Purging process-walkthrough for upholding your extruder’s performances. (Purging is the technical correct term for cleaning the inside of your extruder’s screw, barrel and nozzle, by extruding a certain “purging material”.)
This document contains an introduction to the purging process in Chapter 1, followed by a guideline for which level of purging is most suitable for your situation. Based on that information you will know how to use the cleaning manual/walkthrough in Chapter 3.
In order to understand and apply the entire purging process, it is very useful to know why the extruder should be purged after use of certain materials. In this chapter these reasons will be explained (1.1), along with the theory behind purging (1.2).
1.1 Polymer degradation: the reason to purge
It is always very important to purge your filament maker after use, especially when extruding “engineering polymers” (such as ABS, PA, PEKK, PEEK, etc.). Purging after an extrusion run is important because of the next two reasons, Polymer Degradation and Resin Accumulation.
The answer for why the extruder should be purged mainly lies within the context of ‘polymer degradation’. Polymer degradation is the change in the properties of a polymer under the influence of environmental factors such as heat, oxygen, light or chemicals. For example when filament of the material ABS (acrylonitrile-butadiene-styrene) has been extruded, and you suddenly stop this process without purging the extruder, degradation will occur. This occurs because the machine is still hot and cools down very slowly. The material’s residence time in the heat of the extruder will therefore be too long, which causes the material’s polymer chains to break down, which is known as a form of degradation. In Figure 1 an example is given of how ABS shows degradation over time when left in a still hot extruder of around 240 °C. The first discoloration (second picture) might already show after around 30 minutes.
As a result, this material which is inside the extruder’s screw does not show its original extrusion behaviour anymore during the next extrusion run. This means that some parts might not melt at the same temperature, or not even melt at all which causes the flow of material to get obstructed or clogged. These parts can often only be removed by extruding a special purging material, or in worst case scenario, by removing the entire screw.
When not properly purged after use, resin might accumulate over time in the extruder’s barrel and screw. This also causes the flow of material to stagnate, and contaminate your filament. The flow of material stagnates because this accumulation of old material, which sticks to the extruders’ screw, will block the flow of new material. As a result, polymer degradation will occur because the material’s residence time is too long.
1.2 The theory behind purging
Purging an extruder’s screw can be divided into two different methods, where a specific type of purging material is being extruded to remove the old material. One of these methods is called a ‘Chemical Purge’, which makes use of a chemical reaction to break down the material inside the extruder. The other and most common method is the ‘Mechanical Purge’, which is recommended for your filament maker.
Mechanical purging makes use of physical force, in the form of shear stress (shear stress is external force acting parallel to the flow direction) which depends on the extruder’s screw speed (RPM), and also differences in material melt viscosity (melt viscosity is an indication of how “not-liquid” the molten polymer is. The lower the viscosity, the more liquid it is). Simply described, during extrusion of the purging material the shear stress scrapes/tears the old material from the extruder’s screw and barrel, while the higher viscosity of the purging material pushes everything out. Also the higher the melt viscosity, the higher the possible occurring shear stress. These stresses are visualized in Figure 2 inside a simplified sketch of the flow of material through the barrel, where η stands for viscosity, letter A for the purging material and letter B for the old/ to be purged material. Commercial purging materials such as Devoclean also rely on material affinity – they are attracted to carbon (carbon is the product of burnt polymer) and other residue (ptonline, 2018).
A mechanical purge benefits from high turbulence and agitation caused by a high screw RPM. But a so called “disco purge” provides a highly fluctuating environment of pressure and agitation that allows a purging compound to work its best. A disco purge is done by changing the extruder’s RPM very frequently during the purging process. The specific RPM values and times are not important. What is important is disrupting the flow patterns and establishing new velocities and shear rates. One minute at each RPM is sufficient to do this. At lower RPM’s the polymer that is adhered to the metal of the extruder or stuck in a corner, has a chance to bond with material that is in the mainstream of the barrel and screw. At higher RPM’s the velocity of the material on the walls of the barrel is higher and more of the polymer on the walls can be pushed out. (DuPont, 2005) The best way to do this for your filament maker in our own experience is by changing the screw speed every minute, and using the given RPM values in the following order:
During the purging process this cycle might need to be repeated a few times before the extruder is clean. To find the optimal way to do this for your own filament material and thus purging process, it is recommended to experiment with these RPM values.
