Gearbox and motor selection are important keys to the successful and economically optimized single-screw extrusion process. That is, the motor and gearbox should be selected such that the extruder operates at near 70% motor current load and about 70% of the maximum normal screw speed. This will allow a rate increase in the future and the extrusion of resins that are more viscous while minimizing the capital costs for the motor and gearbox.
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If the gearbox and motor are not selected properly for the application, then the rate of the line can be limited by the extruder by either operating at maximum screw speed or maximum motor current. For most applications, either the most expensive segment of the line or the cooling process should be the rate limiting section of the line, and not the extruder.
This article focuses on setting the maximum screw speed for the extruder based on the maximum motor speed, gearbox reduction and belt sheaves, if used. Motor size depends on the application.
Original equipment manufacturers (OEMs) are typically very good at sizing motors and gearboxes for extrusion applications. But the purchaser has the final decision on the acceptance of the design. I have seen a very large number of extrusion applications that were set up optimally. That is, the motor and gearbox provided the proper speed and torque range for the screw at the target rate, discharge pressure and discharge temperature. An optimal design should also include a future rate increase of up to 20%.
In the last 35 years, however, I have optimized nearly 30 extruders that were configured with less than ideal motors and gearboxes. Most of these cases were because the extruders were purchased for a particular application and then repurposed for a new application that required a different torque and speed for the screw. But there were the occasional specifications of a poor motor and gearbox on new equipment.
I have optimized nearly 30 extruders that were configured with less than ideal motors and gearboxes.
Extrusion applications operate with different discharge temperatures. For polyethylenes (PEs), typical discharge temperatures and screw speeds for some of the most common applications are provided in the accompanying table. The discharge temperatures are controlled in general by the metering channel depth, screw speed and resin viscosity. For example, a cast film process requires that the discharge temperature be near 250oC. The temperature is typically obtained by using a screw with a metering channel depth that is about 6 mm for a 100-mm diameter extruder and a screw speed of about 100 rpm.
Typical discharge temperatures, screw speeds and metering channel depths for 100-mm diameter extruders running PE applications.
For this application, the motor and gearbox (and belt sheave if used) would provide a maximum screw speed of about 120 rpm. For extrusion coating applications, the discharge temperature is typically 300oC and the screw speed during extrusion would be near 220 rpm, and the screw would have a metering channel depth of about 3 mm. The motor and gearbox combination for cast film would not be suitable for extrusion coating because the extruder could only run at the maximum screw speed of 120 rpm, providing about half the required rate. Moreover, the slower screw speed would likely not obtain a discharge temperature of 300oC, even for a screw with a 3-mm deep metering channel.
Recall from high school physics that power for a rotating shaft or screw is equal to the torque multiplied by the rotation speed of the screw. The motor current is directly proportional to the motor torque. A convenient method to estimate the power inputted via the screw is by using this equation:
where P is the power that is required by the screw in hp, Pmax is the name-plate power (hp) for the motor, A is the motor current (amperage) observed during the extrusion, Amax is the name-plate motor current at full load, RPM is the screw speed during extrusion, and the RPMmax is the maximum screw speed that the extruder is capable of running (base speed).
It is easy to see that if the maximum screw speed is 200 rpm and the screw speed used for the extrusion is 100 rpm, the highest power that can be inputted to the screw is half the power from the motor. The maximum screw speed is simply the maximum motor speed divided by the gearbox reduction and belt sheave ratio. The maximum torque at the screw depends on the motor size and the speed reduction by the gearbox and belt sheave.
A gearbox built for extrusion coating was used as a replacement box on a cast film line. The gearbox and motor combination set the maximum screw speed to 200 rpm. The cast film process, however, operated at about 95 rpm, limiting the torque available to the screw. That is, the motor current was running at 98% of the maximum current during operation. The redesign of the screw was complicated because of the lack of torque. The design provided an acceptable rate, but the extrudate temperature was too high.
Typically, a screw designer will increase the metering channel depth to decrease the extrudate temperature to an optimal level. When the metering channel depth is increased, the specific rate increases and the motor current needed for the process will increase. For this line, decreasing the discharge temperature via screw design was not possible because the motor was already running at the maximum motor current.
Purchasing and upgrading extrusion systems should always design flexibility into the line that enables rate increases, alternative resins and process optimizations.
Recently, I examined several cast film extruders that were repurposed on extrusion coating lines. As previously discussed, the processes operate at different temperatures with PE cast film at about 250oC and extrusion coating at 300oC. The extruder had a maximum screw speed of 70 rpm. The motor was operating with 45% of maximum motor current. A new design can provide the proper extrudate temperature at low rate or a higher rate with an extrudate temperature that is too low. The compromise was to design at a low rate that met the high extrudate temperature requirement.
