Printing with Light Weight PLA

Me too,,, but I am watching the results.

An observation ....  Tony is beginning to see some paths into 3D CAD entities.  And some of it by seeing that there are ways to start with 2D and extrude. We started talking about how to add some additional lift to an existing model that currently wants only to fly at roughly the speed of sound so it might work at human response speeds....

We have also been finding what happens to servos when they have not been used for 3.5 years.

I worked with Klippy to fine-tune his new LW-PLA.  We found a lower temperature than I had been using.  Now that I am back in the UK I though I would take the same STL file of the 20mm x 10mm x 6mm open box (two sides are 0.4mm thick, one side is 0.6mm thick and one is 1mm thick  so as to need two perimeters).

I did a PLA base case (A in the figure below) and then a series in LW-PLA stepping up the temperatures as well as testing various flow rates.

What I noticed was that 245C with a 52C bed temperature and a 40% flow rate gave me almost as stiff a print as the 60% flow I was using in the past.

 

I then went back to my section of oval facade which in PLA was 23g and was 12g with my old settings for LW-PLA got down to 8g which my new settings.  I do think that I will design in a channel for some 1.5mm CF rod to stiffen up the edges around the battery opening so as to resist crushing.

Returning to the topic of LW-PLA.  But with a bit of freeCAD.  I have been exploring and learning quite a lot.  After reading about wing plan forms I looked into a possible freeCAD workbench that would support the design of printed tapered wings.  And with my shiny new tapered wing I applied what I learned about making something printable in 'vase mode' and lastly came up with a method that does 'vase' but with a BASE and a TOP!

I will take it in small chunks.  Spoiler - folk doing scale models see the end of this post!

BACKGROUND
Moving from a constant cord wing to a wing tapered in plan-form was an evolutionary step for me (some of you already have the T shirt).For me, a rectangular plan form tapering thickness from root to tip is largely a matter of scaling the airfoil thickness from perhaps 25% of chord to 12% at the tip.  The root depth is certainly helpful with structure.  The varying airfoil thickness - not so sure about the utility of that.Moving beyond a rectangular planform I came across four discussions about planform on the kit planes.com web site:
www.kitplanes.com/design-process-planform-shape/
www.kitplanes.com/design-process-planfor...an-loading-and-drag/
www.kitplanes.com/design-process-planfor...tructure-and-weight/
www.kitplanes.com/design-process-distrib...of-lift-coefficient/
This was new to me. The author seemed to know his stuff and was trying to share that with folk looking to build full size craft. There seemed to be quite a few reasons to venture into a tapered plan form.  This got me thinking about the process I had used to just get a thickness taper from root to tip and the likely additional complexity of implementing a taper in multiple axis.

Earlier freeCAD methods
At a basic level in freeCAD one may ‘loft’ between two or more sketches of airfoils (ribs) that have been cloned, scaled and positioned. Manual scaling and placement of the ‘virtual’ ribs needed to define the wing is time intensive and error prone. Sometimes lofting generates artistic entities rather than viable wings. Adding another taper axis seemed like a lot of extra work.

Curved shapes workbenchI looked for alternative approaches in freeCAD and found a demonstration of using a ‘curved shapes workbench’ to generate a wing with an intentionally complex planform, root-to-tip taper all using the same airfoil.  Having gotten past the strange shape the workbench might actually fit the definition of magic.

Essentially you create three sketches, the first the airfoil, the second the planform and the third the front elevation.  I previously had used the aerotoolbox.com wing plotting tool to get a wing with a 1m (40”) span with an 8” root chord, 3.43” tip cord, plan taper ratio of 0.43 (close to eleptical), aspect ratio of 7 and an area of 228.6 sq”.  This is what I wanted to implement.... 
I wanted to soften up the straight tapered planform so I tweaked the tip with a 4mm radius at the leading edge and a 12mm radius at the trailing edge.I had a Clarke-Y airfoil that I have used many times and I made a scaled clone for the chord with a 14% thickness for the airfoil sketch.  I calculated the thickness at the root and tip and created a polygon sketch for the front elevation. I extended the thickness sketch slightly beyond the planform sketch to prevent clashes. Of course one could use the front elevation sketch to create a winglet or implement a polyhedral.Lastly, I wanted an aileron of 16% of the wing area and created a 4th sketch for that so that I could extruded it and do boolean operations with the wing to ‘carve out’ the aileron. I chose to take the aileron to the tip. The trailing edge of the wing at the aileron would be vertical while I would cut an angle into the hinge edge of the aileron.  In freeCAD the four sketches are shown: 

I then highlighted the airfoil sketch, planform sketch and the thickness sketch (in that order) within the ‘curved shapes’ workbench. It creates a few ‘virtual’ ribs.  I increased the number up to 45 and here is the result.  There is a bit of rounding at the leading edge tip and a rather pleasing trailing edge tip to cut the aileron from.

   



A bit of 'drafting workbench'
To  preserve the initial wing solid I used the ‘Draft’ workbench to clone the wing and then did a boolean operation between the wing and the extruded polygon sketch to cut out the aileron.  I then made a 2nd clone of the wing and did an intersection boolean to get just the aileron. 

