Today’s extruders usually have several different heating (or heating/cooling) zones, which can be individually parameterised with regard to the desired temperature. The temperature control of the extruder is essential for the achievable product quality as well as for the productivity of the plant. Nevertheless, in practice it happens very often that not optimal temperature specifications are the cause of problems or reduced productivity.
In practice, the cylinder temperatures of the extruder are usually parameterised on the basis of an extruder or process setting card or a recipe (digital or analog recipe) as soon as a new process or a new product is started up.
Typical problems that can result from non-optimal cylinder temperatures are inhomogeneities in the melt, dimensional problems with the product, distortion, too long cooling times, low throughput, sagging, black spots, black specs, material degradation, deterioration of mechanical properties and many others. In order to avoid such problems, in addition to initial parameterization, it is often necessary to adjust the cylinder temperatures while the system is running, especially if the production situation or the general conditions have changed.
Exemplary situations in which an adjustment of the cylinder temperatures is often absolutely necessary:
- Changes to the raw material or the
- Material moisture,
- Input temperature of the plastic,
- Throughput, screw speed,
- Clogging of filters,
- Wear on screw or barrel,
- Ambient temperatures,
- etc.
However, it does not only make sense to adjust the cylinder temperatures when actual product defects occur. In very many cases the initially set temperature profile is just sufficient, but by no means optimal. In such a case, targeted optimization of zone temperatures could increase product quality and increase process robustness or even productivity.
The machine operator often has this freedom (or even the specific task) to adjust the cylinder temperature profile individually during system operation for optimization purposes, but due to the fact that the optimization of the zone temperature is very time-consuming and can only be carried out with a certain amount of know-how or experience, this optimization task is often neglected or not carried out at all. Especially due to the fact that a deterioration of the parameterization during an ongoing process can also lead to the production of waste materials, also reduces (understandably) the zeal of the machine operator for optimization and leads to rather cautious and restrained changes to the plant.
Basically, a distinction must be made between the initial parameterization of the zone temperatures (e.g. when starting a new process) and the optimization parameterization (while the system is running).
While the initial parameterization is absolutely necessary and in most cases can also be taken from a prescribed process control card or a recipe, the optimization parameterization is often regarded as a bonus (so to speak add-on) and therefore often neglected.
In addition, it often happens that the initial parameterization (even if it was meticulously worked out at some point) no longer perfectly matches the actual production situation due to changes or adjustments not being made to the recipe. This creates a production situation that contains hidden potentials.
But: For various reasons, the optimal setting of zone temperatures is not a trivial process, since:
- the effects of a temperature change are so slow that often no clear correlation can be recognized (many minutes up to the hour range)
- the temperatures displayed by the zone controllers do not usually correspond to the actual melt temperature, but (e.g.) are strongly dependent on the position of the temperature sensor in the cylinder (the machine operator must know his system)
- there are usually at least 3-4, but sometimes up to 10 or more different zones, which influence each other through heat transport mechanisms (today mostly: approx. 4-6 zones)
- a concrete zone temperature optimization is therefore time (and cost) intensive, which is why such optimization processes are often not wanted and why often little experience is available.
Initial parameterization of the zone temperatures of the extruder:
The typical procedure for the initial setting of the temperature profile is usually based on various material-specific parameters and the selection of a typical characteristic curve shape. Important material-specific values are the softening temperature of the material, the glass transition temperature (especially for amorphous materials), the crystallization temperature (for semi-crystalline materials) and the processing temperature. These values can be used to derive various sampling points, which can then be supplemented depending on the selected course of the temperature profile.
Feed zone
First we consider the temperature of the feed zone: In the feed zone (often externally cooled, especially the grooved barrel) it is important that the plastic cannot melt, otherwise bridging may occur or the grooved barrel may become clogged. In addition, it is the task of the feed zone to let the air escape from the inside of the extruder. Depending on the material being processed, this air may contain volatile components which could condense and contaminate the material if the temperatures in the feed zone are too cold.
For the above-mentioned reasons, a temperature that is significantly below the softening temperature of the material is selected here. Typical values for the feed zone during the processing of standard thermoplastics are 20°C-60°C. However, higher temperatures (60°C-95°C) are sometimes deliberately set, especially if the torque load of the screw drive is to be reduced or the plant throughput is to be very high.
Zone 1
The parameterization of the 1st cylinder zone behind the feed area is carried out according to the following aspects: After feeding and conveying the material (solids transport), the main task of the extruder is to melt the material by friction (dissipation). The set temperature should therefore be above the softening temperature or even above the crystalline melting temperature of the material.
In order to make optimum use of the dissipation power (motor power) available at the same time, Zone 1 should be parameterised in such a way that the motor load is maximised. As a rule of thumb, it can therefore be applied: Temperature slightly above the melt temperature, unless the motor power is insufficient.
Zone 2 to zone “n-1
The description of the parameterization of the zones between the first and the last zone is given below, as this results in interpolation between the two grid points zone 1 and zone “n”.
Nozzle (Zone “n”)
The die temperature usually corresponds to the manufacturer’s specifications for the processing temperature of the polymer, i.e. the desired target temperature to be reached at the end of the extruder (or die).
If a processing temperature is not specified, the following rule of thumb applies in many cases: Processing temperature of typical semi-crystalline materials: 50°C – 75°C above the melting temperature / processing temperature of typical amorphous materials: 100°C above the glass transition temperature. (after Dr. Chris Rauwendaal)
Attention: The set temperature does not correspond to the desired melt temperature in very many cases! The real mass temperature is usually higher than the adjusted set points. An intentional negative correction can therefore be useful in certain cases.
Zone 2 to zone “n-1
Depending on the basic temperature curve selected (see following figure), the temperatures between the first and the last zone of the extruder are usually parameterised with as uniform temperature jumps as possible.
In most cases, one of the following three courses is selected for the temperature profile.
Rising temperature profile: As the temperature profile rises, a continuous temperature increase is set from the temperature of zone 1 to zone “n”.
Constant temperature profile: With the constant temperature profile, a constant cylinder temperature is set (either starting at zone 1 or zone 2) over all zones, which generally corresponds to the processing temperature.
Temperature profile with peak: For the temperature profile with peak, a rising temperature profile is set first, which takes its maximum value at a level above the target temperature, but then drops towards the last zone and thus reaches the target temperature.
The statement as to which of the temperature profiles shown above represents the optimum is not generally valid and depends on the processed material, the extruder, the wear of the system and many other boundary conditions. In addition, the temperature control of the extruder in practice is often a kind of “personal signature” of the machine operator/production company and part of its individual “production philosophy”.
In addition to the initial design of the temperature control, the “optimisation of the cylinder temperature control” (manual and automatic) is becoming increasingly important.
For this purpose, in addition to the manual possibilities, automatisms already exist today which, on the basis of self-learning mathematical functions, can automatically identify and autonomously set optimal temperature profiles.
The basic approach of temperature optimization at extruder cylinders is described in a following article. The basic methods and functions of the fully automatic zone temperature optimization (ZTO) of SHS extruder controls (for retrofitting on existing machines) and the virtual assistance systems of SHS will also be discussed there.
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