In 3D printing, the extrusion temperature, i.e. the optimum melting temperature of the filament plastic used, is of considerable importance. Manufacturers usually specify a recommended temperature range for this. Depending on the material, this can be very low, such as with Polylactic Acid (PLA), at ~210 °C or very high, such as Polyetheretherketone (PEEK) at ~400 °C.
Within the range specified by the manufacturer, the print quality can be improved enormously by specifically investigating different extrusion temperatures.
In principle, this test should be carried out once for each new roll of filament.

The heat tower is the tool of choice for this purpose.

Heat Tower

The Heat tower is an extremely useful tool for testing several print quality parameters under the influence of different extrusion temperatures. These include knobs, overhang, bridging and stringing a. I highly recommend the heat tower design by Thingiverse user “gaaZolee”:

How do you tell your Creality Ender 3 Pro to change all few layers the extrusion temperature?

The solution is very simple with Prusa Slicer. First, the heat tower is simply imported as an .stl file. The general extrusion temperature is set to the highest value directly at the heating bed and the whole tower is sliced once at a layer height of 0.2 mm.
The first temperature change starts at layer 58, then every 50 layers. For this purpose, the desired layer height is selected on the right via the layer height slider and an individual G-code is inserted by right-clicking (Fig 1).

Fig 1: The desired layer height is selected on the right via the layer height slider and an individual G-code is inserted by right-clicking.

The extruder temperature can be changed with the G-code command “M104”. Followed by “S” under the desired target temperature.

M104 S245
Fig 2

Thus, layer by layer, the temperature is changed fully automatically. After all G-code commands have been inserted, a small “G” is displayed at the corresponding positions in the layer height. Once created and sliced, the file can be used universally for all new filaments in the temperature range shown (Fig 2).

Assessment of the examination

Melting and extrusion of polymers in 3d printing represents plastic deformation by exceeding the glass transition temperature Tg. When the glass transition temperature Tg is exceeded, a solid glass or polymer changes to a rubbery to viscous state and can be extruded through the nozzle of the 3d printer.

In amorphous plastics, the glass transition separates the brittle energy-elastic region below (glass region) from the soft entropy-elastic region above (rubber-elastic region). The transition to the flow region (region of plastic deformation) is not abrupt, but continuous. This continuous transition represents the temperature range of the extrusion temperature of the filament specified by the manufacturer. As a result of the fluctuating input of thermal energy into the liquid matrix, the harmonicity of molecular oscillations is constantly disturbed and temporary voids (“free volume”) are created between the elements, the number and size of which depend on the temperature. If the temperature is too high, this will result in a poor print image.

The printed heat tower shows (Fig 3) significant defects in the temperature range 220-250 °C. knobs, strong stringing and failures between the individual layers. Unusually for PETG filament, the best result is between 215 °C and 220 °C. Still not perfect, but the print image can be further improved by retracting the extruder. At temperatures below 215 °C, the print image deteriorates again. Here, the extrusion temperature is too close to the glass transition temperature.