Park Head for Print Core Temperature Transitions?

[3DukeEngineering 2]

Hey guys, does anyone know if there is an option to park the print head in or near the prime tower while one print core cools down, and the other warms up? I see there are start printing and stop printing temperature that can be adjusted in Cura for each print core but I don’t think that adjusting these achieves what we want. I have noticed when the temperature transitions on particular layers that don’t require much extrusion, the head will park itself until the minimum temps are meet to carry on printing. All we were trying to do is let one nozzle cool to idle temp while the other heats to printing temp but we don’t want this to happen while printing. 

 

The reasoning behind this: we mostly create 3d printed patterns for metal casting purposes, and for many of our models we use breakaway for support roofs to achieve a detailed finish on overhangs. While the temperature transition between the print cores is happening there can be very minimal oozing and under extrusion that occurs. While this oozing and under extrusion has no major visual effect on the print, at a microscopic level it tends to create very small breaches in the outer walls or shells.

 

The nature of ceramic slurry used in the casting process is very dense, approximately 2.5 x the density of water. The amount of pressure the slurry exerts on the print when submerged causes the slurry to make its way inside the print through these microscopic holes. This goes undetected until the casting is completed and results in poor quality or even failed castings. I will generally attempt to patch these holes but identifying them is an extremely tedious process and can take hours!

[geert_2 557]

Not a solution, but maybe a workaround: if you would print in PLA, have you tried smoothing it with acetone or other chemicals?

 

See the thread and photos of user cloakfiend on acetone-smoothing of PLA on this forum: he has done hundreds of models with excellent results.

 

I also have done a couple of tests on colorFabb PLA/PHA: a quick brush-on of acetone tends to fill the little gaps and layer lines. But it does not fill larger gaps, nor removes features or defects like blobs and strings. See the photos.

 

Top orange: untreated, has small openings due to minor underextrusion.

Bottom orange: acetone-smoothed by brushing on acetone: all tiny openings are filled.

Green: silicone-impressions of the orange models: they show the irregularities well.

 

Left: acetone-smoothed

Center: heat-treated with heatgun

Right: untreated

 

Same as above. Don't use heat-treatment with a heat gun: this causes the surface to melt (which is the purpose), but it also causes the entrapped bubbles below to expand and explode, creating craters. Similar to printing wet nylon. The surface gets worse instead of better.

 

Don't leave PLA/PHA in acetone: it penetrates the plastic and damages it. Just a quick brush-on, and then let dry thoroughly. This little block (10mm wide x 3mm high, solid) has been sitting in acetone for about 1 hour, if I remember well.

 

[3DukeEngineering 2]

@geert_2, thanks for the info.

 

So we typically chemical vapor polish most of the prints to be used as casting patterns. As we use PLA the polishing chemical we use is Chloroform. We can achieve some phenomenally smooth prints this way, and I had always just assumed it would play a part in sealing up any small holes but upon further inspection, it doesn't really.

 

What is interesting is that anything printed on the UM2's generally don't have issues with small holes. Only parts printed on the UM3s which leads me to believe the heat up and cool down process between nozzle switching is introducing some small anomalies. 

 

I was always under the impression that acetone had no major affect on PLA, and in cases where it did soften the material, it was usually due to the additives and modifiers that are used and mixed into the filament when manufactured. But I have found cloakfiend's original thread, its rather large so will sit down and have a good read through. Then probably test it out myself.

 

I also read through some gcode and managed to identify the m104/m109 extruder temperature change commands which appear to just be slipped in between a bunch of g1/g0 travel commands, but I don't seem to be able to identify any reasoning behind where Cura chooses to insert the temp change commands. This leads me to believe there is currently no settings within Cura to let me achieve what I want. But I am very keen to find a solution for this, at least to test results so if anyone has an idea please let me know.

 

until then, here's some polished eye candy for you all. (casting made from solid bronze)

 

 

 

[3DukeEngineering 2]

So I read the very first part of cloakfiend’s post and I was correct in stating PLA basically goes untouched by Acetone. It is only some brands that can be polished because they add modifiers to their PLA blends to improve the material characteristics and printing processes.

 

Whilst I would love to switch to a brand that utilises modifiers in their PLA so it can be smoothed with acetone, for investment casting this is not an option 😞

 

The reason we use PLA for investment casting comes down to the characteristics of the plastic when it is being burnt out of the ceramic mould. Typically PLA burns very cleanly and without residue, it does leave behind some ash content but this can be washed or blown out of the ceramic mould. 

 

Typically any PLA that has been blended doesn’t burn cleanly at all. The modifiers melt/burn and tend to leave behind all sorts of sticky residue in the mould that is impossible to remove and this ruins the metal casting usually with porosity.

