Extrusion is a field which has naturally generated it's own terminology and this is an attempt to put together a small selection of the words used and what they mean.
General extrusion line layout
The components of the extrusion line are relatively similar whatever type of extruder is used and a typical layout is shown diagrammatically below. The line consists of the basic extruder (drive, gearbox and screws), the extrusion die, the calibration units, the haul-off, the saw (or other cutting device) and finally the treatment devices for final finishing and handling. Major advances have taken place in all components of the extrusion line in the past decade but perhaps the most important have been in the output rates possible and in the command and control segments of the extruder. The advances in output rates have been driven by the demands of processors and the improved control systems have resulted from the availability of low cost computer processing power.
· The extruder drive is electrical in operation and is geared via a thrust bearing to produce the rotational movement of the extruder screw.
· The polymer feed to the screw is from the feed hopper and the feed may be by gravity, metering screw or simple conveying spiral.
· The extruder barrel and screw are of high strength steels and are protected from wear and corrosion by a variety of hardening and coating treatments such as nitriding and hard chroming.
· The barrel and screw are zoned into between 3 and 7 sections which are individually heated and cooled depending on the material and process parameters. The multiple functions of the extruder screw are given in more detail below.
· The die channels the polymer melt from the front of the screw to form the basic shape of the desired product.
· The calibration units stabilise the form of the output to the detailed shape whilst the polymer is being cooled.
· The haul-off provides the dragging force to overcome the frictional forces in the calibrators and to pull the profile through the calibrators.
· The saw / cutter cuts the profile to the desired length.
· Additional operations may be performed in the line or at the end of the line depending on the operation.
The functions of the extruder screw
The extruder screw has the following basic functions:
· To bring the feedstock into the extruder and to move the material along the screw whilst at the same time compressing it and removing volatiles.
· To soften the melt by heating it (both from internally generated shear forces and externally applied heat).
· To mix the melt and produce a homogeneous melt without impurities.
· To apply the constant pressure (free of pulsation) required to force the material through the die.
These functions, at least for the single screw extruder, are generally achieved at different sections of the extrusion screw as the material progresses along the barrel and the functions are illustrated below.
Extruder key dimensions
In absolute terms probably the most referenced number for extruder specification is the L/D ratio (barrel length/barrel diameter) as this defines many of the operating characteristics of the extruder for all types of extruders The L/D ratio is a major factor in the effectiveness of the extruder and of the types of material that it can process. For most extruder types the L/D ratio has increased as technology has advanced. The limitation to high L/D ratios is the torque available from the motor (longer screws mean higher friction) and the capacity of the thrust bearings of the extruder. As advances have been made in these areas then the L/D ratios have steadily increased from L/D's of around 15 to up to 30 in 1998.
Basic extruder types
There are many different types of extruders and the illustration below attempts to divide these into a logical order.
Definitions
Adiabatic Extrusion
A method of extrusion in which the sole source of heat is the conversion of the drive energy through the viscous resistance of the plastics mass in the extruder.
Barrel
The portion of the extruder surrounding the screw or plunger.
Barrel Liner
A sleeve forming the internal surface of the barrel.
Compound
Plastics material as prepared for a further manufacturing operation particularly for extrusion, moulding or calendering.
Compression section
Part of the transition section in which there is a reduction in screw channel volume.
Cooling tank
A tank commonly containing water through which extrusion is continuously passed for cooling.
Crosshead die
An extrusion die which produces an extrudate the axis which is at an angle to that of the extruder barrel.
Cure
The process of cross-linking a plastics material.
Decompression section
In a two-stage extruder that part in which there is an increase in screw channel volume.
Die
A part or assembly of parts held, contained or fitted to the extruder head to form the melt to the desired profile.
Die plate
In moulds, the main support for the punch or mould cavity.
Discharge section
The flighted portion of the screw at the discharge end in which the melt is forced towards the die.
Draw down
Reduction of thickness of plastics emerging from the die by control of haul off speed.
Drive
The entire system used to supply mechanical energy to the screw.
Dry blend
A free flowing mixture of resin or compound and other ingredients as prepared for a further manufacturing operation particularly for extrusion or moulding.
Embossing
The production of embossed film or sheeting.
Embossing roll
A roll having a patterned surface used to produce embossed sheeting.
Extrudate
The product of an extrusion process.
Extruder head
A component which may be attached to the discharge end of the extruder barrel to house the die.
Extruder size
The nominal inside diameter of the extruder barrel.
Extrusion
A continuous shaping of plastic material by forcing it, as a melt, by pressure through a die.
Extrusion coating
A method of coating in which molten plastics is fed direct from an extruder die into a nip-roll assembly together with the substrate.
Extrusion pressure
The pressure of the melt at the discharge end of the screw.
Gelation (compounding)
A stage in compounding material, at which it first becomes a coherent mass.
Haul off or caterpillar
A device for taking away extrudate continuously from the die.
Heating zones
Parts of the barrel, head and die arrange for independent temperature control.
Internal mixer
A mixer consisting of specially shaped rotors operated in a closed chamber.
Length-to-diameter ratio (L/D ratio)
Effective screw length divided by the screw diameter and commonly expressed as a ratio to unity.
Masterbatch
A compounding ingredient resin and additives with the additives present in higher concentration than are required in the final plastics material.
Matting
The process of rendering a polished surface uniformly dull.
Melt
Extrusion material when heated to a plastic condition.
Metering section
The flighted portion of the screw at the discharge end, in which the melt is forced at a controlled rate towards the die.
Outer die ring
The part of tubing die which shapes the external surface of a tube.
Post cure
To continue the cure of a moulded article by a subsequent heating process.
Processing aid
An additive to, or a component in, a resin to facilitate processing.
Ram extruder
A machine consisting essentially of a barrel with means of temperature control, in which a plunger operates to force material as a melt to a die.
Ram pressure
Used colloquially for the total force applied by a hydraulic ram, equal to the hydraulic pressure multiplied by the ram area.
Screen pack
Wire gauzes supported by the breaker plate and used for filtering the melt and increasing back pressure.
Screw
A helically grooved rotating member housed in the barrel of a screw extruder.
Screw diameter
The diameter developed by the rotating flight land about the screw axis.
Screw extruder
A machine consisting essentially of a barrel with means of temperature control and housing one or more rotating Archimedean screws which convey plastics materials from feed aperture and deliver it as a melt under pressure to a die.
Take up (extrusion)
A device for reeling extruded material.
Torpedo (extrusion)
A device at the discharge end of the screw for completing mixing and homogenising of the melt.
Transition section
The flighted portion of the screw between the feed and discharge sections in which the extrusion material becomes a melt.
Travelling saw
A saw which travels with the extrudate while cutting it to length.
Twin screw extruder
An extruder with a pair of screws working together in a common barrel.
Two-stage extruder
A screw extruder so designed that the pressure of the extrusion material drops substantially, part way along the screw.
Two-stage screw
A screw for use in a two stage extruder comprising a decompression section before the final metering section.
Vacuum sizing
A process using a sizing die with vacuum applied to the external surface of the extrudate.
Vacuum tank
A cooling tank operating under reduced pressure to control the dimensions of the extrudate.
Vent
A hole or groove in a mould provided to allow air or volatile matter to escape during the moulding operation.
Vented extruder
A two-stage screw extruder having an opening part way along the barrel for the removal of air and volatile matter from the plastics material.
Weld line
A mark formed by union of two or more streams of material flowing together.
