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PLA has a heat capacity of 1.8-2.1 J/g-K, while PETG 1.1-1.3 J/g-K. This means that each gram of PLA needs more energy to heat up. I assume no "melting latent energy", since we talk about plastics.

The density is about the same.

Still, printing speed for PETG is said to be kept at max at 60 mm/s, while PLA can easily go up to 100 mm/s.

Why is PETG supposed to be printed slower than PLA?

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  • $\begingroup$ Have you looked into the viscosity? $\endgroup$ – 0scar Apr 14 at 13:34
  • $\begingroup$ @0scar The only link I could find: researchgate.net/publication/… $\endgroup$ – FarO Apr 14 at 13:59
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    $\begingroup$ @0scar but indeed PLA at 230°C seems less viscous than PETG at 255°C, judging by the oozing while I warm up the nozzle. $\endgroup$ – FarO Apr 14 at 14:02
  • $\begingroup$ Isn't PETG significantly denser? $\endgroup$ – R.. GitHub STOP HELPING ICE Apr 15 at 4:43
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    $\begingroup$ @CarlWitthoft: You have to scale J/g/K by g/mm³ to get J/mm³/K, the necessary melt rate relative to volume extruded. I think after that scaling PLA and PETG are very close, and the difference is then entirely other things like viscosity and layer bonding issues. $\endgroup$ – R.. GitHub STOP HELPING ICE Apr 15 at 16:19
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The density of PLA is around 1.25 g/cm³ and the density of PETG is around 1.38 g/cm³. When you're talking about the amount of energy needed to melt a particular volume (which is what your extrusion units are) rather than mass, you need to scale the heat capacities (with units of $\frac{\mathrm J}{\mathrm g\cdot \mathrm K}$) by the density to get $\frac{\mathrm J}{\mathrm{cm}^3\cdot \mathrm K}$. This brings their volumetric heat capacities somewhat closer: 2.25-2.63 vs 1.52-1.79 (about 47% higher for PLA rather than your figure of about 62%), but with PLA still higher.

You also have to account for heat loss to the environment. PLA is typically printed around 200°C or 210°C at most; PETG in my experience requires 250°C to reach low enough viscosity to be printable at any speed. Assuming an ambient 20°C, the rate of heat loss should be something like 25% higher for PETG. So the hotend has that much additional energy needed to begin with.

Beside that, PLA is printed at temperatures where it's still extrudable and able to bond even if the temperature drops significantly below the nominal nozzle temperature (down to 180°C, maybe even slightly lower), whereas PETG has trouble with increased viscosity and poor bonding right away if temperature drops.

Going broader still, PETG seems to need to keep its heat longer after being extruded in order for layers to bond well. (As evidenced by the need to lower fan or turn it off completely.) A slow-moving nozzle both provides heat (from the proximity of the nozzle itself) to slow the cooling, and reduces air flow across the part (by not causing as much air flow itself just by moving).

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  • $\begingroup$ Given the values you calculated, and assuming 25->250°C for PETG, 25->225°C for PLA, you can also calculate the maximum flowrate: 0.02 mm^3/s every 10 W of heater cartridge for PLA and 0.031 mm^3/s per 10 W of heating cartridge for PETG. Hotends usually have around 30 W, which would result in 0.06 mm^3/s PLA and 0.94 mm^3/s for PETG. This assuming that, reasonably, almost all the power goes to the filament. Somehow this doesn't match the experience, since I successfully print PLA at over 5 mm^3/s (60 mm/s * 0.6 mm * 0.2 mm). $\endgroup$ – FarO Apr 16 at 14:51
  • $\begingroup$ I can post this as new question if you think it's worth $\endgroup$ – FarO Apr 16 at 14:54
  • $\begingroup$ @FarO: "and 0.94 mm^3/s for PETG" is off by a factor of 10, but otherwise your calculations look right, and I'm not sure where the error is. A question on computing volumetric extrusion rate limits from hotend wattage would be interesting I think. $\endgroup$ – R.. GitHub STOP HELPING ICE Apr 16 at 16:08
  • $\begingroup$ OK I'm dumb. Densities are g/cm³ not g/mm³. $\endgroup$ – R.. GitHub STOP HELPING ICE Apr 16 at 16:21
  • $\begingroup$ Well I also didn't notice that. So it's 31 mm^3/s PLA per 10 W hotend and 94 mm^3/s PETG per 10 W hotend, minus losses. It would look like that viscosity is the main limiting factor in general, together with short permanence in the heater. $\endgroup$ – FarO Apr 16 at 16:50

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