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Extrusion Definitions Clertral


Aquaculture feed : extruded fish feed can have various characteristics depending on the process parameters - floating, slow sinking or sinking products.

Average residence time : the average time the product is contained inside the barrel, as it is conveyed and processed by the screw elements.

Barrel : linked modules housing screw elements on shafts and the material being processed.

Barrel temperature profile : important process parameter ; directly measured with thermocouples ; it may affect the residence time, the die temperature and pressure and the shear history of the product.

Bi-colour/bi-flavour technology : process achieved with twin screw extruder. In the extruder, after the product is mixed and cooked, it is divided in two flows ; colouring and flavouring is added to each flow through ports in the barrel. Then the product is extruded through a specially designed die which merges the product flows into one product with two distinct colours and flavours.

Breakfast cereals : (Often called RTE cereals or ready-to-eat cereals).
Processed by twin-screw extrusion in two variations : - Expanded (or puffed), which are cooked, formed, dried or toasted, coated if desired and cooled for packaging (crisp rice, puffed corn) - Pellets, which are tempered, cooked, formed into strands, then cut into pellets, flaked, toasted, coated if desired and cooled for packaging (corn or wheat flakes).

C. Chamber : intermeshing screws which create "C" chambers which act to convey the material through the barrel to the die.

Caseinate : the casein, milk protein, is chemically modified by extrusion in order to obtain caseinate.

Cellular structure : the expansion of a material can be increased by foaming of the extrudate. In the barrel, the water included in the material may become extremely hot in the extruder barrel. Then, steam is released at the die, expanding the viscous food material and creating a cellular structure to the products.

Channel : the area define by the barrel, the heart of the screw and 2 threads adjacent to this one.

Channel width : space between 2 screw threads.

Co-extruded products : products with a crisp extruded outer "shell" and soft filled center. The outer tube (shell) is produced by a cooker extruder and the filling is injected by a pump into the tube using a special die.

Co-extrusion : process of creating a two-textured product with an expanded material, which is filled with a filling prepared separately and pumped from a holding tank into the expanded material at the die.

Compression zone : the location where the compression ratio increases rapidly and most of the mechanical energy is dissipated. Then the temperature of the ingredients rises quickly and transforms the material from a granular state to a viscous state.

Conjugate screw : self-cleaning screw.
The intermeshing flights of one screw sweep the bottom of the channel of the adjacent screw.

Conveying zone : section of an extruder or a screw characterized by large pitches which are not full and which allows to convey the material.

Cooking : processing of raw materials using only few seconds at high temperatures. This process preserves the ingredients while destroying a high percentage of microorganisms.

Cooking extruder : extruder
A screw extruder employs single or twin screws rotating within a barrel to push the material forward and through a specially designed orifice called a die.

Corotating screws : extruder screws which turn in the same direction (in opposition to the counter-rotating screws which turn in the opposite direction).

Crispness : crispiness
Textural property developing a characteristic sound and mouthfeel when crushed.

Cutter : a cutter is fixed at the end of the extruder, facing the die face to slice the extruded product.

Degassing : degassification, venting.
Evacuation of air, steam or volatile compound through an orifice located in the barrel. The degassing can be forced by using a vacuum pump.

Detoxification : inactivation of non-nutritional factors or toxins present in raw materials under the influence of thermic treatment.

Die : located at the end of the extruder, it contains one or more openings through which the extruded product flows.

Die plate : The barrel is capped with a die plate containing one or several orifices through which the extruded product is forced.

Die pressure : pressure generated by the material before its entrance into the die.

Dry basis : mass basis of a material at 0% moisture.

Dynamic viscosity : physical property of a product indicating its resistance to the flow.

Eight-shaped channel : method used in corotating twin screw extruders to transfer the material from screw to screw.

Expansion : puffing
Increase of the extruded product volume as the product exits the die due to the vaporization and which is influenced by the die design and by the viscosity of the mass.

Expansion ratio : ER
Cross-sectional diameter of undried extruded product divided by the diameter of the die opening. It allows to estimate the modifications of the extrudate dimensions at the die exit.

Extrudate : dough strand, rope.
Extruded product.

To extrude : process of shaping by forcing materials through a smaller orifice, the die, by pressure.

Extruded product : product obtained with an extruder or a cooker-extruder.

Extrusion : process which involves conveying a material by one or two screws through a closed barrel and forcing the material to flow through a little shaped orifice under pressure.

Extrusion cooking : continuous process which consists in cooking, during a very short time, a raw material or a mix of dry or hydrated raw materials, controlled conditions of temperature, pressure or shearing thanks to one or two screws. When the mass is cooked, the product is shaped by the die.

Feed rate : quantity of material introduced in the machine by time unit. (kg/h)

Feeder : auxillary equipment controlling the raw material supply at the main port or an a secondary port of the barrel.

Feeding zone : section of an extruder or an extrusion screw in which the feeding of raw material is carried out.

