In-process monitoring techniques are of paramount importance for optimisation of the injection moulding process. Real-time measurements provide valuable information of the quality of the moulded parts and operating conditions which are required for the success of manufacturing finished plastic products of industrial or consumer applications.
Pressure transducers, rapid dynamic response cavity thermocouples and position sensors are among the most widely used in the plastic injection moulding industry and are thought to result from the ability of these to describe the whole process in detail.
The processes that take place inside the mould are widely acknowledged as being decisive for the quality of the finished product. For example, monitoring of cavity melt pressure data enable examination of the injection moulding, compression and holding processes (Figure 1). The profile of the cavity pressure curve provides detailed information concerning weight, shape, burrs, shrinkage cavity, cavitation, contraction and deformation. Correctly controlled, it guarantees a sustainable quality improvement and a reduction in scrap.
Cavity melt pressure measurements are known to be highly affected by the correct positioning of the cavity pressure sensor. Compared to measurements near the gate, measurements far away from the gate have been reported to be more favourable when special quality problems must be monitored at the end of the flow path*.
In-process quality is concerned with the relationship between pressure and temperature profiles. Combined sensors for pressure and temperature are particularly suited to use for the monitoring of parts with a tendency to shrinking and deformation.
Therefore, the use of real-time monitoring techniques appears to be crucial for the analysis of the injection moulding process and examination of its efficiency, leading to manufacturing process improvements by direct sensorisation of the mould cavity.
What is injection moulding?
Injection moulding is one of the most widely used manufacturing processes in the plastic industry due to its quick cycle of production, material and colour flexibility, low labour costs, design flexibility and low waste. Injection moulding is the preferred process for manufacturing thermoplastic and thermosetting plastics parts of diverse industrial sectors such as packaging, electrical and electronics and automotive, etc.
This process converts solid plastic pellets to viscous masses by thermal conduction and pressure which are then injected into a mould where the melt acquires its final shape by cooling down. The process cycle can vary, typically from a few seconds to minutes, depending on the size of the moulds and materials used.
How does an injection moulding machine work?
The main components of an injection moulding machine include: the injection unit, the mould and the clamp unit (Figure 1).
– Injection unit: in this unit the polymer is gradually melted by simultaneous
application of external heating from the barrel and pressure.
– Mould: the mould comprises a cavity and a cooling system. Here, the
molten polymer is injected under pressure into the cavity and solidifies by
controlled cooling, acquiring its final form.
– Clamp unit: this unit is essential to maintain the pressure inside the mould
and keep the mould closed during the cycle of injection.
Figure 1: Schema of an injection moulding machine
Photo from the PhD thesis of Ryan Gosselin “Injection de mousses composites bois/plastiques d’origine post-consommination” http://theses.ulaval.ca/archimede/fichiers/22718/ch01.html
An injection moulding machine consists therefore of an injection unit which brings the feedstock into the screw and moves the material along the screw, forcing the molten polymer into the mould tool. After solidification, the clamp, which holds the two halves of the mould together, opens and ejects the final product.
Exploring the main components of an injection moulding machine
The injection unit includes the hopper, barrel, heater bands, rotating screw with checking ring (non-return valve), and a hydraulic system (motor).
The polymer is fed into the injection moulding machine through the hopper which is used to store the feedstock. The melting of the polymer is carried out by heaters and internal heat from viscous energy dissipation via shearing by the rotational motion of the screw. The polymer is then conveyed forward along the screw, achieving the required uniformity at the end of the barrel. The check ring prevents the plastic from being flown back, generating the required pressure and compression of the molten polymer before injection.
The mould comprises two parts: one fixed (cavity side) and one moving (core side). The stationary side is commonly called the female part of the print and it is directly in contact with the sprue and the runner system, where the molten polymer is injected from the injection unit. On the opposite side, the moving part contains the ejector system. This is needed to eject the moulded part and it is represented by the male part of the print.
Figure 3: Photo of a mould closed (a) and opened (b)
The cooling system removes heat by blowing air or circulating a cooling fluid (generally water) through the channels inside the mould. When the temperature of the molten material is reduced, the moulded part is then ejected from the mould.
The principal function of the clamp unit consists in keeping the mould closed with enough pressure during the injection phase and opens it when the plastic part is at the required cooling temperature (Figure 4).
Figure 4: Two clamping system showing opened and closed mould a) electric mechanic b) hydraulic
The screw plays an important role in the plastic injection moulding process as it is used both to plasticise the polymer and to inject it into the mould. The major functions of the screw are achieved at different sections along its length, which are: feed zone, compression and metering zone (Figure 5).
The feed zone brings the feedstock into the injection machine and moves the material along the screw. The polymer is then conveyed forward and compressed before reaching the compression zone, where it is gradually melted by simultaneous application of external heating from the barrel heaters and also from the internal viscous shear (75% of the supplied heat is in the form of viscous shearing by the motion of the screw). Finally, in the metering zone, which is confined to the last few turns on the screw, the uniformity of the melt is increased before injection.
Figure 5: Injection screw description
The purpose of the check ring is to prevent melt loss as the screw moves forward during the injection part of the cycle. The check ring acts as a non-return valve; the ring is forced back against the screw tip seat and seals against it. When the screw stops pushing, the check ring is then allowed to move forward and the screw brings new material forward for the next cycle (Figure 6).
Figure 6: Check ring schema a) screw bringing new material for the next cycle b) check ring sealing
Welcome to the first blog of the PREVIEW project which will appear as a regular feature on the project website.
Over the last few years the injection moulding process in the plastics sector has changed due to increased requirements for diversity and personalisation. This has led to the need for a more flexible mould which can be used in different types of machine. Another issue is the fact that short production runs require a high number of mould changes which leads to an increase in downtime and scrap. In fact, scrap produced during the set up phase accounts for 40% overall.
Although attempts have been made to improve productivity by using process parameters and statistical process control systems based on data acquired from sensors inside the mould, fewer than 3% of new moulds are equipped with these sensors. Reluctance to use such sensors is mainly due to anticipated cost increases related to wiring, proprietary interfaces and external devices.
PREVIEW can help resolve these issues.
Preview (PREdictive System to Recommend Injection Mould Setup) has received funding from the European Union’s Horizon 2020 research and innovation programme. The project aims to provide the injection moulding industry with a process control system which will result in a reduction in mould set up time, a reduction in scrap and energy consumption and an increase in productivity and flexibility.
In short, PREVIEW is a middleware solution that facilitates easy, ubiquitous, holistic and fast sharing of product and process information across the entire injection production process.
Want to know more?
To obtain more information about PREVIEW, register for updates on the ‘Contact’ page of the website, join our LinkedIn Group – “PREVIEW – Predictive System to Recommend Injection Mould Setup in Wireless Sensor Networks” or follow us on Twitter @PREVIEW_EU.