Trouble shooting

Category: Literatur
Published on Tuesday, 22 January 2013 14:48
Written by Administrator
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Successful processing of rigid PVC requires an understanding and proper
application of several factors which were discussed earlier in this manual
including :

a. Additive selection and use

b. Blending procedure
c. The nature of single and twin-screw extruders
d. Matching formulation to the Extruder
e. Extruder parameter/operation

 

The following list of problem and possible remedies reflect our own experiences
Accumulated over years of processing trials both in our own and in customer facilities.
The suggested remedies have proven useful during many runs on a wide variety of single
and twin screw extruders.

How ever, it should be realized that, as with automobiles, no two extruders of the same
design run exactly the same. For the example, differences in flight clearance with
varying amounts of crew and barrel wear will affect the temperature profile settings
required to run a given formulation. Therefore, the remedies described should be considered
as suggestions, with the need for possible adjustment for differences in machine behavior
kept in mind.

 

EXTRUSSION
 
IN THE HOPPER THROAT
 
1. "Bridging" or interupted powder flow to the screws, causing erratic flow out of die or in complete filling of mold                   
(a) Powder blend too warm(freshly mixed)let stand for 24 hours
(b) Possible moisture contamination    
(c) Crammer necessary for single screw extrussion to force feed screw
(d) Vibrator on hopper helpfull, especially with highly filled blends
 
2. Powder Level in extruder throat gradually decreases                    
(a) Powder gradually builds up on feeder screw flight through static charge, or compression, which decreases volume transport to the extruder. Feeder screw should be cleaned periodically
 
3. Momentary interruption of powder flow to the screws                
(a) Quickly slow down haul-off(since flow out of die slows down), and turn off vacuum lines. If done quickly, the line may hold together, gradually increase haul off speed as flow from die increases and turn on vacuum again as normal powder feed is resumed
 
IN THE BARREL
 
1. High amps, material to fused at vent "Hot" extrussion with glossy and wavy I.D              
(a) Screw temperature maybe too high. Screw oil temperature should be +/- 20 F of the metering zone of the barrel for twins, and perhaps 100F lower than matering zone for single screw machines
(b) Rear Barrel temperatures too high reduce especially zone 1 & 2; front barrel temperature may be too cold, increase zone 3 & 4
(c) Check barrel and screw temperature controllers for accuracy
(d) As an immediate answer in twins, reduce feed to starve screws, while waiting for temperature 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 temperature here 10 - 20 F
(b) Adapter orifice may be too small; use larger diameter orifice.
(c) Melt may be to stiff coming from screws-increase matering 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 the barrel
(b) Fussion occurring too far down the barrel-increase barrel and screw temperature with higher temp setting in zone 1 & 2 to cause fussion of powder earlier
 
4. Low back pressure caused by low melt viscosity               
(a) Barrel and screw temperature too high, reduce heat and check controllers
(b) Orifice in adapter too large, use smaller diameter orifice
 
5. Fluctuating amps and/or back pressure              
(a) Frequent fluctuation indicate a non homogeneous powder blend. Check blend room
(b) A sudden change to a new but steady amps and back pressure level indicate a bulk density change in the  powder blend-checking blending and resin, bulk density, particle size, percent volatility, resin absorption properties
 
6. Vacuum at vent sucking powder              
(a) Screws not full(no seal between vent and throat)-increase feed to flood screws
(b) Rear barrel zone too cold, increase heat in zone 1 & 2 and screw oil
 
7. Powdered, or semi fused material extruding out of vent, with flood feeding        
(a) Generally barrel temperature profile out of ballance and all zone temperature ussually need an increase for twin screw but a decrease for single
(b) Crammer RPM to high in single screw, forcing material into barrel faster than it can exit through adapter slow crammer RPM
(c) May be a sign of worn screws and barrel. Reshim screw forward if possible as a temporary solution. 
 
