This page provides you with all the basic information about plastic extrusion. As you go through the article, you will learn more about:
Plastic extrusion, also known as plasticating extrusion, is a continuous high volume manufacturing process in which a thermoplastic material -- in the form of powder, pellets or granulates -- is homogeneously melted and then forced out of the shaping die by means of pressure. In screw extrusion, the pressure comes from the screw rotation against the barrel wall. As the plastic melt passes through the die, it acquires the die hole shape and leaves the extruder. The extruded product is called extrudate.
An extruder typically has four distinct zones:
In this zone, the flight depth remains constant. Flight depth refers to the distance between the major diameter at the top of the flight and the minor diameter at the bottom of the screw flight.
In this zone, the flight depth begins to decrease, causing the thermoplastic material to be compressed and start the plasticization process.
In this section, the flight depth stabilizes. To guarantee that the material is thoroughly melted and evenly blended, a specialized mixing element may be employed.
In this zone, the flight depth is reduced compared to the mixing zone but remains constant. Additionally, the pressure in this area forces the molten material through the shaping die.
The melting of the polymer mixture is driven by three main factors:
Heat transfer involves the movement of energy from the extruder motor to the extruder shaft. The melting of the polymer is influenced by factors such as the screw profile and the residence time of the material within the extruder.
Friction, which affects the melting process, arises from internal resistance within the powder, as well as factors like the screw profile, screw speed, and feed rate.
The temperature of the extruder barrel is controlled by three or more independent temperature controllers to ensure consistent processing conditions.
In the United States and Canada, a wide array of machines is available for producing plastic extrusions. These machines play a crucial role in modern industry, enabling the efficient manufacture of a diverse range of plastic products. These products serve key sectors such as construction, automotive, packaging, and consumer goods, driving both economic growth and innovation. Below, we explore some of the leading machines in this field.
The Davis-Standard DS-RE series extruders are renowned for their versatility and dependability. With a broad range of processing capabilities, these extruders accommodate various materials, making them a popular choice across industries with diverse plastic extrusion requirements. Based in the United States, Davis-Standard is a leading manufacturer of plastic extrusion equipment.
Cincinnati Milacron offers a variety of extrusion machines designed for specific applications, such as profile and pipe extrusion. Their machines are well-regarded for their precision and advanced control systems, making them a popular choice among manufacturers in the United States and Canada.
Milacron's PAK series extruders are highly regarded for high-performance plastic extrusions. Engineered for maximum energy efficiency, they offer extensive customization options, making them an excellent choice for industries aiming to enhance productivity and sustainability.
KraussMaffei Berstorff is renowned for producing high-quality extrusion lines. Their machines are celebrated for their innovative features, superior product quality, and consistent performance, making them a preferred choice for a wide range of plastic extrusion applications.
NFM/Welding Engineers, Inc. specializes in extrusion technology, providing a range of machines suited for diverse plastic extrusion processes. Their extruders are recognized for their durability, precision, and exceptional engineering.
Please note that the plastic extrusion industry is constantly evolving, and new models and manufacturers may have emerged since this posting. For the latest information, it is advisable to consult industry-specific publications, visit manufacturers' websites, or seek guidance from field experts.
Before the main extrusion process, the polymeric feed is blended with various additives such as stabilizers (for heat, oxidative, and UV stability), color pigments, flame retardants, fillers, lubricants, and reinforcements. This mixture enhances product quality and processability, helping to achieve the desired property profile specifications.
For some resin systems, an additional drying process is necessary to prevent polymer degradation from moisture. Even resins that typically do not require drying may need it if they have been stored in cold environments and then exposed to warmer conditions, causing moisture condensation on their surface.
Once mixed and dried, the polymer and additives are gravity-fed into the feed hopper and then through the extruder throat.
A common issue with handling solid materials like polymer powder is flowability, which can lead to material bridging inside the hopper. To address this, intermittent injection of nitrogen or another inert gas can be used to disrupt polymer buildup on the hopper surface, ensuring a consistent material flow.
The material then flows into the annular space between the screw and the barrel, and is guided by the screw channel. As the screw rotates, it conveys the polymer forward, subjecting it to frictional forces.
The barrels are heated with a gradually increasing temperature profile. As the polymer mixture moves from the feed zone to the metering zone, the combined effect of frictional forces and barrel heating plasticizes, mixes, and kneads the material.
As the melt nears the end of the extruder, it first passes through a screen pack, which filters out any contaminants and prevents clogging of the die plate hole. The melt is then forced through the die to acquire its shape, cooled, and pulled away from the extruder at a constant velocity.
