The following list of problems and possible remedies reflects my own experiences over several hundred extrusion runs, both lab and production facilities.
It must be realized that, like automobiles, no two extruders of the same design will run exactly the same.

For example, differences in flight clearance with varying amounts of screw and barrel wear will affect the temperaure setings required to run a given formulation. Therefore, remedies described should be considered as suggestions, with the need for possible adjustments for differences in extruder behavior kept in mind.

PROBLEMS AND POSSIBLE REMEDIES

A - IN THE HOPPER & EXTRUDER THROAT

 1.  Bridging, or interrupted powder flow to screws, causing erratic flow out of die 
 (a) Powder blend too warm (freshly mixed) - let stand for 24 hours.
 (b) Possible moisture  contamination.
 (c) Crammer necessary for single-screw extrusion to force-feed screw.       
 (d) Vibrator on hopper helpful,especially with highly filled blends.

 2.  Foreign objects in blend.  
 (a) Always use hopper magnets for metallic contamination!  Nonmetallic materials should pass through with minimal damage to extruder screws and barrel.

 3.  At a given feeder setting, and during long extrusion runs, powder level in extruder throat gradually decreases - resulting in starved screws. 
 (a) Powder gradually builds up on feeder screw flights through static charge, or compression, which decreases volume transport to extruder.  Feeder screw should be cleaned periodically.

 4.  Momentary interruption of powder flow to screws, causing several empty screw flights - generally seen as a drastic drop in amperage.  
 (a) Quickly slow down haul-off (since flow out of die slows down), and turn off vacuum to prevent powder in vacuum lines.  If done quickly, the line may hold together.  Gradually increase haul-off sped as flow from die increases and turn on vacuum again as normal powder feed is resumed.

B - IN THE EXTRUDER BARREL

 1.  High amps, material too fused at vent - generally results in a “hot”    
     extrusion with glossy and wavy I.D. caused by high melt viscosity,      
       or early fusion in barrel (in twin screw).

              (a) Formulation may need a higher external/internal lubricant ratio.
                  Decrease calcium stearate 0.2phr and/or increase wax 0.1-0.2phr.

              (b) Screw temperature may be too high.Screw temperature should be about
                  the same as the metering zone of the barrel for twins, and perhaps 100
                  degrees F lower than metering zone for single screw machines.

              (c) Rear barrel temperatures too high,reduce especially zones 1 & 2;
                  front barrel temperature may be too cold,increase zones 3 & 4.

              (d) Check barrel and screw temperature controllers for accuracy.
       
              (e) As an immediate answer in twins, reduce feed to starve screws,
                  while waiting for temperatures to cool.

2.  High back pressure caused by resistance of material being pushed
            through adapter, head and die.         

              (a) Check adapter,head and die heat zones and controllers:
                  May be too cold. Increase temperatures here by 10-20 degrees F.

              (b) Adapter orifice may be too small; use larger diameter orifice.

              (c) Formulation may be under lubricated, causing frictional drag in die.
                  Increase external lubricant 0.1-0.2phr (especially the polyethylene wax).

              (d) Melt may be too “stiff” coming from screws - increase metering zone
                  and screw temperature.  Check temperature controllers.

3.  Low amps, material too “powdery” at vent.           
             
               (a) Screw not full--increase feeder to flood-feed screws and
                   create more "work" in barrel.

               (b) Fusion occurring too far down the barrel - increase barrel and screw
                   temperatures with higher temperature settings in zone 1 and 2 to
                   cause fusion of powder earlier. 

               (c) Formulation over - lubricated on the external side. 
                   Increase calcium stearate and/or decrease Wax by 0.1-0.2 phr.

               (d) Add 0.8 phr process aid to promote fusion.

4.  Low back pressure.   
            
           
               (a)  Melt viscosity is too low--barrel and screw temperatures too high.
                    Reduce heats and check controllers.

               (b)  Formulation has too much externallubrication; 
                    reduce Wax level 0.1-0.2 phr.
       
