Troubleshooting Single-Screw

Extrusion – Top 10 List

Mark A. Spalding

The Dow Chemical Company, Midland, MI

Gregory A. Campbell

Castle Research, Jonesport, ME

Goals Provide a list of practices and skills

that every single-screw extrusion

engineer should know.

Top Ten

List

1. Know how to calculate the expected

rate for a single-screw extruder.

Calculation of the rotational and pressure flows in the

metering channel.

The metering channel controls the rate.

Calculation routines are available from SPE or you can easily

construct a spreadsheet.

Estimate the pressure profile.

Campbell, G.A. and Spalding, M.A., “Analyzing and Troubleshooting Single-Screw Extruders,”

Hanser Publications, Munich, 2013.

Hopper

Discharge

Cross-sectional

view of the extrudate

Meter

1. Know how to calculate the expected

rate for a single-screw extruder.

0

4

8

12

16

20

0 4 8 12 16 20 24

Axial Position, diameters

Press

ure, M

Pa

Solids

ConveyingMelting Metering

Proper operation

Improper operation

771 kg/h

544 kg/h

152 mm diameter extruder

100 rpm screw speed

2. Channels that are designed to

operate full must be pressurized.

Channels that are only partially filled operate at

zero pressure.

Partially filled channels create stagnation zones.

Stagnation zones will cause some resin to

degrade.

Degradation products will eventually contaminate

the final product.

2. Channels that are designed to

operate full must be pressurized.

Flight TipsStagnant and

Degraded Resin

Resin Flow

RegionFlow Direction

Film Sample

Hyun, K.S., Spalding, M.A., and Powers, J., "Elimination of a Restriction at the Entrance of Barrier

Flighted Extruder Screw Sections," SPE-ANTEC Tech. Papers, 41, 293 (1995).

3. The flight radii should be between

0.5 and 2.5 times the channel depth.

Small flight radii create a stagnant region

between the flight edge and the screw root.

The stagnant region will cause the resin to

degrade and result in degradation products

in the final product. Barrel Interface

Screw Root

Degradation

3. The flight radii should be between

0.5 and 2.5 times the channel depth.

The degradation at the flight radii were caused by low

flow or stagnant regions due to Moffat eddies.

Aspect ratio=2, Top wall moving

x

y

Moffat eddy Enlarged

0.8 0.9 1.00.0

0.1

0.2

y

x

3. The flight radii should be between

0.5 and 2.5 times the channel depth.

Injection

Molded Part

Flight radii design.

R2H

R1

0.5 < < 2.5RH

Mitigates Moffat Eddies

4. Melting of the resin is the primary

method for mixing.

The best mixing occurs in the melt film between

the solid bed and the barrel wall.

The shear stress is very high in the melt film.

A secondary mixing section is generally needed

for most applications. Melt film

Melt pool Barrel side

Screw side Solid bed

4. Melting of the resin is the primary

method for mixing.

100 parts white ABS to

1 part black ABS.

Single-flighted screw.

Benkreira, H., Shales, R.W., and Edwards, M.F., “Mixing on Melting in Single-Screw Extrusion,” Int. Polym.

Process., 7, 126 (1992).

Campbell, G.A. and Spalding, M.A., “Analyzing and Troubleshooting Single-Screw Extruders,” Hanser

Publications, Munich, 2013.

5. All screws will discharge solid resin

if the screw is rotated fast enough.

As a screw is rotated faster, a speed will be reached

where solid resin is discharged with the extrudate.

Solids in the extrudate can look like a poorly

mixed system.

A secondary mixer or a solids trap is needed for

most applications.

5. All screws will discharge solid resin

if the screw is rotated fast enough.

Hopper

Discharge

Cross-sectional

view of the extrudateExtrude mixture with 99

parts white pellets with 1

part black pellets.

View extrudate sections.

5. All screws will discharge solid resin

if the screw is rotated fast enough.

Mixing Flight

Clearance, (u + )

In Flute

Out Flute

Mixing

Flight

Wiper

Flight

Flow

a) axial mixer

Wiper

Flight

Mixing

Flight

b) mixer cross section

Flow

Maddock-style mixers are

excellent secondary mixers

for trapping and dispersing

solid polymer fragments.

6. High-performance screws use

specially designed channels to trap

and melt solids.

High-performance screws can operate at higher

screw speeds, higher rates, and lower discharge

temperatures as compared to a conventional

screw with a mixer.

High-performance screws have deeper metering

channels.

Several different types of high-performance

screws are available.

6. High-performance screws use

specially designed channels to trap

and melt solids.

Common commercially available high-performance screws that

employ this technology include:

Wave screws.

Energy Transfer (ET) screws.