Following cooling, additional processes such as flame treatment, printing, cutting, annealing, and deodorization may be applied. The extrudate is then inspected for quality and, if it meets all specifications, is packaged and shipped.
Today, there are several extruder designs available on the market, which can be categorized into two main types based on their mode of operation:
The primary distinction between the two types of extruders lies in the mechanism used to move the material. Continuous extruders utilize rotating parts, whereas discontinuous extruders use reciprocating components.
Continuous extruders can be further classified into two categories:
In the polymer extrusion industry, single screw extruders are the most prevalent continuous extruders. They are favored for their low cost, simple design, durability, reliability, and favorable performance-to-cost ratio.
A typical single screw extruder features three distinct geometrical zones:
The three zones in a single screw extruder are defined by the screw's constant pitch but varying channel depth. As the screw moves from the feed zone to the metering zone, the channel depth decreases linearly, creating a compression effect. Screws with a single compression section are typically referred to as single-stage. The length of each zone, as well as the maximum and minimum channel depths, can vary even within screws of the same length and diameter, allowing for different screw profiles.
The thermochemical environments inside the extruder can vary based on the following factors:
These factors influence local heat conduction, heat dissipation, velocity profile, and residence time within the extruder.
The typical layout of single screw extruders includes:
Twin screw extruders are typically categorized as continuous multiple screw extruders. They feature two interlocking Archimedean screws, which is how they get their name. The design of twin screw extruders includes numerous classifications due to various adjustable parameters, such as rotational direction and degree of intermeshing.
Twin screw extruders can be broadly classified into two main types, each of which can be further subdivided:
One example of an extruder with more than two screws is the planetary roller extruder. This type resembles a single screw extruder, especially in the feed section. However, the difference becomes evident in the mixing section. In the planetary roller section, six or more planetary screws revolve around a central screw, known as the sun screw. These planetary screws intermesh with both the sun screw and the barrel, which features helical grooves to accommodate their flights. The planetary roller section is typically connected to the feed section via a flange-type connection.
Planetary roller extruders offer intensive mixing through the rolling action of the planetary screws, sun screw, and barrel. This design is particularly advantageous for processing heat-sensitive compounds with minimal degradation.
Common applications of planetary roller extruders include rigid or plasticized PVC extrusion. They can also be integrated with standard extruders to enhance their mixing capabilities.
Disk extruders are continuous extruders that use disks or drums rather than screws to convey the material. Often referred to as screwless extruders, these machines operate primarily based on viscous drag transport. They are less common in the industry compared to screw extruders.
Compared to screw extruders, disk extruders are now less commonly used in the industry.
The stepped disk extruder, also known as a slider pad extruder, features a stepped disk positioned slightly away from a flat disk. When one of the disks is rotated with the melt in the axial gap, pressure is generated at the transition between different gap sizes.
Stepped disk extruders can be used continuously if they include exit channels in the stepping disk. However, a major drawback is their maintainability; the complex design of the flow channels makes cleaning challenging.
The drum extruder uses a rotating drum and barrel to carry out the extrusion process. The polymeric material is introduced into the annular space between the drum and the barrel. As the drum rotates, the material is transported along the barrel's circumference. A wiper bar, positioned before the drum completes a full rotation, scrapes the melt from the drum and directs it to the exit, eventually leading to the extruder die.
Diskpack extruders perform basic polymer processing operations with efficiency comparable to other machinery. They share features with single screw extruders and drum extruders, functioning as single screw extruders with zero helix angle and very deep flights.
The material is fed into the axial gap between thin disks on a rotating shaft. The melt moves with the disks, and before completing a full rotation, a channel block closes off the space between the disks. This channel, similar to the wiper bar in drum extruders, directs the flow to the outlet channel.
Diskpack extruders can also incorporate mixing blocks and spreading dams to enhance processing efficiency.
Unlike other disk extruders that rely on viscous drag transport, elastic melt extruders utilize the elastic properties of the polymer melt to move the material and generate the necessary die head pressure.
In elastic melt extruders, shearing deformation of the material between stationary and rotating plates creates uneven normal stress distributions in all directions. These stresses produce a centripetal pumping action, driving the continuous extrusion of material through a central opening. This mechanism is why elastic melt extruders are also known as normal stress extruders.
Ram extruders, also known as plunger extruders, operate in a discontinuous manner. They are robust, positive displacement devices capable of generating very high pressures.