When is purging necessary?
To know if purging after an extrusion run is necessary, please consult the supplier of your material/granulate, but in almost all cases purging is highly recommended to ensure the machines’ endurance. In our own experience there are only a few materials which don’t need purging (if the extrusion process went as it should, and on the condition that these are just raw materials, without any additives). So far these are: PLA; HDPE; Devoclean. In terms of maintenance, periodical purging is also highly recommended depending on how well the extruder still performs and how often it is used. This is once a week in general if the machine is used every day (ptonline, 2018).
2. Different types of purging
Different types of filament materials call for different ways of purging the extruder. The way of purging the extruder is based on the melt temperature, viscosity, and the use of additives such as colorants, fibres and fillers. Also the state of the machine plays a part, for example if the extruder is filled with burnt material, heavier purging might be necessary. The ways of purging can be divided into three different levels. Generally level 1 purging with HDPE can be applied when there are no additives used, there is no burnt material inside the extruder and the to be purged material has a temperature range within 180 to 280 °C (for more information see 2.1). Level 2 purging with Devoclean Mid-Temperature (2.2) can generally be applied to all situations if the to be purged material has a range of 180 – 320 °C, and level 3 purging with Devoclean High-Temperature (2.3) for a range of 280 – 420 °C. Below these levels will be fully explained, so you will know exactly which level of purging is necessary for your extrusion material.
2.1 Level 1 – Light purging
Light purging with HDPE (180- 280 °C) is often considered when the machine is still in a decent state (not filled with burnt material), but it has to be purged after an extrusion run of a certain type of material. Light purging can be done using the material HDPE (High Density Polyethylene), which is often used as a purging material in the plastics processing industry. This is because of its favourable high melt viscosity, its compatibility with many materials and wide temperature range. This material usually has a melting point around 180 °C, and a decomposition (burning) point around 320 °C. When HDPE is used as a purging material, it can be used within a temperature range of 180 °C – 280 °C. The maximum temperature for using HPDE as a purging material is 280 °C, because at this point the melt viscosity starts to decrease which makes it more and more unsuitable as a purging material. HDPE can be bought on the 3devo website:
|Advantages of level 1 purging||Disadvantages of level 1 purging|
|Relatively cheap||Not suitable for high temperature materials such as PEEK|
|Easier transition to the next material (the HDPE also easily comes out of the extruder)||Does not perform well purging out additive filled materials (with colorants, fibres)|
|Very little fumes/smoke produced by the HDPE||Does not perform well purging out burnt materials|
|Requirements for a level 1 purge|
|The extruder does not contain burnt material. (You will know if the extruder contains burnt material if your filament contains black specks, or if the extruder generates unusual amounts of smoke/fumes.)|
|Maximum melting temperature of the old material (which will be purged out of the machine) is: 250 °C|
|Extruding is possible within the following temperature range without burning the material: 180 – 280 °C|
|Devoclean||PC/ABS (not just PC)||PS||PPS|
|PE (LDPE or HDPE)||PP||PA 12||Polyester alloy|
2.2 Level 2 – Standard purging
Standard purging can be done with Devoclean Mid-Temperature (180 – 320 °C), and can generally be used for any situation and material as long as it stays within its temperature range of 180 – 320 °C. If you have read the previous chapter these temperatures might be familiar, because the extrusion range of HDPE is also 180 – 330 °C. This is because Devoclean Mid-Temperature is based on HDPE. But then it has certain materials “grafted” on the existing polymer, which basically means that the HDPE gets synthesized and enhanced by adding certain chemicals. This blend of HDPE for example has a very favourable melt viscosity for purging all the way up to its decomposition (burning) point of 320 °C, in contrast to pure HDPE which has a decrease in melt viscosity above 280 °C. The disadvantage of this is that it is also harder to extrude out of the machine, when starting with a new material. With Devoclean Mid-Temperature the transition to a new material often takes quite long, and a lot of new material needs to be extruded before the Devoclean Mid-Temperature is fully removed from the machine. To speed up the transition to a new material it is recommended to purge out the Devoclean Mid-Temperature with HDPE. The transition to a new material is way faster when starting with HDPE in the extruder, because the melt viscosity is much lower. Devoclean Mid-Temperature can be bougt on the 3devo website:
As mentioned below, Devoclean Mid-Temperature can be used in any situation as long as it is used within a temperature range of 180 – 320 °C.
