Trouble-Shooting

Probable Cause/s

• Set temperatures are very low.
• Heater/s not working.
• Screen pack is blocked.

Remedy

• Increase the set temperature.
• Replace.
• Change the screen pack.

Probable Cause/s

• Hopper is empty.
• Motor stopped.

Remedy

• Load the raw material.
• Remove cause.

Probable Cause/s

• Loose connection or disconnected wires.
• Heater failed.
• Heater bands loose on the body.
• Zone fuse out of order.
• Thermocouple is damaged.
• Thermocouple is out of place.

Remedy

• Tighten connection or reconnect wires.
• Replace.
• Tighten them.
• Replace.
• Replace.
• Mount it properly.

Probable Cause/s

• Charge point is very hot

Remedy

• Increase cooling water flow and / or decrease water temperature.

Probable Cause/s

• Charge points are too hot.
• Drive belt are loose.
• Screw is worn out.
• Screen pack is too dirty or obstructed.

Remedy

• Increase flow of cooling water and / or decrease water temperature
• Tighten them.
• Replace.
• Change the screen pack.

Probable Cause/s

• Hopper is empty.
• Waxy melt plug is formed in extruder due to previous excessive thermal exposure.

Remedy

• Charge the hopper with material.
• Empty the hopper. Feed heavy duty (lower MI material than one in use) material or thick wastage film belt and try till melt exits or drive gets overloaded. If this fails, remove the screw, clean it and restart.

The most important factors the affect the physical properties of film are:

Probable Cause/s

• Frost line-height.
• Blow-up ratio.
• Take-off speed.
• Extruder output.

Remedy

• All these variables have to be balanced to produce film with good physical properties and uniform gauge at an economical speed.

Probable Cause/s

• Spiders / spiral in die.

Remedy

• Increase material temperature.
• Increase extruder pressure.

• Abrasive components in polymer matrix i.e. filler, reinforcements, additives, etc.

Probable Cause/s

• Abrasive Wear
• Adhesive Wear
• Laminar Wear
• Surface – Fatigue Wear

Remedy

• Screw OD is worn-out but not flight flanks and the root diameter, than problem can be caused by contact between screw and barrel
• one could put hard facing on the tip of the flight.

Corrosive Wear

Probable Cause/s

• Corrosive wear is more frequent with non filled polymers such as PVC or PVC2.
• Hygroscopic material such as ABS, PA, PET, PMMA.
• Metal to metal wear occur at start up, or misalignment, or wrapped barrel or wrapped screw, or in twin-screw extruder.

Remedy

• This can reduced the magnitude of the problems.
• Corrosive wear can usually identified by pitted and worn surface.
• To eliminate the corrosive component from compound.
• Use corrosion resistant materials.
• Bimetallic barrels provide better resistance than nitriding barrels.
• Screw hardening by nitriding and nickel / chrome plating can increase the life.

Probable Cause/s

• Thermal Polymer is exposed to an elevated temperature in an inert atmosphere under extrusion of other compounds.
• Mechanical Molecular scission induced by the application of mechanical stresses.
• Chemical Processes which are induced under the influence of chemicals in contact with a polymer such as acids, bases, solvents, reactive gases, etc.

Factors affecting rates of degradation:

• RTD.
• Stock temperature and temperature.
• Deformation rate and deformation rate distribution.
• Presence of solulaytic agents, oxygen and other degradation promoting agent.
• Presence of antioxidants and other stimulates.

The chance of degradation in the extruder can be reduced by marking following changes to process:

Remedy

• Reduce RTD and achieve narrow RTD.
• Reduce stock temperature and avoid high pick temperature.
• Eliminating degradation promoting subs.

If degradation occurs by thermo-oxidative mechanism, air should be excluded from the extruder. This can be done by putting a nitrogen blanket on the feed hopper, vent port or at the die depending upon where air is introduced.

If the degradation occurs by chemical reaction with metal surfaces of screw and barrel, a non-corrosive material of construction has to be selected for screw and barrel.

Probable Cause/s

• Air being dragged in with particular material from the feed hopper under normal conditions.
• The compression of solid particulate material in feed section will force the air out of the solid bed. However under some circumstances the air can not escape back.
• To feed hopper and travel with polymer until it exits from die.
• As the air pocket exit from the extruder the sudden exposure too much lower ambient pressure may cause the compress air bubble to burst in explosive manner.

Remedy

• There are a number of possible solutions to air entrapment.
• The first approach should be to change the temperature in the solid conveying zone to achieve a more positive compacting of the solid bed.
• Often a temperature increase of the first barrel section reduces the air entrapment. However, in some cases, a lower temperature causes n improvement.
• In any case, the temperature in the solids conveying zone are important parameters in the air entrapment process.
• It should be realized that both the barrel and screw temperature are important. Thus, if a screw temperature adjustment capability is available, it should definitely be used to reduce the air entrapment problem.
• The next step is an increase in the die-head pressure to alter the pressure profile along the extruder and to hopefully achieve a more rapid compacting of the solid bed.
• The die-head pressure can be increased by simply adding screens in front of the breaker plate.
• Another possible solution is to starve feed the extruder; however, this will probably reduce to output somewhat and requires additional hardware i.e. an accurate feeding device.
• The aforementioned recommended solutions can be implemented rather easily. However, if these measures do not solve the problem, more drastic steps have to be taken.
• One possibility that needs to be explored is a change in particle size or shape if this is a reasonable option, it will most likely solve the problem.
• A rather safe solution is to utilize a vacuum feed hopper system; however, these systems are rather complex and expensive. Another possible solution is to use a grooved barrel section.
• Pressure development in a grooved barrel section causes a rapid compacting of the solid bed and therefore, a much reduced chance of air entrapment.
• Instead of grooving the barrel, one can opt for reducing the friction on the screw which would have a similar effect. A coating that might be used for this purpose is described by Luker. Air entrapment is also often successfully eliminated by vented extrusion using a multi-stage extruder screw.
• Increasing the compression ratio of the screw is also likely to reduce.
• Air entrapment it should be noted that bubbles in the extrudate are not only signs of air entrapment, but it may also be an indication of moisture, surface agents, volatile species in the polymer itself, or degradation.
• Thus, before concentrating on solving an apparent air entrapment problem one should make sure that the problem is indeed caused by air entrapment. In some cases, the pellets contain small air bubbles within the pellet itself. In the case, one of the few possible solutions is vented extrusion; most of the other recommended solutions will not work in this situation.