There are a number of respected publications on the market devoted to the extrusion process but many focus on the theory, rather than the reality, which makes them impractical for operators or too academic for educational and training purposes. By contrast, our Extrusion Guide Book (EGB) is intended to provide practical down to earth answers to questions posed by operators regarding the extrusion of thermoplastic materials. The various topics will cover the basics, “How to extrude?” (covering the most common materials such as PVC, PE, PU, PET), FAQs, troubleshooting guides and web links.The most basic and common extruder for the extrusion of thermoplastic materials is the single screw extruder. As the screw can be considered as the heart of the single screw extruder, it will be our first topic.
A) Material preparation: mixing, blending & drying
B) Feeding of extruder: from starve to force
C) Extruder: extrusion of material
D) Tooling: shaping, calibration & cooling
E) Secondary processes
Single screw extruders – screw basics
Single screws are typically classified in two basic different designs: the “three-zone screw” (also referred to as “metering screw”) and the “barrier screw”, both with or without a mixing section (distributive or dispersive mixing). The importance of mixing becomes obvious when considering the high number of additives that can be processed such as processing aids (slip additives, heat stabilisers, lubricants), agents (anti-block, antistatic, anti-fogging), anti-oxidants, plasticisers, pigments and fillers among others. Most single screws are nitrided to provide a basic wear resistance, but they are also available in a wide range of surface coatings to provide increased wear resistance (stellite or colmonoy) or chemical resistance (chromium plated). Due to the scale and complexity, mixing elements, surface coatings of screws and screw wear will be separate topics.
The screw, whether featuring metering or barrier design, has the following basic tasks:
- Conveying of the material forward (and to generate the necessary pressure in front of the die)
- Heating and melting (by bringing shear into the material)
- Mixing and homogenising of the melt
Channel depth: distance from the top of the flight to the root
Channel: space between flights
Trailing flight flank: back edge of flight
Pushing flight flank: front edge of flight
Pitch: distance between consecutive flights
Helix angle: the angle flights make from a line perpendicular to the screw shaft
Screw diameter: distance between furthest flights across the screw shaft
Root diameter: distance from the channel bottom on one side to the channel bottom on the opposite side
Length: distance from either keyway, hopper or first flight to screw tip
This screw, typically between 20D and 28D long, has three different sections, that are the feed section, compression (or transition) section and metering section. In the case of a vented extruder, that is equipped with a vacuum degassing, the screw would be longer and have two additional zones: a decompression zone before the degassing and second metering zone. The three-zone screw is commonly used for the low to medium output range, and also when shear-sensitive materials (e.g. PVC) are processed.
The feed section has deep flights to receive the material from the feed throat (feed port) and has the main task of conveying the solids. This zone is of particular importance as the overall output rate of the extruder is directly connected to the solid conveying rate in this zone. In order to get the solids conveyed, the material must “stick to the barrel and slip on the screw”. To achieve this, proper temperature control is very important. For instance, if the feeding zone is set to the wrong temperatures, it is possible that the material is just moving around the screw below the feed throat without any tendency to move forward. The feed section is the zone where many problems can occur, and where a skilled operator can push up the output through the right temperature control of the screw root, feed section/cylinder and barrel. The feed section is usually 4-8 flights long.
The flights of the compression section change gradually from deep to shallow. Here the material is compressed and melted over a length of 5D to 10D. The metering section is the last section and has the shallowest flight depths and is typically 6-10 flights long. This section is needed to build up the necessary pressure before the die.
If a three-zone screw is not capable of completing the melting process and higher output rates are required, a “barrier screw” can be employed. This screw has a secondary flight (barrier flight) in the channel which is undercut and permits the passage of only fully molten plastic. Many different executions of this barrier design are available today, but all aim to provide higher output rates at low melt temperature. Introduced more than 50 years ago, today this screw design has become the leading and most widely used screw design. A barrier screw, especially when employed in connection with a grooved feed bush/section, can improve significantly the solid conveying rate (and thereby the output) of materials which have a poor COF (coefficient of friction) and which are difficult to process on conventional three zone-screws.
What is the screw compression ratio?
The compression ratio is the ratio between two depths, the feed channel depth and the metering channel depth (compression ratio = feed channel depth / metering channel depth).
Channel depth (feed section): 20.5mm
Channel depth (metering section): 6.2mm
Compression ratio: 3.3:1
The compression ratio of a three-zone screw can be considered as the most basic parameter when processing different polymeric materials with different viscoelastic properties. This ratio is typically between 1.5:1 and 4.5:1. Some polymers simply run better on screws with a 2.5:1 compression ratio, while other materials process better on screws with a 4:1 compression ratio. Often so-called general purpose (GP) screws with a ratio of about 2.5-3:1 are employed which are suitable for a wider range of materials.
This compression ratio refers to the depths of the channels. It is important to mention that this compression ratio does not indicate how much shear the screw brings into the material, and two screws with the same compression ratio can feature significantly different output rates. And this depth compression ratio has no meaning for the barrier screw. Another approach is the “volumetric compression ratio” which is explained in detail by Timothy W. Womer in Basic Screw Geometry: “Things Your Screw Designer Never told You about Screws”.
How is the L/D ratio defined?
The L/D ratio is the screw length divided by the screw diameter. The term diameter is clearly defined, but not the length of the screw, and companies are interpreting the length in their own way. Screw manufacturers usually indicate the overall screw length starting from the keyway, others measure from the front side or centre of the feed throat (feed port), still others measure only the effective screw length (flighted length) where flights exist. L/Ds for profile extruder screws, where flexibility in output has priority, are typically from 24:1 to 30:1, while high output screws, for instance barrier screws for pipe and sheet, can be 35/38:1.
Surging, resulting in output variations, is the most common problem associated with single screw extruders. And as the single screw extruder, unlike the twin screw extruder, is not a positive displacement device, feed variations result directly in output variations. Most of the surging problems result from the feed zone/section.
Extrusion: The Definitive Processing Guide and Handbook | Harold F. Giles, Jr., John R.Wagner, Jr.,Eldridge M. Mount, III
Basic Screw Geometry: “Things Your Screw Designer Never told you about Screws” | Timothy W. Womer – Oct 1, 2007
Fundamental Concepts of Extrusion Design | Lou Piffer (Davis-Standard Corp.)
Extrusion Principles, Product Application & Research Centre (Mumbai)