the theory and practice of extruder system purging · tappi place conference 2007 17 purging...

1

2007 PLACE Conference

September 16-20St Louis, MO

The Theory and Practiceof Extruder System Purging

Presented by:John D. VansantDuPont Packaging

Tappi Place Conference 2007

2

What Worry about Purging?

•Effective transitions / Set-ups•Cleaning out the gels•Proper shut down•Preventing corrosion•Coming in quickly on specification•Avoiding heat seal problems•Good manufacturing practice


3

Velocity Profile - Flow Through a Pipe

* Shear Stress (force) increases with Shear Rate

Low Shear High Shear


4

Purging Theory – Velocity and Shear Rate

•High velocity / RPM’s•Shear rate needed to move material away at the metal surfaces


5

Effect of Viscosity on Purging

ηA>>ηBηA<<ηB AB B AB B

Don’t make large steps down in viscosity

Low shear

Ineffective Purge Effective Purge


6

Purging Theory – Consider Viscosity

•High viscosity = low Melt Index•Large steps up in Melt Index = slow purging


7

Melt Viscosity vs. Temperature

temperature

log

mel

t vis

cosi

ty

IonomerAcid CopolymerLDPE

190°

C


8

Purging Theory – Temperature & Viscosity

•Consider how the viscosity of different materials responds to temperature changes

• Some materials decrease in viscosity faster with temperature

• Use this to your advantage, not disadvantage

•Use temperature differentials to aid in purging

• Cool the barrel

• Keep the rest hot


9

Effect of Shear Rate

log shear rate

Short-chain branched

Linear

Long-chainbranched

Constant Mw

log

mel

tvis

cosi

ty


10

Purging Theory – Shear Thinning

•Linear polyethylenes don’t shear thin as much as LDPE•Purging into linear resins – high RPM’s•Purging out of linear resins – moderate RPM’s


11

Dead Spots


12

Purging Theory – Dead Spots

•Circulation dead spots can occur at higher throughput (RPM)•Low throughput doesn’t move cross-linked material out of dead spots (not enough shear)•The best throughput rate may be somewhere in the middle, but hard to predict


13

Purging Theory – Resin Compatibility

•Resins that are more compatible (similar) purge more easily•Resins that adhere to one another purge more easily

Note: Gels can not be effectively removed without adhesion. To accomplish this, short (5 minute) periods of very low (~10) RPM’s should be incorporated into the purging process.


14

Purging Theory – Resin Compatibility

Some resins should not be directly mixed:•EVOH & Nylon 6•EVA and Acid Copolymers (>230°C)•EVOH/Nylon and tie resins


15

Purging Theory – Resin Stability

Some resins crosslink (form gel) if run too hot

•EVA

•EAA

•EMAA

•EVOH

•Even PE (from oxidation)


16

Purging Theory – Purge Compounds

•Typically have very high viscosity to push hard (high shear rate / shear force)•Often contain abrasive materials to scour die surfaces, etc.•May contain blowing agents – foaming helps to reach dead zones such as:Breaker plates

Valved adapters

Coextrusion elbows

Remember that you must purge out the purge compound


17

Purging Practice

You must run a wide range of RPM’sNever just “let it drool” – be active in purgingPurge compounds can help clean up a very dirty

extruder, but it takes time to purge out the purge compounds

We prefer the Disco Purge Procedure as the standard transition purge

Rule of thumb: Drop the temperature before purging into a copolymer resin, and purge out completely before increasing the temperature


18

Disco Purge Method

0102030405060708090

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Minute

% o

f Max

imum

RPM


19

Disco Purge Procedure*MAXIMUM EXTRUDER RPM = __ RPM

1st min: 30% of max = 0.3 × __ RPM = __ RPM

2nd min:90% of max = 0.9 × __ RPM = __ RPM

3rd min: 50% of max = 0.5 × __ RPM = __ RPM

4th min: 15% of max = 0.15 × __ RPM = __ RPM

5th min: 70% of max = 0.7 × __ RPM = __ RPM

6-10 min’s: 10% of max = 0.10 × __ RPM = __ RPM

(small extruders 20% of max = 0.20 × __ RPM = __ RPM )

minutes 11-15: Repeat the cycling steps of the first 5 min.

Continue with additional 10 minute cycles until the web looks clean at all RPM’s

developed by Al Soutar


20

Disco Purge Method

Varies the shear rate to change flow patterns

Slow periods allow resins to bond together

Slow periods interrupt flow circulation

Often effective without purge compounds

Minimizes time and resin over “let it run”

Does not scour tough die deposits

Not always effective in removing tough gels


21

The information herein is intended for guidance only. While believed reliable, DuPont neither warrants or guarantees its completeness, efficacy or safety in use. It is intended for use only by competent personnel. The user accepts full responsibility for its use and fully indemnifies and holds DuPont harmless from any loss, claim, damage, or injury arising out of its use regardless of cause, unless user shows such to have been caused directly by ill will or willful misconduct of DuPont. Also, no license is granted or implied, and no recommendation to infringe any patent is made.

22

Thank YouPRESENTED BY

John D. VansantDuPont [email protected]

Please remember to turn in your evaluation sheet...

the theory and practice of extruder system purging · tappi place conference 2007 17 purging...

Documents