Reliable testing technology for plastics processing

In plastics processing, only precise temperature control ensures high, consistent product quality. The process is monitored using non-contact, infrared temperature measurement methods based on pyrometers for pinpoint measurements and infrared cameras for overall measurements.

In plastics processing where thermal processes play a particularly important role, temperature is the crucial measurement parameter. Only precise temperature control ensures high, consistent product quality. Micro-Epsilon provides suitable measurement technology from a single source for almost any material or polymer.

The process is monitored using non-contact, infrared temperature measurement methods based on pyrometers for pinpoint measurements and infrared cameras for overall measurements. Non-contact measurement technology in the plastics processing industry based on modern, infrared measurement systems offers many advantages. Very hot measurement objects, as well as difficult-to-access or rapidly moving objects can be easily detected at extremely fast measurement and response times.

Modern, infrared measurement systems from Micro-Epsilon are suitable for a wide range of applications, from frozen foods to molten metals. Depending on the product series, they detect a temperature range from -40 °C to +2200 °C. These values have to be determined in real-time and enable, if required, the immediate adaption of process parameters in order to ensure high product quality and to avoid unnecessary rejects. Infrared cameras enable the documentation of temperature behavior over the whole material surface, while pyrometers measure a point. Their advantage is that they are available with different wavelengths, which means temperatures of even very thin plastic films can be determined, where for example, long-wave thermal imaging cameras are operating at their limits due to the material’s transmissivity.

The measuring principle
Every body with a temperature above absolute zero of -273.15 °C (= 0 Kelvin) emits electromagnetic radiation proportional to its own temperature on the surface which is the so-called “intrinsic radiation” regardless of whether the object is ice or hot steel. Part of this radiation is infrared radiation and can be used for temperature measurements. This radiation penetrates the atmosphere and is focused by a lens (input optics) in the infrared measurement system onto a detector element, which in turn generates an electrical signal proportional to the radiation. The signal is amplified, digitally processed and converted into an output size proportional to the object temperature. The measured value can be shown on a display or output as an analog signal, which enables easy connection to process control systems.

The three most important factors in IR temperature measurement are emissivity, transmissivity and reflection. The emissivity of a body indicates how much radiation it emits compared with an ideal heat radiator which is a black body. The transmissivity is relevant for thin plastic films and varies with the wavelength. It is inversely proportional to the thickness whereas thin material is more permeable than thick plastic films. Optimal temperature measurement is possible at wavelengths where the transmissivity is independent of the thickness, close to zero. Polyethylene, polypropylene, nylon and polystyrene are, for example, IR-impermeable at 3.43µm. The temperatures of these measurement objects can be determined using the thermoMETER CTP-3. The temperature range of the thermoMETER CTP-3 extends from 50°C to 400°C. However, polyester, polyurethane, Teflon, FEP and polyamide are impermeable at 7.9µm. Here, the thermoMETER CTP-7 is used, which operates precisely within this wavelength range. Without cooling, this robust thermoMETER provides precise measurement values in ambient temperatures up to 85 °C. With thicker (> 0.4mm) and pigmented films, a wavelength between 8 and 14µm can be selected for temperature measurements. The emissivity is between 0.9 and 0.95.

IR temperature sensors of the thermoMETER CT series have a modular design and can be used for a wide variety of applications in non-contact temperature measurement. From low temperatures prevalent in cooling chains or laboratories, to the highest temperatures in hot molten metals and blast furnaces, these IR sensors measure precisely and reliably. Due to their compact design, the temperature sensors can be integrated in applications where installation space is restricted, for example, in machine building, manufacturing of extremely small devices or OEM applications with multiple infrared measuring positions. Fast response times, high precision and high resolution are distinctive features of the thermoMETER product group. Particularly with temperature-critical applications, IR sensors from Micro-Epsilon are the preferred choice in order to achieve reliable measurements.

