Infrared Thermography is a technique that can be employed in the plastic industry for process optimization and quality improvement, as well as in the development of new tools. In the thermoplastic injection process, a large amount of information about the transformation process can be collected by taking thermographic images of the recently injected pieces that are still in the mould or that have just been extracted.
ABS piece and mould.
Infrared Radiation
Infrared rays were discovered in 1800 by Frederick William Herschel. He noticed that the heat going through the coloured filters, with which he observed the sun, was dependant on the colour of the filter. Then, Herschel decided to shine the light through a crystal prism in order to generate a spectrum (the rainbow) and he measured the temperature of each colour. He observed that the temperature was higher on the red side, where there was no visible light to the human eye. This radiation was called "caloric rays", and finally became known as infrared radiation.
Around 1880, Samuel P. Langley invented the first infrared radiation detector (bolometer) which could detect radiation by the temperature increase produced by a heat absorbing body. In the 1980's the first micro-bolometers appeared which are currently used as detectors in thermographic cameras.
All objects that have a temperature superior to absolute "Zero" (0 Kelvin or -273.15 ºC); emit waves in the infrared band. When the temperature of an object increases, it emits more energy in a lower wavelength.
Infrared radiation, visible light and ultraviolet light are energy forms in the electromagnetic spectrum which differ in wavelength.
Thermopraphic Cameras
The human eye can only see a small range of wavelengths (from 0.4 till 0.75 µm); nevertheless thermographic cameras can detect infrared energy that is non-visible to the human eye. The normal range of temperatures these cameras can register is between -20ºC and 500 ºC, but this range can be extended from -40 ºC at the bottom end up to 2000ºC at the top end. The cameras transform the infrared energy into an image with a colour map which shows the temperature of the object at each point. This makes them very versatile and gives them an infinite range of applications both, in the plastic sector as well as other sectors.
One of the most important parameters when measuring temperature with a thermographic camera is the emissivity. It indicates the capacity of an object to emit infrared radiation. This capacity can be measured and the values go from zero, for non-emitting materials, to one for black objects. There are several variables that affect the emissivity of an object, such as the wavelength, the visual field, the geometrical form and the temperature.
If high precision is required when measuring the temperature of an object, we need to introduce the emissivity value of the object into the thermographic camera or into the analysis software. These already have the algorithms required to correct the temperature according to the emissivity values.
Applications in the Plastic Industry
Infrared Thermography is a technique that can be employed in the plastic industry for process optimization and quality improvement, as well as in the development of new tools.
In the thermoplastic injection process, a large amount of information about the transformation process can be collected by taking thermographic images of the recently injected pieces that are still in the mould or that have just been extracted, or even of the mould surface itself:
- Temperature deviation in the critical points (injection points, insertions, thicker areas of the piece, etc.)
- Detection of hot spots, caused by thermoplastic material friction in some area of the mould.
- Effectiveness of the temperature control.
- Effectiveness of the mould tempering system.
- Appropriate heat distribution both in the piece and the mould.
- Study of the temperature changes on the mould surface until the process becomes stable.
- Optimization of cooling times.
- Validation of results obtained using Simulation Programmes.
- As a practical case study to see the possibilities that thermography can offer, AIJU undertook a thermographic study of the injection process on a mould for a toy hobby-horse.
As can be seen in the figure, it is a one cavity mould with the feeding system by pouring cold with three entrance points on the workpiece. The test was undertaken using a Demag Ergotech machine of 110 Tn and Wittmann cooling equipment.
Thermography can be a useful and versatile tool for mould making and plastic transformation companies as it can provide a wide range of information about the behaviour of both, the mould and the material, and consequently process optimization. With this in mind, AIJU is tightly collaborating on different Research and Development projects with interested companies in the sector.