<p>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 <em>volume</em> (which is what your extrusion units are) rather than mass, you need to scale the heat capacities (with units of <span class="math-container">$\frac{\mathrm J}{\mathrm g\cdot \mathrm K}$</span>) by the density to get <span class="math-container">$\frac{\mathrm J}{\mathrm{cm}^3\cdot \mathrm K}$</span>. 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.</p>
<p>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.</p>
<p>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.</p>
<p>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).</p>

CC BY-SA 4.0
2020-04-16T14:51:49.067Z
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).
FarO
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2020-04-16T14:54:22.000Z
I can post this as new question if you think it's worth
FarO
CC BY-SA 4.0
2020-04-16T16:08:34.537Z
@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.
R.. GitHub STOP HELPING ICE
CC BY-SA 4.0
2020-04-16T16:21:57.383Z
OK I'm dumb. Densities are g/cm³ not g/mm³.
R.. GitHub STOP HELPING ICE
CC BY-SA 4.0
2020-04-16T16:50:58.487Z
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.
FarO
CC BY-SA 4.0
2020-04-16T17:12:11.637Z
Where did you get 31 and 94? Looks like 20 and 31. I think you're also underestimating the contribution from heat loss to the environment. You can get an idea of the magnitude of this number by measuring the average power used by the printer to maintain hotend temperature without extruding anything.
R.. GitHub STOP HELPING ICE
CC BY-SA 4.0
2020-04-16T19:10:17.817Z
Sorry I already multiplied by 3, for 30 W, plus a typo for PLA. Good suggestion about measuring no-extrusion power. However, due to PWM it's not so easy, power meters oscillate between minimum and full power.
FarO
CC BY-SA 4.0
2020-04-17T18:22:21.183Z
@FarO: Right, 20 vs 31 (per 10W) or 60 vs 93 (for 30W). Not 31 vs 94. Re: PWM, you need to either measure the duty cycle or use a proper watt-hour meter rather than just an instantaneous reading.
R.. GitHub STOP HELPING ICE
CC BY-SA 4.0
2020-04-15T17:21:47.283Z
2021-08-06T05:46:02.137Z
2021-08-06T05:46:02.137Z
R.. GitHub STOP HELPING ICE
<p>I'm adding this answer to somewhat challenge the findings of my original answer, and the premise of the question: PETG does not need lower print speeds, and can even be printed at higher speeds than PLA under some conditions due to reduced need for cooling. You can see this from some of the &quot;#speedboatrace&quot; entries printed with PETG. So what was really going on with the original claim and my agreement with it?</p>
<p>I think my original answer is still somewhat true: it's likely that it takes more hotend power to melt PETG at a rate that can be successfully extruded <em>and bonded</em> than to do the same for PLA. But there are other factors at play in the perception that &quot;PETG has to be printed slow&quot;.</p>
<p>FarO did not specify details of the printer(s) in question, but I found the big limiting factor for my Ender 3 printing PETG was the stock extruder, which presumably was skipping bad to begin with, and even worse with Linear Advance, trying to keep the filament under high pressure to compensate for its compressibility. Since replacing the extruder with a direct drive one, I've had no problem printing PETG at the same speed as PLA, and both can print much faster than I ever could with the stock bowden extruder.</p>

CC BY-SA 4.0
2021-04-23T07:40:30.577Z
Link to the entries? I always see ABS, which is even faster to print
FarO
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2021-04-23T07:51:45.473Z
You may add a mention of the "melting index" and two TDS: https://polymaker.com/Download_File/TDS/PolyLite_PETG_TDS_V5.1.pdf and https://polymaker.com/Download_File/TDS/PolyLite_PLA_TDS_V5.1.pdf
FarO
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2021-04-23T07:52:18.787Z
Also, those TDS show a much smaller speed difference than I thought. You could mention it.
FarO
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2021-04-23T00:19:09.407Z
2021-04-23T00:19:09.407Z
R.. GitHub STOP HELPING ICE
<p>PLA has a heat capacity of <a href="https://www.sd3d.com/wp-content/uploads/2017/06/MaterialTDS-PLA_01.pdf" rel="nofollow noreferrer">1.8-2.1 J/g-K</a>, while <a href="http://www.matweb.com/search/datasheet_print.aspx?matguid=4de1c85bb946406a86c52b688e3810d0" rel="nofollow noreferrer">PETG 1.1-1.3 J/g-K</a>. This means that each gram of PLA needs more energy to heat up. I assume no &quot;melting latent energy&quot;, since we talk about plastics.</p>
<p>The density is about the same.</p>
<p>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.</p>
<p>Why is PETG supposed to be printed slower than PLA?</p>
<p>Edit: a link to a more recent question may be of interest: <a href="https://3dprinting.stackexchange.com/questions/10173/power-consumption-of-filament-extrusion/10175?noredirect=1#comment30444_10175">Power consumption of filament extrusion</a></p>

