There are various types of plastic extrusion machines out there. Depending on the end product and other needs, a manufacturer will use one type of the machine and not the other. In this section are details on the different plastic extruder types together with their construction, application, and how they work.
Plastic Extruder Types
The different types of plastic extrusion machines are grouped by their design, specifically in the part that compresses the molten plastic. In light of that, the main extruder types today include screw and ram extruders. A screw extruder is characterized by a shaft with a helical surface, while a ram extruder uses a plunger assembly. More details about the ram and screw extrusion machines below.
Screw extruders are the most common types of plastic extrusion machines. In their simplest construction, they consist of these parts: a drive, feeder, a rotating screw inside a barrel, and an orifice at the end. The helical surfaced screw (which is the most crucial part) serves to heat, mix, and press the plastic melt against the die.
Plastic screw extruders exist in a broad range of configurations, from the small laboratory extruder to large industrial lines. The screw itself varies greatly in terms of size and design. The typical plastic extrusion machine screw is designed with varying diameters, called extrusion zones, along its length.
Most screws consist of 4 extrusion zones; the feed, melting, compression, and metering zone. In a vented extruder, a decompression zone (larger screw diameter) is added to allow gases and moisture to leave the plastic melt. Generally, screw extruders are manufactured in 3 different types: single-screw, twin-screw, and multi-screw.
Single Screw Extruder
The plastic extrusion machine has just one screw that moves the polymer through the barrel. Single screw extruders are the most popular in the category owing to their low price and fewer maintenance costs. They’re also more reliable in their operation and generally simple in construction.
In terms of application, the single-screw, plastic extruder is best employed in processes that do not need a lot of mixing, especially those that contain no additives or compounding needs. In some cases, these extruders come equipped with protrusions to enhance mixing, such as pins.
Twin Screw Extruder
The twin screw extruder uses two rotating screws instead of one. These types of plastic extrusion machines best fit applications where compounding is required such as when extruding UPVC plastic, and the heating and extrusion stages must happen simultaneously. Also, when extruding materials that contain additives and must mix well.
A twin screw extrusion machine can be either co- or counter-rotating and intermeshing or no-intermeshing. A co-rotating extruder has both screws rotating in the same direction, which allows for high screw speeds and low-wear characteristics. Counter-rotating types mix materials better and are most suited for compounding.
Multi Screw Extruder
A multi-screw extrusion machine uses more than 2 screws to extrude plastics. These types of extrusion machines are not very popular today, but can still be found in the market. A common variation of the multi-screw extruder is planetary roller type. These feature one main screw (sun screw) and several other screws (6 or more) arranged in a planetary fashion toward the end.
Planetary roller screw extruders function much like the single screw types, the only difference being in the multiple screws in one end. A feed system is usually positioned above the roller section, which results in a thorough mixing of the materials. As such, these extruders are best used with UPVC materials or in conjunction with other extruder types.
Ram extruders are the simplest types of plastic extrusion machines as they do not contain any screws or barrels. Instead, a plunger or piston moves back and forth under the action of a hydraulic or pneumatic system. The material is pushed through the die as the plunger moves forward, creating pressure that forces the material through the die.
A ram extruder will typically have these parts: a feed system, a drive system, which is either hydraulic or pneumatically operated, the ram itself, a heating system, and a die. A control system is used with the machine to regulate the rate of resin feed, temperature levels, and ram operation.
Ram extrusion is a very powerful process and these extruders are most suited for processing dense plastics. The ram extrusion process also mostly fits applications where there’s no need for mixing, blending, or heating of the material. A ram extruder can be either a single or multi-ram configuration. Ram extruders are further divided into the following: vertical and horizontal types.
Vertical Ram Extruder
Vertical ram extruders are the most common types of the extrusion machine due to their usability and versatile characteristics. For example, they allow easy resin feed systems and a mandrel that’s easy to position in the die. Their design also allows for compact machines that can be used in smaller spaces.
Vertical ram extruders have their disadvantages, too. For instance, these types of plastic extrusion machines are not easy to cool. They’re also not the best option when the application involves extruding long parts such as pipes. Instead, they’re best used for profiles and other shorter forms. They’re also more prone to leaking at the die.
Horizontal Ram Extruder
Horizontal ram extruders use the same parts found in the vertical types. However, they have their components mounted horizontally, which provides several advantages: the horizontal design makes them easy to cool as well as maintain. They’re also more effective when it comes to producing long forms than the vertical extruder types.
The advantages notwithstanding, the horizontal ram extruder will usually be a large machine that requires more space to install. These types of the ram extruders are also not as easy to feed with resin as the vertical type. When the die has to be used with a mandrel, centering it can be a challenge.
Different types of plastic extrusion machines mean different designs and machine structures. Consequently, that translates into different output characteristics and performance features for different applications. When choosing the type of extruder for your project, consider the design and performance differences. That way, you can be sure to get an extrusion machine that will deliver exactly what you want and need. Learn about the things that can go wrong in an extruder next.