Beyond the Screw and Barrel: Other Critical Components When Selecting an Extruder

Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with over 30 years’ experiences of plastic pipe extrusion equipment, new environmental protection and new materials equipment. Since its establishment Fangli has been developed based on user’s demands. Through continuous improvement, independent R&D on the core technology and digestion & absorption of advanced technology and other means, we have developed PVC pipe extrusion line, PP-R pipe extrusion line, PE water supply / gas pipe extrusion line, PVC-O pipe extrusion line, RTP pipe extrusion line, which was recommended by the Chinese Ministry of Construction to replace imported products. We have gained the title of “First-class Brand in Zhejiang Province”.

How do people typically go about selecting an extruder? Beyond analyzing your own needs, you also need to develop a thorough understanding of the supplier and of the extruder itself.

Most companies already have a fairly clear picture before purchasing a new extruder: whether they need a twin-screw or a single-screw extruder, and what materials they intend to process. Since the required throughput varies with product specifications, one can first determine the screw diameter by referring to “Screw Diameter and Product Specifications,” and then select the extruder model and size accordingly.

Once the type and model of the extruder have been determined, choosing the equipment manufacturer is another issue that deserves attention. The selection can be made from multiple perspectives, such as product quality and after-sales service.

Screw Speed

Screw speed is the most critical factor affecting the throughput of an extruder. Increasing the screw speed not only raises the conveying rate and output of the material, but more importantly enables the extruder to achieve high output while maintaining good plasticizing quality.

In the past, the main way to increase extruder output was to enlarge the screw diameter. Although a larger screw diameter does increase the amount of material delivered per unit time, an extruder is not a screw conveyor. In addition to conveying material, the screw must also compress, mix and shear the plastic in order to plasticize it. With the screw speed held constant, a screw with a large diameter and deep channels is less effective at mixing and shearing the material than a screw of smaller diameter.

Modern extruders therefore increase their throughput mainly by raising the screw speed. Conventional extruders typically ran at 60–90 rpm (revolutions per minute; hereinafter the same), whereas speeds of 100–120 rpm are now common, and high-speed extruders reach 150–180 rpm.

Raising the screw speed while keeping the diameter unchanged increases the torque exerted on the screw; when the torque reaches a certain level, the screw runs the risk of being twisted off. However, by improving the screw material and the manufacturing process, rationally designing the screw geometry, shortening the feed zone, increasing the material flow rate and reducing extrusion resistance, the torque can be reduced and the load-carrying capacity of the screw enhanced. How to design the most rational screw that maximizes the screw speed within the capacity of the screw requires a large number of trials by specialists.

Screw Geometry

Screw geometry is a major factor affecting extruder throughput. Without a rational screw design, simply raising the screw speed in an attempt to increase the extrusion output runs counter to the laws of physics and will not succeed.

High-speed, high-efficiency screws are designed on the basis of high rotational speeds. Such screws exhibit relatively poor plasticizing performance at low speeds, but as the screw speed increases, the plasticizing quality gradually improves and reaches its optimum at the design speed. At this point, the screw delivers both high throughput and acceptable plasticizing quality.

Barrel Design

Improvements in barrel design mainly concern temperature control of the feed zone and the arrangement of the feed pocket. The entire length of this separate feed zone forms a water jacket, whose temperature is controlled by an advanced electronic control system.

Whether the water jacket temperature is properly maintained is of great importance to stable operation and efficient extrusion. If the temperature is too high, the material softens prematurely, and the surface of the pellets may even melt, weakening the friction between the material and the inner wall of the barrel and thereby reducing the extrusion thrust and output. However, the temperature must not be too low. If the barrel is too cold, the resistance to screw rotation becomes excessive; when this exceeds the load capacity of the motor, the motor may be difficult to start or the screw speed may become unstable. Advanced sensors and control technology can be used to monitor and regulate the water jacket, automatically maintaining its temperature within the optimal process range.

Gear Reducer

With gear reducers of essentially identical construction, the manufacturing cost is roughly proportional to their external dimensions and weight. A larger and heavier reducer means more material consumed in manufacturing and larger bearings, which increases the manufacturing cost.

For extruders with the same screw diameter, high-speed, high-efficiency models consume more energy than conventional ones. The motor power must be doubled, and the frame size of the reducer must be increased accordingly. However, a higher screw speed implies a lower reduction ratio. For reducers of the same size, those with a lower reduction ratio have larger gear modules than those with a higher reduction ratio, and their load-carrying capacity is correspondingly greater. Therefore, the increase in the volume and weight of the reducer is not linearly proportional to the increase in motor power. If the extrusion output is used as the denominator and the weight of the reducer is divided by it, the quotient is small for a high-speed, high-efficiency extruder and large for a conventional one.

