Injection moulding is one of the most important and efficient manufacturing techniques for polymeric materials, with the capability to manufacture high value-added products. It is an extremely complex, non-linear process and therefore needs a sensor technology that has optimum sensitivity to raw material variation.
Zero-defect production with 100 percent quality is the supreme goal of every injection moulding operation – in sectors ranging from automotive supply and medical technology to electronics. Reliable process control is the key to achieving this goal.
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Implementing a computer integrated manufacturing (CIM) system
Integration of injection moulding machines into a computer integrated manufacturing (CIM) system requires reliable process monitoring allowing statistical process control (SPC) to be implemented. CIM systems are concerned with computer control and linking together of all functions in the manufacturing environment. Successful implementation of CIM requires accurate, reliable and meaningful information relating to all aspects of the manufacturing process.
In the injection moulding process, an accurate and reliable product quality measurement is achievable through in-line process measurements.
Optimizing injection moulding process measurements
In the laboratory environment, process measurements derived from the mould cavity are of fundamental importance. However, they are not always practically suited to the manufacturing environment, or cost–effective, so compromises have to be made. In the manufacturing environment, sensors must be robust enough to survive continual tool and nozzle changes, combined with the high–pressure cyclic moulding operations.
Accurate cavity pressure measurement is crucial for cost-effective quality assurance to professional standards. Piezoelectric sensors and systems, such as those supplied by Kistler, measure and analyse the cavity pressure during the injection moulding process, while monitoring the process and separating rejects. These systems control the injection moulding process and balance the hot runner system in line with the mould cavity pressure – no matter which machines, moulds and peripherals are in use. All relevant quality data is fully documented.
Kistler’s cavity pressure systems ensure a full-range approach – from the actual measurement through to comprehensive documentation of cavity pressure. Customised monitoring of tolerance limits by piezoelectric sensors means that process deviations are detected at an early stage, so unnecessary rejects are avoided.
Injection pressure is an important process parameter, since it directly relates to the degree of filling of the mould.
Measurement of the polymer melt state at the nozzle is attractive from a manufacturing viewpoint, where a single sensor (most likely pressure or temperature) would provide useful information independent of the mould type being used. Accurate and reliable process information derived from the machine but away from the nozzle area would be even more attractive.
Measurements of hydraulic pressure are recommended as they are simple to carry out and provide important information about the injection and feeding.
In the first instance, such measurements would be hydraulic injection pressure and screw displacement, being relatively simple, low-cost and easy to implement.
Hydraulic injection pressure is a measurement that is incorrectly perceived as being an insensitive assessment of relative changes to polymer melt viscosity. In fact, hydraulic injection pressure measurements can be as sensitive and precise as nozzle melt pressure measurement. Hydraulic injection pressure sensors are cheaper, typically less than 20 percent of the cost of a nozzle pressure sensor. They are located away from the nozzle, and therefore less susceptible to being damaged by external forces, such as blow-back, or by the internal forces of the polymer melt.
Nozzle melt pressure shows qualitatively the same tendencies as hydraulic injection pressure. However, measuring hydraulic pressure is preferable to measuring nozzle melt pressure, which cannot reliably be done during continuous operation.
Extrusion lines
In extrusion, calendaring and blown film operations, Micro-Epsilon supplies systems suitable for thickness profile measurement of blown films (non-contact and contact measurement 10µm to 500µm), thickness measurement in flat film extrusion lines (thickness 30µm to 6mm), non-contact thickness measurement of thermoformed flat films (thickness 0.1mm to 6mm) and non-contact thickness measurement in hot melt calendar lines (thickness 30µm to 6mm).
For thickness profile measurement of blown films, the systems in the BTS 8104 family are designed as reversing systems and are based on capacitive technology. They are directly applied immediately behind the calibration cage on the bubble and therefore offer a very fast and efficient control. Adaptive reversing speed allows an ideal adjusting of the measurement to each step of the extrusion process. Therefore, these systems present the basis for optimum film production.2
For thickness measurement in flat film extrusion lines (thickness 30µm to 6mm), the systems in the FTS family series offer thickness measurements with extreme stability and accuracy. Applying them in extrusion lines provides reliable measurement results in high precision and thus creates the basis for controlling the production process and ultimately the quality achieved.
Applying similar systems and hot melt calendar lines in thermoformed flat film lines provides high precision and reliable measurement results.
Reference:
- Phil Coates (University of Bradford), Speight, Hull & Peters. In-line process monitoring for injection moulding control. Proceedings of the Institution of Mechanical Engineers Part E Journal of Process Mechanical Engineering 1989-1996 (vols 211(2) 115-128 May 1997. DOI: 10.1243/0954408971529601
- Micro-Epsilon: Thickness profile measurement of blown films
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This is most important and it is clear that our Mold engineering technologies must include the right areas in the cavity for sensing the pressure and global standardized tips must be included . Those tips to sense must be made with a vision of consistency and not calibration of flow profiles,
This way , part proved and life time production out of the proven mold could be the objective of the Mold design engineering. Can we which companies are looking at such consultants to conceptualize and convert into a training material .