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2020-04-14T13:34:48.273Z
Have you looked into the viscosity?
0scar
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2020-04-14T13:59:10.257Z
@0scar The only link I could find: https://www.researchgate.net/publication/334170527_Effect_of_printing_speed_on_thermal_and_mechanical_properties_of_rapid_prototyped_samples_for_shipping_packaging/figures?lo=1
FarO
CC BY-SA 4.0
2020-04-14T14:02:00.490Z
@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.
FarO
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2020-04-15T04:43:23.667Z
Isn't PETG significantly denser?
R.. GitHub STOP HELPING ICE
CC BY-SA 4.0
2020-04-15T09:25:16.587Z
@R..GitHubSTOPHELPINGICE not really https://real-filament.com/datasheets/MSDS/EN/Real%20filament%20PLA.pdf https://real-filament.com/datasheets/MSDS/EN/Real%20filament%20PET-G.pdf
FarO
CC BY-SA 4.0
2020-04-15T14:51:59.197Z
And just for fun,  try a test print or two to see if faster speeds w/ PETG lead to stringing, or gaps, etc.
Carl Witthoft
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2020-04-15T14:53:07.247Z
@R..GitHubSTOPHELPINGICE   Density is irrelevant.  Liquid Hg is far denser than these plastics but has very low viscosity & will flow like crazy.
Carl Witthoft
CC BY-SA 4.0
2020-04-15T16:19:56.257Z
@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.
R.. GitHub STOP HELPING ICE
<pla><print-material><petg>
Why does PETG require slower speed?
FarO
# Why does PETG require slower speed?

(Asked by: username_0 on 2020-04-14)

PLA has a heat capacity of [1.8-2.1 J/g-K](https://www.sd3d.com/wp-content/uploads/2017/06/MaterialTDS-PLA_01.pdf), while [PETG 1.1-1.3 J/g-K](http://www.matweb.com/search/datasheet_print.aspx?matguid=4de1c85bb946406a86c52b688e3810d0). 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?

Edit: a link to a more recent question may be of interest: [Power consumption of filament extrusion](https://3dprinting.stackexchange.com/questions/10173/power-consumption-of-filament-extrusion/10175?noredirect=1#comment30444_10175)

**Tags:** pla, print-material, petg
> **Comments:**
> **username_1**: Have you looked into the viscosity?  
> **username_0**: @0scar The only link I could find: https://www.researchgate.net/publication/334170527_Effect_of_printing_speed_on_thermal_and_mechanical_properties_of_rapid_prototyped_samples_for_shipping_packaging/figures?lo=1  
> **username_0**: @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.  
> **username_2**: Isn't PETG significantly denser?  
> **username_0**: @R..GitHubSTOPHELPINGICE not really https://real-filament.com/datasheets/MSDS/EN/Real%20filament%20PLA.pdf https://real-filament.com/datasheets/MSDS/EN/Real%20filament%20PET-G.pdf  
> **username_3**: And just for fun,  try a test print or two to see if faster speeds w/ PETG lead to stringing, or gaps, etc.  
> **username_3**: @R..GitHubSTOPHELPINGICE   Density is irrelevant.  Liquid Hg is far denser than these plastics but has very low viscosity & will flow like crazy.  
> **username_2**: @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.  

---

## 2 answers

### Answer by username_2 on 2021-04-23. Score: 5 | Accepted answer

I'm adding this answer to somewhat challenge the findings of my original answer, and the premise of the question: PETG does not need lower print speeds, and can even be printed at higher speeds than PLA under some conditions due to reduced need for cooling. You can see this from some of the "#speedboatrace" entries printed with PETG. So what was really going on with the original claim and my agreement with it?

I think my original answer is still somewhat true: it's likely that it takes more hotend power to melt PETG at a rate that can be successfully extruded *and bonded* than to do the same for PLA. But there are other factors at play in the perception that "PETG has to be printed slow".

FarO did not specify details of the printer(s) in question, but I found the big limiting factor for my Ender 3 printing PETG was the stock extruder, which presumably was skipping bad to begin with, and even worse with Linear Advance, trying to keep the filament under high pressure to compensate for its compressibility. Since replacing the extruder with a direct drive one, I've had no problem printing PETG at the same speed as PLA, and both can print much faster than I ever could with the stock bowden extruder.

> **Comments:**
> **username_0**: Link to the entries? I always see ABS, which is even faster to print  
> **username_0**: You may add a mention of the "melting index" and two TDS: https://polymaker.com/Download_File/TDS/PolyLite_PETG_TDS_V5.1.pdf and https://polymaker.com/Download_File/TDS/PolyLite_PLA_TDS_V5.1.pdf  
> **username_0**: Also, those TDS show a much smaller speed difference than I thought. You could mention it.  

---

### Answer by username_2 on 2020-04-15. Score: 3

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).

> **Comments:**
> **username_0**: 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).  
> **username_0**: I can post this as new question if you think it's worth  
> **username_2**: @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.  
> **username_2**: OK I'm dumb. Densities are g/cm³ not g/mm³.  
> **username_0**: 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.  
> **username_2**: Where did you get 31 and 94? Looks like 20 and 31. I think you're also underestimating the contribution from heat loss to the environment. You can get an idea of the magnitude of this number by measuring the average power used by the printer to maintain hotend temperature without extruding anything.  
> **username_0**: Sorry I already multiplied by 3, for 30 W, plus a typo for PLA. Good suggestion about measuring no-extrusion power. However, due to PWM it's not so easy, power meters oscillate between minimum and full power.  
> **username_2**: @FarO: Right, 20 vs 31 (per 10W) or 60 vs 93 (for 30W). Not 31 vs 94. Re: PWM, you need to either measure the duty cycle or use a proper watt-hour meter rather than just an instantaneous reading.  

---

