EXTRUSION 

Baby Kurlakose 

In rubber processing, extruder is mainly used for shaping the rubber compound into the desired profile before it is finally processed.  The compound is pushed out through the die either by the action of a ran or by a rotating screw inside the barrel.

In the former case, the extruder is called as the ram extruder and the latter is known as the screw extruder.  The ram extruder, which is at lower temperatures and for short runs.  In the ram extruder, it is easy to control the conditions while extruding difficult compounds.  But due to the high operating costs and lower output, ram extruders have been replaced by screw extruders.  Now a days, the screw extruders are used mainly for the production of tubings, windscreen channels, tyre treads and for the insulation of wire and cables.  It is also used as a continuous mixer by a jousting the shape of the screw and a strainer by fixing a stainless steel sieve at the head, respectively. 

The extruder consists of a feed hopper, cylindrical barrel, rotating screw, head attachment and a die.  The screw is driven by an electric motor reduction gear systems.  The compound to be shaped is fed into the machine through the feed hopper. It is better to have slightly slanting position, for easy feeding and quick take up of the compound.  The width of compound strip fed is slightly less than the width of the feed hopper and the thickness of the strip should be slightly less than are equal to the depth of the flight of the screw.  The barrel is usually made of hardened steel and is jacketed for the circulation of steam or cold water.  Heating of the barrel is necessary in the early stages, but as the material is worked in, temperature.  In vacuum extruders, provision will be made at the barrel for connecting it to a vacuum device.    

The screw of the extruder is perfect spirally, cut and is coaxial with the barrel.  The pitch of the screw often decreases gradually from the driven and to the face and.  This helps to build up constant pressure in the head and to eliminate air trapped inside the compound.  Since the screw is not expected to chew the compound at the clearance between the barrel and the screw, this clearance is kept very small, usually in the range of the to one half thousand this of an inch per inch of the diameter of the screw.  The screw is hollowed out to circulate cold water for controlling the temperature.  For using the extruder to produce different product and to have better production control variable speed to the screw is desirable. 

The head attachment of the extruder varies in shape according to the purpose for which it is used.  For diverse use of the machine, detachable head attachments are provided.  The design of the head is very important to get free movement of the compound at equal pressures and speeds from all sides of the head into the die.  Any point within the head where the compound does not more is known as a dead spot’.  Compound gets scorched  at these points and small portions which break away from these give bits of scorched compound into the extrudate.  The head of the extruder used for the tyre tread production should be designed is such a way that the flow at the center line of the screw is restricted to some extends so as to get uniform pressure on the dig.  Without this, the rubber compound would tend to flow very quickly in the middle of the die and slowly at the extreme width of the die, thus causing the edges of the extrudate to break and result in a saw to form.  Provisions for heating and cooling should also be provided at the head attachment for better control of the temperature. 

The die of the extruder shapes the compound into the desired profile.  For better shape and finish of the extrudate, the design of the die is very important.  The die is the hottest part of the extruder and is usually heated initially by a gas flame.  The cross-sectional area of the die should never be lower than 5% less or reader than 30% more of the cross sectional area of the extrudes.  Accessories required for the extruder depends on the tyre of product manufacture.  The extradite coming out from the die is usually carried to the next stage of processing, through, conveyor systems.  For proper control of the dimension of the product it is desirable to have variable speed conveyors.  Cooling of the extrudate is done by impression in water, or by a spray of cold water.  The moisture is subsequently removed by a current of air blown over it. Tale is applied at the inside of the tubings while they are being extruded and tale is applied at the outer surfaces ofter placing them  on rotating tables. 

Depending on the design of the screw and barrel, extruders can be of three types namely, hot feed, cold feed and vacuum extruders.  The screw of the vacuum extruder consists of three zones namely, the feed zone, there is provision at the barrel for connecting it to a vacuum device.  In the vacuum zone, the screw is either deeper or more widely cut or the cylinder in that zone is slightly helps to remove an trace of moisture on entrapped air from the compound.  Hence it is used in shaping articles for open steam cure, hot air cure, molten salt cure and fluidised bed cure.  Since moisture, and air trapped in the compound is removed during vacuum extrusion, the product will be normally free from porosity.  Depending on the design of the screw, the extruder may be used for hot or cold feeding of the compound.  The important differences between the two types of compound.  The important differences between the two types of extruders are given below: 

Hot Feed Extruder                             Cold Feed Extruder    

I. Machine Factors 

(a) Has got a short barrel                  The barrel is long

(b) The length to diameter ratio          The length to diameter ratio of the of the screw is low ie. In the range           screw is high ie. In the range of 20 : 1

     of 5 : 1 

2) Rough Surface or poor finish of the extrudate 

Rough surface of the extrudate can be due to poor dispersion of the compounding ingredients, scorching of the compound during extrusion, very high Mooney viscosity of the polymer, low green strength of the polymer, very low temperature of extrusion, low pressure of extrusion and the to bigger die size.  But by proper adjustment of these factors good finish of the product can be obtained. 

3) Wavy edges 

This may be due to any one of the factors like high temperature of extrusion, high pressure of extrusion, lower viscosity of the compound and very should die area for a big extruder. 

4) Low rate of extrusion

 Low rate of extrusion may be due to any one of the factors like high viscosity of the compound, low rate of feeding, lower speed of the extruder, lower temperature of extrusion and bigger die section. 

5) Porosity 

Porosity in the extrudate may be due to the presence of excess moisture in the compounding in radiant use of highly volatile compounding ingredients, presence of entrapped air, very low viscosity of the compound and due to under feeding of the extruder.  Proper drying of the fillers before using, addition of materials like calcium oxide in the compound and addition of materials like calcium oxide in the compound and use of vacuum extruder can reduce porosity in the extradites.

 6). Collapse of the Material 

Collapse of the extrudate occures when the quality of the polymer used is poor, when the viscosity of the compound is very low and when the processed material is recycled several times.

By elimination the above mention factors which causes these defects, it is possible to get high output and good quality extrudates.

Extrusion