Inline profile thickness measurement systems for plastics

Paul Boughton

Plastics processing companies and extruders of blown plastic films can now source complete, inline precision measurement and inspection systems from a single UK supplier.

Micro-Epsilon’s Systems Group specialises in the development, manufacture and installation of a comprehensive range of non-contact, inline and offline measurement and inspection systems for process monitoring and quality control applications. These include specific measurement systems for plastics processing applications, including systems for measuring thickness and profile of blown film, extruded plastic sheet, strip and plates.

Technical advantages include fully automatic calibration. The system periodically re-measures against a known reference. The system also has integrated measurement sensors in the framework and mechanical parts, which measure the thermal expansion of the system. The software then compensates accordingly.
Micro-Epsilon’s range of measurement systems for plastics comprises the following:

Part of Micro-Epsilon’s thicknessCONTROL family, the FTS 8102 is a C-frame system that measures the thickness profile of plastic strip and sheet. C-frame systems are modular in their design, offering flexibility and high performance. These systems can be installed in plastic extrusion and calendar lines, providing reliable, high precision measurement results that create a basis for controlling the production process and product quality.
C-frame systems measure differentially. An application-specific (confocal, capacitive or laser triangulation) displacement sensor is integrated in the upper and lower flanges of the C-frame. The thickness of the target material is derived from the distance between the two sensors and the resulting signals. By combining highly efficient signal processing algorithms and visualisation software, the system is able to achieve accuracies to +/- 1µm. These systems are therefore suitable for use in extrusion lines for cast film or deep drawn film; blown film lines after collapsing; melting calendar lines; and extrusion lines for plastic profiles and plates.
The C-frame can be used as a traversing thickness measurement system on linear axes in order to ensure full width measurements of the target. The control and analysis software is developed in-house by Micro-Epsilon specifically to match the needs of the plastics industry, providing all the required functions in order to continuously record and evaluate production quality. This includes graphical user interfaces and the ability to store multiple product libraries. Also, the software supports and displays Statistical Process Control (SPC) parameters and other ‘Process Capability’ analysis and reporting methods such as Six Sigma ĈPK and ĈPM. Various fieldbus interfaces are available, which enable integration to the customer’s production control systems.
Fully automatic calibration ensures that measurements are not affected by temperature, enabling the system to be installed as a fixed, inline measurement system in harsh production environments. All sensor technologies used are non-contact, wear-free and do not use isotopes or X-rays. The C-frame system provides long-term measurement reliability at an affordable price. C-frame systems can be used to measure strip widths up to 1250mm and strip thickness from 10µm up to 50mm.
For larger material widths up to 4,000mm, Micro-Epsilon’s FTS 8101 O-frame systems are able to measure plastic thicknesses from <100µm up to 10mm at accuracies of ± 5µm.
O-frame systems measure differentially and incorporate many of the same features, functionality and software as C-frame systems. An application-specific displacement sensor is mounted on a mechanical carriage in the upper and lower flanges of the O-frame, which detects the upper side of the target material. This works in combination with an eddy current sensor in a housing, which detects the underside of the opposite target via a measuring roll. The thickness of the target material is the difference between the two sensor signals. This provides the advantage of ensuring a ‘true thickness’ measurement of the product irrespective of rollers and bearing wear, which would cause errors in conventional measurement systems. In addition, a sophisticated thermal compensation algorithm corrects for changes in the measured thickness, by compensating for thermal expansion of mechanical parts.
Micro-Epsilon’s thicknessCONTROL BTS 8104 system measures profile thickness of non-conductive blown film, including film used for packaging, agricultural applications, waste bin liners, carrier bags, freezer bags and shrink covers.
Designed as reversing systems, the BTS 8104 is based on capacitive measuring technology. Capacitive sensors are mounted directly behind the calibration cage on the film bubble, therefore offering very fast, efficient control of the production process. Adaptive reversing speed enables measurements to be adjusted according to each stage of the extrusion process.
The BTS 8104.NC system, for example, is based on a patented pneumatic positioning control system that utilises non-contact air bearing technology. These bearings enable very precise positioning and compensate for any movement on the film bubble. The system is ideal for measuring highly adhesive surface protection films on the bubble very close to the blow head.
The BTS 8104 system measures film widths from 255mm up to 3,600mm (double flat) and film thicknesses from 5µm up to 300µm.
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