1

Product Application & Research CentreMumbai

EXTRUSION PRINCIPLES

Introduction

Extrusion of thermoplastics is a process in which the material is melted by external heat /frictional heat and conveyed forward by a screw to the opening of the die, which givesthe shape of the required product. Extrusion process is a continuous process by whichmany products like Films, Raffia tapes, Pipes, Sheets, Mono filaments, Fibre andFilaments can be manufactured.

Extruders

Extruders comprise of Hopper, Barrel/Screw and Dies. Figure 1 shows the componentsof a modern extruder

Hopper

All the extruders have an opening in the barrel wall at the driven end, through which theplastic granules enter the extruder. The hopper, a simple sheet - metal enclosure, ismounted above that opening and holds about an hour's capacity material. Hopper isprovided with heating system, if the material has to be preheated before entering theextruder.

3

Screw

This is the heart of the extruder. Screw conveys the molten polymer to the opening of thedie after properly homogenising the molten polymer. Figure 2 illustrates a typical screwconfiguration.

There is considerable variation in the design of the screw for various materials, the mostimportant variable being the depth of the channels. Despite much desire for universalscrew, it is advisable to use a different design for each material to achieve the best results.So for optimum process of PVC, PP/PE following screw designs are advised.

PVC Screws

Since PVC is thermally unstable material, the screw (Fig 3) for processing this materialhas deeper channels, little or no metering, hard chrome plated, conical nose to preventmaterial hangup. Screw diameters vary from 30mm to 140mm, L/D ratios vary from 18to 22 for single screw and from 16 to 18 for twin screw extruders. Compression ratiosare between 1.5 to 2.2:1 for twin screw / single screw extruders. Venting unit is presenton the extruder to remove any volatiles.

4

PE/PP Screws

PE/PP screws (Fig. 4) are more or less similar. The screws are designed to have shallowchannels, sudden compression and long metering zones. Screw L/D varies from 24:1 to33:1 and the diameters from 20 mm to 250 mm and the compression ratios from 2.5 to3.1

Barrier - Type Screws

In some cases, even an optimally designed screw is not capable of completing the meltingprocess. In such cases, the extrudate will contain unmelted plastic. This can beprevented by building a barrier or secondary flight in the channel (Fig. 5). The barrierflight is undercut and permits the passage of only fully molten plastic. The design of thebarrier screw ensures complete melting of the polymer solids.

There are different designs of barrier screws now available i.e. Maillefer / Uniroyal,Hartig, Barr I and II etc, each with its own characteristics.

5

Mixing Heads

The metering section of a standard screw is not a good mixer. Smooth laminar flowpatterns are established in the channel which do not mix dissimilar elements in the melt.Mixing devices are frequently installed in screws to distrupt these flow patterns andimprove melt homogenisation.

Pin mixers (Fig. 6) / Du Pontmixer are the examples ofmixers which operates with lowshears. They operate byinterrupting and recombining theflow many times. They can beinstalled easily on existingscrews to improve theperformance.

Other type of mixers are Maddock(Union Carbide) (Fig. 7) and Egan(Fig. 8). These mixers operatesubjecting the melt to high shearingforces. Maddock screw operates likea barrier type of screw, it separatesout any unmolten particles andhomogenise the melt. Egan mixeralso operates like a barrier screw,helical splines contribute to forwardflow and helps in homogenising themelt.

6

Breaker Plate / Screen Pack

Breaker plate with screen packs (Fig. 9) inserted are kept in the adapter which is a jointbetween the end of the extruder and the beginning of the die. This assembly has severalfunctions

• Arrest the rotational flow of the melt and convert into axial flow• Improves melt homogenity by splitting and recombining the flow• Improves mixing by increasing back pressure• Removes any contaminants and unmelt

Screen packs are made up of series of screens of different mesh, with the coarest screenplaced against the breaker plate to support the finer screens.

