Thermography in the Plastic Industry
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 1980 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.
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.
Figure 2 shows a table with the emissivity values of some materials:
• Materials : Emissivity (ε)
• Black Object : 1
• Human Skin : 0.98
• Water : 0.98
• Asbestos : 0.95
• Ceramic : 0.95
• Mud : 0.95
• Cement : 0.95
• Fabric : 0.95
• Gravel : 0.95
• Paper : 0.95
• Plastic : 0.95
• Rubber : 0.95
• Wood : 0.95
• Copper (rusty) : 0.68
• Stainless Steel : 0.1
• Copper (polished) : 0.02
• Aluminium (polished) : 0.05
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 temperature controller.
• 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 a thermographic study of the injection process on a mould for a toy hobby-horse.
It was 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.
The superficial temperature of the pieces obtained, which vary according to the cooling times established, could be observed:
- After 15s cooling, it was 1100, 1100 & 82’2 in three strategic points;
- After 30s cooling, 107’2, 83’8 & 67’1 in the same points, and
- After 60s cooling, 72’7, 57’6 & 52’3.
It could be appreciated that the widest areas of the piece had higher temperatures than the rest. According to the material used and the injection conditions, you can determine the time required to remove the piece from the mould without any difficulty, validate the refrigeration system of the mould and determine the cause of some problems in pieces such as shrinkage. In this practical case the influence of the cooling time was studied to optimise the process conditions, but a similar study can be done, applying it to other variables like the material temperature, the mould temperature, the injection speed (shear load problems), etc.
The experimentation showed the changes of the superficial temperature of the mould after several injection cycles, in such a way that it can be used to determine the times that a mould takes to become stabilised and consequently, to obtain pieces with the same properties, a basic aspect for technical pieces with high quality requirements.