PlasticsExtrusion molding

PVC pipe

Gas pipe

Straw

Ink tube

Profiles

Extrusion Molding

• It is process to make parts of continuous length and uniform cross-section.

• A characteristic that often differentiates extruded from injection-molded

plastics is the viscosity of the plastic at normal processing temperatures.

• Extruded plastics often have a higher melt viscosity, which allows the

extrudate to retain the shape imparted to it by the die while the extrudate is

in the quenching stages.

Tubes

Extrusion Molding

• The machine used to extrude materials is very similar to an injection

molding machine.

• A motor turns a screw which feeds granules of plastic through a heater.

• The granules melt into a liquid which is forced through a die, forming a

long 'tube like' shape.

• The shape of the die determines the shape of the tube.

Hoses Wire coating

Extrusion Molding

• The extrusion is then cooled and forms a solid shape.

• The tube may be printed upon, and cut at equal intervals.

• The pieces may be rolled for storage or packed together.

• Shapes that can result from extrusion include T-sections, U-sections,

square sections, I-sections, L-sections and circular sections.

StripsRods

Extrusion Molding

Extrusion Molding

Extrusion Molding

• Screw extruders working on then same principle as the ordinary domestic mincing machines are used.

• A plastic extruder consists essentially of a barrel or cylinder fitted with a screw.

• The die is fitted into the extruder head at the front of the machine; the feed hole is at the back end.

Curtain tracks

Tape

Extrusion Molding

• The extruder head and barrel are heated and with the screw rotating, plastic material is fed in, carried along the barrel by the screw and forced through the die in the extruder head.

• A screw extruder requires a powerful motor to drive the screw and a strong thrust bearing to take up the back pressure.

• The screw is usually canalled so that water or oil can be passed through it for the purpose of temperature control.

Shopping bagsAngle sections

Extrusion Molding

Extrusion Molding

Extrusion Molding

• Screw extruders usually have three or four heat zones along the length of the barrel each of, which can be separately controlled at predetermined temperatures.

• Heating is affected by means of electric resistance heaters on the barrel and nozzles, or by passing steam or hot oil through ports built into the machine.

• Electric heating is most usual.• Different materials require different temperature settings at the heat zones

and for a given material the temperature settings vary from one type of machine to another.

• The following list gives an indication of the temperature ranges from feed hopper to die employed with the important thermoplastic materials.

Extrusion Molding

• Cellulose Acetate (Plasticised) 21oC ___ 204 oC• Cellulose Acetate Butyrate (Plasticised) 21 oC ___ 204 oC• P.V.C. (Unplasticised) 168 oC ____ 191 oC• P.V.C. (Plasticised) 152 oC ____ 177 oC• Polyethylene 135 oC ____ 190 oC• Nylon 190 oC ____ 290 oC

Sheets

Extrusion Molding

Extrusion Molding

Extrusion Molding

Extrusion Molding

Pipes/Tubes:• The die for producing a tube or pipe consists of a circular orifice with a

carefully centered mandrel or torpedo, which is held, in the die orifice by, supports at the back end.

• Generally the mandrel is constructed so that compressed air can be blown through it into the hot extorted tube with the object of preventing the tube from collapsing.

Strings

Extrusion Molding

Extrusion Molding

• At the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the melt.

• The screens are reinforced by a breaker plate (a thick metal puck with many holes drilled through it) since the pressure at this point can exceed 5000 psi (34 MPa).

Screenpacks

Extrusion Molding

• The screen pack/breaker plate assembly also serves to create back pressure in the barrel.

• Back pressure is required for uniform melting and proper mixing of the polymer, and how much pressure is generated can be 'tweaked' by varying screen pack composition (the number of screens, their wire weave size, and other parameters).

Breaker plates

Extrusion Molding

Wires:• The coating of wire with thermoplastic material is carried out by using a

special die, which extrudes the plastic material at right angles to the screw direction.

• Wire is continually fed through the die from which it energies evenly coated with the plastic material.

Film tape

Extrusion Molding