A diverse range of applications are possible for temperature monitoring in production processes:

Injection molding
In the production of injection-molded plastic parts, thermal imaging cameras enable the monitoring of product quality, particularly with regards to stability and accuracy of fit. The inspection of the cooling process is a critical factor that ensures the material densities within the injection-molded parts are consistent. Inhomogeneous cooling can cause different material densities and can have adverse effects on the material characteristics. Also, incomplete molded parts that remain undetected by a visual inspection are immediately recognized. For monitoring, a component is conveyed directly in front of the thermal imaging camera during the production process using an automatic handling system for removal and storage of components, which modern injection molding systems are normally equipped with. The inline thermography system which is used for component testing is the moldCONTROL thermal imager, which enables fast, continuous and cost-effective quality inspection of molded plastic parts directly in the processing line. This system solution includes a thermoIMAGER infrared camera, a ready-for-use industrial PC, the moldCONTROL software and a communication interface for machines. The moldCONTROL thermal imager can be installed into existing removal systems and machine control systems at moderate cost. The primary advantages of this inline thermography system are the early recognition of quality fluctuations and, based on the measured values, faster production start-up along with optimal tool temperature adjustment in order to reduce waste.

Blown film extrusion
With blown film extrusion, the temperature of the tubular film must be measured precisely at different points in order to ensure high product quality, consistency and to minimize waste. The position of the frost line is a decisive factor, which if detected precisely, avoids blocking of the take-off rollers.

Thermoforming
In thermoforming of plastic sheets and films made from thermoplastics, the material is heated in the molding machine until the material is plasticized. When a predefined temperature is achieved, the material is sucked into a predefined mold via a vacuum. The heating time depends both on the material itself and on the material surface. Dark plastics, for example, can be heated more quickly than light-colored plastics, which means reliable temperature control is critical. Otherwise, the temperature would have to be detected in diverse, expensive test runs, which can be avoided using non-contact temperature monitoring.

(Injection)Stretch blow-molding
This technique is intended for thermoplastics such as PET, PVC and PP, which are often processed into bottles. Hollow plastic parts are used for the production of PET bottles. The preforms are first heated to temperatures between 80°C and 120°C. The plastic material becomes viscous and is clamped into a mold. During the so-called compensation times, the preforms are not heated further and their temperature is compensated for across the entire wall thickness. In the second stage, the actual molding process takes place in a blowing wheel of the stretch blow molder. Finally, the finished molded bottle is cooled with water. In particular, the process where the bottles are heated to a suitable processing temperate must be controlled in order to ensure high quality molding of the preforms. Infrared cameras enable the monitoring of temperature across the entire preform surface.

Flat film and sheet extrusion
In the extrusion process of flat film and sheets, molten material is pressed through large, slotted nozzles and further processed in a calender, where the extruded parts are cooled down in several steps. Infrared measurement is required at several points in order to control the film temperature and to ensure smooth processing. Therefore overheating, cracks and surface defects become visible. Fast defect detection is an indispensable instrument that helps to prevent high rejection rates and costs.

Summary
As decisive factors for quality and smooth production, temperature measurements are vitally important, particularly in plastics processing. Depending on the respective application, temperature can be determined point by point using pyrometers or covering the whole surface area using an infrared camera. The advantages of these systems are fast response times and high precision temperature detection directly in the processing line. This is how infrared temperature measurement helps to minimize waste and its associated high costs. Possible system control errors are determined in real time, enabling fast intervention measures.

Micro-Epsilon is a full-service provider that offers suitable measurement technology for almost any material used in the plastics industry. The infrared temperature sensor product range includes both pyrometers and infrared cameras. Depending on the application, these point sensors and infrared cameras can be combined, which enables high standards of quality in the production process. Micro-Epsilon offers cross-product consulting, sales and support services from a single source.

Author:
Dipl.-Ing. Manfred Pfadt
Product Manager
Micro-Epsilon Messtechnik GmbH & Co.KG