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PROCESS CONTROL | IN-LINE MEASUREMENT


Figure 1: Measuring melt index with an in-line rheometer gives real-time melt index data that can show precisely when material is outside of specification limits. This can be used for more speedy product changeovers


Source: Goettfert


as quickly as possible. “Deviation in process stability is displayed instantly, making drift or fluctuations in the process immediately apparent. Compared to lab-based testing, in-line measurement offers a range of advantages both in terms of response time and a wealth of information that cannot be gained by a few lab measurements covering a total production run,” says Eker. He adds that laboratory tests performed on solid samples taken at a specific time can also be correlated to data from in-line measurements if they are accurately time stamped. In-line real-time measurement forms a critical component of the “Industry 4.0” concept of process control enabled by digitisation and data analysis. “The more ambitious manufacturers in the polymer industry are already adopting this new manufacturing philosophy with very good results in terms of costs as well as increased quality and supply reliability,” says Eker. He says that in-line measurement has been demonstrated to be especially helpful in compounding and processing of recycled material, where the stability and possible range of input materials is particularly challenging. “The recycling companies are confronted with a


mass of materials in different qualities and with different additive contents. From this they are then supposed to produce high-quality products competing against pure polymers – an extremely challenging task,” Eker says. “In-line process measurement helps the recyclers maintain process stability, weed out finished product that has been polluted by unwanted raw materials, and confirm to the customer that the product delivered matches the quality specified in the customer’s order. Our in-line technology has been successfully used in recycling for over five years and has shown its value in delivering top output quality in this difficult environment.” ColVisTec argues that recyclers get significant


34 COMPOUNDING WORLD | November 2020


returns when they adopt in-line testing and other technologies. Eker says that, in order to be able to deliver what future markets requires, the plastics recycling sector has no choice but to adopt these better process and product monitoring technologies.


In-line rheology The benefits of process and product monitoring also apply to in-line rheology testing. Melt index (MFI) is a widely-used quality control test because it correlates directly to polymer properties. In-line melt-flow measurement promises real-time quality control of the melt in the extruder under processing conditions. Various types of in-line melt-flow measurement systems are available, says Tim Haake, General Manager of Goettfert. Some take a continuous bypass stream, measure it, and discharge the sample as a waste stream, but he says that the more sophisticated (and also more expensive) approach is use a return melt rheometer that directs the sample stream back to the extruder. This avoids material loss and the need to handle a waste stream. Haake says that care must be taken in setting up an in-line rheometer if it is to correlate melt index results with those from laboratory equipment. “One needs to find the right setup for die size, pump speed and pressures,” he explains. “Sometimes it is possible to work with a single drive and single capillary to cover the required range of measurements. If you run a high melt and the pump is also running fast, the results are often quickly available. However, emulating the forces seen in a melt indexer with a low melt will result in slowly running pumps, with a response time too slow for users trying to optimise their process.” One solution is to use a rheometer with an extra pump designed solely to speed up the flow through the bypass. This gives a fast, reliably


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