In plastics processing, the devil is in the detail – and one such detail is selecting the correct temperature for the plasticising system. This is not just a number entered into the control panel of an injection moulding machine; it is a real parameter that determines the quality of the finished part, the stability of the process, and the service life of the mould.
Although plastics distributors provide recommended temperature ranges in their technical data sheets (Fig. 1), experience shows that these are not a universal solution. The range is wide for amorphous plastics, while the margin of error is much smaller for semi-crystalline polymers.
Fig. 1
This is where the challenge for the process engineer begins. Does the cylinder temperature set on the control panel correspond to the actual temperature of the plasticised alloy? Definitely not. During operation of the injection moulding machine, the system becomes a dynamic laboratory in which the heat of friction during the plasticisation phase, the temperature profile set by the operator and the degree to which the dosing capacity is utilised all change the temperature, often in unpredictable ways.
Therefore, conscious management of these variables is not so much a matter of following the recommendations in the technical data sheet, but rather the art of interpreting them. In practice, selecting the temperature of the plasticising system is a strategic decision — striking a balance between theory and reality, where every degree has its consequences.
Melt temperature verification procedure
Accurate melt temperature measurement is the foundation of a stable process, and the numbers leave no room for doubt. Only an immersion thermocouple can provide a reliable result, unlike infrared pyrometers or thermal imaging cameras, which can underestimate the temperature by 30–50 °C when the sprue cools down quickly.
Full stabilisation of the system is required for the procedure: the machine should run in automatic mode for 15–20 minutes before the operator performs the overmoulding and immerses a suitably selected thermocouple. The figure-of-eight movement of the sensor in the melt is not an additional gesture here, but a necessity: lack of mixing results in rapid heat absorption by the probe and an incorrect, underestimated reading.
It is important to record the maximum temperature and refer to the data in the technical data sheet. This demonstrates the significant differences in requirements even within the same group of polymers. For instance, homopolymer polypropylene (PP-H) is typically processed at temperatures ranging from 220 to 250 °C, whereas PP-R copolymer requires temperatures between 230 and 270 °C. Using a single ‘universal’ setting can lead to technological disaster: at 270 °C, PP-H can degrade and, at 220 °C, PP-R resists plasticisation so much that the screw may break.
Procedure for measuring the temperature of the plastic melt using a thermocouple:
Machine stabilisation
Allow the machine to run in automatic mode for about 15–20 minutes to allow the system to reach a stable operating temperature.
Thermocouple selection
Select a thermocouple with an appropriate size and measuring range matched to the volume of the overmoulding to be performed.
Preheat the thermocouple
Preheat the thermocouple tip, for example by placing it near a plastic injection moulding that has already been made. This reduces the effect of cooling on the accuracy of the measurement.
Prepare the injection moulding
Move the thermocouple away and perform the plastic injection moulding to obtain fresh melt for measurement.
Temperature measurement
Immerse the tip of the thermocouple in the melt and move it in a figure-of-eight pattern (see Fig. 2). A lack of movement can result in an underestimated temperature reading due to the thermocouple cooling rapidly and absorbing heat from the melt.
Recording values
Read and record the maximum temperature value indicated by the thermocouple.
Fig. 2
Profiling the temperature of the plasticising system
Profiling the temperature of the plasticising system is one of the main parameters of the injection moulding process and directly affects the quality of the products and the stability of the entire production cycle. The plasticising unit is equipped with at least three heating zones and allows precise shaping of the temperature profile. Each zone – feeding, compression and dosing – requires an individual approach.
In the feed zone, where the granulate comes directly from the feeder, temperatures must be kept in the lower range to prevent premature plasticisation, which could block the throat and disrupt air transport.
In the compression zone, the plasticising system is responsible for properly plasticising and deaerating the material, so the temperature must be raised in a way that ensures a smooth transition of the granules into the melt phase.
In the dosing zone, the aim of temperature profiling is to homogenise the melt in terms of temperature and rheology. This results in repeatable moulded parts and fewer surface defects.
The differences are fundamental from the point of view of the type of plastic. Amorphous plastics require less energy, so the temperature profile of the plasticising system can be maintained linearly and stably.
Conversely, semi-crystalline polymers require higher thermal energy in the feed zone, necessitating a so-called ‘hump’ profile: initial temperatures are increased, then lowered in subsequent zones to prevent material degradation due to excessive exposure to high temperatures.
This temperature system improves the plasticity of the material and is also important for glass fibre reinforced plastics. Higher initial temperatures reduce the friction between the fibres and the screw surface, reducing erosion and extending the service life of the plasticising system. The result is lower operating costs and greater process reliability.
A properly selected temperature profile for the plasticising system is a prerequisite not only for product quality, but also for the maintenance strategy and economic optimisation of the injection moulding process. Every degree Celsius matters, from the initial settings to the final values in the metering zone. This is why optimising temperature parameters for amorphous and semi-crystalline plastics is becoming one of the most important tools for those responsible for the injection moulding process.