Filling ratio : volume used by the material divided by the volume available in the extruder or in a part of the extruder.

Fish : see aquaculture feed.

Flash vaporization : transformation into steam of part of the water contained in the extruded product which is due to the material expansion.

Gap : distance between screws caps and between screws and barrel.

Gelatinization : conversion of raw starch into a cooked and digestible material by the application of heat and moisture.

Glass transition : temperature zone where occurs the transition from the glassy state to the rubber state, due to the variation of the mobility of the molecules or the macromolecule segments.

Glassy state : vitreous state
State of a amorphous mass under its temperature of glass transition.

Gravimetric feeder : very precise equipment, composed of a volumetric feeder and scales, which delivers a constant mass of product.

Grinding : process leading to the decrease of the particle sizes of a dry product.

Headspace : air gap.
Adjustable distance between the screw end of an extruder and the die plate.

Heating/cooking fluid : liquid used for the thermic regulation of the barrel and which circulates into an enclosure surrounding it.

HTST process : ( High Temperature Short Time)
Continuous process submitting a product to a high temperature during a very short time to sterilize it and/or cook it (the product keeps its nutritional qualities).

Inductor : induction coil device generating heat to the extruder barrel by means of induction.

Instant flour : precooked flour
Cereal based product obtained by cooking-extrusion at a moisture of 15 or 25 % which is then dried and ground (it is a starch base).

Instant soup : soup base.
Similar process as the instant flour.

Intermeshing screws : screws used in a twin screw extruder. The flights of one screw engage the channel of the other screw.

Kneading : action realized in cooking-extrusion to obtain a mixing of ingredients due to the mechanical work of the screw elements.

Kneading disc : mixing disc
Elements of a screw section inserted on the splined shaft between the screw sections of a twin screw extruder inside the machine, food materials pass from one disc to another, which provides a mechanical kneading action.

Kneading zone : location where the compression ratio increases rapidly and most of the mechanical energy is dissipated rising quickly the ingredient temperature. The mass is transformed from a granular state to a continuous plasticized mass.

Leakage : flow of a product moving through the gaps between screws and between screws and barrel.

Longitudinal expansion index (LEI) : ratio of the exiting velocity of the extruded product after expansion to its velocity in the die orifice.

Loss-in-weight feeder : see gravimetric feeder.

Low moisture extrusion : a product is extruded at low moisture when its water content is below 30 %. It requires less drying of the product, for reduced energy consumption.

Melting : plasticization
Material processing step without water excess, which is characterized by the crystalline structure fading of the starch and the secondary and tertiary structures of the proteins.

Metering zone : transition zone, melting zone, shearing zone.
See kneading zone

Mixing discs : see kneading discs.

Modular screw : screw which is the result of the assembly of different elements (having a fixed length and a fixed thread for each element) fitted together on splined shafts.

Moisture constent : water quantity.
Ratio of the water quantity to the whole mass (moist base).

Number of threads : number of threads contained in a section perpendicular to the screw axle.

One lobed disk : see kneading disc

Output flow rate : throughput
Material quantity exiting the extruder in relation to a time unit (kg total (or dry) weight/h).

Pet food : dry or semimoist product which incorporates cereals and animal proteins to create a "ready to use" product for the pets.

Pitch : rectilinear distance covered by the product following the screw thread during one revolution. The distance is measured parallel to the rotation axle.

Positive pumping : drag flow is the forward pumping action of the extruder as a result of the relative motion between the screw and the barrel.

Preconditioning : process which takes place in chambers prior to extrusion. Under controlled temperature and pressure conditions, the ingredient mix is preheated, moisture is increased with steam and/or water, and held to allow the moisture to penetrate and soften the individual particles.

Product pressure : pressure exerted by the material in the full sections of the extruder barrel.

Radial expansion index : ratio of the diameter of the extruded product to the die diameter.

Ready to eat cereals : RTE cereals
Cereal based products for breakfast obtained by extrusion cooking and which are ready for use.

Residence time : time required for the material to advance along the screws (elapsed time between the entrance and the exit of a particle inside an extruder).

Residence time distribution (RTD) : parameter which characterizes the statistic distribution of the residence time of the material in a process equipment working on a continuous basis.

Reverse screw element : screw element with a pitch designed to convey product back to the screw preceding it, creating a filled barrel section.

Rheological behaviour : behaviour of the flow of a solid or liquid material.

Rubbery state : state of an amorphous mass above its temperature of glassy transition.

RVA : Rapid Visco Analyser. Laboratory equipment enabling to measure the cooking degree of starch.

Screw configuration : screw profile
General profile of the screw resulting from the change of its geometric characteristics and/or the organization of successive screw segments insuring different functions.

Screw element : extrusion screw segment, rotating in a tightly barrel, conveys, heats and transforms food ingredients into a continuous plasticized mass.

Screw speed : number of revolutions of the screw(s) in relation a time unit.

