Automation of thermochromism compensation enables 100% inline inspection in plastics production
31-08-2017
Tackling thermochromism with colour and temperature sensors
In injection moulding processes, products are usually moulded with colour pellets supplied in batches and even supplied from different manufacturers. Therefore, it is essential to continually monitor precise colour values of these products to ensure they are exact. In order to provide 100% inline control of the product colour during injection moulding of plastic parts, the SKZ institute has developed and tested a method to correlate the difference in colour of an object at different temperatures - thermochromism behaviour, using colour spectrophotometers and infrared temperature sensors from Micro-Epsilon. Colour measurements carried out with a colorCONTROL ACS7000 spectrophotometer and temperature measurements using the CT series infrared temperature sensors, generate a master curve at several different temperature levels that describes the thermochromic behaviour of each specific colour or shade. This allows colour values measured on a warm product to be converted to the actual room temperature colour values.
The pre calculated ‘cooling curves’ are stored in the system software and allow the colour values determined during the de-moulding process, which is typically 60-80°C to be converted into a reference temperature, for example, 20°C. This enables the colour to now be automatically checked earlier in the process, which reduces cycle time, optimises productivity and increases yield, compared to conventional sample checks.
Inline quality control and colour inspection in injection moulding
Different inspection techniques can be skilfully combined, enabling easy, flawless in-process monitoring. In injection moulding, for example, the new moldCONTROL inline thermography system and the colorCONTROL ACS7000 inline colour measurement system, can be combined to measure and inspect during production. Here, a moulded component is conveyed directly in front of an infrared camera during the production process. Modern injection moulding systems are normally equipped with an automatic handling system for removal and storage of components. The positioning of the component in front of the camera enables a precise time window to be met when recording thermographic images. This ensures that the thermographic images from one video image to another can be compared. During automatic removal of the components, a “good/bad” component decision must be made within a short time frame. The aim here is to prevent problems that may disrupt further processing and to take any corrective measures as quickly as possible, to avoid any unnecessary additional costs.
Defect detection using thermal imaging
The inline thermography system moldCONTROL detects variations in quality by using a high speed and high resolution infrared thermal imaging camera. It records the entire component in up to six different views and examines it. The principle is simple: a thermoIMAGER TIM thermal imaging camera records the infrared radiation emanating from the mould and visualises it. The temperature distribution provides a quality statement about incorrect temperature control of the mould, malfunction of the tool temperature, visible geometry errors and hidden defects. The software displays the reference image, the IR image of the component and the difference between the two images. The identified temperature differences provide the basis for a Pass/Fail decision. Temperature alarm limits can be freely specified using the operating SKZ software. Defective components can be sorted and removed depending on these settings.
100% inline quality control for plastic products
By combining the colorCONTROL ACS7000 inline colour measurement system with correction for thermoschomism and MoldCONTROL thermaI imaging camera system for geometrical defect detection, 100% inline quality control of moulded parts is now possible using technology from Micro-Epsilon.