|Advantages of level 2 purging||Disadvantages of level 2 purging|
|Performs very well purging out additive filled materials, especially colorants||Not suitable for high temperature materials such as PEEK|
|Can be used for any material within the temperature range of 180- 320 °C||Slow transition to the next material (it does not come out as easily because of its high viscosity)|
|Leaving it in the extruder for a long time after the machine is shut down has a great cleaning effect and enhances the screw’s life span||Has a tendency of producing a lot of smoke when extruded at high temperature and high RPM|
2.3 Level 3 – Heavy purging
Heavy purging is often done with Devoclean High-Temperature (290 – 420 °C), which is the most powerful high temperature grade for hard-to-clean materials. This is often used when a high temperature (above 290 °C) engineering polymer has been extruded in your filament maker and it has to be removed from the machine. Devoclean High-Temperature is a purging material built up in basically the same manner as Devoclean Mid-Temperature, but then based on different materials and with the additive of glass fibre to enhance stresses and cleaning ability. Also, Devoclean High-Temperature can be used for any situation and material as long as it stays within its temperature range of 290 – 420 °C. But Devoclean High-Temperature does come with a limitation, which is that it may not stay in a hot extruder for longer than half an hour (30 minutes), and cannot be left in your filament maker after shutdown. This automatically requires you to purge out the Devoclean High-Temperature using Devoclean Mid-Temperature or other applicable purging materials. Devoclean High-Temperature can be bought on the 3devo website:
|Advantages of level 3 purging||Disadvantages of level 3 purging|
|Very suitable for high temperature / super-engineering materials such as PEEK or PEKK||Cannot be left in a hot extruder for longer than 30 minutes|
|Can be used for any material within the temperature range of 290- 420 °C||Cannot be left in extruder after shutdown|
|Performs very well purging out any additive filled materials||Relatively expensive|
|Relatively low smoke and odour production|
3. Purging manual/walkthrough
In this chapter the entire purging process of ‘How to purge your filament maker’ be explained. This purging process can be divided into four steps. These steps are: preparations (3.1); starting the purging process (3.2); during the purging process (3.3); and ending the purging process (3.4). The last step; ending the purging process (3.4) has different approaches for the three levels of purging explained in paragraph 1.3. The first three steps can be applied to all levels of purging/purging materials.
Before purging your extruder, there are a few preparation steps that should be taken in order to let the purging process go safe and smoothly.
NOTE1: Do not shut down the machine before the purging process is completed.
- The first step is gathering all the necessary tools and materials for the purging process. These items are:
- At least 350 grams of purging material (Depending on which level of cleaning will be applied as discussed in paragraph 1.3, the type of material which is to be purged out of the machine, and the machine’s state.)
- A piece of paper or cardboard
- Multitool (or a side cutter and pincers)
- Make sure that the area is properly ventilated, and for level 2 and 3 purging a fume hood is also highly recommended to guarantee your safety. Wearing a gas mask is also a good option.
- Turn off the hopper sensor in the settings menu, so the hopper can be run completely empty before putting in the purging material.
- Turn off the filament fans (set as 0% in settings menu), and point them as in Figure 3.
- Cover the filament sensor with the piece of paper or cardboard as in Figure 3, and hold it in place with a piece of tape.
3.2 Starting the purge
Once the preparations in paragraph 3.1 are taken, the purging process can begin.
NOTE1: Do not shut down the machine before the purging process is completed.
NOTE2: During this purging process, do not change the machine settings relative to the settings used in the (previous) extrusion run, unless stated else.
- First let the hopper run completely empty, until the stage that there is few to none granulate of the previous material inside the hopper, as in Figure 4. (Do make sure that the hopper does not remain completely empty for too long, not more than approx. 3 minutes)
- Now put in a small portion (around 60 grams) of purging material, so more of the previous material can be flushed out of the hopper and the feeding section of the screw. (Example in Figure 5) Temperature settings don’t need to be changed as long as it is within the range of your purging material’s extruding temperature (as discussed in paragraph 1.3).
3.3 During the purging process
Now the first purging material has been put in the machine, the following steps can be taken.
- First wait for the previous small portion of purging material to be flushed away by the extruder, this might take around 15 minutes, depending on the screw RPM or machine state.
