posted July 07, 2001 11:20 PM            

This is a looong post, but hopefully worthwhile for the archives, relating to problems and remedies during PVC extrusion.

Regards, Skip

P.S.to FSD Admin. This post may be too long to keep, so if you want to archive it elsewhere or delete it---OK by me! Skip

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, better to use blend from silo.
(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, - resulting in decreased volume transport to feed zone. 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), turn off vacuum to prevent powder in 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.

B - IN THE EXTRUDER BARREL

1. High amps, material too fused at vent-generally results in a "hot" extrusion with glossy and wvy 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 0.2 phr and/or increase Wax 0.1-0.2 phr.
(b) Screw temperature may be too high.Screw temperature should be about the same as the metering zone of 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 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 0.1-0.2 phr (especially the polyethylene).
(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 external
lubrication; 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 nonhomogeneous powder blend. Check blend room for accuracy, and Brabender curves on batches!!
(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 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 0.1-0.2 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 temperatures usually need an increase for twin screws, but a decrease for single screws.
(b) Crammer RPM too high in single screws, 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 screws. 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 should be about the same temperature. 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 a 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 wax, the late working lubricant which functions in the head and die.

5. Uniform yellowing, or discoloration of material out of die
(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-tosmooth again, with some surging.
(a) Too much cooling on barrel coolin 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 or bridging in hopper.

D, Downstream, can come later!!

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.

[This message has been edited by 2beeps (edited 07-08-2001).]

posted July 25, 2001 06:12 AM            

Thanks, in advance

posted July 25, 2001 12:01 PM            

First, rather than continue posting here, which can get very long, the entire booklet will be archived in the new encyclopedia section of the "plastics.com" website.

Now, your questions:

High amps---higher drive motor amperage occurs when trying to turn screws in a higher viscosity, or stiffer, melt.

I.D. & O.D.---inside and outside diameter (of pipe or hollow profile).

phr---parts per 100 parts PVC resin, of additives.

Rear barrel near hopper, front barrel near die.

A crammer is a vertical screw in the hopper to maintain a positive force of material to the feed zone of the screw---used mainly in single screws. A dosing (or starve) feeder is a horizontal metering of material from hopper to feed zone --mostly used on twin screws.

Accuracy in the blend room certainly means accuracy in weighing resin and all additives,
as well as following correct blending sequence and temperatures.

posted October 17, 2007 10:53 AM            

The extruded material breaks very often. My question is about temperatures, if is it correct to work higher than with powder and how much degrees moreless.

What is the temperatures profile of the 4 zones?

Thank you and excuse my english,