Extrusion is one of the most important processes in plastics manufacturing. In this process, plastic granulate is melted, homogenized, and pressed through a shaping die to continuously produce products such as pipes, tubes, films, or profiles.
The quality of these products depends heavily on stable process conditions—particularly the pressure in the extruder screw and the temperature control in the heating zones. Even small deviations can affect the melt viscosity and lead to dimensional deviations, surface defects, or even material damage.
Fluctuating pressure and temperature profiles not only impact product quality but also lead to scrap, machine downtime, and unnecessary costs.
This article explains the causes of pressure and temperature instability, the effects on production—and how these issues can be avoided using modern sensor technology and process monitoring.
Why are stable pressure and temperature conditions so important?
Extrusion is a continuous process in which the melt is forced through a shaping die under high pressure. For the material to flow evenly and be accurately formed, pressure and temperature must remain stable. Even slight fluctuations affect the viscosity of the plastic melt—i.e., its flow properties.
If the viscosity is too low, the material becomes too fluid, the forming process loses precision, and the product becomes unstable. If it is too high, the melt becomes difficult to convey, resulting in pressure spikes, mechanical overload, or even damage to the die.
Stable process conditions are especially critical for the following product groups:
Pipes and Tubes: When producing supply lines, heating tubes, or medical tubing, wall thickness, roundness, and inner diameter must be precisely maintained. Pressure or temperature fluctuations can lead to dimensional deviations, which may compromise sealing and functionality.
Technical Profiles: Window frames, cable ducts, or sealing profiles often feature complex cross-sections. Even minor irregularities in melt delivery can affect geometry, fit, and stability—resulting in waste or the need for rework.
Films and Sheets: In the packaging industry—and in technical applications such as solar films or construction panels—uniform layer thickness is essential. Unstable temperatures can cause waviness, sink marks, or thickness variations.
Depending on the industry, the requirements for process stability can be particularly high:
In medical technology, plastic products must not only be precisely formed but also free from particles and produced under defined temperature conditions. Any deviation may compromise the safety of the final product.
In the automotive industry, dimensional accuracy is important—but so is long-term resistance to heat, pressure, and chemical exposure. Faulty extrusion can compromise the functionality of entire assemblies.
In the packaging industry, process efficiency is just as important as quality: High throughput rates and consistent product quality are only possible with stable extrusion conditions.
Stable pressure and temperature conditions are the backbone of efficient and error-free extrusion. Losing control over these two parameters not only results in quality losses, but also leads to downtime, scrap, and rising costs.
Common Causes of Instability
Despite state-of-the-art extrusion systems, pressure and temperature fluctuations still occur regularly in practice. These instabilities directly affect product quality and process reliability. The causes are often technical or material-related—and in many cases, they can be easily resolved if detected early.
The following table provides an overview of common sources of error and their consequences:
| Cause | Description | Typical Consequence |
|---|---|---|
| Inaccurate or outdated sensors | Pressure and temperature sensors with insufficient resolution or worn tips deliver unreliable readings. | Incorrect process control, sluggish regulation, quality issues |
| Inadequate temperature control | Heating bands or control circuits operate unevenly or respond too slowly. | Viscosity fluctuations, local overheating or undercooling of the melt |
| Fluctuating material properties | Especially with recyclates, composition, moisture, or melt viscosity can vary significantly. | Unstable feed pressure, uneven output, inconsistent product dimensions |
| Wear in screw or die | Abrasion, deposits, or damage alter the conveying behavior and pressure build-up. | Irregular melt flow, pressure fluctuations, blockages |
| Missing or incorrect calibration of measuring instruments | Sensors provide incorrect readings if they are not regularly tested and adjusted. | Incorrect process data leads to misinterpretation and faulty control |
Practical tip: Many of these causes can be detected early through regular maintenance, modern sensor technology, and targeted process monitoring—helping to prevent long-term damage and production downtime.
