Key Questions to Ask When Ordering Single Screw Extruder

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.

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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.

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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.

According to the different production materials, product specifications and material consumption of customers, it is necessary to select the diameter of the screw first, and then further select the specification and model of the extruder. We need to consider many factors when selecting plastic extruder. Only by considering these factors can we ensure that the selected equipment meets the use requirements.

 

1. Screw

Screw is the main factor affecting the productivity of extruder. The performance of the screw determines the productivity, plasticizing quality, dispersion of additives, melt temperature, power consumption of an extruder. It is the most important part of extruder, which can directly affect the application scope and production efficiency of extruder. The rotation of the screw produces extreme pressure on the plastic, so that the plastic can move, pressurize and obtain part of the heat from friction in the barrel. The plastic can be mixed and plasticized in the moving process of the barrel. When the viscous melt is extruded and flows through the die, it can obtain the required shape and form.

 

2. Screw speed

Screw speed not only affects the extrusion speed and amount of material, but also makes the extruder achieve high output and good plasticizing effect. In the past, the main way to increase the output of extruder was to increase the diameter of screw. Although the material extruded per unit time will increase with the increase of screw diameter. However, in addition to extruding materials, the screw also extrudes, stirs and shears the plastic to plasticize it. On the premise of constant screw speed, the mixing and shearing effect of the screw with large diameter and large screw groove is not as good as that of the screw with small diameter. Therefore, modern extruders mainly improve production capacity and product quality by increasing screw speed.  

If the screw diameter is unchanged and the screw speed is increased, the torque borne by the screw will increase. When the torque reaches a certain degree, the screw is in danger of being twisted and broken. However, by improving the material and production process of the screw, reasonably designing the screw structure, shortening the length of the feeding section, increasing the flow rate of the material and reducing the extrusion resistance, the torque can be reduced and the bearing capacity of the screw can be improved.

 

3. Barrel structure

The improvement of the barrel structure is mainly to improve the temperature control of the feeding section. Whether the temperature of the water jacket through the barrel is reasonable is very important for the stable work of the extruder and efficient extrusion. If the temperature of the water jacket is too high, the raw material will soften prematurely, and even the surface of the raw material particles will melt, which will weaken the friction between the raw material and the inner wall of the barrel, thus reducing the extrusion thrust and extrusion volume. However, the temperature should not be too low. The barrel with too low temperature will make the screw rotation resistance too large, which will cause difficulty in starting the motor or instability of speed when exceeding the bearing capacity of the motor. Advanced sensor and control technology are used to monitor and control the water jacket of the extruder, so as to automatically control the temperature within the range of the best process parameters.

 

4. Reducer

On the premise of basically the same structure, the manufacturing cost of the reducer is roughly proportional to its overall size and weight. The large shape and weight of the reducer means that more materials are consumed in manufacturing, and the bearings used are also relatively large, which increases the manufacturing cost.

For extruders with the same screw diameter, high-speed and efficient extruders consume more energy than conventional extruders, double the motor power, and correspondingly increase the base of the reducer. But high screw speed means low reduction ratio. For the reducer of the same size, compared with the reducer with large reduction ratio, the gear module increases and the capacity of the reducer to bear load also increases.

 

5. Motor drive

Extruders with the same screw diameter, high-speed and efficient extruders consume more energy than conventional extruders, and the motor power also needs to be increased. During the normal use of the extruder, the motor drive system and heating and cooling system are always working. The energy consumption of transmission parts such as motor and reducer accounts for 77% of the energy consumption of the whole machine; Heating and cooling account for 22.8% of the input energy consumption of the whole machine; Instrument electrical accounts for 0.8%.

The extruder with the same screw diameter is equipped with a larger motor, which seems to consume electricity, but if calculated by output, the high-speed and efficient extruder is more energy-saving than the conventional extruder. If the power consumption of heaters, fans and other devices on the extruder is also considered, the difference in energy consumption is also greater. Extruders with large screw diameters should be equipped with larger heaters, and the heat dissipation area will also increase. Therefore, for the two extruders with the same capacity, the new extruder with high conversion efficiency has a smaller barrel, the heater consumes less energy than the traditional large screw extruder, and saves a lot of electricity in heating.

In terms of heater power, the power consumption of the extruder heater is mainly in the preheating stage. During normal production, the heat of material melting is mainly converted by consuming the electric energy of the motor. The conductivity of the heater is very low and the power consumption is not large.

 

6. Shock absorption measures

High torque extruders are prone to vibration. Excessive vibration is very harmful to the normal use of the equipment and the service life of the parts. Therefore, multiple measures must be taken to reduce the vibration of the extruder and improve the service life of the equipment.

The motor shaft and the high-speed shaft of the reducer are the most susceptible parts of the extruder to vibration. First of all, the high rotation extruder should be equipped with high-quality motor and reducer to avoid becoming the vibration source due to the vibration of motor rotor and reducer high-speed shaft. Secondly, we should design a good transmission system. Pay attention to improving the rigidity, weight and quality of processing and assembly of the frame, which is also an important link to reduce the vibration of the extruder. A good extruder does not need to be fixed with anchor bolts when in use, and there is basically no vibration. This depends on the sufficient rigidity and self weight of the frame. In addition, we should strengthen the quality control of the processing and assembly of various components.

 

7. Instruments

The production operation of extrusion can only be reflected by instruments. Therefore, precise, intelligent and easy to operate instruments and meters will make us better understand their internal conditions, so that production can achieve better and faster results.

 

 

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