Dies

Various types of dies are used for processing PVC and PE/PP. They can be eitherAnnular dies or Flat dies (T-Die or Coat hanger dies). Details of various dies are given inAnneure - 1

Annular Dies

PVC Dies

PVC is thermally unstable, so itrequires perfect flow channels. Aspider mandrel (Fig. 10) die is usedwhich helps in dividing the melt andfulfils the task of betterhomogenisation and allows the melt toflow smoothly.

7

PE/PP Dies

For processing PE/PP a die with spirals areused as illustrated in Fig. 11. As the plasticflows from the entry point it spirals aroundthe mandrel section of the die. From theFig. 10, it is clear that the land depthbetween the spiral section and the wallincreases as the material progresses throughthe die. As a result, the distribution aroundthe die periphery is made uniform in order tocontrol the gauge of the extruded tube.

Flat Dies

There are two types of Flat Dies. One is T-Type of flat die (Fig. 12) and the second oneis Coat - Hanger type of die (Fig. 13). In coat hanger type of die, the design is oftriangular preland section, which gives balanced pressure leading to uniform flow of thematerial. This minimizes the adjustment required to obtain uniform sheet thickness.

8

Auxiliary Equipment

Once the product in the molten form (Films, Sheets, Pipes, Fiber setc., ) comes out of thedie, it is cooled by chilled water/air/rollers etc., and wound/cut by various means. Insome process like BOPP, Raffia tapes, Fibre etc. after cooling initially they are reheatedby passing them through ovens and stretched to obtain required mechanical and opticalproperties. Various down stream equipment used in different process are given briefly inthe following table.

Product Process Quenching Stretching Stabilisation

Winding /Cutting

BOPP Tenter frame Chilled roll/Chilledwater

MDO/TDOHot air Oven

Oven Rolls

DoubleBubble

Chill water Nip rollersand inflatingthe bubble

Air Rolls

TQPP Invertedbubble

Chill water Nip rollersand inflatingthe bubble

Air Rolls

Cast Flat film Chill rolls Rollers Air RollsRaffia Flat film

then cut intotapes

Water Goddetrollers andHot air oven/ hot plate

Oven Bobbins

F&F Spinning Chill air Goddets andhot plate /hot rollers

Air Bobbins

Sheet Flat Rollers -- Air Rolls / Cutleng.

Pipes -- Vacuumsizing andchill water

-- Water Cut leng.

9

ANNEXURE 1

VARIOUS DIES WITH DIFFERENT DIE GAPS FOR PP/PE/PVC APPLICATIONS

SECTOR POLYMER APPLICATION TYPE OF DIE DIE GAP(MM)

REMARKS

Film Polypropylene BOPP - Flat Coat hanger 2 - 2.5 - Blown Spiral mandrel 1.75 - 2 Double BubbleTQPP - Blown Spiral mandrel 1 - 1.5 Downward ExtrusionCast - Flat Coat hanger 0.8 - 1.2

Polyethylene LLDPE - Blown Spiral mandrel 1.8 - 2.5 - Cast Coat hanger 1.5 - 1.8LDPE - Blown Spiral mandrel 0.8 - 1HDPE - Blown Spiral mandrel 0.8 - 1

PVC PVC - Blown Spider mandrel 0.8 - 1Raffia Polypropylene PP - Flat Coat hanger 0.6 - 0.7 Die widths can be adjusted

with decklesPolyethylene PE - Flat Coat hanger 0.7 - 0.8 -do-

Extrusion Coating Polypropylene PP - Flat Coat hanger 0.5 - 0.6 -do-Polyethylene PE - Flat Coat hanger 0.6 - 0.7 -do-

Sheet Polypropylene Sheet - Flat Coat hanger 0.8 - 10 Die manifolds can be changedand make sheets from 0.750mm to 8 mm thick sheets

Coat hanger 0.3 - 1 For making thinner gaugesheets / films

Pipes Polyethylene/Polypropylene

Small bore tubings /Pipes

Spiral mandrel 0.25 - >5 Depending on the class of thepipe thickness varies

PVC Pipes Spider mandrel >1 Depending on the class of thepipe thickness varies

10

11