This article details the issue that arises when trying to push DevoClean MidTemp out with PLA, and how HDPE is the perfect solution.
The issue: viscosity difference
The issue
This is a very common scenario:
We are using the Filament Maker ONE to extrude 3devo PLA.
We notice some dark specs on the PLA filament so we decide to carry out a Standard purge.
This means adding MidTemp (or MT) into the machine without any changes to the settings (PLA presets).
After 30 minutes, and maybe some Disco purge, we conclude that the MidTemp purge is successful. There are no black specs or any discoloration in the MidTemp output.
We are ready to go back to extruding our PLA.
This is what happens:
 Small amount of remaining MT visible in the hopper. |
 Pure MT is extruding from the nozzle at PLA preset settings. |
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After 5-10 minutes it becomes clear that our MidTemp is not extruding. There's no burning, no smoke, no error messages, but the output just hangs there. |
The screw is rotating and the PLA pellets are still in the hopper but they are not being pushed forward. The flow has stopped and the material is exposed to heat for too long. |
We vacuum out any remaining MT to ensure an effective transition.
We put a small amount of PLA to start the transition from MT to PLA. .jpg?width=688&height=387&name=6%20N%20stopped%20MT%20(PLA%20failing%20to%20push).jpg)
If we do not do anything now, the PLA will continue to degrade and burn!
This is the issue of viscosity difference.
The FM ONE machine presets for 1.75mm PLA are 170℃, 185℃, 190℃, 170℃ on Heaters 4 to 1 (hopper to nozzle). This means PLA is melting sufficiently and coming out molten. In comparison these temperatures are at the lower end of MidTemp's thermal range, meaning MidTemp is much more viscous and flows slower.
Therefore, as indicated in the graph below:
1. MidTemp can purge and push PLA out efficiently, even if PLA is degraded, however...
2. PLA really struggles to push MidTemp out, and our transition fails.
3. At this point, we have to purge with MidTemp. We turn all 4 heaters up to 200℃ to ensure everything melts sufficiently, and the stuck material doesn't continue to degrade inside the barrel. For the same reason, we also turn up the RPM to 5.
Fixing the stuck PLA
This is what we do:
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If there is still no flow, using a rectangular wooden stick can help. We force feed the MT to add additional pressure and get the flow back. |
After 5 minutes of active force feeding, we finally see some output. There is gas, smoke, spitting molten plastic, and solid lumps. |
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We make sure that the fume hood is active. |
And the protective screen is in place. Safety first! |
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Since the flow is back, we can stop the force feeding at the hopper. We just have to make sure to keep feeding MT into the hopper. |
Now we see a consistent output, that is colored a dark yellowish brown. This is a mixture of the degraded PLA and MT. |
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After 30 minutes there is less gas and smoke, and the color of the output is brighter. |
After 45 minutes, the MT is consistent, fluffy, and almost white. We add a Disco Purge. |
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After 55 minutes, the MT is looking good with some minor discoloration and specks. |
After 65 minutes, the MT is pure white and we can safely switch off the machine. |
We turn all heaters to 200℃ and the RPM to 5. We fill the hopper with MT. 
Summary
- There was MT in the machine, and we tried to push it out with PLA. This did not work.
- Due to the material properties at these operating temperatures and the viscosity difference, the PLA got stuck and there was no output for 20 minutes.
- To fix the situation, we changed the settings (200℃ H1-H4, 5RPM) and force fed MT for at least 5 mins.
- This solved the flow and pushed out the mixture of degraded PLA and MT.
- With the added use of a Disco Purge, it took an additional 60 minutes to get pure MidTemp out.
The solution: HDPE
Since the issue arises from viscosity difference at these relatively low temperatures, HDPE works as a perfect transitional material.