               (c)  Orifice in adapter too large, use smaller diameter orifice.   


5.  Fluctuating amps and/or back pressure.           
           
               (a)  Frequent fluctuations indicate a non-homogeneous, non uniform powder
                    blend. Check blend room operation!!

               (b)  A sudden change to a new but steady amp and back pressure
                    level indicates a bulk density change in the powder blend.
                    Check blending and resin: bulk density, particle size,  percent volatility,
                    plasticizer absorption properties.

  6.  Vacuum at vent sucking powder. 
             
             
               (a)  Screws not full (no "seal" between vent and throat)--
                    Increase feed rate to flood screws.

               (b)  Rear barrel zones too cold,increase heat in zones 1 and 2.

               (c)  Formulation may have too much external lubricant, which delays
                    fusion.  Reduce wax 1.1-0.2 phr.
       
               (d)  Add process aid to promote fusion.

         7.  Powdered, or semi-fused material extruding out of vent, with flood feeding.           
             
             
               (a)  Generally a barrel temperature profile out of balance.
                    Rear zone (1&2) temperatures usually need an increase
                    for twin screws, but a decrease for single screws.

               (b)  Crammer RPM too high in single screw,forcing material into barrel
                    faster than it can exit through adapter; slow crammer RPM.

C - OUT OF THE DIE

 1.  Material lumpy, low gloss (also “surging” in single screw). 
            Low impact, poor acetone test.        

                (a)  Material too cold--not fully fused.
                     Barrel and screw temperatures too low--increase heats in twin screw.

                (b)  Decrease screw temperature in single screw to provide more
                     friction and shear.

                (c)  Formulation may be over-lubricated.
                     Decrease external lubricant (Wax)0.1-0.2 phr.

                (d)  Add, or increase, process aid to promote fusion.
 

 2.  Wavy I.D. (with gloss in single screw, low gloss in twin screw).       
    Possible grey-brown “streaks”on O.D.           
             
             
                 (a)  Material flow unbalanced in twin screw extruders: screw and
                      metering zone barrel temps should be about the same.
                      Generally a little higher heat in both will help smooth out the I.D.

                 (b)  In single screw extruders, reduce heat in front barrel and screw which
                      should create more friction, work, and smoother flow.  Also higher
                      crammer RPM will create more shear.

                 (c)  Slight over lubrication - may require slight reduction in wax level.

 
        3.  Wavy, glossy I.D. in twin screw. 
             
                                   
          (a)  Material too hot entering die--reduce screw and metering zone
                      temperatures (possibly the entire barrel temperature profile
                      should be reduced).

  4.  Rough “orange peel” I.D. or O.D. sometimes starting at spider line -       
      leading to eventual burn.         

                  (a)  Reduce spider and cone heat zones (some heads have air-cooled spider legs).

                  (b)  Formulation may be under-lubricated - increase (.05-.10 phr) low
                       molecular weight polyethylene, the late working lubricant which
                       functions mainly in the head and die.                functions in the head and die.

            
 5.  Uniform yellowing, or discoloration of material.         

                  (a)  Overall temperatures too high--reduce heats and check controllers.

                  (b)  Discoloration combined with “orange peel” roughness and/or
                       high amps indicates under-lubrication. Increase external lubrication.

                  (c)  Check stabilizer use level, was it miss-weighed?

6.  Visible spider lines on I.D. with little or no gloss.         

                  (a)  Spider and die heat zones too cold. Increase heat carefully.

                  (b)  Possibly material too cold entering head and die -
                       increase barrel and screw heat.

 7.  Erratic flow out of die: smooth-to-rough-to-smooth again, with        
     some surging.            
            
            
           (a)  Too much cooling on barrel cooling zones, which cycle on and off,
                       resulting in "temperature shock".Decrease cooling water flow
                       to barrel cooling oil.   
 

        8.  Nonuniform wall thickness around pipe circumference.          

                  (a)  Bushing is not centered. Adjust die bolts for even flow out of die.