OUT OF THE DIE
 
1. Material lumpy, has bumps on I.D. Low gloss low impact.        
(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) Check vacuum system for proper operation
(d) Check for moisture leaks in silo and transfer lines
 
2. Wavy I.D(with gloss in single screw and low gloss in twin       
(a) material flow unbalanced in twin screw extruders; screw and matering zone barrel should be about the same temperature, generally a little higher heat in both will help smooth out oh the I.D.
(b) In single-screw extruder, 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 or combination product level
 
3. Wavy, Glossy I.D in twin screw                     
(a) Material too hot entering die-reduce screw and matering zone temperature (possibly the entire barrel temperature profile should be reduced)

 

4. Rough"orange peel"I.D. or O.D. leading to burn     
(a) Reduce spider and cone heat zones. (Some heads have air cooled spider legs)
(b) Check all scales in blend room for proper operation
 
5. Uniform yellowing, or discoloring of material      
(a) Overall temperatures too high reduce heats and check controllers
                                                       
6. Visible spider lines on I.D. with little or no gloss       
(a) Spider and die heat zones too cold. increase heat carefully.
(b) Possible 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. Non uniform wall thickness around pipe             
(a) Bushing is not centered. Adjust die bolts for even flow out of die circumference.
(b) Check feed auger. Clean as needed
 
9. Increasing or decreasing wall thickness            
(a) Initially, adjust haul off speed, then check for feed variations due to bulk density changes, build upof material on feed screw or bridging in hopper 

IN THE SIZING, COOLING TANK AND HAUL OFF
 
1. Thin wall pipe in vacuum sizing collapsing,or jumming up, or drowing 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 three inches should be a sufficient gap
(c) Material may be too hot out of die-reduce temperatures.

 

 
2. No uniform O.D appearance (glossy and dull)or rough areas        
(a) Possible "plate out" build-up (lubricants, pigment, etc) on sizing sleeve. Mineral spirits added drop wise on hot melt just as it enters vacuum-sizing sleeve usually would remove most of the plate out.
(b) On large pipe die and head may not be uniformly heated; generally several hours are required for a "heat soak" for a large mass of metal, also check heater bands.
 
3. O.D not smooth and glossy                          
(a) Not fully sized-increase vacuum or increase air pressure on plug to insure full contact with sizing sleeves
(b) Check accuracy of blend room scales. possible misblend of forward  
4. Pipe not around                                    
(a) Check for full sizing as above for sufficient vaccum or pressure
(b) Cooling is not cool enough and haul off compressess 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 maybe worn out. Replace plug some use a double plug -one before and one after the plug 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-at least 75% of the cooling tank length.
(b) If I.D has no gloss, I.D of the material too cold-increase screw and matering zone heat and also increase mandrill heat slightly, if available.
(c) Haul-off speed to high- decrease speed slightly
(d) Increase extruder feed and speed
 
6. Pipe slips in haul-off                             
(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 plug, so line will have to be broken and restrung. 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. Profile or pipe too thick                          
(a) Haul-off too slow-speed up slightly

 

 
 
IN THE SAW
 
1. Pipe or profile jamming                            
(a) Saw blade too slow 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 or chamfer saw.
 
 
BLOW MOLDING
 
IN THE HOPPER THROAT
 
1. "Bridging" or interupted powder flow to the screws, causing erratic flow out of die or in complete filling of mold                  
(a) Powder blend too warm(freshly mixed)let stand for 24 hours    
(b) Possible moisture contamination    
(c) Crammer necessary for single screw extrussion to force feed screw
(d) Vibrator on hopper helpfull, especially with highly filled blends
 
2. Powder Level in extruder throat gradually decreases                    
(a) Powder gradually builds up on feeder screw flight through static charge, or compression, which decreases volume transport to the extruder. Feeder screw should be cleaned periodically
 
3. Momentary interruption of powder flow to the screws                   
(a) Quickly slow down haul-off(since flow out of die slows down), and turn off vacuum lines. If done quickly, the line may hold together, gradually increase haul off speed as flow from die increases and turn on vacuum again as normal powder feed is resumed

 

 
IN THE BARREL
 
1. High amps, material to fused at vent  "Hot" extrussion with glossy and wavy I.D              
(a) Screw temperature maybe too high. Screw oil temperature should be +/- 20 F of the metering zone of the barrel for twins, and perhaps 100F lower than matering zone for single screw machines
(b) Rear Barrel temperatures too high reduce especially zone 1 & 2; front barrel temperature may be too cold, increase zone 3 & 4
(c) Check barrel and screw temperature controllers for accuracy
(d) As an immediate answer in twins, reduce feed to starve screws, while waiting for temperature 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 temperature here 10 - 20 F
(b) Adapter orifice may be too small; use larger diameter orifice.
(c) Melt may be to stiff coming from screws-increase matering 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 the barrel
(b) Fussion occurring too far down the barrel-increase barrel and screw temperature with higher temp setting in zone 1 & 2 to cause fussion of powder earlier
 
