Screw Parts

This description of screw parts is going to be a work in progress for some time to come.  Verbal descriptions of screw parts can only go so far in conveying the parts.  To try to more fully convey what the parts are, examples have been drawn up to help with the descriptions.  The screw elements have been drawn in a very general manner and are intended to convey the basics concepts of screw geometry, not the geometry of any given extruder manufacturer.  The geometries are not quite correct, but are close enough to convey the concepts to an audience wanting to learn about food extrusion.  The geometries used were for the purpose of ease of drawing the parts.

The focus of this page is twin-screw extrusion.  Single screw extruders will have some parts and geometries not used or not possible with twin-screw extruders.  This page will not show all of the parts possible or available with twin-screw extrusion, but will focus on the most common parts. 

The basic use of measure on an extruder is the diameter of the screw part.  Using the diameter of the screw as the basic unit of measure allows extruder lengths and screw profiles to be described using a single set of terms and measures from the smalllest extruder to the largest.  This can be best observed by looking at a screw element in the axial direction (from the "front" or the "back" of the screw).  In many of the images below, there is a sphere 1D in diameter to make visualization of the length of pieces in Diameters more easy. 

The length of screw parts are described in terms of diameters.  In general, the helical screw parts will be 0.5D, 1.0D, or 1.5D.  Other lengths are possible and available, depending on the manufacturer. 

The pitch of the screws are expressed in terms of the number of diamters it takes for the flight to wrap 1 full revolution around the screw (see images below).  The most common pitches tend to be 0.25D, 0.5D, 0.75D, and 1.0D.  The 0.25D pitch tends to be a single-flight screw, this has not yet been drawn up for this site.

Below are images of the screw parts of different pitches.  On the left and right are the sphere 1D in diameter for comparison.  The spheres make it relatively easy to see these parts are 1.5D in length.  The screw element on the left has a pitch of 0.5, the middle screw element has a pitch of 0.75, and the screw element on the right has a pitch of 1.0.  Using the screw element on the right (1.0 pitch), you can follow the flight on the bottom of the image as it rotates around the element (hiden behind the piece for part of the revolution).  When 1 full revolution around the element is completed, the distance along the element is 1.0 D, so the point where full rotation is completed is equal in height to the diameter of the sphere (1.0 D).  The 0.75 Pitch piece will complete a full rotation of the flight in 0.75 D along the screw (or 2 rotations for the 1.5 D length).  The 0.5 Pitch will make 1 full rotation around the screw in 0.5 D along the screw (or 3 rotations for the 1.5 D length.

Image of screw pitches on 1.5D parts
Image of screw pitches
The length of these conveying pieces are typically 0.5 D, 1.0 D, and 1.5 D.  The pieces for the three pitches are below (1.0 D pitch, then 0.75 D pitch, then 0.5 D pitch).

Images for 1.0 D pitch (also call "full pitch")
Image of Full Pitch Screws

Images for 3/4 pitch:
Image of 0.75 Pitch Screws

Images for 1/2 pitch:

Conveying elements put work into the extudate slowly, paddles (shearlocks) put work into the profile more rapidly.  A paddle is essentially an oval piece that is a poor conveying element, even when configured as part of a group of elements set to "forward conveying".  Forward conveying is essentially lining the paddles up so the general direction of the progression of the longest dimension of the paddles (lobe) continue the direction of the conveying elements.  Neutral conveying is essentially setting the paddles so from one paddle to the next in the profile, the lobes are offset by 90 degrees.  Reverse conveying is essentially lining up the paddles so the general direction of the progression of the lobes are opposite the direction of the conveying elements.  Generally, paddles will be built up in sets equal to 0.5 D in length.  For this web page, the length of each paddle is 1/8 (0.125) D and blocks of 4 paddles were created.  Generally, for 0.5 D in length, a block of elements will be offset by 90 degrees, so for the forward and reverse conveying paddles, each paddle is offset by 30 degrees from the paddle upstream. 

The easiest way to demonstrate this is to show an animation of the paddles as they would rotate in the extruder.  For this animation, the general direction of flow in the extuder is from left to right.  One way to look at the direction of conveying of the paddles is to look at the top or bottom of the group of paddles.  If the "wave" that comes around goes from left to right, the parts are forward conveying (direction of extrudate flow).  If the "wave" that comes around is right to left, the parts are reverse conveying (opposite direction of extrudate flow).  The middle set of paddles are offset by 90 degrees, so will have no tendency to convey one direction or the other.

Animated .gif of the paddles

The screw elements are built up on the extruder shaft, resulting in the screw profile being used.  The animation below is an arbitrary screw profile for an arbitrary extruder length - it is not meant to be any particular profile for any particular extruder manufacturer or for any particular product.  The profile has a length for full pitch (1.0 D) screws feeding into forward paddles.  Coming out of the forward paddles, there is another set of full pitch (1.0 D) screws feeding into a set of three-quarters pitch (0.75 D) screw elements.  Coming off of those screws, there is a set of forward paddles, followed by a set of reverse paddles.  Coming off of those paddles is a full pitch (1.0 D) screw element feeding a half pitch (0.5 D) screws, which would then feed the die on the end of the extruder.  Not shown in the animation is the barrel surrounding the screws. 

Animation of a rotating screw profile.