Screw with trapezoid profile : screw on which the thread section (parallel to the rotation axle) is a trapezoid.

Shaping : forming
Result of the configuration of the orifice (die) through which the material is extruded and the relative speeds of the cut-off knife and the dough strand.

Shear rate : rate of deformation of the material (within the shear field).

Shearing : mechanical deformation of the material due to the screw movement in the barrel.

Single screw extruder : extruder with barrel in which there is only one screw.

Single screw feeder : the controlled metering of the feeder is realized by one screw which turns with a variable speed.

Slot : opening in the threads which allows the material to advance toward the end of the machine.

Snacks : extruded products composed of a cereal based mix and obtained by cooking-extrusion. They are dried or toasted. They can be coated, flavoured and/or coloured.

Specific mechanical energy (SME) : mechanical energy supplied by the motor and in relation to the mass unit of the material.

Texture : attribute of a substance resulting from a combination of physical properties and perceived by the senses of touch, sight and hearing.

Texturized vegetable proteins (TVP) : extruded product which is obtained from vegetable proteins imitating the fibrous texture of the meat products and destined to replace them.

Thermomechanical treatment : addition of thermic and mechanical operations applied to the material during the extrusion-cooking (shaping, kneading, cooking and/or cooling).

Twin screw (cooker) extruder : machine with 2 parallel screw shafts running side by side in a double-bored barrel which allows advanced mixing and processing of products.

Twin screw feeder : the controlled metering of the feeder is realized by 2 screws which turn at variable speed.

Venting : see degassing

Vibratory feeder : volumetric feeder which vibrates to introduce the material in the extruder.

Volumetric expansion (VEI) : ratio of the volumic mass of the product in the die to the one of the extruded product.

Volumetric feeder : volumetric feeder is not as precise and expansive as a gravimetric feeder. It delivers a constant volumetric output and the delivered mass by time unit can vary due to the packing down (compression) of dry materials in the feeder hopper.

Weigh-belt feeder : gravimetric feeder where the control of the feeding rate is done by a belt which allows control of the mass output of dry ingredients.

Wet basis : mass basis of a material at its stable moisture content.


Amorphous : irregular arrangements of polymer chains.

Alloy : mix of polymers compatibles or not.

Compound : semifinished product resulting from a mix of a thermoplastic and one or several additives. It is used by plastics transformers.

Devolatisation : operation which consists of eliminating the volatile materials (monomers, moisture, ...) contained in the thermoplastics.

Dispersive mixing : micro-mixing, dispersion, agglomerates break-up.

Distributive mixing : macro-mixing, homogenisation.

Filler : additive used to improve some properties of thermoplastics (impact strength, temperature resistance, dimensional stability...) and/or reduce the price.

Masterbatch : mix of an additive (with a high concentration) into a thermoplastic matrix. Used in dilution in a plain material.

Melting : transition from solid state to liquid state.

Partially cristalline : regular and periodic molecular arrangements.

Pellitizing : compounds and masterbatch forming step.

Pigment : organic or inorganic product in powder form used to colour plastic materials.

Reinforcement : additive used to improve the mechanical properties of thermoplastics.

Reactive extrusion : extrusion technique which uses the twin screw extruder as a continuous reactor in order to complete chemical reactions.


Bivis : aquipment specifically developped by CLEXTRAL using the mechanical principle of a twin screw extruder and used for the pulp procesing.

Bleaching : chemical impregnation done with bleaching chemical products. Twin screw technology ensures TCF (Total Chlorine Free) bleaching, using hydrogen peroxyde or sodium hydrosulphite.

Chemical impregnation : the mixing between the chemical reactive and the material accomplished by injection of reactives in the barrel, is very efficient. In the case of alkaline cooking, the efficient chemical impregnation caused by the bi-vis process decreases the chemical reaction time.

Densifier : equipment developed to continuously feed the twin screw machine. Particularly used for cotton fibers.

Dwell tank : equipment located after the twin screw machine to enable the chemical reaction to develop after the fibers are impregnated. Often used for cooking and bleaching applications.

Effluent: liquid streaming from the filtration sections of the machine. The effluent volumes are very low during the washing step with a twin screw machine.

Fiber cutting : the twin screw machine is adapted to the pulping of cellulose fibers, including long textile fibers (cotton, flax, hemp, ...). The screw profile can be tailored to significantly reduce the fibers length.

Filtration / extraction : the screw profile can be tailored to increase the pressure on the material in certain sections. In these sections, the barrel can be fitted with filters which separate the fibers from the associated fluids.

Mechanical treatment : different screw profiles exert various mechanical actions on the material.

Washing : after chemical treatment minerals and dissolved minerals are efficiently washed from the materials in several steps that internately inject water in the barrel, then extract the effluent through filters fitted on the barrel. Current or countercurrent washing may be achieved.

Wood defibering : screws are available with a specific geometry to defiber wood shavings to the specifications of the traditional paper industry.