Understanding Shear Heating, On-Site Troubleshooting, and Corrective Actions
The barrel is set to 220°C, but once the extruder reaches stable operation, the measured melt temperature rises to 230°C or even 240°C.
In some cases, the temperature in the downstream barrel zones continues to climb. The heaters have already switched off, the cooling system is running, yet the temperature still refuses to come down.
The first reaction on the production floor is often to lower the barrel temperature settings or increase cooling. But if the excess heat is not coming from the heaters, simply lowering the setpoint may have only a limited effect.
The first question should be: Where is the heat actually coming from?
The barrel temperature setpoint is not the same as the actual melt temperature.
I. Why Can the Melt Reach 240°C When the Barrel Is Set to 220°C?
The temperature control system regulates heating and cooling in each barrel zone. The final melt temperature, however, also depends on what happens to the material inside the screws.
During twin screw extrusion, the motor continuously supplies mechanical energy to the screws. As the polymer is conveyed, plasticized, mixed, sheared, compressed, and rubbed against surrounding surfaces, part of that mechanical energy is converted into heat.
This is generally referred to as mechanical heating or shear heating.
Even after the barrel heaters have stopped supplying heat, the screws may still be continuously transferring mechanical energy into the material.
1. Screw Speed and Feed Rate
Changing screw speed affects several conditions at the same time, including shear rate, screw fill, residence time, and mechanical energy input.
It is therefore too simplistic to assume:
Higher screw speed → higher motor current → higher melt temperature.
For some shear-thinning materials, increasing screw speed reduces the apparent viscosity, and motor current may actually fall. At the same time, however, the higher shear rate can still result in a higher melt temperature.
Shop-floor note: Motor current alone cannot be used to determine how much shear heat is being generated.
Feed rate also has to be considered together with screw speed. The relationship between throughput, screw fill, specific mechanical energy, and melt temperature is often more important than any single operating parameter.
2. Material and Formulation
Different polymers and formulations respond differently to mechanical energy.
Viscosity, filler loading, additives, lubrication, and general processing behavior all influence how much mechanical energy is converted into heat.
A change in raw-material lot, lubricant system, or filler content can therefore change the melt temperature even when the machine settings remain unchanged.
Insufficient lubrication may increase processing resistance and mechanical heating. However, continuously adding more lubricant is rarely a complete solution. Product properties and formulation limits must also be considered.
3. Screw Configuration
The number, angle, and arrangement of kneading blocks and other mixing elements directly affect plasticization, dispersion, distributive mixing, and mechanical energy input.
An overly aggressive screw configuration can generate excessive heat.
A configuration with insufficient shear, on the other hand, may result in poor melting or inadequate dispersion.
A good screw configuration is not the one that creates the highest possible shear. It is the one that provides enough mixing and shear to complete the required process without unnecessary energy input.
4. Screen Pack and Die Resistance
A blocked screen pack, restricted flow passage, or material build-up in the die can increase resistance in the melt flow path.
If melt temperature rises together with a noticeable increase in die pressure, the downstream flow path should be checked before other process parameters are changed.
II. When the Temperature Will Not Come Down, What Should Be Checked First?
Avoid repeatedly lowering the barrel temperature setpoints.
A more effective approach is to troubleshoot the system in a defined sequence, starting with temperature control and then moving through the process conditions.
Step 1: Check the Temperature Control System
First confirm that the temperature control system itself is operating correctly:
- Has heater output actually stopped?
- Is the cooling system operating?
- Is air cooling or cooling-water circulation functioning normally?
- Is the temperature measurement reasonable and stable?
If the heaters are already off and cooling is operating properly, but the temperature continues to rise, the additional heat is likely being generated by the extrusion process itself.
Step 2: Compare with Normal Operating Conditions
Do not judge the situation from one number in isolation.
Compare the current operating condition with a known stable production condition:
- Has screw speed changed?
- Has feed rate changed?
- Is throughput stable?
- Are motor current and torque different from normal?
- Has die pressure changed?
- Has the appearance or consistency of the extrudate changed?
An increase in motor current does not automatically mean that material viscosity has increased.
Poor plasticization, changes in screw fill, or increased downstream resistance can all affect motor load.
Likewise, a reduction in motor current does not necessarily mean that mechanical heating has decreased.
Step 3: Check the Material
Confirm that feeding is stable and check whether there has been any change in:
- formulation;
- raw-material lot;
- filler or additive content;
- lubrication;
- feeding behavior.