 

The only exception we have come across is Polymaker Polycast. This material is PLA based but is polishable with IPA. It also burns extremely cleanly with fantastic results. The only down side is it totally crap for very intricate parts. It’s quite prone to stringing witch isn’t such a big deal but the adhesion between layers is extremely poor. For many basic shapes it’s perfectly fine, but for anything with intricate support where you need to use a bit of persuasion to remove it, you tend to break prints.

 

just thought I would leave this info here for anyone else who comes across it. (I find it fascinating 🤓)

[johnse 31]

On 9/12/2019 at 4:17 PM, 3DukeEngineering said:

Hey guys, does anyone know if there is an option to park the print head in or near the prime tower while one print core cools down, and the other warms up? I see there are start printing and stop printing temperature that can be adjusted in Cura for each print core but I don’t think that adjusting these achieves what we want. I have noticed when the temperature transitions on particular layers that don’t require much extrusion, the head will park itself until the minimum temps are meet to carry on printing. All we were trying to do is let one nozzle cool to idle temp while the other heats to printing temp but we don’t want this to happen while printing

 

I don’t know if there is a setting for this, but perhaps a workaround? Try adding a second object, just a simple tower that uses both materials (much like the prime tower) that prints after your actual part on each layer. This is often used as a cooling tower when printing parts that have small features, like the point of a cone. Between the cooling tower at the end of a level, and the prime tower at the start, the hot end temp and flow rates should be stable for the primary object.

 

You should be able to search for part order and cooling tower to to find relevant threads about these topics.

 

[geert_2 557]

It was also my understanding that PLA-smoothing with acetone was due to modifiers, not the PLA itself. But for making our silicone moulds, it works well enough, and it doesn't destroy details. I haven't tried chloroform yet.

Have you tried burning colorFabb PLA/PHA Natural (=uncolored)? When I tried burning it in a spoon in a bunsen burner a couple of years ago, it left a little bit of black powder dust, but not much. It could easily be wiped off. Contrary to for example PET which left a thick, glossy, enamel-like coating which was very hard to remove.

 

For the tine holes in UM3 prints: could it be that these are caused by tiny PVA support strings embedded in the model? And then when the support is washed away, it leaves a hole? Just guessing, I don't have an UM3, so no experience.

 

PS: beautiful bronze casts by the way. The grey one, is that also a cast, or a painted print?

 

Edited September 16, 2019 by geert_2

[3DukeEngineering 2]

@geert_2, we haven't specifically tried any colorfabb materials. Unfortunately its not readily available in our country and is in the upper price bracket. I may order a spool as I have a job coming up where I need to purchase 15kg of steelfill from them.

 

As for the holes, we don't use PVA. My personal opinion of PVA is that its terrible lol! My favorite is Ultimaker breakaway, and I use it solely as a support interface layer, it works wonders. But you are correct, about the tiny strings embedded into the walls of models. This happens both with PVA and Breakaway. The PLA when dissolved does leave holes. Also, because the PLA and breakaway don't adhere to each other, when there is a string of breakaway embedded in the wall, the string as actually not watertight even though its embedded. Fortunately these areas are easy to visually identify and patch up. What I have found with the UM3, is in preparation to switch to the secondary nozzle the PLA nozzle starts to cool while its still printing. This seem to have an under extrusion effect at the end of each layer before the nozzle switch. The under extrusion is so microscopic that you cant see it, but when I leak test the parts in water I can see bubbles forming in a row across particular layers.

 

I have some further testing to do. I think I will increase the number of walls, change the wall printing order to print the outside first. And I will experiment with a secondary sacrificial prime tower in the hope of the nozzle cool down / warm up sequence taking place while printing this.

 

For the interim to repair my current prints, I will be brushing the PLA models with chloroform using a sponge tipped brush much like cloakfiend has done with acetone. I had a bit of a play last night and I think the mechanical brush strokes seem to displace the surface layer and (hopefully) seal over the tiny holes.

 

As for the casting photos, grey on the left is straight from the printer and the grey centered is chloroform vapor polished 😉 

[3DukeEngineering 2]

I wanted to resurrect this issue again as time has passed, Cura has had some revisions and this still drives me nuts.

 

Does anyone know a UM contact they could tag that my able to look into my original post, as I think I have an idea that could be fairly easy to implement and would also fix other users issue of secondary nozzle ooze in their prints. If just to hear some imput from UM would be great.

 

The idea is to have a prime tower consisting of an inner and outer rings/perimeters. The outer perimeter of the prime tower is printed during the nozzle warm up phase as is it does currently, then the model layer is printed as per normal, but as the nozzle enters the cool down phase in preparation to switch nozzles it would return to the prime tower and print the inner perimeter before retracting to a parked state. Each nozzle would print the prime tower both before and after the part layer whenever a nozzle switch is going to happen.

 

The idea is to have the print head printing the prime tower during the warm up/cool down phase of both nozzles rather than having temperature changes of either nozzle taking place while the print head is printing/hovering above the part.