
Twin-screw extruders are widely used in manufacturing, especially in the processing of polymers and plastics. They are highly versatile and efficient, but they can also present several problems during operation. These issues can lead to low-quality output, production delays, and even equipment damage.
Some common problems include extruder overload, screw element wear, material degradation, and venting issues. For instance, too much material feeding can cause extruder overload, while screw elements can wear out over time due to friction with the processed material. Material degradation can occur from high temperatures and shear rates, resulting in thermal and mechanical degradation. Venting problems can also arise, with material exiting the extruder vent openings due to high vapour velocity.
Additionally, twin-screw extruders may encounter issues with broken strips, unstable discharge, black spots on pellets, unstable mainframe current, and high start-up current. These problems can be caused by factors such as excessive feed rate, high screw speed, worn or damaged screws, overheating, and inconsistent material feed.
To address these challenges, operators must implement practical solutions, including adjusting feed and screw speeds, replacing worn components, optimising screw design, and improving ventilation and cooling systems.
What You'll Learn
Screw elements wearing out
Causes of Screw Element Wear:
- Friction with the Material: Prolonged operation of the twin-screw extruder can lead to wear and tear on the screw elements due to constant friction with the material being processed.
- Abrasive or Corrosive Materials: Using abrasive or corrosive materials can accelerate the wear and tear of screw elements.
- Improper Screw Design: An inappropriate screw design, such as incorrect pitch, diameter, or length, can result in increased wear and tear.
- Excessive Operating Temperatures: Operating at high temperatures can also contribute to the deterioration of screw elements.
Solutions and Preventative Measures:
- Regular Maintenance: Implement a schedule for regular inspections and maintenance of the screw elements. Replace any worn or damaged parts promptly to prevent unexpected breakdowns.
- Use Abrasion-Resistant Materials: Opt for screw elements made from hardened steel, tungsten carbide, or other wear-resistant materials to prolong their lifespan.
- Optimize Screw Design: Ensure that the pitch, diameter, and length of the screw elements are appropriate for the specific material being processed. This can reduce wear and improve efficiency.
- Reduce Operating Temperature: Consider lowering the barrel temperature or adjusting the cooling system to decrease the operating temperature. This will help minimize thermal degradation and wear on the screw elements.
- Anti-Seize Compound: When assembling elements onto the shaft, apply an anti-seize compound sparingly. This will help prevent seizing and make future disassembly easier.
- Proper Cleaning and Storage: After removing screw elements, clean them thoroughly and inspect for any damage. Store them safely to prevent damage or debris accumulation before the next use.
- Improve Material Distribution: Ensure that the material is evenly distributed across the screw channel. This can reduce the risk of wear by minimizing localized friction or pressure.
- Slow Feeder Speed: Reduce the feeder speed if too much material is being fed into the extruder. This will decrease the load on the screw elements and reduce wear.
By following these instructions and adopting a proactive maintenance approach, you can effectively manage the problem of screw elements wearing out in your twin-screw extruder, minimizing downtime and improving the overall efficiency of your operations.
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Material degradation
High Processing Temperatures
The temperature inside the barrel of a twin-screw extruder is crucial for melting and processing the raw material. However, excessively high temperatures can lead to material degradation. To prevent this, it is important to reduce the processing temperature. This can be achieved by adjusting the barrel temperature and utilizing an efficient cooling system to maintain the desired temperature range.
Shear Rates
The shear rates within the twin-screw extruder also play a significant role in material degradation. High shear rates can cause mechanical degradation of the polymer material. To mitigate this issue, it is necessary to reduce the shear rate by adjusting the screw speed or feed rate. Additionally, using low-shear screws can help decrease the shear rate and prevent material degradation.
Thermal and Mechanical Degradation
The combination of high temperatures and shear rates can result in thermal and mechanical degradation of the polymer material. This degradation can compromise the integrity and properties of the final product. To combat this, the use of polymer stabilizers is recommended. Polymer stabilizers are additives that protect the polymer material from the adverse effects of high temperatures and shear rates, thus preventing thermal and mechanical degradation.
Versatility of Twin Screw Extruders
Twin-screw extruders are highly versatile machines that can process a wide range of materials, including polymers, plastics, rubber, food products, and pharmaceuticals. However, the diverse requirements of these materials in terms of temperature and shear rates can pose challenges. It is important for operators to have a thorough understanding of the specific needs of the material being processed to prevent material degradation.
Optimizing Performance
To optimize the performance of twin-screw extruders and minimize material degradation, manufacturers should consider the following:
- Screw design: The type and configuration of screws play a crucial role. Co-rotating screws are suitable for high mixing and kneading actions, while counter-rotating screws provide unique mixing and shearing capabilities. Intermeshing screws enhance material conveyance, melting, and mixing.
- Temperature control: The barrel is typically divided into multiple zones, each with independent temperature control. This allows for precise management of the temperature profile, which is crucial for processing temperature-sensitive materials and preventing material degradation.
- Venting and vacuum zones: These zones facilitate the removal of volatile components and air from the material, improving the overall quality and consistency of the final product.
