Extrusion, Problems

On-line Ultrasonic Technology for Monitoring of Polymer Processing

On-line Ultrasonic Technology for Monitoring of Polymer Processing

The Industrial Materials Institute (IMI) of the National Research Council of Canada (NRC) elaborates strategies for improved materials processing. The methods hinge strongly on process instrumentation and rely on development of new sensors for process monitoring and control. For the polymer industry, IMI has developed a powerful technique that uses ultrasonics for on-line rheological, physical, and chemical characterization.

The new technique utilizes low energy ultrasonic waves to sense the behavior and the properties of polymers during actual processing.

Probes, fitted to the flow channel, generate pulses of ultrasound. In propagating through the polymer, the characteristics of the pulses are modified depending on the properties of the melt.

The instrument gauges the characteristics of pulse propagation (sound velocity, sound attenuation and reflection coefficient), and can provide important information on: viscosity, molecular weight, composition of blends or alloys, as well as concentration and dispersion of additives during polymer extrusion, and on flow front arrival, end of filling, part detachment and solidification during injection molding.

The method exploits results from extensive research on ultrasonic characterization of polymers, and work is ongoing to evaluate new applications.

A practical Tool for the Polymer Industry

The technique is non-invasive and therefore causes no disruption to the process.

The ultrasonic sensors involve no moving parts For polymer extrusion, these sensors are designed for insertion in standard pressure ports. The instrument was designed to be rugged and reliable; its operation and maintenance require no special skills. They can be operated in pulse/echo mode which requires only one side access.

A personal computer houses all the electronics and provides automated real time measurement and data analysis.

Potential Uses and Benefits

Potential uses relate to manufacturing processes such as extrusion, compounding and injection moulding. In contrast to usual temperature or pressure sensors, the ultrasonic technique provides real time characterization of actual material properties, allowing rapid optimization of end-product quality.

Quality Control

The technique ensures consistent quality of production, by indicating off-specs in flow behavior, composition, concentration and dispersion of fillers and additives, screw wear and misalignment thus minimizing the requirements for off-line quality control.

Process Control

By providing immediate information on material properties, processes such as extrusion, compounding and injection molding can be optimized with respect to additive concentration and throughput or with respect to energy costs.

The ultrasonic method helps to:
* minimize start-up problems and start-up time
* speed up attainment of uniform production
* maintain standard quality of end-product

Applications

The development of each application results from specific studies which allow to analyze the dependence of ultrasonic propagation characteristics on the material or process characteristics.

Concentration/Dispersion of Fillers

Ultrasonic response of filled polymers depends strongly on the concentration, particle size and dispersion of fillers. By continuously monitoring formulation and dispersion, ultrasound can greatly improve quality and consistency.

The technique has been demonstrated on several widely used filler systems. Concentration of calcium carbonate in polypropylene, can be monitored to 0.5 wt%. accuracy, on-line and in real time.

Composition of Polymer Blends

When polymers are mixed, the ultrasonic response has been shown to depend strongly on composition, thus making ultrasound measurements a powerful tool to monitor polymer blending operations.

It can be particularly valuable, for example, in plastics recycling via polymer melt blending when exact composition is unknown or variable. Adjusting the feed of different product streams according to ultrasound variations can help optimize product consistency.

Molecular Weight Distribution

Conditions in continuous reactors or reactive extrusion processes can be adjusted according to changes in ultrasonic characteristics of the polymer to achieve improved quality and consistency.

For example, during peroxide degradation of polypropylene, on-line measurement of ultrasonic velocity can be used to monitor molecular weight. The information can then be applied to a feedback loop to control the process by modifying initiator feed rate.

Extrusion Flow Instabilities

Numerous experiments have demonstrated that the ultrasonic technique probes the viscoelastic properties of the polymer as it flows through a die. Moreover it was shown that ultrasonic results depended on the nature of the flow regime.

For instance, the technique can discriminate between laminar flow, sharkskin, surging and melt fracture. It can be used to predict the onset of one of these regimes during extrusion of profiles or film. Thus, in this mode, the method serves to optimize processing and at the same time ensures a uniform quality of the end product.

As another example, the technique probes the interface between the polymer and the die, and can be used to measure the effectiveness of processing aids that are often added to materials such as polyethylene.

Extrusion of Polymer Foams

Manufacturing of polymer foams involves delicate control of materials properties and processing parameters. Knowing solubility of physical blowing agents and phase diagrams may serve to guide process design. The ultrasonic technique provides a unique and non-invasive tool to monitor in real time phase separation in polymer/blowing agent mixtures and bubble nucleation. The method can serve to determine the optimal conditions of the foaming process or to investigate new blowing agents.

Screw Wear and Misalignment Monitoring during Material Extrusion

The flow behavior in the extruder is strongly affected by the geometry of the screw. The friction, or corrosive nature of some extruded materials can cause significant screw wear. Furthermore, the torque exerted on the screw shaft by highly viscous materials during extrusion, or an erroneous assembly of the extruder may deform the screw shaft in such a way that it deviates appreciably from the central position along the axial direction of the barrel. The screw wear and misalignment can result in poor quality products. The ultrasound technique constitutes a fast and economic way for in-line monitoring of the screw status. This provides an extra safeguard on the quality control.

Injection Molding of Polymers

Injection molders always seek new methods to optimise the molding process and to reduce cost. The non-invasive ultrasonic sensors attached to the external surface of the mold can be operated in a pulse/echo mode to monitor molding process in-line. The ability of ultrasound to detect the local flow front arrival, the end of filling, the solidification of the polymer melt, the detachment of the part, the solidification, the temperature of the mold and the molded part can lead to low cycle time, long mold life, energy saving and product consistency during injection molding

Working with Industry

At IMI, the collaboration of polymer processing and ultrasound technology specialists has uncovered outstanding potential in using on-line ultrasonic measurements. The work so far has shown that measurement of ultrasonic velocity and attenuation in a flowing polymer melt has numerous potential applications.

Other exciting new applications for ultrasound technology are now emerging from the laboratory that gave polyethylene producers the DS-500, the ultrasound based automatic density determination system that became an ASTM standard method.

Technology Transfer

The on-line monitoring of polymer processing using ultrasonics technology is available to the Canadian industry. Companies with R&D needs can benefit from several progressive technology transfer opportunities. The work can be carried out in the form of:
* a joint R&D project with an integrated approach which could involve other expertise available at IMI in modelling and control
* a feasibility study for process or product validation
* technical support involving specific or unique expertise or equipment

Information

To learn more about these and other technologies, to benefit from NRC's R&D resources and give your firm a technological advantage, you are invited to contact IMI representatives.
Dr. Cheng-Kuei Jen
Group Leader
Characterization and Ultrasonic Sensors
Tel.: (450) 641-5085
Fax: (450) 641-5106
E-mail: Cheng-Kuei.Jen@cnrc-nrc.gc.ca

Ngoc Huynh, Eng., M.A.Sc., M.B.A.
Business Development Officer
Modelling and Diagnostics Section
Tel. : (450) 641-5135
Fax : (450) 641-5106
E-mail: Ngoc.Huynh@cnrc-nrc.gc.ca