In the extrusion process,polymer is propelled continuously along a screw through region of high

temperature and pressure where it is melted and compacted ,and finally forced through a die shaped  to

give the final object .

Extrusion is used to make three main types of products:

(1) Standard profile shapes such as rod,pipe,and irregular cross sections.

(2) Extrusions around wire and cable as a protective coating.

(3) Film to be used alone and as coating for paper, cloth,and other surfaces.

Each of these types can be made in varying sizes depending upon the size of the machine. Extruders are

designated as to size by the diameter of the rotating screw.They range from large industrial extruders

with screw diameters up to 6 in. or 8 in.,down to the laboratory model with a 3/4 in.diameter

screw.Extrusion is used only for the processing of thermoplastic resins.

The screw of an extruder is divided into several sections.each with a specific purpose(Chung 1977).The

feed section picks up the finely divided polymer from a hopper and propels it into the main part of the

extruder . In the compression section,the loosely packed feed is compacted,melted,and formed into a

continuous stream of molten plastic.Some external heat must be applied,But much is generated by

friction.The metering section contributes to uniform flow rate,required to produce uniform dimensions in

the finished product, and builds up sufficient pressure in the polymer melt to force the plastic through the

rest of the extruder and out of the die.Since viscous polymer melts can be mixed only by the application

of shearing forces(their viscosity is too high to allow turbulence or diffusion to contribute appreciably to

mixing),an additional working section may be needed before the die.

Modern trends in extruder usage include the twin-screw or multiple-screw extruder,in which two screws

side by side to opposite direction,providing more working of the melt,and the vented extruder, having an

opening or vent at some point along the screw that can be opened or led to vacuum to extract volatiles

from the polymer melt.

1.Coextrusion

Films or sheets consisting of layers of two or more different polymers can be produced by mixing the

molten streams from a like number of extruders in a multimanifold die .This process can be used to

combine in a sight polymer .For example,high strength ,low oxygen permeability,and heat sealability.

2.Film Extrusion

The extrusion of film can be carried out either by casting as in sheet extrusion (see below) or by the

Blown-film process.In the latter a tubular die is used,From which a hollow tuble of product is extruded

vertically upward toward a film tower. The tube is blown into a thin cylindrical film by air introduced

through the die and trapped in the film bubble .At the top of the tower,The bubble (by now cool) is

collapsed ,and subsequently slit into a flat film.

3.Pultrusion

In this variation of extrusion ,material is pulled rather than pushed through a die or mold. The process is

used to produce continuous lengths of fiber-rein-forced thermosetting resin. The reinforcing materials are

continuously pulled through a bath of liquid resin that saturates each individual fiber. Excess resin is

removed by a forming guide,and the mix is then pulled through a heated mold that shapes and cures it .

4.Other Extrusion Processes

In sheet extrusion, the product from a sheet die is passed between rolls that control its thickness and

apply a desired surface finish by polishing and embossing , Wire and cable extrusion employs dies

through which bare copper or aluminum wire is passed. And in which it is coated with plastic

insulation .Extrusion rates can be as high as 8000 ft /min.


2017年02月15日

How Surface Treatments Keep Molds Operating Longer
Co-injection molding

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