Ram extruders are beneficial for batch operations such as injection molding and blow molding. However, they have two major drawbacks: limited melting capacity and uneven temperature distribution of the melt.
One variant is the single ram extruder, which is commonly used for general-purpose molding. It is also employed in the extrusion of ultrahigh molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE).
Multi ram extruders are designed for continuous operation. Some models feature four plunger-cylinders—two for plasticating and two for pumping—connected by a complex shuttle valve system. Other designs include twin ram extruders with cylinders arranged in a V-shape.
This chapter will discuss the key functions of various components in a single screw extruder, including:
The extruder drive is responsible for rotating the screw and supplying the necessary torque to the screw shank. There are three primary types of drive systems:
The thrust bearing assembly resists the axial forces exerted on the screw due to the die head pressure. The actual force is determined by the die head pressure, with the load calculated by multiplying the screw's cross-sectional area by the die head pressure.
The assembly is typically located where the screw shank connects to the drive’s output shaft.
There are various types of thrust bearings like ball thrust bearings, cylindrical roller thrust bearings, spherical roller thrust bearings, tapered roller thrust bearings, etc. Each type has its advantages and disadvantages.
In some extruders, the barrel and feed throat are connected. The barrel encases the screw, while the feed throat is the entry point for the material.
For extruders with this design, the feed throat is often equipped with a water-cooling system to prevent premature melting of the polymer at the entry point. This cooling system helps avoid issues such as melt sticking to the surface and restrictions in material flow.
When designing the feed section of the extruder, the following factors should be considered:
The feed hopper is used to introduce material into the extruder, typically through gravity flow. However, some materials do not flow easily and may cause bridging inside the hopper if not properly managed. To ensure smooth material flow, additional devices such as vibrating pads and stirrers are often incorporated into the feed hopper.
Two common issues that may arise when using a feed hopper are:
The extruder screw is the most important component of the extruder. Basically, a screw is a cylindrical rod with changing diameter and helical flights around it. This component is responsible for the conveying, heating, and mixing of the material.
In most extruders, the outside diameter of the screw, measured from flight tip to flight tip, remains constant. The radial clearance relative to the screw diameter typically ranges from 0.0005 to 0.0020, with an average ratio of 0.001.
Medium carbon steels are commonly used for screws, but other materials include low carbon steel, stainless steel, tool steel, nickel-based alloys, and hard-facing materials.
The die is a crucial component of the extruder, as it shapes the material as it exits. When the exit opening of the extruder barrel does not align with the entry of the die, an adapter is necessary to connect the two. However, if the die is designed specifically for the extruder, no adapter is needed.
A breaker plate is used to interrupt the spiraling motion of the melt and redirect it into a straight-line flow. If the spiraling motion is not controlled, it can extend to the die exit and cause distortions in the extrudate. Additionally, breaker plates support filter screens within the extruder.
The primary purpose of screens in an extruder assembly is to filter out any foreign materials from the melt. Screens can also enhance melt homogenization and increase die-head pressure. Typically, breaker plates support the coarsest screen, with finer screens arranged sequentially behind it.
Four types of filter media are commonly used as screens in extruders:
For melts with high levels of contaminants, screens can become clogged quickly and require frequent replacement. To minimize downtime, automatic side-plate screen changers are often employed, allowing screen replacement while the extruder remains in operation.
The pressure drop across the screen is a critical parameter and is closely monitored. If the pressure drop exceeds a predefined threshold, it triggers an automatic screen change. A hydraulic piston ejects the used screens while a new set is positioned against the breaker plate. In some systems, additional filtering units are installed downstream for further cleaning if necessary.
Heating and cooling systems regulate the extruder's temperature during both startup and normal operations. Common methods for heating include electric, fluid, and steam heating.
Electric heating is favored for its low maintenance, cost-effectiveness, and efficiency. It also accommodates a wide range of temperatures, making it suitable for various applications. Each barrel of the extruder is equipped with its own electric heater, which is controlled independently to maintain a consistent temperature profile along the extruder.
Electrical heaters can be categorized into two types:
Fluid heating systems provide even temperature distribution across the heat transfer area, preventing localized overheating. However, they typically operate at lower temperatures, with a maximum operating temperature usually below 250°C. Some major drawbacks of fluid heating systems include:
Steam is effective for heating due to its high specific heat, making it a good choice for extruders. However, achieving and maintaining high temperatures with steam can be challenging, as it requires high operating pressures. Steam heating is now less common in extruder applications due to the following drawbacks:
In heating the polymer, the screw action accounts for 70 to 80% of the total energy input. This can lead to localized internal heat generation in the melt, potentially raising the process temperature above the set point. To manage this, cooling systems are employed to reduce the temperature when it exceeds the target. Forced-air cooling using blowers is a commonly utilized method for this purpose in extruders.
