Thermochromism compensation enables 100% inline inspection in plastics production

Author : Louise Dodd, Marketing Manager at Micro-Epsilon

07 March 2018

Naturally, for consumer devices, colour (or multiple colours) is a key consideration, both from a branding perspective and to make the product as appealing as possible to consumers. However, verifying correct colour consistency via inline inspection is challenging for manufacturers. This piece discusses a potential solution for production lines.

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Plastics are a vital component in the electronics industry, with many consumer electronics devices utilising injection moulded plastics in their construction – from micro-sized components to complete product bodies. Versatile, durable and lightweight, plastic also provides insulation and prevents harmful elements, such as water or heat, from damaging the electronics.

It is generally understood that the effect of temperature will change the observed colour of a product. Even with a temperature difference of just 20°C, colour deviations of more than 2 ?E units can occur for certain colours. This effect is known as ‘thermochromism’. Therefore, it is usually necessary to allow the product to cool before a precise colour measurement can be taken.

In a drive for 100% inline inspection and reduction of product wastage to zero, Micro-Epsilon has been working closely with SKZ (Süddeutsche Kunststoff-Zentrum, the largest plastics institute in Germany) to develop a working solution that compensates for the effects of thermochromism.

Tackling thermochromism with colour and temperature sensors

In injection moulding processes, products are normally moulded with colour pellets supplied in batches, sometimes from different manufacturers. It is therefore essential to continually monitor precise colour values of these products to ensure that they are exact.

In order to provide 100% inline control of 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 (thermochromic behaviour) – using Micro-Epsilon’s colour spectrophotometers and infrared temperature sensors.

Colour measurements, carried out using 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 (typically 60 to 80°C) to be converted to a reference temperature, for example, 20°C. This enables the colour to be automatically checked earlier in the process, reducing cycle times, optimising productivity and increasing yield, compared to conventional sample checks.

100% inline quality control of plastic products

Different inspection techniques can also be skilfully combined, enabling easy, flawless in-process monitoring. With injection moulding, for example, Micro-Epsilon’s new moldCONTROL inline thermography system, and their colorCONTROL ACS7000 inline colour measurement system, can be combined to measure and inspect both colour and geometrical defects during production. 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.

By combining Micro-Epsilon’s colorCONTROL ACS7000 inline colour measurement system with correction for thermochromism, with the moldCONTROL thermal imaging camera system for geometrical defect detection, it is now possible to achieve 100% inline quality control of moulded parts.

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