2.2 The Basic Structure of the Single-screw Extruder

The single-screw extruder consists of one Archimedes screw rotating in the heating sleeve.The size of the single-screw extruder is usually expressed by the screw diameter and its basic structure mainly includes transmission device，charging device，the sleeve，screw，etc.，as illustrated in Fig.2-2.

2.2.1 Transmission Device

The transmission device is the part which drives the screw to rotate，and it is usually composed of the electric motor，the deceleration mechanism，and the bearing.In order to ensure the quality of products，there are several basic requirements for the transmission device.Firstly，in normal operating conditions，no matter whether the load of the screw changes or not，the rotational speed of the screw should remain unchanged.Because if the screw speed changes in the extrusion process，it will cause the pressure fluctuation of the plastic material flow.Therefore，it will be difficult to maintain the stability of the product quality.Secondly，the screw speed of the same extruder can be adjusted in order to extrude different extruded products or different plastics.To meet these requirements，it had better use infinitely variable speed transmission device.There are three ways toachieve infinitely variable speed.① Commutator motor or DC motor，which is both

a drive device and a shifting device.②Mechanical friction transmission drove by constant speed motor，such as the CVT apparatus of a gear transmission.③Motor-driven oil pump sending the oil to the hydraulic motor and changing the fuel discharge of the pump in order to change the speed of the extruder screw.

Transmission device should have a good lubrication system and a rapid braking device.

2.2.2 Feeding Device

The kinds of lastics which are used to be processed are various，such as granular plastic，powdery plastic，ribbon plastic and so on.The feeding device generally applies a hopper whose capacity is able to accommodate for materials at least 1h.A hopper should have devices used to cut off the material flow，calibrate material quantity and drop remnant.A better hopper has devices such as timing，feeding quantitatively，inherent drying or preheating.Besides，it also adopts feeding under reduced pressure，which is known as vacuum feeding device，and is particularly suitable for the hygroscopic plastics processing and powder material processing.With the improvement of extrusion equipment and technique，the supply amount of power feeding is more and more.Powder and pellets are gravity-fed from the hopper into the feeding hole，but with change of the height of the material layer，it may cause the change of the feed rate at the same time.It may produce the phenomenon of“bridging”，which may cause material shortage.A setting agitator or screw forced feeder conveyor in feeding device can overcome this disadvantage（shown in Fig.2-3）.The feeding hole can be rectangular or round but the rectangular is used more widely.Its long sides are parallel to the axis，with 1-1.5 times the length of the screw diameter and 7 °-15 ° the tilt angle on the feeding side.Around feeding hole，it should be provided with the cooling jacket（shown in Fig.2-4）to eliminate the high-temperature cylinder from transferring heat to the hopper and avoid plastics in hopper being tacky because of rising temperature，thus resulting in uneven feeding or blocked stream.

2.2.3 Sleeve

Sleeve is one of the main components of the extruder，where the process of plastic plasticizing and pressurizing are carried out.During extrusion，the pressure of cylinder is up to 55 MPa，and the operating temperature is generally 180 ℃-300 ℃，so the sleeve can be regarded as pressure and heating container.The material manufactured the sleeve should be of high strength，fastness，wear and corrosionresistance，which is generally composed of steel shell and alloy steel liner.Its outside sets the part of heating and cooling device，each with a thermocouple and automatic meter respectively.The heating methods include resistance heating and induction heating.The latter could heat better and cool more conveniently，but it is costly.

Extruder can also be heated by usable oil and steam，and it owns the advantage of uniform heating under a certain range of temperature，but it is rarely used now because of the complicated apparatus，and the limited temperature control range.There will be a risk of increasing the products contamination.

Usually the sleeve is also provided with a cooling system，whose main role is to prevent the plastics from overheating，or rapid cooling when stopping in case of resin degradation or decomposition.The sleeve is generally cooled by air or water.Some of the fins of extruder sleeve or heater aim to increase the efficiency of the air cooling.As for the cooling efficiency，cooling with cold water through copper tubes embedded in the sleeve is effective.However，it easily leads to rapid coo-ling，occurring fouling，rusting and other negative phenomena.

2.2.4 Screw

The screw is the key part of the extruder.Through its rotation，the plastics within the sleeve will move，generating a supercharger and a portion of the heat（friction heat）.Screw geometric parameters，such as diameter，aspect ratio，each segment length ratio，the spiral groove depth and so on，have a significant impact on the operating characteristics of the screw.Therefore，the basic parameters of the screw and its role will be introduced briefly in the following passage.The screw structure is shown in Figure 2-5.