4. Low back pressure caused by low melt viscosity              
(a) Barrel and screw temperature too high, reduce heat and check controllers
(b) Orifice in adapter too large, use smaller diameter orifice
 
5. Fluctuating amps and/or back pressure              
(a) Frequent fluctuation indicate a non homogeneous powder blend. Check blend room
(b) A sudden change to a new but steady amps and back pressure level indicate a bulk density change in the powder blend-checking blending and resin ; bulk density, particle size, percent volatility, resin absorption properties
 
6. Vacuum at vent sucking powder              
(a) Screws not full(no seal between vent and throat)-increase feed to flood screws
(b) Rear barrel zone too cold, increase heat in zone 1 & 2 and screw oil
 
7. Powdered, or semi fused material extruding out of vent, with flood feeding         
(a) Generally barrel temperature profile out of ballance and all zone temperature ussually need an increase for twin screw but a decrease for single
(b) Crammer RPM to high in single screw, forcing material into barrel faster than it can exit through adapter slow crammer RPM
(c) May be a sign of worn screws and barrel. Reshim screw forward if possible as a temporary solution. 

 

 
OUT OF THE DIE
 
1. Material lumpy, has bumps on I.D. Low gloss low impact.       
(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) Check vacuum system for proper operation
(d) Check for moisture leaks in silo and transfer lines
 
2. Wavy I.D(with gloss in single screw and low gloss in twin       
(a) material flow unbalanced in twin screw extruders; screw and matering zone barrel should be about the same temperature, generally a little higher heat in both will help smooth out oh the I.D.
(b) In single-screw extruder, 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 or combination product level
 
3. Wavy, Glossy I.D in twin screw                     
(a) Material too hot entering die-reduce screw and matering zone temperature (possibly the entire barrel temperature profile should be reduced)
 
4. Rough"orange peel"I.D. or O.D. leading to burn     
(a) Reduce spider and cone heat zones. (Some heads have air cooled spider legs)
(b) Check all scales in blend room for proper operation
 
5. Uniform yellowing, or discoloring of material      
(a) Overall temperatures too high reduce heats and check controllers
                                                       
6. Visible spider lines on I.D. with little or no gloss       
(a) Spider and die heat zones too cold. increase heat carefully.
(b) Possible 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. Non uniform wall thickness around pipe circumference             
(a) Bushing is not centered. Adjust die bolts for even flow out of die
(b) Check feed auger. Clean as needed
 
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 or bridging in hopper 


 
 
INJECTION MOULDING
 
1. Short shots                                 
(a) Shot size too small
(b) Material melt temperature too low
(c) Injection pressure too low
(d) Injection velocity too slow
(e) Mold temperature too low
(f) Sprue, runners, and/or gates too small
 
2. Dull streaks, flow lines
(a) Melt temperature too low
(b) Runners too small
(c) Inadequate cold slug well
(d) Mold temperature too low
(e)Injection speed to slow
 
 
3. Sink marks
(a) Shot size too small
(b) Injection pressure too low
(c) Hold pressure too low
(d) Hold time too short
(e) Cooling time too short
(f) Mold temperature too high
 
 
4. Warpage
(a) Mold temperature too high
(b) Melt temperature too high
(c) Insufficient hold time
(d) Injection speed too fast
(e) Insufficient cooling time
 
 
5.Poor knit lines
(a) Mold temperature too cold
(b) Injection speed too slow
(c) Melt temperature too low
(d) Poor venting
 
6.Lamination
(a) Purging compound left in barrel
(b) Mold temperature too low
(c) Melt temperature too low
(d) Injection speed too fast
(e) Gate size too small
 
7. Blush marks at gate
(a) Mold temperature too cold
(b) Injection speed too fast
(c) Sprue and nozzle diameter too small
(d) Insufficient cold slug well
(e) Moisture in compound
 
8. Burn streaks in center of sprue
(a) Front zone temperature too high
(b) Screw speed too high
(c) Excessive back pressure
(d) Compression ratio too high
 
9. Silver streaks on part
(a) Injection pressure too high
(b) Injection speed too fast
(c) Melt temperature too high
(d) Poor venting
 
10.Burn streaks at gate
(a) Injection speed too fast
(b) Injection pressure too high
(d) Shear burning due to low melt temperature