Maximum Efficiency: Non-Contact Temperature Measurement of Plastics

Temperature is the critical measurement parameter in plastics processing where thermal processes play a specifically vital role. High, consistent product quality is guaranteed only by precise temperature control. Micro-Epsilon offers ideal measurement technology from a single source for nearly any polymer or material.

The process is monitored with the help of non-contact, infrared temperature measurement techniques based on infrared cameras for overall measurements and pyrometers for pinpoint measurements. In the plastics processing industry, non-contact measurement technology based on new, infrared measurement systems provides numerous advantages. It is possible to easily detect very hot measurement objects and also objects that are difficult to access or move very fast, at tremendously fast measurement and response times.

Micro-Epsilon’s modern infrared measurement systems are appropriate for a broad range of applications, from frozen foods to molten metals. Based on the product group, they detect a temperature range from −40 °C to +2200 °C. These values must be determined in real time and, if needed, should enable the immediate adaption of process parameters for ensuring high product quality and avoiding unnecessary rejects. Infrared cameras allow the temperature behavior over the entire material surface to be documented, while pyrometers measure a point. Their benefit is that they are available with different wavelengths, which allows determining temperatures of even very thin plastic films, where for instance, long-wave thermal imaging cameras operate at their limits due to the transmissivity of the material.

The Measuring Principle

All bodies with a temperature above absolute zero of −273.15 °C (= 0 K) emit electromagnetic radiation proportional to their own temperature on the surface, which is called as “intrinsic radiation” irrespective of whether the object is ice or hot steel. Infrared radiation forms a part of this radiation and can be used for temperature measurements. This radiation penetrates the atmosphere and is focused onto a detector element with the help of a lens (input optics) in the infrared measurement system, where the detector element generates an electrical signal proportional to the radiation. The signal is then amplified, digitally processed, and converted into an output size proportional to the temperature of the object. The measured value can be shown on a display or output as an analog signal, which allows easy connection to process control systems.

Emissivity, transmissivity and reflection are the three most significant factors in IR temperature measurement. The emissivity of a body signifies the amount of radiation emitted by the body compared to a typical heat radiator which is a black body. The transmissivity is pertinent for thin plastic films and changes with the wavelength. It is inversely proportional to the thickness, meaning thin plastic films are more permeable compared to thicker ones. Optimal measurement of temperature can be performed at wavelengths where the transmissivity is independent of the thickness, close to zero.

For instance, polypropylene, polyethylene, polystyrene and nylon are IR-impermeable at 3.43 μm. The thermoMETER CTP-3 can be used to determine the temperatures of these measurement objects. It has a temperature range of 50–400 °C. However, Teflon, polyester, FEP, polyurethane and polyamide are impermeable at 7.9 μm. In this case, the thermoMETER CTP-7 is used, which works precisely within this wavelength range.

The CTP-3 and CTP-7 thermometers

Ambient radiation penetrates thin plastic films falsifying the natural radiation of the film itself. The CTP-3 and CTP-7 thermometers are perfectly suited to transparent film types as they ignore penetrating radiation and enable high-precision temperature measurement.

Without cooling, this robust thermoMETER offers precise measurement values in ambient temperatures of up to 85 °C. For thicker (> 0.4 mm) and pigmented films, a wavelength in the range of 8–14 μm can be chosen for temperature measurements. The emissivity is between 0.9 and 0.95.

The thermoMETER CT series of IR temperature sensors have a modular design and can be used for a broad range of applications in non-contact temperature measurement. From low temperatures that are common in laboratories or cooling chains to the highest temperatures in blast furnaces and hot molten metals, these IR sensors provide reliable and precise measurement. Thanks to their compact design, the temperature sensors can be incorporated in applications with limited installation space, such as in the manufacture of very small devices, machine building, or OEM applications with multiple infrared measuring positions. The characteristic features of the thermoMETER product series are high precision, high resolution, and fast response times. IR sensors from Micro-Epsilon are the preferred choice to obtain reliable measurements, specifically with temperature-critical applications.

A Diverse Range of Applications Are Possible for Temperature Monitoring in Production Processes

Injection Molding

During the manufacture of injection-molded plastic parts, thermal imaging cameras allow the monitoring of product quality, specifically with respect to stability and accuracy of fit. The inspection of the cooling process is a crucial factor that guarantees the consistency of material densities within the injection-molded parts. Inhomogeneous cooling can lead to variations in material densities and can have undesirable effects on the material characteristics. In addition, unfinished molded parts that remain undetected upon visual inspection are immediately identified.