1.Screw Diameter（D）and Aspect Ratio（L/D）

Screw diameter is one of the basic parameters of the screw，commonly used to represent the size of the extrusion.Users should select screw diameter according to the shape and size of the product and other requirements of productivity.In addition，other parameters such as the screw length，the depth of groove and the width of the screw flight are closely related to the diameter，which are mostly represented by diameter ratio.

Another important parameter to characterize screw is the ratio of effective length（L）and diameter，i.e.the aspect ratio （L/D）.If the screw is only taken as the way of materials transportation，the aspect ratio of the screw mainly determines its volumetric capacity.Besides，the aspect ratio also affects the rate of heat transition from the sleeve wall to the material，thus affects the heat generated by shearing the energy input and the ratio of the quantity of power and the extrusion amount.Therefore，increasing the aspect ratio can prolong the residence time of the plastics on the screw to plasticize more uniformly and can accelerate the screw speed to increase the extrusion amount.Currently，the aspect ratio has been increased，but a too long screw will bring some difficulties for manufacturing and assembling.Generally，aspect ratio is around 25.

2.Functions of Each Segments of Screw

According to the operation of the plastic on the screw，it can be divided into feeding segment，compression and metering segment.The general screw，sometimes referred as a standard screw or a metering type screw，has a thread angle of 17.6°，and its screw pitch equals to the diameter of the screw.Each segment has different functions.

The feeding section refers to a distance extending from the plastics inlet to forward，and it is about 4-8 times the length of the screw diameter.In this section，the plastics is still solid.The main function of it is to grab material from the hopper and transfer it to the compression section and to heat the material at the same time.Because of the low density of the material，the spiral groove is deep，and the screw in the feed section（h1）is 0.10-0.15 times the length of the screw diameter，in addition，in order to create the best transport condition for the plastics，it requires to reduce the friction between material and the screw and increase the tangential friction between the material and the sleeve.In this case，the surface of the sleeve which is in contact with the plastics should open longitudinal groove（see Fig.2-5），and increase the smooth extent of screw surface and cool the center of the screw with water.

Compression section（transitional section）is the middle section of screw.Apart from heating and forwarding，plastics is gradually compacted and softened from granular solids to continuous melts as well as discharges entrained air to the feeding section.Accordingly，the depth of spiral groove is gradually reduced to the screw spiral groove depth（h）in the metering section，which is not only conducive to the quality of the products，but also the warming and the melting of the materials.Typically，the ratio between the volume of a spiral groove in feeding section and the volume of a spiral groove in metering section is called screw compression ratio. For this constant pitch screw，the compression ratio can be expressed by（h1/h2），whose value is 2-4.Generally，it depends on the types of plastics，the aggregation state when it is fed and the shape of extrusion products.

Metering section （homogenization section）is the last section of the screw，it is of 6 to 10 times of the length of the screw diameter.It is called metering section，because the function of it is to plasticize melt evenly and extrude material flow from the flow channel of the nose and mouth die quantitatively and in constant pressure.The depth of this spiral groove is relatively shallow，with the depth of 0.02 to 0.06 times of the screw diameter.

3.Helix Angle of the Screw and the Screw Flight Width（e）

The size of helix angle is relevant to the shape of the materials.Different material shapes require different helix angles of the feed zone.It can be proved that a spiral angle of 30° is suitable for fine powder plastics both in theory and experiment；a spiral angle of 15° is suitable for box materials；a spiral angle of 17.6° is suitable for ball and columnar materials.In the metering section，according to the formula derivation，a helix angle of 30° produces a maximum yield.However，considering the screw manufacture，it is easy to process when the pitch is equal to the diameter and the helix angle is 17.6°.Besides，it has slight effect on the yield，and the spiral direction of screw is generally right.

Screw flight is generally 0.08-0.12 times of the width of the screw diameter，but the bottom of the spiral groove is wider and its root uses arc transition.

4.Shape of the Screw Head

The screw head is usually a blunt tip cone［shown in Fig.2-7（a），（b）］to avoid decomposition of the materials because of long stagnant in the screw head.If it is a axial displacement screw，the screw head can also play the role of pressure adjustment.The screw head can also be torpedo-shaped［shown in Fig.2-7（c）］，and called torpedo head or leveling head.The gap between leveling head and the sleeve is usually smaller than the spiral groove depth，whose surface can be opened to trench or rolled into special patterns.It benefits the mixing and heating of plastics and it is conducive to increase the pressure of the feeding stream and to eliminate pulsation phenomenon，commonly used to squeeze plastics of a large extrusion viscosity and poor thermal conductivity or plastics with an apparent melting point.