- Die formation: The die shapes the molten material into the desired form. It is important to ensure that the die is properly designed and maintained to prevent material degradation and achieve the desired product quality.
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Venting problems
Operational Issues
Operational problems occur when the polymer or raw material exits the extruder vent openings. This usually requires operator intervention to clear out the vent ports or may even necessitate a shutdown of the production line. This issue is often caused by high vapour velocity, resulting from too much volume or insufficient open area. The solution is to increase the vent area, possibly requiring additional vent ports.
Another cause of operational problems is the presence of unmelted polymer at the first side feeder, which can result in filler coming out of the associated atmospheric vent. To address this, the screw design in the upstream part of the screw should be modified to ensure complete melting of the polymer.
Vacuum vents positioned near the pelletizing die are designed to release residual vapours. However, if the melt reaches the vacuum vent, it can block the opening, leading to porous pellets with voids. This issue can be resolved by decreasing the pressure, increasing the pumping length of the extruder, or installing a melt pump to pressurise downstream equipment. Worn screws should also be replaced to prevent chronic vent flooding.
Residual Volatiles/Gases
If insufficient quantities of volatiles are removed during the venting process, modifications are typically required in the screw design and/or barrel configuration. This issue can be addressed by reducing the throughput to increase the average residence time of the melt under vacuum. Alternatively, the vacuum section can be extended upstream by relocating the melt-sealing elements, or the vacuum level can be increased to the maximum.
Decreasing the degree of fill in the vacuum section by increasing the conveying-element pitch will result in thinner melt layers, facilitating the diffusion of gases. Reconfiguring the machine with additional vacuum vents can also enhance the removal of volatiles.
In summary, venting problems in twin-screw extruders can be effectively managed by addressing operational issues and ensuring sufficient removal of residual volatiles/gases. These solutions help maintain efficient production processes and improve the quality of the final product.
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Extruder overload
To resolve this issue, the operator or maintenance manager should check the extruder step by step to identify and eliminate the causes of the overload alarm. The solution is to slow down the feeder speed to reduce the amount of material intake.
Another reason for extruder overload is worn-out screw elements. After extended use, the twin screw extruder's screw elements may wear out due to friction with the material, reducing the extruder's feeding capacity. When the feeding rate remains unchanged, the extruder can quickly become overloaded. The solution is to remove the screw from the extruder, inspect the worn-out screw elements, and replace them if necessary.
In some cases, hard materials or metals may accidentally enter the extruder due to incorrect operation. This can cause the extruder load to increase suddenly. The solution is to remove the screw, check for and remove any hard materials or metals, and clean the barrel before reinserting the screw. If any screw elements are severely damaged, they must be replaced. Additionally, the shaft alignment should be checked, as the load on the screw and shaft will significantly increase when hard materials or metals enter the extruder.
An extruder heater fault can also lead to overload issues. If some heaters are not functioning correctly, specific zones of the twin screw extruder will not heat up, preventing the material from melting properly. This will cause an increase in the extruder ampere, triggering an overload alarm. To address this, the heaters should be checked using the temperature controller and solid-state relay to identify and replace the faulty heaters.
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Motor overheating
To address this problem, it is crucial to take immediate action to prevent further damage. Here are some practical solutions:
- Ensure Proper Ventilation: Adequate ventilation in the room housing the twin-screw extruder pelletizer is essential. Proper ventilation helps dissipate heat and prevents the motor from overheating.
- Inspect the Cooling System: Check the cooling system, which includes a heat exchanger, a water pump, and hoses, to ensure it is functioning correctly. Make sure the heat exchanger is clean and free of any clogging or blockage in the hoses.
- Reduce Load on the Motor: Monitor the load on the motor to ensure it does not exceed the motor's capacity. Overloading the motor can lead to overheating. Adjust the feed rate or screw speed to reduce the load and prevent overheating.
- Install a Thermal Protection Device: Implement a thermal protection device on the motor. This device will detect when the motor is overheating and automatically shut it down, preventing further damage.
Remember, it is essential to address motor overheating promptly to avoid disruptions in production, decreased productivity, and increased costs associated with equipment damage and downtime.
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Frequently asked questions
Some common problems with twin-screw extruders include:
- Extruder overload
- Screw elements wearing out
- Hard materials/metals falling into the extruder
- Extruder heater fault
- Raw material components not being well mixed or dispersed
- Venting problems
- Broken strips
- Unstable discharge
- Black spots on pellets
- Unstable mainframe current
- High start-up current
- Wear on screw elements
- Inconsistent pellet size
- Low output rate
- Material degradation
- Motor overheating
Solutions will depend on the specific problem, but may include:
- Slowing the feeder speed
- Changing screw elements
- Adjusting the temperature
- Improving material distribution
- Using high-quality materials
- Ensuring consistent material feed
- Optimizing screw design
- Reducing operating temperature
- Regular maintenance and inspections
- Adjusting the screw speed
- Improving ventilation and cooling
Twin-screw extruders offer several advantages over single-screw extruders, including:
- Superior mixing and homogenization capabilities
- High output rates and throughput
- Versatility in material processing
- Precise temperature control
- Improved melt quality
- Reduced residence time
- Modular design
- Energy efficiency