Dictionary of Food Extrusion Terms

Conveying Element: In general, this would be any screw element that conveys product forward in the extruder. More specifically, it is typically a twin-flight screw with a wide pitch (about 1.0D).

Cut-flight Screw Element: A screw element that has channels cut out of the flights to allow more back-flow (mixing) in the element.

Dependent Variable: A parameter related to the extruder that the operator has no direct control over. One example the extruder exit temperature.

Diameter(s): A frequently used dimensionless expression of measures. It is easier to relate a 50 mm extruder and 100 mm extruder with L:D of 25 than it is to realize that a 750 mm long 50 mm diameter extruder and a 1500 mm long 100 mm diameter extruder are proportionally the same length. It is unclear if extruder manufacturers give the model size in terms of screw diameter or barrel diameter. In practice, there is only a minor difference between the two measures, so the error is minimal.

Extrudate: The stuff being pushed through the extruder.

Feed Screw: See Conveying Element. In general, this may refer to any screw element used in a “feed” section. This will tend to be a twin-flight screw with a wide pitch (1.0 D) intended to convey extrudate forward rapidly.

Independent Variable: A parameter related to the extruder that the operator has direct control over. One example is screw speed.

Kneading block: See Paddle.

L:D: A dimensionless way to express the length of an extruder. For example: a 15:1 L:D (or just 15 D) extruder has a length 15 times the diameter of the extruder.

Machine Direction: Typically refers to the direction of the extrudate travel. Machine direction is the direction the extrudate travels, as opposed to the transverse direction.

Orifice Disk: A circular screw element that is a fraction of a diameter in length. On a twin-screw extruder, the elements must be paired so that there is one large and one small diameter disk on each screw to block most of the flow area of the barrel. Provides a point where there is a large resistance to flow in the screw profile.

Paddle: An oval shaped screw element. Typically a fraction of a diameter in length. Paddles can be arranged in neutral, forward, or reverse conveying direction. Paddles can provide intense mixing.

Pitch: The distance forward a screw element will convey (unrestricted) for one revolution (360°). Typically expressed as a dimensionless number, as a fraction of screw diameter. Example: ½D Pitch screw (may also be expressed as 0.50D Pitch) screw will convey forward a distance equal to ½ of the diameter for each revolution of the screw.

Rated Screw Speed: If the drive motor is turning at it's rated speed, this is the speed at which the extruder will turn. Typically there is a gear-down between the drive motor and the extruder screws.

SME: See Specific Mechanical Energy

Specific Mechanical Energy: Mechanical Energy input per unit mass. This is a measure that tends to be useful in scale-up since it is independent of scale.

Specific Thermal Energy: Thermal Energy input per unit mass. This is a measure that tends to be useful in scale-up since it is independent of scale.

STE: See Specific Thermal Energy

Steam Lock: See Orifice Disk

Steam Quality: A measure of how much of the water in steam is vapor. Example: 75% steam quality means that 75% of the steam is in vapor form, 25% in liquid form. Alternative expression is that the enthalpy of the steam at saturation temperature/pressure is 75% of the way from the enthalpy value of the liquid water to the enthalpy value of the water vapor.

Transverse Direction: Typically refers the direction at 90º relative to the direction of the extrudate travel. See also Machine Direction.