ABS on PETG

I have been using PET tape on my heated bed for a long time now. It works very well as long as I clean it with acetone about every 100 hours. It does need a high temperature (145°C) for the first layer with some types of ABS though .

It seems to last forever, the only failure mode is that large thick objects with sharp corners can defeat the adhesive and raise blisters at the corners near the edge of the bed. I solve that by building little heat shields to keep the corners warm. I am always on the lookout for something better though. It would be nice to get rid of the lines where the tape butts against itself.

A friend gave me a sheet of 1mm thick PETG to try. I clipped it onto my heated bed, and thinking it would behave like PET tape, I ran a build using the same temperatures.

Big mistake, PET has a glass transition at 75°C so it went soft and floppy. The object stuck to it very well and was hard to remove, but after getting a knife under one corner, it peeled cleanly. However it left an impression in the PETG.

The base of the object is flat but the filaments are more ridged because they sank into the sheet rather than being squashed.

When the sheet cooled down it warped badly, so that was the end of that experiment. I did have a small offcut though so I tried again at 70°C.

This time the object warped badly. It stayed stuck to the PETG but it warped the sheet. The adhesion was less and the object was easily peel-able. The PETG warped where the object was but the rest of it stayed flat. The heat of the object must have been enough to tip it over its glass transition locally. It left an impression, but not as deep as the first time.

The filaments on the bottom were squashed tighter, not as smooth as when using tape.

So a failed experiment. It is a shame because at high temperatures it bonds very well but, unlike PC, it still peels, but it is no good if it doesn't remain rigid. Wikipedia does say that PETG has a lower melting point than PET. It doesn't mention how it affects Tg, but it gives the Tg of PET as 75°C. Odd then that PET tape doesn't go soft at 75°C. My next trial will be Mylar, which is another form of PET (BoPET).

Frequency limit

I currently do my infill on Mendel at 36mm/s. The machine can go faster but the extruder flow rate maxes out at about 40mm/s when extruding ABS at 0.6mm, so 36 is a good safety margin for reliability and quality.

Although the speed is limited there is no real limit on how fast it can change direction. Suppose you make something 2.4mm wide with 0.5mm filament. E.g. a Mendel spring: -

Each wall will be 0.6mm wide leaving a 1.2mm gap in the middle. That gets filled with a zigzag infill where the head moves to within 0.3mm of each wall, so the head moves about 0.6mm on each stroke. At 36mm/s that makes 30 complete oscillations every second. 30Hz is a pretty high frequency for a mechanical system!

What actually happens is my y-axis starts to resonate. Over a few cycles the amplitude of the oscillation builds up and the infill overshoots the outline leaving a serrated edge.

The torque of a stepper motor is zero at rest and increases as it is displaced, so in that respect it behaves like a spring. That springiness together with the inertia of the rotor gives a resonance at hundreds of Hertz, known as mid band resonance. When the load is rigidly coupled, as in this case, the mass of the load brings the resonant frequency down.

As I don't get any missed steps I think the springiness might actually be in the belt rather than the motor. Timing belts have metal cables in them so that they don't stretch, but that makes them stiff, so they don't like to bend round a tight radius. That means the belt has some springiness being pulled round the pulley. A bigger pulley would be better but that would reduce the effective stiffness of the motor, so might actually make things worse. A lighter bed would be good but I haven't found a way to ensure it is flat without going to 6mm tooling plate.

I fixed the problem in software by slowing down the infill that has a high frequency content. I examine each infill path, one axis at a time, and convert it into a list of lengths between changes in direction. I then find the shortest wavelength over three cycles (less than three cycles is not long enough for the resonance to build up). I do this for X and Y directions and save the shortest of the two wavelengths. When I extrude the path I work out the frequency from the pre-calculated wavelength and the desired speed. I then compare that with a limit for each machine and reduce the speed if the frequency limit would be exceeded. I could have a separate frequency limit for each axis but I don't like the idea that the orientation of an object affects how it builds, so I pick the worst axis when deciding the limit.

I set the frequency limit to 20 Hz on my Mendel and 16 Hz on HydraRaptor. HydraRaptor does not show the overshoot problem, but it makes horrible growling noises and shakes the house. The machines make more interesting noises now because each infill run that hits the limit is extruded at an arbitrary lower speed. The overshoot is completely cured.

The builds are a bit slower and in some cases a long infill path will be slowed down by a short section that is high frequency, often a section between a hole and the outline. A more complicated solution would be to isolate the high frequency section and extrude the rest of the path at full speed.

Tip top top layer tip

When I first started printing on my Mendel I found it difficult to get the top layer infill solid and meeting the edges. It behaved differently to HydraRaptor, but since it was a different bot and extruder and I had also changed to a different type of ABS and updated Skeinforge it was hard to work out what the problem was.

The first problem I identified was backlash caused by the filament dragging on the carriage. I fixed that by switching from basket feed to spool feed, see hydraraptor.blogspot.com/2010/07/bit-of-drag.html. That made a big improvement but I also set the "Infill Perimeter Overlap" ratio to its default value of 0.15, where previously I had used 0, and also increased the amount of plastic above the theoretical 100% value.

That is the way it stayed until very recently when I made a discovery about Skeinforge. A new parameter had appeared when I updated: "Infill Interior Density over Exterior Density" ratio, which defaults to 0.9. This seems like a good idea to make inner solid layers a bit less dense. It helps if the bottom layer is a bit too low by giving somewhere for the excess plastic to go. As I was using a little excess plastic anyway it seemed a good idea.

I had noticed that some outer surfaces are never well filled even when other surfaces on the same object are. Here is an example in the bottom of the well in this bracket.

I only realised recently that this was because the 0.9 is applied to some exposed surfaces, not just to internal ones. I set the value to 1.00 and things got a lot better. Not only does it fix the problem above, but it helps to make the other top surfaces solid. I normally use three solid layers to get a good surface on top of sparse infill. But with the first two at only 90% the top layer is still lacking in plastic. That is why I had to use a higher flow rate than theory predicted. Once I got rid of this parameter I could reduce the flow rate and still get a solid top surface. In fact, I can get a reasonable top surface with only two solid layers now.

Another side effect of having the flow rate too high to compensate for the layers below being only 90% was that the top layer was being forced in. When the infill goes from two different directions and meets in the middle I was getting a ridge because the plastic would be being forced into a channel that was a bit too small for it.

Yet another issue I had noticed was that some side walls were inexplicably lumpy. I.e. not in positions where the filament starts or stops. Examining the slices I realised that it was caused by the infill displacing the outline. This was because I had a 15% overlap. Since I made the inner solid layers solid I found I don't need this any more and those bumps have gone away.

So in summary I was using excess flow rate and infill overlap to compensate for inner solid layers (and some outer ones) not being 100% solid. The side effects were lumpy walls and ridges on the top surface.