        9.  Increasing or decreasing wall thickness.         
             
               
                  (a)  Initially, adjust haul-off speed, then check for feed variations due to
                       bulk density changes, build-up of material on feed screw (static)
                       or bridging in the hopper. 

        Most of the above problems - and possible remedies - in the extruder are
 influenced by the quality of powder blend being fed to the extruder.  Blend
 consistency is MOST important.  Since PVC resin generally comprises 90-
 95% of the formulation, resin molecular weight, molecular weight distribu-
 tion, bulk density, particle size, percent volatility and plasticizer absorption
 properties must be correct for the application - and consistent.  Also, all
 additives - lubricants, stabilizers, etc., - must be of consistent quality. 
 Otherwise, many of the above described remedies will be of little use in
 overcoming these problems.

D - IN THE SIZING, COOLING TANK AND HAUL-OFF

 1.  Thin wall pipe in vacuum sizing collapsing, or jamming up, or       
     drawing down too thin.         
             
                 (a)  Too much vacuum creates a drag on sizing sleeve--reduce vacuum slightly
                      for smooth flow over sizing sleeve.

                 (b)  Too much distance between die and vacuum tank - two to three
                      inches should be a good clearance.

                 (c)  Material may be too hot out of die -reduce temperatures.

 2.  Nonuniform O.D. appearance(glossy and dull) or rough areas.       
            
            
          (a)  Possible "plateout" build-up (lubricants,pigments,etc.) on sizing sleeve.
                      Mineral spirits added dropwise on the hot melt just as it enters
                      vacuum sizing sleeve usually will remove most of the plateout. 

                 (b)  On large pipe, die and head may not be uniformly heated; generally
                      several hours are required for a “heat soak” for large mass of metal.
                      Also check heater bands and controllers.

 3.  O.D. not smooth and glossy - some pock marks.         
            
            

                  (a)  Not fully sized--increase vacuum,or increase air pressure on plug
                       to insure full contact with sizing sleeve. 

        4.  Pipe not round.    
             

                  (a)  Check for full sizing, as above,for sufficient vacuum or plug pressure.

                  (b)  Cooling water is not cold enough,and haul-off compresses pipe.  Use
                       chiller, or reduce haul-off pressure.

                  (c)  On large pipe, spray cooling, or cascade cooling is more efficient
                       than a flooded tank.

                  (d)  If this occurs after a long run, plug may be worn out.  Replace plug -
                       some use a double plug - one before and one after the haul-off, to
                       insure no loss in air pressure for sizing.                  

      
 5.  I.D. exhibits occasional tearing. 
             
             
           (a)  Plug too close to die--pulling material apart before it is cooled
                       sufficiently. Use longer chain---i.e.,75% of the cooling tank length.

                  (b)  If I.D. has no gloss, I.D. of material too cold - increase screw and
                       metering zone heat and also increase mandrel heat slightly, if available.

                  (c)  Haul-off speed too high - decrease speed slightly.

 
        6.  Pipe slips in haul-off, causing material to accumulate in sizing sleeve.         
             
             
    (a)  Increase air pressure on haul-off to provide a tighter grip on pipe.
                       If not corrected quickly, material build-up in sizing sleeve will not pass
                       over plug, so line will have to be broken and re-strung. Also, keep outer surface
                       of pipe dry before entering haul-off. Wet pipe will slip.
 

         7.  Pipe or profile too thin.   

 
                  (a)  Haul-off too fast--slow down slightly

         8.  Pipe or profile too thick.   

                  (a)  Haul-off too slow--speed up slightly.

E - IN THE SAW

 1.  Pipe or profile jamming.  
             
                                        
                       
               (a)  Saw blade too slow returning to original cut position.
                    Adjust speed of blade cut and return to occur before
                    saw table reaches end of forward travel. Adjust saw
                    table return to occur before next cut.(This applies
                    to both a chop saw and radial-cut/chamfer saw.)