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The above sections in combination are known as the flighted length of the screw.
Screws are generally described by their L/D ratio and compression ratio.
L/D ratio: is calculated by dividing the flighted length by the Outer diameter (OD) of
the screw. E.g. - Screw of 2.5” (inches) and has flighted length of 6” the L/D ration of
screw would be 24:1.
Compression ratio: This is obtained by dividing feed depth by metering depth. E.g. –
if screw has feed depth of 0.600 and metering depth of 0.200 then screw has
compression ratio of 3:1.
Though general-purpose screws are available, different plastics require differently
calibrated extrusion screws. The length-to-diameter ratio, pitch, length of each zone
and helix angles of a screw must all be matched to the plastic type being used. The
screw must also be designed to suit a specific type of die.
Functions of extruder screw:
Getting the material from the hopper into the extruder and move the material along
the screw
Soften the plastic by heating (internal frictional heat and external heating)
Proper mixing to produce a homogeneous melt without impurities
Compression and removal of moisture and degassing
Meter the melt into the die area and apply the constant pressure (free of pulsation)
required to force the material through die.
For more detailed explanation on extruders and screw (Kindly Refer –understanding feed screw &
barrel explained in earlier chapter
TYPES OF PLASTIC EXTRUSION TECHNIQUES
PIPE OR TUBE EXTRUSION: Annular shaping die used for the extrusion of pipe or
tubing consists of a die body with a tapered mandrel and an outer die ring which
control the dimensions of the inner and outer diameters, respectively.
Since the product involves thicker walls it is advantageous to cool the extrudate by
circulating water through the mandrel as well as by running the extrudate through a
water bath. Pipe passes through a vacuum sizing tank which lowers the air pressure
outside so the greater pressure inside pushes tube surface out against sizing tool.
Profiles can be sized by puller speed ratio, vacuum tank for submerged sizing or dry
calibrator stand. Formed pipe is pulled to the winder or a cut off device with the aid of
haul off device. Commonly produced are medical tubing's, drinking straws,
agriculture, building and construction pipes, others.
PROFILE EXTRUSION: Profile extrusion is similar to pipe extrusion except that the
sizing mandrel is obviously not necessary. A die plate, in which an orifice of
appropriate geometry has been cut, is placed on the face of the normal die assembly.
The molten polymer is forced through orifice, pulled through a cooling or calibration
tank, and cut to size. Many types of desired shape plastic cross-sections both rigid
and flexible are produced like windows, decking, sliding, fencing and railing with
either single or twin screw extruders.
Draw down ration (DDR): is the ratio of the cross sectional areas of the extruded

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