Troubleshooting

Tracking down the source(s) of problems in extrusion systems can be a difficult task.  This page is meant to be a starting point for places to look in the process that may have issues.

Before going to the effort of looking for difficult to confirm causes, first look at the base system itself.  Is something in the system running incorrectly?
Check:

• Mixing system
• Does the scaling system consistently deliver the correct weight of ingredients (if manual, what sort of check system is there for inventory?)
• Is the mixer filled to the level the manufacturer recommends?  (neither too full nor too empty)
• Has any sort of analysis ever been performed to ensure the mixing time is suitable?
• Does the mixing system leak?  (if the first ingredient in leaks out the bottom, problems will result)
• Feeder
• If volumetric:
• Does the screw turn at the RPM setpoint? Or if on a potentiometer, does it run at a steady rate if the setting is unchanged?
• Is there a substantial difference in feed rate when the feeder is full versus when it is empty?  
• If this is a problem, try to keep the feeder at a consistent level of fill.  Frequently, keeping the feeder as full as possible will give the most stable feed rate.
• If loss-in-weight:
• Does the system deliver the setpoint consistently?
• During refill (when the system has to go into volumetric mode), is there a spike in feedrate when the feeder comes out of volumetric mode?
• If yes, try increasing the setpoint where the feeder will call for a refill.  This problem is similar to the volumetric refill problem above.
• Preconditioner (if present)
• In-feed
• Is there an issue with build-up on the feed screw.  If so, clean the screw and address the cause.  This is likely to be too much steam in the preconditioner.
• Mixing paddles
• If they are adjustable, are they adjusted properly?
• In general, the first third of the preconditioner paddles should be forward conveying, the middle third should be neutral, the final third should be reverse conveying.  This may vary by location or manufacturer.
• Water injection
• At the typical water addition rate, is there a good spray fan going into the preconditioner?
• If the water is not well distributed in the preconditioner, the system will tend to run poorly.  The spray nozzle may need to be cleaned, replaced, or a nozzle of a suitable size may need to be installed.
• Does the injection system deliver to the setpoint?  Or if the system is volumetric, does it deliver a consistent feed rate over time?
• Steam injection (if present)
• Is a consistent steam quality being delivered?
• This is rather difficult to measure.  One indirect measure of how the system is performing is to monitor the preconditioner exit temperature.  If it is unstable, there is at least one problem with the system.
• To try to deliver consistent quality: insulate the steam line and install a steam trap just upstream of injection to the preconditioner.
• Note: it also has to be upstream of the mass flow meter, if present (otherwise expelled condensate will be counted as part of the mass flow into the preconditioner.  
• Is a consistent feed rate being delivered?
• Preconditioner speed
• Is it consistent?
• Blend exiting the preconditioner
• The largest agglomeration should be about the size of a pea.  If the particles are larger,  it may be an indication that there is too much water (and steam) or poorly distributed water in the preconditioner.
• Ideally, the blend should be wet enough to compress and hold it's shape if squeezed by hand, but dry enough that it will break back apart easily if tapped by a finger.
• If steam is used, keeping the system at or above 140 F (60 C) will generally be enough to prevent microbial growth.
• Extruder
• Water injection
• Does the injection system deliver to the setpoint?  Or if the system is volumetric, does it deliver a consistent feed rate over time?  
• Steam injection 
• Is a consistent steam quality being delivered?
• This is rather difficult to measure, there is no easy indirect measure as above.
• To try to deliver consistent quality: insulate the steam line and install a steam trap just upstream of injection to the preconditioner.
• Note: it also has to be upstream of the mass flow meter, if present (otherwise expelled condensate will be counted as part of the mass flow into the preconditioner.  
• Is a consistent feed rate being delivered?
• Screws
• Do the screws turn at a consistent speed when the control system is set to a constant value?
• If not, the issue is deeper than will be covered in the web page.
• Screw wear
• Are screw or barrel parts excessively worn?  Refer to manufacturer standards.
• Have the screws and or barrel recently replaced?  If so the extruder may perform slightly less work on the extrudate.  It may be necessary to add some paddles or other work element to make up for the extruder having become a more efficient pump.
• Barrel
• Are barrel temperature setpoints reasonable?  Is the system trying to maintain the temperature the extruder would run at without active heating or cooling?
• If the answer is "no" consider either changing the setpoints or changing the amount of work in the screw.
• Is barrel heating and cooling functional?
• The plumbing could be plugged (fluid) or the heating elements (electrical) could be unplugged or burned out.
• Die
• Is wear on the die at a reasonable level?  
• If the die is too worn, replace it.

This guide will be expanded in the future.