Hot extrusion vs cold extrusion Process

 
Hot extrusion is a hot working process, which means it is done above the material’s recrystallization temperature to keep the material from work hardening and to make it easier to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses that range from 230 to 11,000 metric tons . Pressures range from 30 to 700 MPa , therefore lubrication is required, which can be oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions. The biggest disadvantage of this process is its cost for machinery and its upkeep.
The extrusion process is generally economical when producing between several kilograms and many tons, depending on the material being extruded. There is a crossover point where roll forming becomes more economical. For instance, some steels become more economical to roll if producing more than 20,000 kg.
Cold extrusion is done at room temperature or near room temperature. The advantages of this over hot extrusion are the lack of oxidation, higher strength due to cold working, closer tolerances, better surface finish, and fast extrusion speeds if the material is subject to hot shortness.
Materials that are commonly cold extruded include: lead, tin, aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel.
Examples of products produced by this process are: collapsible tubes, fire extinguisher cases, shock absorber cylinders and gear blanks.

Extrusion Applications for Food

With the advent of industrial manufacturing, extrusion found application in food processing of instant foods and snacks, along with its already known uses in plastics and metal fabrication. The main role of extrusion was originally developed for conveying and shaping fluid forms of processed raw materials. Present day, extrusion cooking technologies and capabilities have developed into sophisticated processing functions including: mixing, conveying, shearing, separation, heating, cooling, shaping, co-extrusion, venting volatiles and moisture, encapsulation, flavor generation and sterilization.[21] Products such as certain pastas, many breakfast cereals, premade cookie dough, some french fries, certain baby foods, dry or semi-moist pet food and ready-to-eat snacks are mostly manufactured by extrusion. It is also used to produce modified starch, and to pelletize animal feed.
Generally, high-temperature extrusion is used for the manufacture of ready-to-eat snacks, while cold extrusion is used for the manufacture of pasta and related products intended for later cooking and consumption. The processed products have low moisture and hence considerably higher shelf life, and provide variety and convenience to consumers.
In the extrusion process, raw materials are first ground to the correct particle size. The dry mix is passed through a pre-conditioner, in which other ingredients may be added, and steam is injected to start the cooking process. The preconditioned mix is then passed through an extruder, where it is forced through a die and cut to the desired length. The cooking process takes place within the extruder where the product produces its own friction and heat due to the pressure generated (10–20 bar). The main independent parameters during extrusion cooking are feed rate, particle size of the raw material, barrel temperature, screw speed and moisture content. The extruding process can induce both protein denaturation and starch gelatinization, depending on inputs and parameters. Sometimes, a catalyst is used, for example, when producing texturised vegetable proteins (TVP).

Hydrostatic extrusion

In the hydrostatic extrusion process the billet is completely surrounded by a pressurized liquid, except where the billet contacts the die. This process can be done hot, warm, or cold, however the temperature is limited by the stability of the fluid used. The process must be carried out in a sealed cylinder to contain the hydrostatic medium. The fluid can be pressurized two ways:
Constant-rate extrusion: A ram or plunger is used to pressurize the fluid inside the container.
Constant-pressure extrusion: A pump is used, possibly with a pressure intensifier, to pressurize the fluid, which is then pumped to the container.
The advantages of this process include:
  • No friction between the container and the billet reduces force requirements. This ultimately allows for faster speeds, higher reduction ratios, and lower billet temperatures.
  • Usually the ductility of the material increases when high pressures are applied.
  • An even flow of material.
  • Large billets and large cross-sections can be extruded.
  • No billet residue is left on the container walls.
The disadvantages are:
  • The billets must be prepared by tapering one end to match the die entry angle. This is needed to form a seal at the beginning of the cycle. Usually the entire billet needs to be machined to remove any surface defects.
  • Containing the fluid under high pressures can be difficult.
  • A billet remnant or a plug of a tougher material must be left at the end of the extrusion to prevent a sudden release of the extrusion fluid.

Where to buy extruder?

Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes are its ability to create very complex cross-sections, and to work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms parts with an excellent surface finish.
Drawing is a similar process, which uses the tensile strength of the material to pull it through the die. This limits the amount of change which can be performed in one step, so it is limited to simpler shapes, and multiple stages are usually needed. Drawing is the main way to produce wire. Metal bars and tubes are also often drawn.
Extrusion may be continuous or semi-continuous. The extrusion process can be done with the material hot or cold. Commonly extruded materials include metals, polymers, ceramics, concrete, modelling clay, and foodstuffs. The products of extrusion are generally called “extrudates”.
Extruder Process
The process begins by heating the stock material (for hot or warm extrusion). It is then loaded into the container in the press. A dummy block is placed behind it where the ram then presses on the material to push it out of the die. Afterward the extrusion is stretched in order to straighten it. If better properties are required then it may be heat treated or cold worked.
The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. One of the main advantages of the extrusion process is that this ratio can be very large while still producing quality parts.
process
Where to buy extruder?
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How choose the right extruder motor?

 
Extruders must operate dependably despite the high temperatures, plastic dust and, in many cases, space constraints that all put demands on electric motors. The question is: How can processors maximize the dependable operation of the electric motors that drive extruders? The question applies to all extruded products, whether pipe, profile and tubing, sheet or film.
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He does not see particular performance advantages for any one motor type, regardless of the type of product being extruded. He says that the company designs a 15 percent safety factor into its extruders for the amount of torque required, regardless of the type of motor used. “As things wear, as the life of the extruder goes on, you will need to turn up the speed of the extruder and take advantage of that safety factor,” he says.
Proponents of the laminate-frame AC motor design — which uses sheets of steel that are stacked to create the core of the motor — cite features that they say make it particularly well suited for extrusion applications. The motors can be installed on original equipment and used as retrofits.
• Low profile, making them a good choice for situations in which space is an issue.
• High power density with the ability to run at a low temperature.
• Low inertia, with the ability to perform at an extended speed-versus-torque range, and high overload capacity at zero speed.
• The versatility to run different resins through the same extruders.
Heat is the enemy of electric motors. Of the three temperature-rise classes, which reflect safety margins — H (a rise of 356 degrees Fahrenheit), F (311 degrees Fahrenheit) and B (266 degrees Fahrenheit) — Class B motors are the largest. Of the three major insulation classes — the insulation that a motor requires for a certain temperature rating — motors with Class H insulation can operate at the highest temperatures.

Plastics extrusion is a high-volume manufacturing process in which raw plastic is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weatherstripping, fencing, deck railings, window frames, plastic films and sheeting, thermoplastic coatings, and wire insulation. It is generally used by list of pvc extrusion manufacturers to produce different types of plastic materials.
process
A picture of an extruder manufacturing line is shown above. First, raw material, in the form of small plastic pellets, is placed in the hopper. The hopper rests on top of the barrel. The barrel is a heated hollow steel cylinder, sort of like a really thick pipe. An auger-type screw rotates inside of the barrel. The screw’s rotation takes the plastic pellets and pushes them forward, into the barrel. As the pellets move towards the front of the barrel, frictional and electrical heat from the barrel melt the plastic. After the plastic is melted, the rotating screw continues to act as a pump and forces the molten plastic through a die. 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. Once cool, the product can be rolled up, cut into sections, packaged, or can go on to secondary operations.
Extrusion machinery can be complicated to operate, but the overall process is relatively straightforward. The heart of the machine is the screw, which sometimes is referred to as an auger. The screw is turned by a gearbox, which is powered by a motor. It is enclosed in a tight, heated barrel, which helps to provide friction.
The thermoplastic pellets are delivered into the machine through a hopper. The hopper is located at the rear of the barrel/screw assembly and the pellets drop into the barrel from there. As the screw turns, it slowly drags the thermoplastic pellets forward. The heat from the friction of the screw turning inside the barrel–along with external heating–melts the plastic as it moves forward in the barrel. The melted plastic is pushed into a section designed to meter the plastic for the next stage in the process. It also may be subjected to pressurized pumping at this phase of the process.
Once the plastic has entered into metering section of the barrel, it is ready to be extruded into a die. The die is attached to the barrel and it represents the final shape or profile that the plastic is intended to take. The plastic is forced into the die. As the plastic moves forward into the die, it will be separated by a mandrel, which is centered in the extrusion channel.
Pressurized air is forced though the mandrel structure as a means of keeping the plastic from collapsing as it moves through the die. As the plastic leaves the die, it will enter into a vacuum environment. Inside the vacuum, there are sizing rings meant to keep the plastic in the desired shape. The vacuum environment also will be filled with water as a means of cooling the extruded plastic. After the extruded plastic has passed through the water-filled vacuum environment it can be cut or spooled as appropriate.