There are several types of dies used for different extrusion applications, including film and sheet extrusion, pipe and tubing extrusion, blow film extrusion, profile extrusion, and coextrusion.
Films and sheets differ primarily in thickness, with films being thinner (less than 0.5 mm) and sheets thicker (more than 0.5 mm). In film and sheet extrusion, three types of dies can be used:
Pipes and tubing differ primarily in diameter, with tubing having a smaller diameter (less than 10 mm) compared to pipes, which generally have larger diameters. In-line dies and crosshead dies are commonly used to extrude annular products.
Blown film extrusion commonly uses a spiral mandrel die due to its versatility across various materials and operating conditions. Spiral mandrel dies are known for their excellent melt distribution and operate efficiently at low pressures.
Alternatively, conventional crosshead dies can be used. However, compared to spiral mandrel dies, crosshead dies are more susceptible to weld lines.
In the plastic industry, certain applications require specific extrudate shapes. For these cases, profile dies are used to create shapes other than the typical rectangular or annular profiles.
In coextrusion, a single die is used to join two or more polymers that are simultaneously extruded.
Three types of dies are commonly used in coextrusion:
A calibrator is a sizing die used in pipe extrusion to define the final dimensions of the pipe. It directly contacts the extrudate, cooling and reinforcing it as it exits the extrusion die to ensure it holds the correct shape.
To prevent the extrudate from collapsing, a vacuum can be used to keep the calibrator securely in place against the extrudate. This vacuum-assisted device is known as a vacuum calibrator.
Pellets produced from compounding extrusion can serve as feedstock for additional extrusion processes such as injection molding or rotomolding. Compounding polymers with various ingredients—such as flame retardants, stabilizers, release agents, mineral fillers, and colorants—creates new formulations that meet the specific requirements of different applications.
The construction industry is a major consumer of plastic sheets, which are commonly used for applications such as glazing in doors and windows, bulletproof sheets, and protective coverings.
In addition, plastic sheets serve as protective covers for walkways and sound barriers in commercial and industrial settings. They are also used in refrigerator liners, food containers, decorative panels, and lamination applications.
Cast films, in contrast, are primarily utilized in the food, agricultural, and packaging industries. Specifically, cast films are used for:
Extrusion coating involves applying a polymer to a substrate to enhance its properties. This process improves the quality of the product by providing benefits such as better heat sealability for packaging, increased tear and crease resistance, improved appearance, enhanced chemical resistance, and superior printing capabilities.
Common substrates for plastic coating include paper, polyester, metal foils, cellophane, paperboard, cloth, and other plastics. Extrusion coating and lamination are widely used in various applications, including dairy packaging, juice cartons, carpet coating and backing, frozen food containers, oven-safe paperboard trays, and heat-seal layers for general packaging.
Plastic extrusion is often used to coat wires and cables for insulation and protection. This coating process typically employs a crosshead extrusion method.
The common polymeric materials used for this application include:
Pipes and tubes are extensively used across various industries. Specifically, these plastic extrusion products find applications in:
Material selection for pipes and tubing is based on their intended applications. Pipes are typically made from polyethylene, rigid polyvinyl chloride (PVC), and nylon. In contrast, thermoplastic resins are commonly used for producing tubing.
Tubing can be either flexible or rigid. For flexible tubing, materials such as elastomers, cross-linked polyethylene (PE), flexible polyvinyl chloride (PVC), and polyurethane are commonly used. Rigid tubing, on the other hand, is usually made from commodity resins.
The sports industry is a major user of monofilament fiber, which is used in tennis, badminton, racquetball, and squash racket strings. Additionally, synthetic ropes made from monofilament fibers are widely used in construction, fencing, greenhouses, and orchards.
Extrusion blow molding is commonly used to create large, irregularly shaped hollow parts. The automotive industry is a significant consumer of extrusion blow molding products, including components for bumpers, knee bolsters, air conditioning systems, side view mirrors, and door handles.
Extrudates are not limited to rectangular or annular shaped products. They can also take the form of a solid shape. Rods, slabs, square rods, square tubes, etc. are some of the common stock solid shapes.
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