Then inspect the extrudate itself.
Is discharge continuous? Is the strand or melt surface normal? Is there any sign of uneven plasticization or incomplete melting?
These observations often provide useful clues about what is happening inside the extruder.
Step 4: Check Flow Resistance
Inspect the screen pack, melt passages, and die for blockage or material build-up.
If die pressure is significantly higher than its normal operating level, downstream resistance should be ruled out first.
Where necessary, verify the situation by measuring the actual melt temperature independently.
III. If Excessive Mechanical Heating Is Confirmed, What Should Be Adjusted?
A common question is:
If shear heating is too high, should the feed rate simply be reduced first?
Not necessarily.
If screw speed remains unchanged while feed rate is reduced, screw fill will decrease.
Under some operating conditions, this can increase the amount of mechanical energy applied per kilogram of material. The result may be lower throughput without a meaningful reduction in melt temperature.
1. High Die Pressure: Check for Restrictions First
If die pressure has risen abnormally, first inspect:
- the screen pack;
- melt flow passages;
- the die;
- possible material build-up.
Restore a smooth discharge path before making major changes to screw speed, feed rate, or screw configuration.
Otherwise, parameter adjustments may only mask the actual cause.
2. Normal Die Pressure: Review Screw Speed and Feed Rate
When downstream pressure is normal, review whether screw speed and feed rate are properly matched.
Change one variable at a time whenever possible.
During adjustment, monitor several parameters together:
- melt temperature;
- torque;
- motor load;
- die pressure;
- throughput;
- product quality.
Do not optimize the process based on a single number.
3. Abnormal Discharge or Plasticization: Check the Formulation
Recheck:
- the lubrication system;
- raw-material lot;
- additive and filler levels;
- feeding stability;
- material condition.
Not every processing problem should be attributed to machine settings.
Changes in formulation or raw-material behavior can alter viscosity, energy consumption, pressure, and melt temperature even when the extruder itself has not changed.
4. Persistent High Melt Temperature: Evaluate the Screw Configuration
If excessive melt temperature is a recurring problem rather than a temporary disturbance, the screw configuration should be evaluated.
Check whether:
- too many kneading blocks are being used;
- mixing elements are concentrated too heavily in one section;
- kneading angles are more aggressive than necessary;
- the overall shear level exceeds what the process actually requires.
The objective is not to eliminate shear.
Shear is necessary for melting, dispersion, distributive mixing, and many compounding processes.
The real objective is to reduce mechanical energy input that does not contribute effectively to melting or mixing.
5. Cooling Must Work Properly, but Cooling Alone Is Not the Solution
The cooling system needs to function correctly, but cooling only addresses the question:
How can the excess heat be removed?
Process optimization addresses a different question:
Why is so much heat being generated in the first place?
Both matter.
If excessive mechanical heating is caused by an unsuitable combination of screw speed, feed rate, flow resistance, formulation, or screw configuration, increasing cooling capacity alone will not correct the underlying process condition.
IV. Recommended On-Site Troubleshooting Sequence
| Sequence | What to Check | Key Question |
|---|---|---|
| ① Temperature control | Heating, cooling, temperature measurement | Are heating, cooling, and temperature sensing operating normally? |
| ② Operating conditions | Screw speed, feed rate, throughput, current, torque, pressure | What has changed compared with the normal production condition? |
| ③ Material | Formulation, material lot, feeding, plasticization | Has the material or its processing behavior changed? |
| ④ Flow resistance | Screen pack, flow passages, die | Is downstream restriction causing pressure and temperature to rise? |
| ⑤ Screw configuration | Kneading and mixing intensity | Is the screw configuration applying more shear than the process requires? |
| ⑥ Process adjustment | Screw configuration, screw speed, feed rate, cooling | Adjust only after the actual cause has been identified. |
Conclusion
An abnormal melt temperature is a result of what is happening inside the extrusion process.
When the melt temperature refuses to come down, repeatedly asking how much further the barrel temperature setpoint should be reduced is usually not the most useful starting point.
A better question is:
Where is the extra heat coming from?
Start with the temperature control system.
Then compare the current operating condition with normal production, check the material, and rule out excessive downstream resistance.
Only after these factors have been evaluated should screw speed, feed rate, cooling, or screw configuration be adjusted.
In twin screw extrusion, melt temperature is rarely controlled by barrel temperature alone. Understanding how mechanical energy is introduced, converted, and dissipated through the process is the key to finding the real cause of excessive melt temperature.