Transitioning from MidTemp to PLA: The solution

Overview

The issue: viscosity difference

The solution: HDPE 

Step by step process

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The issue: viscosity difference

This problem arises any time there is MidTemp in the machine and we try to transition to PLA.

This can happen with 3devo PLA or other low temperature material. The combination of low viscosity properties and low operating temperatures (for MidTemp) means it is very difficult to push MidTemp out of the machine. 

Summary of Part 1:

1. There was MT in the machine, and we tried to push it out with PLA. This did not work.
2. 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. 
3. To fix the situation, we changed the settings (200℃ H1-H4, 5RPM) and force fed MT for at least 5 mins. 
4. This solved the flow and pushed out the mixture of degraded PLA and MT. 
5. With the added use of a Disco Purge, it took an additional 60 minutes to get pure MidTemp out.

The solution: HDPE

All the issues that arise from the MT to PLA transition: stopped flow, no output, degrading material, low pressure etc, are fixed by the use of HDPE. High-density polyethylene or HDPE for short is a very durable and forgiving material:

• it has a relatively high melt viscosity
• a wide temperature range
• it's also relatively cheap
• produces very little smoke or fumes when melted
• it can act as an excellent transitional material between two otherwise incompatible materials
• it is not great at removing degraded or burnt particles, therefore not always suited for a purging material

Due to its properties it works well for pushing out the foamy and equally dense MT. This can be done at PLA preset temperatures, without making any changes to the heater settings. 

Step by step process

After extruding PLA, we carry out a Standard Purge. This means MidTemp pushing out any remnants of PLA from the barrel without any changes to the heater settings. We are using the PLA preset temperatures. 

In this close up we see a mixture of MT and PLA, which means our Standard Purge is almost done. 

We see pure white MT exiting the nozzle. 

Now we want to go back to PLA. Since we know the issue of viscosity difference, we will use a small amount of HDPE to transition from MT successfully back to PLA. 

Step 1: MT to HDPE

We do not change the heater settings.

We can increase the screw speed to make the transition faster.

However, it is important to monitor the motor current on DevoVision to ensure there are no issues or signs of inconsistent pressure. 

Here we see a mixture of MT and HDPE. The material is a little bit fluffy (MT) and a little bit translucent (HDPE). There is no flow issue or clogging that we experienced with the PLA. 

5 minutes later, in this close up we see HDPE extruding with some small specks of MT. 

Another 5 minutes later, we see pure HDPE exiting the nozzle. We are ready to transition to PLA. 

Step 2: HDPE to PLA

Once again, no changes to the settings. 

We simply continue with changing the input materials, making sure to transition efficiently. 

After 5 minutes, we see a mixture of PLA and HDPE. Both materials are semi-crystalline and have a slightly translucent color when extruding. However, PLA has a yellowish hue to it. 

After another 5 minutes, we see PLA extruding with some with some white HDPE specks. 

Finally, we get pure PLA. 

Close up of pure PLA.

Summary

1. There was MT in the machine, and we wanted to go to PLA. 
2. Due to the material properties at these operating temperatures, and the viscosity difference between MT and PLA, HDPE is the perfect transitional material between the two.
3. Without any heater changes, we begin our transition from MT to HDPE. 
4. After 10-15 minutes, we see pure HDPE extrude, so we begin our transition from HDPE to PLA. 
5. After another 10-15 minutes, we see pure PLA extrude.