Potential Consequences for Operations
Instabilities in pressure and temperature during extrusion affect not only product quality—they also have operational and economic consequences. What begins with an inaccurate sensor or a faulty heating controller can quickly lead to production interruptions and dissatisfied customers.
The following table summarizes typical effects:
| Problem | Effect on the Product | Effect on Operations |
|---|---|---|
| Dimensional and shape deviations | Inaccurate wall thickness, warped profiles, defective films | Increased scrap, customer complaints |
| Viscosity issues due to temperature fluctuations | Incomplete shaping, material failure (e.g. cracks) | Rework, delayed deliveries |
| Pressure fluctuations due to sensor errors | Inconsistent material flow | Line disruption, productivity loss |
| Accumulation of minor process errors | Inconsistent quality | Machine failure, increased maintenance |
| Misinterpretation of process data | Process deviations remain undetected | Financial losses due to inefficiency |
How to Prevent Instabilities
To reliably prevent pressure and temperature instabilities in extrusion, a combination of modern measurement technology, proactive maintenance, and well-trained personnel is required. The following measures help increase process reliability, reduce scrap, and ensure long-term product quality:
| Measure | Benefit for the Extrusion Process |
|---|---|
| Use of precise, temperature-compensated pressure sensors | Avoids measurement errors despite temperature fluctuations – ensures stable pressure control throughout the process |
| Continuous temperature monitoring along the entire line | Enables rapid detection of heating zone issues – allows targeted control and consistent melt viscosity |
| Viscosity measurement as an early warning system (e.g., with ViscoIndicator) | Detects material fluctuations in real time – enables immediate process adjustments |
| Regular maintenance, calibration, and operator training | Increases equipment availability, reduces downtime – enables faster response from personnel |
| Capture and evaluate process data (e.g., via digital interfaces) | Trend analysis, root cause investigation, and process optimization – foundation for automated quality assurance |
- Use of precise, temperature-compensated pressure sensors
High-quality pressure sensors with temperature compensation provide reliable readings even under challenging environmental conditions. This prevents incorrect control actions in the presence of significant temperature fluctuations and enables precise control of the extrusion pressure—essential for consistent product quality and dimensional accuracy.
- Continuous temperature monitoring along the entire line
Complete temperature monitoring at critical points such as the screw, heating zones, and die is key to detecting local deviations early. Hot spots, cold zones, or faulty heating elements can be identified before they affect melt viscosity or product shaping.
- Viscosity measurement as an early warning system (e.g., with the ViscoIndicator)
Viscosity is a sensitive indicator of changes in material flow. With continuous monitoring—such as using a ViscoIndicator—material fluctuations or temperature issues can be detected in real time. The operator can respond immediately, preventing scrap or quality defects.
- Regular maintenance, calibration, and operator training
Technical instabilities often arise from wear or insufficient calibration of measurement instruments. Scheduled maintenance and regular calibration ensure system functionality. Operator training is equally important to correctly interpret fault patterns and stabilize processes effectively.
- Capture and evaluate process data (e.g., via digital interfaces)
Consistently capturing and analyzing process data—such as pressure, temperature, and viscosity—lays the foundation for proactive process optimization. Digital interfaces like OPC UA or Modbus allow for centralized evaluation of this data and support automated quality control in line with Industry 4.0 standards.
In Conclusion
In plastic extrusion, small details often determine success or failure. Even slight deviations in pressure or temperature can alter melt viscosity—with significant impacts on dimensional accuracy, shaping, and process stability.
Modern sensor technology therefore plays a central role in quality assurance. Accurate pressure and temperature measurement, combined with real-time viscosity monitoring, provide the transparency needed to quickly respond to deviations.
Those who continuously monitor and regularly evaluate these measurement parameters not only increase the efficiency of their production but also significantly reduce downtime, scrap, and rework. In short: Reliable sensor technology makes extrusion not only more stable—but also more economical.
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