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					                                                    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.
.

				
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