Instead, they use disks or drums to facilitate the extrusion process. In some cases, disk extruders are also called screwless extruders. Most disk extruders are operated based on viscous drag transport. Stepped disk extruder, also known as slider pad extruder, has a stepped disk placed a small distance from a flat disk. Pressure is generated at the transition of one gap size to another when one of the disks is rotated with a melt in the axial gap. Stepped disk extruders can be used in a continuous process if there is a provision for exit channels in the step disk.
One major drawback of this extruder is maintainability. The intricate design of the flow channels makes it hard to clean. Drum extruder utilizes a rotating drum and barrel to perform the extrusion process.
The polymeric material is fed into the annular space between the drum and the barrel. The material is carried along the circumference of the barrel as the drum rotates. Before the end of one full rotation, there is a wiper bar that scrapes the melt from the drum and directs the flow to the exit and then to the extruder die. Diskpack extruder is capable of basic polymer-processing operations that is as efficient as existing machineries.
It has similar features like the single screw extruder and drum extruder. Diskpack extruders can be treated as single screw extruders with zero helix angle and very deep flights. The material is fed in the axial gap between thin disks on the rotating shaft. The melt moves with the disks and before one complete rotation, a channel block closes off the space between the disks.
Like the wiper bar in drum extruders, this channel directs the flow to the outlet channel. Unlike other disk extruders which operate based on viscous drag transport, elastic melt extruders make use of the elastic properties of polymer melt to move the material and build the required die head pressure.
During shearing deformation of the material between a stationary and rotating plates, uneven distribution of normal stresses will develop in the melt in all directions. These stresses cause a centripetal pumping action which drives the continuous extrusion of material through a central opening.
Due to this mechanism, elastic melt extruders are also commonly referred to as normal stress extruders. Ram extruders, also known as plunger extruders, are used in a discontinuous operation. They are simple and tough positive displacement devices which can generate very high pressure. Ram extruders are advantageous in batch operations like injection molding and blow molding. However, this type of extruder also has two major drawbacks such as its limited melting capacity and poor equal temperature distribution of the melt.
One type of ram extruder is the single ram extruder, which is used for general purpose molding. Multi ram extruders are designed to operate in a continuous manner. Some designs have four plunger-cylinders —two for plasticating and two for pumping which are connected by an intricate shuttle valve.
Some multi ram extruders, on hand, are designed as twin ram extruders with cylinders in a V-shape. The major functions of different components of a single screw extruder will be discussed in this chapter. These include:. The extruder drive is responsible for the screw rotation.
It supplies the required torque to the extruder screw shank. There are three main drive systems:. Thrust bearing assembly resists the axial forces acting on the screw which is brought by the die head pressure.
Hence, the actual force is determined by the die head pressure. The assembly is normally placed at the connection of the screw shank and the output shaft of the drive. 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 own advantages and disadvantages. Some extruders have a barrel and feed throat which are connected. Basically, the barrel is the part that encloses the screw while the feed throat is where the material enters.
For extruders with this design, the feed throat is usually designed with a water-cooling system to avoid premature melting of the polymer at the feed entry. This is also to prevent further problems like melt sticking on the surface and material flow restriction. The feed hopper is used to feed the material into the extruder. The feeding is usually done by gravity flow. However, some materials do not easily flow and if not aided with additional devices, it may cause bridging inside the hopper.
Additional devices like vibrating pads and stirrers are incorporated in the feed hopper to ensure good flow of material. 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, from flight-to-flight tip, is constant. The radial clearance to screw diameter ratio is 0. Medium carbon steels are the most common material used in screws. The die is usually a piece of steel with the shape of the desired part machined into it. Once the melted plastic exits the die, it is shaped like the finished product.
Next, it is pulled through some sort of cooling apparatus, which usually cools with air or water. The die is what gives the final product its profile or shape and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape.
Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile. The product must now be cooled which is usually achieved by pulling the extrudate through a water bath.
Plastics are excellent thermal insulators and are therefore very difficult to cool quickly. Today we will learn about extrusion process working, types, application, advantages and disadvantages. Extrusion is a metal forming process in which metal or work piece is forced to flow through a die to reduce its cross section or convert it into desire shape.
This process is extensively used in pipes and steel rods manufacturing. The force used to extrude the work piece is compressive in nature. This process is similar to drawing process except drawing process uses tensile stress to extend the metal work piece. The compressive force allows large deformation compare to drawing in single pass. The most common material extruded are plastic and aluminum. Extrusion is a simple compressive metal forming process. In this process, piston or plunger is used to apply compressive force at work piece.
These process can be summarized as follow. In this type of extrusion process, metal is forced to flow in the direction of feed of punch.
The punch moves toward die during extrusion. This process required higher force due to higher friction between billet and container. In this process, metal is flow toward opposite direction of plunger movement. The die is fitted at opposite side of punch movement.
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