For the monitoring process, a component is conveyed directly in front of the thermal imaging camera at the time of the production process with the help of an automatic handling system for removal and storage of components, which are usually fitted in advanced injection molding systems. The moldCONTROL thermal imager is the inline thermography system used for component testing and allows continuous, rapid and cost-effective quality inspection of molded plastic parts directly in the processing line.

Injection Molding

The moldCONTROL is an inline thermography system for detecting changes in quality in the injection molding process. The compact thermal imager detects the entire part and inspects it to predefined parameters during the production process. A good/bad decision is made based on the identified reference values.

This system solution includes a thermoIMAGER infrared camera, a ready-to-use industrial PC, the moldCONTROL software, and a communication interface for machines. The moldCONTROL thermal imager can be incorporated into prevalent removal systems and machine control systems at a reasonable cost. The main advantages of this inline thermography system are the early recognition of quality fluctuations and, based on the measured values, a more rapid production start-up along with optimal tool temperature adjustment to minimize the production of waste.

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.

Blown Film Extrusion

Using blown film extrusion, the tubular film’s temperature should be measured precisely at various points to guarantee consistency and high product quality, as well as to reduce waste. The position of the frost line is a deciding factor that can prevent blocking of the take-off rollers if detected precisely.

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.

Thermoforming

Thermoforming of plastic sheets and films made of thermoplastics involves heating the material in the molding machine until the material is plasticized. On achieving a predefined temperature, the material is sucked into a predefined mold through a vacuum. The heating time is dependent on the material itself as well as on the material surface. For instance, dark plastics can be heated more rapidly compared to light-colored plastics, which implies that reliable temperature control is crucial. Else, the temperature would have to be detected in different, expensive test runs, which can be avoided by non-contact temperature monitoring.

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 the predefined temperature is achieved, the material is sucked into a predefined mold via a vacuum.

(Injection) Stretch Blow-Molding

This method is meant for thermoplastics such as PVC, PET, and PP, which are usually processed into bottles. PET bottles are produced by using hollow plastic parts. First, the preforms are heated to a temperature ranging from 80 to 120 °C. The plastic material turns viscous and is clamped into a mold. During the so-called compensation times, the preforms are not heated anymore and their temperature is compensated for over the whole wall thickness. In the second stage, the actual molding process occurs in a blowing wheel of the stretch blow molder. Lastly, the complete molded bottle is cooled with water. Specifically, the process in which the bottles are heated to an appropriate processing temperature should be controlled to ensure high-quality molding of the preforms. The temperature over the entire preform surface can be monitored using infrared cameras.

(Injection) Stretch Blow-Molding

(Injection) Stretch blow molding is intended for thermoplastics such as PET, PVC, and PP, which are often processed into bottles.

Flat Film and Sheet Extrusion

Extrusion of flat film and sheets involves pressing the molten material through large, slotted nozzles and further processing it in a calender, where the extruded parts are cooled down in a number of steps. It is necessary to perform infrared measurement at many points to control the film temperature and to ensure smooth processing. As a result, cracks, overheating, and surface defects become visible. Rapid detection of defects is an essential instrument that aids in preventing high rejection rates and costs.

Flat Film and Sheet Extrusion

In the extrusion process of flat films and sheets, molten material is pressed through large, slotted nozzles and further processed in a calendar, where the extruded parts are cooled down in stages.

Summary

Since temperature measurements are decisive factors for quality and smooth production, they are crucially important, specifically in plastics processing. Based on the particular application, temperature can be determined either by covering the entire surface area with the help of an infrared camera or point by point using pyrometers. Fast response times and high-precision temperature detection directly in the processing line are the advantages of these systems. In this way, infrared temperature measurement helps reduce waste and its associated high costs. Probable system control errors are determined in real time, which enables fast intervention measures.

Micro-Epsilon is a full-service provider offering appropriate measurement technology for almost every material used in the plastics industry. The infrared temperature sensor product series comprises both infrared cameras and pyrometers. Based on the application, these infrared cameras and point sensors can be combined, which allows high quality standards in the production process. Micro-Epsilon provides cross-product consulting, sales, and support services from a single source.

injection-molded plastic parts

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.

Non-contact thermometers and IR cameras are used for

  • Thermoforming
  • Extrusion of blow films, flat films, and plastic plates
  • Injection molding
  • Laminating and shaping
  • Plastic welding
  • Coating of plastics

This information has been sourced, reviewed and adapted from materials provided by Micro Epsilon.

For more information on this source, please visit Micro Epsilon.

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