Fusion screws.

DM2 screws.

Campbell, G.A. and Spalding, M.A., “Analyzing and Troubleshooting Single-Screw Extruders,”

Hanser Publications, Munich, 2013.

6. High-performance screws use

specially designed channels to trap

and melt solids.

Feed Transition ET Section

The Energy Transfer (ET) screw is constructed by

positioning an ET section in the metering section of a

conventional screw.

6. High-performance screws use

specially designed channels to trap

and melt solids.

A B A

B

A

B

A

B

A

B

Flow

Flight Undercuts

Channel Peaksa) ET Section

b) Channel Cross

Section

Barrel

Screw

Vbx

Vbz

Barrel

Screw

Barrel

Screw

Vbx

Vbz

6. High-performance screws use

specially designed channels to trap

and melt solids.

30 rpm

33 kg/h

60 rpm

62 kg/h

90 rpm

85 kg/h

120 rpm

110 kg/h

150 rpm

135 kg/h

30 rpm

33 kg/h

60 rpm

62 kg/h

90 rpm

85 kg/h

120 rpm

110 kg/h

150 rpm

135 kg/h

ET

Screw

Conventional

Screw

7. Injection molding screws use the

same design principles as screws

for extruders.

All channels must operate full and under

pressure.

Operational rate is calculated using the mass of

the parts and runner system, the plasticating

time, and the screw speed.

Rotational and pressure flow rates are calculated just

like the procedure for extruders.

8. Flow surging is most often caused

by a temperature control problem in

the feed section.

Proper solids conveying occurs with specific

temperatures at the barrel wall and screw surfaces.

Forwarding forces at the barrel wall must be

maximized and the retarding forces at the screw

must be minimized.

Forces depend on temperature.

Other root causes downstream of solids conveying are known

to cause flow surging.

8. Flow surging is most often caused

by a temperature control problem in

the feed section.

203.2 mm

diameter

extruder.

HIPS resin

Two-stage

screw.

0

5

10

15

20

25

0 200 400 600 800 1000

Time, minutes

Pre

ssu

re,

MP

a

0

200

400

600

800

1000

1200

1400

Mo

tor

Cu

rren

t,

A

CurrentCurrent

P1

P1

P2

Extruder Discharge

Pump Inlet

8. Flow surging is most often caused

by a temperature control problem in

the feed section.

Instability was

caused by a high

temperature at the

screw surface.

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50 60

Time, minutes

Mo

tor

Cu

rren

t,

A

80

120

160

200

240

280

320

360

Scr

ew

Sp

eed

, r

pm

Screw Speed

Motor Current

Cooling On CoolingOff

Cooling On

9. The first-stage metering section of a

two-stage screw must control the rate.

Two-stage extruders typically operate at a rate that is 1.1 to

1.3 times the rotational flow rate of the first-stage meter.

A negative pressure profile exists in the first-stage

meter.

Vent flow will occur if the second stage limits rate.

Vent flow can also occur if the vent diverter is not

designed properly.

9. The first-stage metering section of a

two-stage screw must control the rate.

0

4

8

12

16

0 6 12 18 24 30 36

Press

ure, M

Pa

Axial Position, diameters

Vent Opening

Proper operation,

no vent flow.

9. The first-stage metering section of a

two-stage screw must control the rate.

0

4

8

12

16

0 6 12 18 24 30 36

Press

ure, M

Pa

Axial Position, diameters

Vent Opening

Proper operation,

no vent flow.

Vent Flow

9. The first-stage metering section of a

two-stage screw must control the rate.

A vent diverter is positioned in the vent opening to tuck

molten resin back into the screw channel.

If the diverter is not installed, installed improperly, or

not designed properly, then flow of resin out the vent

opening is likely.

9. The first-stage metering section of a

two-stage screw must control the rate.

Vent diverter Recessed area

a) b)

This edge is flush with the inside barrel wall

9. The first-stage metering section of a

two-stage screw must control the rate.

Vent diverter

Barrel

Vent

Diverter

Vent

Opening

Screw

10. The first time a screw is installed

into an extruder, both the screw and

barrel should be at room temperature.

If the screw has the correct outside diameter and

it is not bent, then it should slide easily into the

barrel.

Never force a screw into a barrel.

Never install for the first time a cold screw into a

hot barrel – the hot barrel is oversize (thermal

expansion) – the screw may slide in easily, but

could expand to bind with the barrel.

Summary A list of ten top practices and skills

were presented.

Single-screw extrusion engineers

should be aware of these practices

and skills.

Campbell, G.A. and Spalding, M.A., “Analyzing and Troubleshooting Single-

Screw Extruders,” Hanser Publications, Munich, 2013.