On a per-unit-output basis, the smaller motor power and reducer weight of high-speed, high-efficiency extruders mean that their machine manufacturing cost per unit output is lower than that of conventional extruders.

Motor Drive

For extruders with the same screw diameter, high-speed, high-efficiency models consume more energy than conventional ones, and the motor power must be increased accordingly. A 65 mm high-speed extruder requires a 55–75 kW motor, a 75 mm high-speed extruder requires a 90–100 kW motor, and a 90 mm high-speed extruder requires a 150–200 kW motor, which is 1–2 times the motor power fitted to conventional extruders.

During normal operation of an extruder, the motor drive train and the heating and cooling system are continuously at work. The drive components, including the motor and the gearbox, account for about 77% of the machine's total energy consumption; heating and cooling account for about 22.8% of the machine's input energy; and instrumentation and electrical equipment account for about 0.8%.

An extruder of the same screw diameter fitted with a larger motor appears to consume more electricity, but when calculated on the basis of output, high-speed, high-efficiency extruders are more energy-efficient than conventional ones. For example, a conventional 90 mm extruder with a 75 kW motor has an output of 180 kg/h and consumes 0.42 kWh to extrude one kilogram of material, whereas a high-speed, high-efficiency 90 mm extruder with a 150 kW motor has an output of 600 kg/h and consumes only 0.25 kWh per kilogram of material — only 60% of the former's specific energy consumption, a significant energy saving. Moreover, this comparison covers only the motor; if the electricity consumed by the heaters, fans and other devices on the extruder is also taken into account, the gap in energy consumption becomes even wider. Extruders with large screw diameters require larger heaters and have greater heat-dissipation areas. Therefore, of two extruders with the same output, the new high-speed, high-efficiency model has a smaller barrel and consumes less energy for heating than a conventional extruder with a large-diameter screw, saving a considerable amount of electricity in heating as well.

In terms of heater power, compared with a conventional extruder of the same screw diameter, a high-speed, high-efficiency extruder does not increase the heater power because of its higher output. This is because the electricity consumption of the heaters is concentrated mainly in the preheating stage; during normal production, the heat required to melt the material is mainly converted from the electrical energy consumed by the motor, and the duty cycle of the heaters is very low, so their electricity consumption is small. This is even more pronounced in high-speed extruders.

Before variable-frequency drive (VFD) technology became widely applied, traditional high-output extruders generally used DC motors and DC motor controllers. This was because DC motors were then generally considered to have better power characteristics and a wider speed range and to be more stable at low speeds. In addition, large-capacity VFDs were quite expensive, which also limited their application.

In recent years, VFD technology has developed rapidly. Vector-controlled VFDs have achieved sensorless control of motor speed and torque, low-frequency performance has improved considerably, and prices have fallen fairly quickly. Compared with DC motor controllers, the greatest advantage of a VFD is energy saving: its energy consumption is proportional to the motor load — the energy consumption increases when the load is heavy and decreases automatically when the load falls. The energy-saving benefit of this over long-term operation is highly significant.

Vibration Reduction

High-speed extruders are prone to vibration, and excessive vibration is highly detrimental to the normal operation of the equipment and to the service life of its components. Multiple measures must therefore be taken to reduce the vibration of the extruder and extend the service life of the equipment.

The parts of an extruder most prone to vibration are the motor shaft and the high-speed shaft of the gear reducer. First, a high-speed extruder should be equipped with high-quality motor and reducer, so that vibration of the motor rotor and of the high-speed shaft of the reducer does not become a source of vibration. Second, a well-designed drive system is required. Improving the rigidity and weight of the machine frame and the quality of machining and assembly in all respects are also important measures for reducing extruder vibration. A well-built extruder runs essentially without vibration even when it is not secured with foundation bolts, which relies on a frame with sufficient rigidity and self-weight. In addition, quality control of the machining and assembly of the various components should be strengthened: for example, the parallelism of the upper and lower surfaces of the frame and the perpendicularity of the reducer mounting surface relative to the frame surface should be controlled during machining. During assembly, the shaft heights of the motor and the reducer must be carefully measured, and the shims of the reducer strictly fitted so that the motor shaft and the input shaft of the reducer are concentric, and so that the mounting surface of the reducer remains perpendicular to the frame surface.

Instrumentation

Extrusion production operations are essentially a “black box” in which the internal conditions cannot be observed directly and can only be reflected through instrumentation. Precision, intelligent and easy-to-operate instruments therefore enable us to gain a better understanding of the internal situation and allow production to achieve the desired results faster and better.

If you need more information, Ningbo Fangli Technology Co., Ltd. welcomes you to contact for a detailed inquiry, we will provide you with professional technical guidance or equipment procurement suggestions.