enough spacing of spray nozzles to assure complete top and bottom coverage and sufficient volume to give a constant, substantial flow across the tread. Cooling conveyors are mainly of stainless steel mesh type or polyamide rod type, which allows water to be sprayed from top and bottom. l. SKIVER UNIT The tread after coming out of cooling chamber is cut to specified length. The skive should be at an angle to facilitate a stronger joining at the building stage. The angle of skive is normally in the range of 22 – 25°. The specified length is preset using either a mechanical cam system or an electric counter system. Two types of cutters are used normally: - one of which moves the cutter along with the extrudate and cuts on the fly; the other stops and loops the extrudate while the knife moves across and cut. It is generally held that the former type cuts faster while the latter more accurately. In both the systems a circular blade rotating at very high-speed cuts the extrudate transversely. The condition of the blade has considerable effect on the quality of the cut. Sharpness of the blades must be maintained to assure the maximum efficiency and quality of cut. Adequate wetting must be maintained on the surface to lubricate the cut and prevent the extrudate from kicking as the knife cuts. Water is used as the normal lubricant. m. AIR BLOWER After skiving the extrudate passes through a water blow off device, which removes the water on either side of the extrudate. The blow off systems is normally a hot air blower. n. BOOKING BALANCE This is also a knife-edge suspended balance as in the case of running weight scale. However the booking balance must have a table long enough to accommodate the longest extrudate run on the line. The balance can be either mechanical or electronic. The read out is in plus or minus units. The weight of the tread as per specification is set on the balance first. The cut extrudate coming to this balance makes the balance to zero reading if it is correct in weight. The treads with correct weight are stored on leaf trucks, off limit treads rejected as scrap. 5. TYPICAL SPECIFICATIONS OF A HOT FED DUPLEX EXTRUDER PARAMETER 10”extruder 8.5” extruder Diameter of screw 9.999” 8.499” Screw-barrel clearance: new 0.020” 0.017” L/D ratio 4.5 : 1 4.5 : 1 Maximum screw speed 75 rpm 76 rpm Drive 200HP(149.14kw) 105HP(111.86kw) Motor RPM 1500 1500 Hopper size 27.5cm X 32.0cm 26.5cm X 28.0cm Screw cooling FCW Barrel cooling FCW Head Type ‘Y’ shape with ‘V’ block opening on top Head opening 16” X 1.6” 43” X 0.75” Head temperature control TCU Head temperature range 125°F – 250°F III. EXTRUSION PROCESS Let us now see the extrusion of a typical cap-base tread using a face-to-face duplex hot feed extruder. The various steps involved in the actual production process are detailed below. 1. PROCESS 1. Start the temperature control unit at least half an hour in advance so as to get the required head temperature as per running code specification. 2. Follow the extrusion specification for the respective product code. 3. Heat the required preforms and dies for at least half an hour in advance of start of extrusion. 4. Clean the mill guides and mill pan. 5. Arrange the specified compounds from the storage area. Follow up of FIFO is desirable 6. Start break down mills and load the specified OK compounds in to the respective mills. 7. Set the break down mill nip as per running code specification. 8. Open mill-cooling water lines. 9. Start the feed mills and transfer the warmed up stocks from the break down mills to feed mills as and when instructed by the operator. 10. Make use of blender rolls in feed mills to get homogeneous mix of the compound. 11. Set the nip and feed width of both feed mills as per the respective specification using feed knives and templates respectively. 12. Arrange / set the line marking unit and printing unit for running code color line identification as per running code specification and for date and shift identification and fill up with paint. 13. Arrange empty leaf trucks/side wall books 14. Set the cushion calendar temperature as per running code specification. 15. Start cushion feed mill and load the compound as per running code specification. 16. Feed cushion compound strips of specified dimensions to the cushion calendar as and when instructed by the operator. 17. Cut out the strips from the feed mills and convey to the extruders using overhead conveyors. 18. As the strips from the feed mills enter the feed hopper, start the screw at a slow speed and increase the speed gradually. 19. When the stock runs smoothly from the head opening, stop the extruder, cut the extrudate, insert the specified preform and die and lock the same properly using die clamps. Apply soap solution on preform and die before insertion. 20. Start the extruder and set the screw R.P.M of extruders and line speed as per guidelines given in running code specification. Lead the extrudate through take away conveyor, check the dimensions and make necessary adjustments. 21. Set the width and gauge of cushion as per running code spec. 22. Apply the cushion to the bottom side of tread. 23. Engage the disc rollers to avoid air traps if any. (Applicable for tread-cushion assembly) 24. Polythene may be applied on bottom side of tread as indicated in the respective specification. 25. Set the linear weight in order to maintain the specified dimensions and weight for the extrudate by adjusting the speed of the shrinkage conveyor. 26. Engage line marking and printing units. 27. Lead treads to cooling conveyor through inclined conveyor. Sidewalls can be by-passed after first cooling conveyor. 28. Set the skiver for the running code. 29. Start the skiver when the extrudate reaches the skiver and then start the blower for removing water drops. 30. Set the specified weight on the weighing scale as per running code specification. 31. Monitor the weight for every extrudate tread/side wall. . 32. Checks dimensions of the extrudate periodically and enter at least one set of readings for each code, in the process control register 33. In case of any variation, perform necessary adjustment in the operations. 34. Book the treads, which are within the specified tolerances in the leaf trucks with the cushion side up with out touching each other. Identify the truck by a tag and transfer to storage area. 35. Book sidewalls (except radial) that are within the specified tolerances in the side wall books. (One pair in each leaf). Identify by a tag and transfer to storage area. 36. Book radial sidewalls in leaf trucks with Polythene underneath and identify by a tag. 37. Non-conforming treads to be slitted. After slitting each compounds to be kept separate on skids with work away tag. 38. Whenever extrudate is going out of specification continuously (say beyond the control of the operator) to be held for disposition with hold tag after intimating the concerned supervisor. The details to be entered in the held up register. 39. If lumps/F.M are observed in the mill, sheet out the compound, identify each sheet with a crayon and allow the sheets to cool. After cooling, stack the sheets on a skid and hold for disposition using a hold tag 40. Operator should record all process parameters as listed in the process register. 41. In case of any abnormalities in the process, the operator should inform the matter to the concerned section in charge. 42. For unit stoppage follow the following general guidelines. 1. Cut the feed strip and stop the screws 2. Cut steam supply to the head and open FCW 3. Remove the preform and dies. 4. Follow push out/clean out procedure. 5. Sheet out all the compounds from mills and allow it to cool. 6. Stop the mill and close cooling water lines. 7. Identify each sheet with crayon for date, shift and code 8. Stop all accessory units, chilled water spray etc after clearing the extrudate from the cooling line. 9. Clean the preform and dies and return to storage racks. .