Gearboxes and motors on cooling extruders used in tandem foam sheet lines must be selected carefully. A schematic of a tandem foam sheet line is shown in the accompanying figure. The first extruder melts the resin and mixes in a physical blowing agent such as supercritical carbon dioxide. The discharge is typically about 235oC for polystyrene (PS). The resin, however, is too hot to foam. The cooling extruder decreases the PS mixture to about 140oC. The cooling extruder is larger in diameter than the first extruder, and the screw rotates very slowly and has channels that are very deep.
Schematic of a tandem foam sheet line equipped with a physical blowing agent delivery system. Source: M.Spalding
A new tandem foam sheet line was designed and installed with a conventional slotted screw in the cooling extruder. This screw runs at a relatively high specific rate and a screw speed of about 18.2 rpm. The motor and gearbox combination provided a maximum screw speed of 20 rpm from a 150 hp motor. This screw, however, could not decrease the material temperature consistently, resulting in a low quality and unusable foam.
A high-performance screw was designed and built that would increase the foam quality and the rate. The high-performance screw operated at a much higher specific rate, causing the screw speed to operate at 11.5 rpm for the same rate. At 11.5 rpm, only about 58% of the motor power could be delivered to the screw. The motor and gearbox were not capable of supplying enough power to the high-performance screw.
There are often belt drives between a gearbox and a motor that can be used to either increase the torque to the screw or increase the maximum speed to the screw. The belt drive sheaves were changed on the cooling extruder such that the new maximum speed was 15 rpm, enabling the input of 77% of the motor power to the screw. Changing the belt sheaves to a maximum screw speed of 15 rpm will decrease the service life of the gearbox because the maximum torque was increased on both the input and output shafts. The OEM should be contacted when a belt drive is changed to make sure that the service life and safety of the gearbox are not compromised.
Purchasing and upgrading extrusion systems should always design flexibility into the line that enables rate increases, alternative resins and process optimizations. Many of these upgrades will require additional screw speed and/or torque. A properly specified gearbox and motor speed will enable additional torque and screw speed to satisfy these optimizations.
About the Author: Mark A. Spalding is a fellow in Packaging & Specialty Plastics and Hydrocarbons R&D at Dow Inc. in Midland, Michigan. During his 37 years at Dow, he has focused on development, design and troubleshooting of polymer processes, especially in single-screw extrusion. He co-authored Analyzing and Troubleshooting Single-Screw Extruders with Gregory Campbell. Contact: 989-636-; ; dow.com.
Plastic extrusion is a kind of continuous process for high volume manufacturing, where a certain thermoplastic material can be made in the form of pellets, powder, or granulates. Thereafter, they are homogeneously melted and moved into shaping die by applying pressure.
For example, screws are made with the molten plastic flowing into the rotating molds. As your plastic melts, it will pass through the die, where it is going to acquire the shape of the die-hole shape, and will then leave the extruder. This extruded product is known as an extrudate.
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A typical plastic extruder will consist of 4 zones:1. Feed zone
The flight depth is constant in this zone. The flight depth is the distance between the main diameter of the screw at the top of the flight and the minor diameter of the screw at the bottom of the flight.
2. Transition zone/compression zone
In this zone, the flight depth begins to decrease. The thermoplastic substance is crushed and starts to plasticize as a result.
3. Mixing zone
The flight depth remains constant in this zone. A particular mixing device may be used to ensure that the material is completely melted and homogeneously blended.
4. Metering zone
The flight depth in this zone is lower than in the mixing zone, but it remains constant. In this zone, the pressure also forces the melt using the shaping die.
What can be created by using plastic extrusion?Plastic extrusion is a technique that presses the molten plastic using a die to create tubes and plastic profiles. The procedure is used to make a wide range of goods and component parts for both industrial and home applications.
Thermoplastics will be utilized to force these plastics into shape because they are the best sort of material for melting and cooling to form a rigid shape. Two of the mainly included thermoplastics for plastic extrusion are polypropylene and PVC. Let us discuss here a few different applications for plastic extruder.
Extrusion of plastic can be used to make a wide variety of pipes and tubes. Tubes can be extruded to transport vast amount of liquid or gases, such as sewerage or water pipes, on one end of the scale.
Tubes with a considerably smaller diameter, on the other hand, can be made for items like medical tubing or straws! Tubing is one of the most common things made by plastic extrusion, and it is also one of the simplest to make because it can be extruded to any length and cut to the desired size.
Extruded tubes can be made not just to a given diameter, but also can be thicker or thinner as needed.
Extruded tubes can be hard or flexible, depending on the type of plastic employed. This means that plastic tubes can be manufactured to remain rigid for use in plumbing and sewage while also becoming more flexible.
Garden hoses, medical tubing, and tubes that transport food, chemicals, or gases in a factory setting will all be made with more flexible plastic extrusions.
