Injection moulding process optimization by online material monitoring
Elodie Bugnicourt(1), Kevin Murphy(1), Oana Ghita(2), Paul Brookbank(2), Raquel Ventura(3)
1 –Innovació i Recerca Industrial i Sostenible (IRIS), Parc Mediterrani de la Tecnologia- Castelldefels, Spain
2- College of engineering, mathematics and physical sciences, University of Exeter, North Park Road, Exeter, Devon EX4 4QF, UK
3- Fundacio privada ASCAMM, Avenida Universitat Autonoma , Parc Tecnologic Del Valles 23, Cerdanyola Del Valles, 08290, Spain
Corresponding author: firstname.lastname@example.org
Injection moulding is a complex process. The properties of the produced parts are directly dependent on the quality of supplied raw materials and process variables including temperature, pressure and speed, among others. OPTIJECT project is based on the feasibility of using spectroscopic probes in injection moulding machines for monitoring material parameters but also processing conditions. This real time control is expected to hold significant benefits for injection moulders in terms of improved quality, higher productivity and cost reductions.
Key words: Injection moulding, plastics converting, spectroscopy, in-line process monitoring, real time materials control
Plastic products are present in all areas of our daily life from toys, kitchen utensils, bottles and electric appliances to car parts and approximately 30% of them are processed by injection moulding making it the major plastic converting technique. A proper control of the process as well as material parameters such as temperature, moisture, color and speed of injection is required to obtain good parts repeatedly. However this can be difficult and currently most adjustments are based on quality control carried out after injecting the parts, generating economic losses due to the amount of scrap generated. A technology that would allow molders to monitor and control the injection process inline would bring substantial benefits due to improved quality control practices, increased productivity associated with reduced scrap and improved cost competitiveness.
Spectroscopic techniques such as Near-infrared (NIR) has been previously used to monitor the degradation, colour (Ghita et al.2008), moisture content (Fornes et al, 2003; Dumitrescu et al, 2005), concentration of fillers (Santos et al, 1998; Reis et al, 2003) and proportion of polymers blends (Fischer et al, 1997; Rohe et al, 1998) during the extrusion and injection of polymers. Although based on this past research, the OPTIJECT project has much greater ambitions of delivering a new tailored spectroscopic device and its control system for the online monitoring of injection process. This will allow for the predicting of a number of material and process parameters while also providing a full methodology of calibration. Currently a few probes are available for the real time monitoring of single machine parameters (temperature T, pressure P, etc.) but there are no integrated techniques giving access to all parameters simultaneously. Even fewer techniques are available for determining the variables related to the material during the process which can predict the compliance of the final part with quality specifications.
The University of Exeter have previous experience of integrating spectrophotonic instrumentation into injection moulding equipment. The colour characterization described here is an early example of tests carried out using NIR spectroscopy to monitor colour concentrations in different polymers matrixes. Tests were first made offline by monitoring injected bars and then online within the injection machine. 20 Spectra were recorded for each concentration between 0.0 and 2.0% with 0.1% concentration increments (weight metering of proportions). Figure 1 shows the effect of the amount of added color on the NIR spectrum: an increased color concentration in the mixture (in this case red masterbatch dispersed in polyethylene terephthalate PET) is reflected by a change in signal absorbance in the indicated areas (Figure 1). The amount of IR radiation absorbed is proportional to the IR active groups in the masterbatch changing the collected signal. The collected spectra are subjected to mathematical pre-processing to reduce the variability between spectra of the same concentration which then go forward to be processed using partial least squares regression to produce a linear predictive calibration (Figure 2).
Figure 2 shows the correlation between the true concentration of a solid red masterbach dispersed into polyethylene terephtalate (PET) as controlled by the dosing system of color and the predicted concentrations calculated from the NIR spectra. Result acquired online reached an excellent correlation with over 99% linear regression. This measurement is expected to be of interest for moulders who often have to rely on the recommendations of their masterbatch suppliers to determine the colour concentration needed and are left with no possible control of the proper adjustment of their colour metering system. Nevertheless, this measurement does not directly monitor the perceived colour as on the final part, therefore complementary experiments are ongoing using VIS spectroscopy in addition to NIR to measure the colour itself according to the CIELAB scale. Related data will be published as well as existing correlations between the colour measured inline in the polymer stream and the final injected part which is the only data currently available to the moulder.
As just seen, thanks to a preliminary calibration, the same principle can be applied to a wide variety of material and process variables (besides colour, expected monitorable parameters via OPTIJECT are listed in Figure 3). A large number of tests are being performed to find the best configuration and suitability of the system to monitor each parameter of interest. It is therefore expected to be able to predict whether the characteristics of the molded parts will be within the desired specifications with an excellent accuracy. In particular, OPTIJECT will give access to the material moisture and degradation levels before the part has left the mould which is a real breakthrough. In addition, regarding the process parameters, as previously mentioned, some probes already exist for the monitoring of some of them (eg. T and P), but often these values are taken at the level of the machine and not within the polymer, therefore OPTIJECT will give a more reliable picture of the effect of the process on the quality of the material. It may also be possible to provide access to additional parameters such as the polymer density as reflected by material processability.
The OPTIJECT technology (scheme in Figure 3) is expected to be able to monitor these parameters by locating NIR and VIS spectroscopic probes in the stream of polymer within the machine nozzle area. A prototype of the OPTIJECT system that can be suited to different injection machines is being designed and built by the company IRIS.
A really important part of the system will also lie in the interface for controlling the system and the data processing (and artificial intelligence behind) that will allow for the flagging of any parameter which is out of specification or even advising the operator on the changes needed. This work is being implemented by the ASCAMM foundation.
During the upcoming months, the full system will be validated in injection molding machines (of various brands and various sizes) of the SMEs moulders within the consortium to assess whether its use results in improved injection process in industrial conditions and the quality of parts produced.
OPTIJECT system development will cover an important gap in the market since, to date, no online monitoring techniques for the injection process which allows for controlling of the material properties in real time exists. In fact, the adjustment of injection machines are based today on the criterion of the operator and controls of final part quality, making it impossible to take corrective actions in real time, resulting in a quite significant percentage of defective parts. It was reported that in some applications, the waste can be as high as 15% which limits significantly the margin of plastic moulders. On the one hand, the OPTIJECT system will allow for the decreasing of this scrap amount and therefore increase moulders’ profitability; also resulting in a lower environmental impact for plastic converting activities. On the other hand, it will respond to the high demand of certain high value added markets in terms of part quality, such as the medical sector that would benefit from an accurate inline control of degradation within the polymer parts obtained as even trace amounts represent a risk for the end user and need to be accurately controlled by the moulder. The consortium hopes that this system will be accessible to SMEs in this sector with an affordable cost within a couple of years.
About the project:
The research leading to these results has received funding from the [European Community's] Seventh Framework Programme ([FP7/2007-2013] under grant agreement n°  through the OPTIJECT project “A novel spectroscopic instrument for in-line monitoring during injection moulding”. This 2 year long project started in January 2011 and is coordinated by Innovació i Recerca Industrial i Sostenible (IRIS – Spain). The participating SMEs have a range of complementary specialisations including industrial testing for injection moulding, Optim Test Center SA (Belgium), micro-injection moulding, Microsystems (UK), sensors, FOS Messtechnik GmbH (Germany), injection machines, Cronoplast S.L. (Spain) as well as software and embedded systems, LNL Technology Ltd. (Turkey). The research work as described in this article is being performed by IRIS, University of Exeter (UK) and Fundació Privada Ascamm (Spain).
For more project information, please visit the project’s website www.optiject.eu
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