Chapter 2 Extrusion Molding

2.1 Outline

Extrusion molding is also known as extrusion or plastic extrusion.In thermoplastic processing field，extrusion is a more changeable，broadly used processing approach which accounts for a large proportion in the plastics processing.The extruded products are all extendable material with the fixed cross-sections，such as a tube，a plate，a thread，a film，the coating of wire and a cable，etc.But the extrusion is limited in thermoplastic processing.Apart from profile extrusion，it can also be used for mixing，plasticization，granulation，coloring and so on.

Pipe extrusion molding technique is shown in Fig.2-1.Plastics is placed into the hopper.With the help of the rotation of the screw，the plastics enters into the feeding tube.Due to the heating of the hopper and the shear friction heat bot from the plastics itself and between plastics and the equipment，the plastics is melted and becomes the fluid state.At the same time，the melted plastics is stirred by the screw，dispersed uniformly and advanced continually.Finally，after the melted plastics is shaped by the nose body，stereotyped by molding unit and cooled，the product comes out.As long as you replace the die head and the corresponding auxiliary equipments，different products are obtainable.

Fig.2-1 Pipe extrusion molding process schematic diagram

（1—screw；2—barrel；3—die cone；4—interface sets；5—nose body；6—core-rod；7—adjusting screws；8—die；9—sizing sleeve；10—cooling tank；11—chain；12—plug；13—traction device；14—clamping device；15—plastic tubes）

From the above phenomena，extrusion process seems complicated superficially，but it is divided into two phases theoretically.The first phase is to plasticize the plastics（i.e.become the viscous fluid），and the plastics is forced through a special mouth die shaped to form a continuum whose cross section is similar to the shape of mouth die under pressure.The second phase is to make the extruded continuous body leave the plastic state and become solid by adopting a proper method，and obtain the desired product.

The plasticization of plastics can not only use heating to make plastics melted（the dry method），but also use solvent to soft plastics sufficiently（wet method），thus，according to the difference between the approaches of plasticization of plastic，the extrusion technique can be divided into dry method and wet method.For dry method，its characteristics are that the plasticizing and pressing can be conducted in the same device and the stereotype of its products can be completed only by using cooling.However，it is easy to arouse the phenomena of uneven plasticization or overheat in the process of heating due to the poor heat-transfer ring capability of the plastics.For wet method，the plastics needs to be soaked in the solvent for a long time.Even though the plasticizing is uniform and the overheating can be avoided，the plasticizing and pressing should be divided into two separate processes.Besides，the setting treatment must adopt a troublesome solvent removal process and the solvent recycling should also be taken into consideration.Based on these disadvantages，it only applies to plastic extrusion of cellulose nitrate and minority of cellulose acetate.

During the extrusion process，it can be divided into continuous method and intermittent method according to different plastic pressurizing methods.The equipment of the former way is a screw extruder；the latter is a plunger-type extruder.The types of the screw extruder includes single-screw extruder and multi-screw extruder，and the single-screw extruder is used more frequently.The main components of the plunger-type extruder are a sleeve and a hydraulically controlled plunger.When operating，firstly，we should pour a batch of plasticizing plastics into the sleeve，then extrude the plastics through extrusion die with the pressure of plunger，and remove the plunger for the next operation after squeezing the plastics.The biggest advantage of the plunger-type extruder is its ability to offer plastics a greater pressure，while its obvious disadvantage is the discontinuous operation as well as its requiring pre-plasticizing material beforehand.Thus，it has fewer applications.It is suitable for the extrusion of polytetrafluoroethylene，ultrahigh

molecular weight polyethylene and so on.

In conclusion，the majority of plastic extrusion is thermoplastic plastics which adopts the dry method to squeeze continuously.