Polymer Screw Extrusion

Most polymeric materials are extruded at least twice in their lifetime, rst through a pelletising die after the reactor and then for nal shaping.In simplest terms, screw extruders are polymer pumps with the capacity to melt the material which they are fedScrew extruders comprise one or two Archimedean screws rotating in a heated barrel.
Polymer Screw Extrusion
The single screw extruder (SSE) is the workhorse of the plastics industry. Polymer resins in the form of pellets, powders or fakes from a hopper to the gap between a rotating screw and a heated barrel. The depth of the con- veying channel in the screw is contoured from large to small in the ow direction, to account for the density change from the particulate solid feed to the molten polymer extrudate, and for pressure development. The SSEs normally have diameters between 25 and 250 mm, and length/diameter ratios between 20 and 36. Usual rotation speeds range from 20 to 150 rev min . A 60 mm diameter machine may deliver up to 200 kg , while a 150 mm diameter machine can exceed 1000 kg.
In the first, or solids conveying, zone of the extruder, the solid polymer particles are compacted together in the screw channel by the rotating action of the screw to form a solid bed of material.At the start of the next extrudersection, the plastication (melting) zone, barrel heaters cause a thin film of molten polymer to form in the gap between the solid bed and the barrel wall. The melt lm is subjected to intense shearing in the thin gap, and because of the extremely high viscosities of molten polymers, high rates of viscous dissipation result. The generated heat melts the solid bed within a short distance of the start of melting. In the last zone of the extruder, the metering section, the polymer melt flow is stabilised in the shallow screw channels, and nally the material passes out through the die at the end of the machine.
Pressurebuildupowing to frictionalforcesin the solids con- veying section of the screw has been modelled by Darnell and Mol (along with some more recent research ) on the basis of force and torque balances, which result in an exponential expression. For forward motion of the solid bed, the friction coeffcient must be larger on the barrel than on the screw. Screws have polished surfaces. The barrel surfaces are sometimes intentionally roughened (as in grooved barrels).
To improve the mixing capabilities of single screw extruders, various types of mixing devices have been developed. Most prominent among these is the Maddock (or Union Carbide) mixer, in both straight and spiral variations.These devices are normally located at the downstream end of a screw, and improve melt quality by reducing temperature non-homogeneities in the polymer stream and increasing the dispersion and distribution of additives. Used increasingly in the extrusion industry are barrier screws and grooved barrels. Barrier screws have a secondary screw ight in the melting section of the screw, which serves to segregate the solid bed from the molten polymer. By independently controlling the dimensions of the solids and melt channels, the melting process can be accelerated and made more stable, there by increasing the extruder output and melt quality. Grooved barrel extruders feature axial
grooves or slots in the part of the barrel immediately following the feed throat. The grooved barrel can signi cantly enhance feeding of resin pellets and flakes, and thereby sharply increase the output of the extruder above the drag ow output of equation.
Twin screw extruders(TSEs) are extruderswith two screws of the same diameter, which turn side by side within the extruder barrel at the same speed. In recent years, twin screw extruders have come into increasingly wider use in applications such as mixing, blending, compounding of thermoplastic polymers with additives, devolatilisation and reactive extrusion.The TSEs offer greater control over residence time distribution (RTD) and mixing than that of single screw extruders, and have superior heat and mass transfer capabilities. The disadvantage of TSEs compared with SSEs is their signi cantly higher capital cost.
There are three major classes of twin screw extruders: corotating intermeshing, counter rotating intermeshing and counter rotating non-intermeshing. The screws are made up of conveying, kneading block and mixing sections. The screw design is frequently modular, which allows for a nearly unlimited number of possible screw con gurations. Kneading blocks comprise several discs staggered at an angle to one another, the motion of which causes intense shearing and has a chopping effect on the material stream. Most of the dispersive mixing (melt homogenisation and solids breakup) and melting that occurs within the extruder takes place in kneading blocks.
Corotating intermeshing twin screw extruders are the most common type of TSE. These machines have the feature that their screws are self-cleaning,owing to their special design.This leadsto narrower RTDs in these extrudersthan in SSEs, making them particularly attractive for use with thermal and shear sensitive materials, as there are no dead spots in which material can collect and degrade. Large twin screw extruders can rotate at more than 1200 rev min RPM with outputs of more than 10 tons/h.