- When few to none granulate is left (as described in step 1 of the previous chapter), put in the next portion of purging material (around 200 grams, example in Figure 6).
- When the purging material is visibly coming out of the extruder, usually mixed with the previous material (example in Figure 7), proceed to the next step. This is usually clearly visible as the structure, colour and surface smoothness of the filament changes.
If this is still not visible after 15-20 minutes (from the moment the first small portion was put in the hopper) increase the temperatures of all heaters with 20°C. If this still doesn’t work, increase the screw speed to 10 RPM to give the flow a boost. As soon as the flow is back, decrease to the original RPM.
- Now the purging process is in motion, we can proceed to “Disco purging”. Simply explained, this is changing the extruder’s screw speed every minute, ranging from 2 to 15 RPM. Try experimenting with random RPM value’s, how you do this exactly does not matter, as long the RPM is changed every minute. The theories and tips behind this are discussed in paragraph 1.2.
- After 10 minutes of disco purging, see if the extruded purging material still contains old material or other contamination. Figure 8 shows an example of signs that the machine is clean as there is only pure purging material (in this case Devoclean) is coming out of the extruder. Figure 9 shows the opposite and requires longer cleaning. If your machine is clean, proceed to the next step. If not, continue with disco purging and add more purging material if necessary.
3.4 Ending the purging process
Now the machine is clean, the purging process can be ended. These endings depend on what purging material has been used, and which level applies to your situation (as discussed in paragraph 1.3). If you used a level 1 purge with HDPE, the cleaning process is finished once step 7 in the previous paragraph has been completed. The following information only applies to level 2 and 3 cleaning.
3.4.1 Ending the purge after a level 2 cleaning
As mentioned in paragraph 2.2, transitioning to a new material with Devoclean Mid-Temperature can take a long time. To speed up this transitioning to a new material, the Devoclean needs to be purged out using HDPE. If you are planning to do another test soon, doing this is highly recommended. How to do this is explained below in this paragraph as step number 8 in the purging process. If you are not going to do any filament making soon, you can leave the Devoclean Mid-Temperature in the machine after shutdown as this has a positive effect on the screw’s lifespan as mentioned in 2.2.
- If you wish to extrude new filament again, step 7 has been completed, and the extruder is clean, the machine can be purged with HDPE to speed up the transitioning process.
To do this, set all heaters to 240 °C and wait until the hopper is completely empty (just as in Figure 4, step 1). Now follow the steps of the purging process again starting at step 2, paragraph 2.2. The machine is clean enough and ready for the next material once only clear HDPE is coming out of the nozzle, without white spots/flakes of the Devoclean. Figure 10 shows an example of a machine which is almost ready for the next filament material. A few spots of old Devoclean are still visible, so this means that Devoclean is still inside the machine’s system. In this case purging with HDPE needs to be continued for a few more minutes. Usually the total transition from Devoclean to pure HDPE takes around 25 minutes and 250 grams of HDPE granulate.
3.4.2 Ending the purge after a level 3 cleaning
As mentioned in paragraph 1.3.3, Devoclean High-Temperature needs to be removed from the extruder after it has been used as a purging material. This can be done using Devoclean Mid-temperature.
To do this, set all heaters to 300 °C and follow the steps of the purging process again starting at step 2, paragraph 2.2. After around 25 minutes and 250 grams of Devoclean Mid-Temperature this process should be completed and the Devoclean High-Temperature will probably be fully removed from the machine.
If you are planning to extrude a material which has a much lower processing temperature (under 260°C), transitioning with HDPE is recommended. For more information about this transition, read paragraph 2.4.1.
Your filament extruder should now be completely clean again and ready for the next filament extrusion run.
DuPont. (2005, August 9). The Disco Purge Procedure for Extrusion Coaters . Retrieved from www.dupont.com: http://www.dupont.com//content/dam/dupont/products-and-services/packaging-materials-and-solutions/packaging-materials-and-solutions-landing/documents/disco_purge_procedure_.pdf
ptonline. (2018). Contamination Development Mechanism and Preventative Measures Using Purging Compound. Retrieved from www.ptonline.com: https://www.ptonline.com/knowledgecenter/purging_compound/purging-compound-basics/preventative-measure
ptonline. (2018). Types of Purging Compound. Retrieved from www.ptonline.com: https://www.ptonline.com/knowledgecenter/purging_compound/purging-compound-basics/types-of-purging-compound