To best suit the required design, cross-section shapes can be extruded in a range of various plastic types. Rain gutters, railing, trims, and seals for windows and doors are examples of profiles, which are forms rather than simple tubes.
Co-extruded elements, such as rubber seals on window trims, can frequently be added to profiles like these. Furthermore, adding another kind of plastic to the expulsion process.
Extrusions don&#;t have to be hollow tubes or profiles and they can be solid shapes as well. Plastic planks and decking, as well as a variety of curved bar stock for industrial or automotive usage can be made in this way.
Solid form extrusion is beneficial for making outdoor furniture, such as benches and fences because the plastic is a weatherproof, low-maintenance alternative to wood that lasts far longer.
Plastic extrusion can also be used to make the insulation that surrounds the electrical cable. This is accomplished by passing a wire through the die and then extruding plastic around it to form an insulating layer.
Other types of cable can also go through this extrusion process to make them waterproof and resistant to corrosion and abrasion, as well as easier to handle.
Extruded sheeting and film can also be used as a protective screen or as part of window glass. Extruded plastic film can also be used to make plastic packaging like blister packs.
Few Different types of plastic extrusion lineThere are several designs of extruders that are available in the market today. We will take up a few important ones here and briefly discuss them.
Single screw extruders are the most prevalent continuous extruders in the polymer extrusion sector because of their numerous advantages, including low cost, simple design, toughness, reliability, and a high performance/cost ratio.
Any standard single screw extruder will have 3 geometrically varying zones:
· Feed zone
· Metering zone
· Transition or compression zone
The consistent pitch of the screw, but variable channel depth creates the three zones. The compression phenomenon is caused by the screw channel&#;s depth decreasing linearly from the feed zone to the metering zone.
The term &#;single-stage&#; refers to screw designs having only one compression portion. For the same screw length and diameter, zone length, as well as maximum and minimum channel depths, may vary. As a result, several screw profiles are feasible.
Generally, twin screw extruder is categorised as continuous multiple screw extruders in general. The name comes from the fact that these extruders contain two Archimedean screws in their design.
There are several classes for twin-screw extruders since there are more design characteristics that may be adjusted in twin-screw extrusion, such as rotational direction, degree of intermeshing, and so on.
The twin screw extruder has 2 varieties and they are also further subdivided as follows:
a. Intermeshing extruders
· Counter-rotating
· Co-rotating
b. Non-intermeshing extruders such as
· Counter-rotating
· Co-rotating
· Co-axial
Tubes and pipes are extruded using this method of extrusion. Air with a positive internal pressure can also be used in this process. After departing the die, the tubes or pipes are dragged into a cooling tank, where they are generally water-cooled.
This type plastic extrusion line can be used to make plastic film tubes using a continuous sheeting process. The melt from the film tube is cooled before it leaves the die, resulting in a semi-solid tube that is blown to a suitable size and film thickness. This method is employed in the production of items like shopping bags.
In this plastic extrusion line plastic sheets or films that are too thick to be blown are extruded using this method. The sheets are drawn and cooled through a sequence of cooling rolls after exiting the die, which helps regulate sheet thickness.
Wire coating is done with this form of extrusion. The wire is drawn into the die&#;s centre in this procedure. When a high level of adhesion between the wire and the coating is required, pressure tooling is employed.
The wire is covered with molten plastic while in the die and is pressured when it exits the die in this manner. Jacketing tooling is utilised if adhesion is not required. The melt covers the wire as it exits the die in this way.
How to select a plastic extruder?The following are a few things that you must see while selecting any plastic extruder:
Reliability and experience are important things to consider when selecting a manufacturer. You must be confident that your chosen manufacturer will be there and able to assist you throughout the extruder&#;s operational lifetime.
A long and successful track record is the best indicator of a manufacturer&#;s dependability and industry experience.
Consider manufacturers who only employ high-quality components from globally recognised brand names when it comes to the numerous components used on an extruder machine. If a component has to be improved, repaired, or replaced in future, there will be no issues locating or integrating a spare part.
The term &#;quality&#; refers not just to the extrusion machine itself, but also to the continual support provided at every step along the road. A good business will follow through on its promise of excellent customer service by offering assistance throughout the purchasing process.
Nanjing Cowin Extrusion Machinery Company Ltd was formed by the optimization and reorganisation of the twin-screw extrusion industry&#;s leading team. It is a multinational, high-tech limited corporation specialising in the production of polymer material processing equipment.
In China, this company is considered to be one of the best extruder manufacturing companies. It was founded in Nanjing, Jiangsu Province, in . The company employs more than 50 people, including ten technical R&D engineers, twenty machining and assembly workers, 18 management and sales teams, and five after-sales support teams.
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