Different Types of Extrusion Machine
Many different industries use extruding machines: the food industry extrudes pasta; the aerospace and automotive industries extrude metal shafts; the plastic industry extrude plastics tubing; and the rubber industry extrude rubber fending, just to name a few.
The extruder machines look different depending on the industry, however, they all operate in essentially the same way. In the simplest form, an extruder consists of four basic components:
- Drive system – consisting of the motor and power transmission units.
- Barrel – where the rubber is fed.
- Transporting unit – typically a ram or screw.
- Head – which holds the die that ultimately turns the material into the desired shape.
In industry, extruders have been categorised in different ways: by transporting unit, such as screw or ram extruders; or by the temperature of the material fed into the extruder, such as hot or cot fed extruders. But the fundamental distinction between two types of extruders is in mode of operation:
- Continuous – delivers rubber in a continuous manner and has a rotating member or screw; here the pressure is produced by a screw.
- Discontinuous – delivers rubber in an intermittent fashion and has a reciprocating ram or screw, these types of extruders are ideally suited for batch type processes such as injection moulding.
Otherwise, it is important to note that extruders are usually designated by the diameter of the extruder barrel or the length to diameter (L/D) ratio.
In a screw extruder, material is continuously fed into the Hopper and heated by the heating components along the barrel until it rises to a temperature that makes it easy to shape. The rotation of the screw forces the material toward and through the die, producing the extrusion of desired length and cross-section. Notice how the shape of the screw gets larger the closer it gets to the die, this compresses and mixes the material, reducing the amount of trapped air and makes the material more homogenous.
There are different variations of screw extruders, these being single-, twin- and multi-screw extruders. As the names suggest, the difference is the number of screws inside the barrel. Each variation has its place and has different advantages and disadvantages. Let’s look at how they compare.
Generally, the advantages of a single-screw extruder are:
- A very high throughput.
- Capable of handling a wide range of materials.
Single-screw extruders do, however, struggle to mix certain materials like pasta and powder, sometimes rendering them unsuitable.
When compared to single-screw extruders, twin- and multi-screw extruders have:
- Higher pumping efficiency.
- Better mixing capabilities.
- More uniform and quicker heating of the material.
- Better handling of high moisture and sticky materials.
- Self-cleaning and higher material use efficiency.
However, these types of extruders come with higher costs, both initial and operation costs. They are also more complex, leading to them being less robust than their single-screw counterpart. Additionally, the single-screw extruder is far superior to twin- and multi-screw extruders at handling materials that are more common.
In ram extruders, a quantity of warm compound is placed in the barrel, the die is attached to the head of the barrel and the ram forces the material through the die to form a profiled section as shown in Figure 2.
The advantages of ram extruders when compared to screw extruders are:
- Extrusions can be carried out at lower temperatures.
- Difficult compounds can be extruded.
- Easy to clean.
- Useful for short runs.
- Useful for compounds which need to be strained through gauge for quality products requiring completely contamination free material.
- Almost no material losses, leading to large savings.
However, a disadvantage of the ram extruder is that it is not a continuous process, meaning that the volume of rubber that is possible to put into the ram is limited.