 

If you wanted to use a standard printed infill pattern you could export the STL for the wing and another for the aileron. Job mostly done.

A later post will cover how I turned this into something that can be vase printed. And then not-quite-vase-printed.

For you SCALE model folk.... 
I found a site which uses this workbench to create a wing that closely resembles that in. A Corsair.  It is ‘sketched’ over the plan and two elevations of the plane to create the three sketches needed to drive the curved shape workbench.  This wold seem to be just the thing for those interested in scaled models.
  

The is the 2nd chunk - Adapting a CAD wing for vase mode printing

I previously had explored tweaking CAD designs so that one could use a ‘vase’ mode of printing.  This involves creating very thin bodies to partly slice through the wing. This results (if you are careful) in the printing path as one continuous line. Details of that technique for creating very light-weight printed ribs are in another video ().

I had previously applied this to a LW-PLA wing created via a ‘loft’ in freeCAD and found that I could get a stiff wing structure via shear webs created in vase mode rather than traditional ribs.  If the shear webs are ~15-20mm apart the result is a rather stiff wing.  Note the alignment of the web with the sleeve for the spar.  

The nice thing about a Clarke-Y airfoil is that the base is flat and so it is simple to ensure the thin bodies being cut from the wing begin 0.8mm above the lower face of the wing.  This is just enough that the LW-PLA will touch but not confuse the slicer.  As with the prior work having the cut-away bodies be 0.1mm thick ensures that the path of the filament goes into the core of the wing and back out creating a shear web that is effectively two layer thick (while the shell of the wing is still one layer thick). 



Following the same pattern with the curved shapes wing I created a series of very thinbodies as well as the main and trailing spar to cut from the wing as seen below.  Most of the webs were at 20mm centres.  The one just aft of the main spar is 25mm.

 

Dealing with servos...

Why - well we need a pocket for the aileron servo.  I previously had incorporated a servo pocket as part of the wing design (see below). Tedious to create and hard to get right. It follows the pattern used by the guys who want to charge for their models. But it requires trusting slightly flexible LW-PLA as the foundation for a servo?  

 

Perhaps better to design a separate standard PLA insert sized for the servo.  I created one and measured it and then adjusted the web spacing in the wing to match that.  A different servo - just tweak the insert design, measure and adjust the web spacing.  And if the model is not using ailerons then just don't use the insert.

 

 

Here is a view of the PLA insert between the webs and with the servo wire fed through.

 

Cutting the resulting half-wing...

The next challenge.... I had a maximum Z height in the Prusa Min printer of 180mm so the design was tweaked so that the resulting body could be cut at 178mm intervals. Some iteration in the CAD was necessary to ensure that the webs worked with the sleeves for the main and trailing spars as well as the taper in the plan and elevation.  Below shows the root, mid and end sections of the half-wing of 500mm.
 
 

What does the vase print look like in detail?  No stringing. No extra stuff inside so easier to shift wiring through the hollows.

 

Of course if one is using a vase print it is only possible to do this one section at a time. AND the other classic issue with printing in vase mode is that you can have base layers but no top layers.

Teaser...  

So how did I get a top on the end wing section - that is for the NEXT post....

   

That is very clever vase printing although each shear web does seem to leave a small ridge on the outer surface of the wing surface.
I will have a go at vase printing like that myself.
Now if only it was possible to vase print horizontally so the filament line was along the wing span. It would be so much stronger and stiffer it might not need reinforcing.  

Thanks for the comment about the vase printing.  LW-PLA does sand easily so those little ridges should be treatable.

About 'horizontal'.  When rotating the printing from vertical to horizontal those almost-sliced-through shear webs would need to become almost-sliced-through ribs.

Moving from shear webs to almost cut-through-ribs probably needs an extra step to accommodate a CF spar. I can imagine after the rib cuts are in place one could 'subtract' a spar shape that would allow a CF spar to slide into the wing passing through the ribs.

Looking back through the posts I did do an early version of horizontal printing with one part of the print before the spar and one aft.  The cut-outs were not optimised very well to work near the spars.

I am rather liking this idea of these thin cut-outs and am exploring it for my centre wing join to enable a 4 degree dihedral.  More on that later.

Next bit of the story....

In playing around I found some design hints as well as printing hints that allow RC parts to be designed to be printed in vase mode but to be printed with the 'vase' toggle off.  The result is almost as clean as vase mode but gets us a number of other benefits.

 

I have been using the Prusa slicer with my Prusa mini.  And LW-PLA needs specific parameters, especially if you are going to use design-for-vase-printing but opt for a non-vase print so you can get a closed top.

Here are the settings I use.  You would find most of these also on Cura (just in different places).

First up the filamen my best temperature from testing is 250C. Your milage might vary. And 50C for the build plate and 0.42 for the flow rate (and it is also the answer to the question of the universe):

 

Next cooling:  off BUT allowed for bridges.  Otherwise it just falls to the bottom.  Note that some slicers will not consider upper surfaces which are sloped as qualifying.  So keep the top as flat as possible unless you might have to find other ways to kick on the fan just when you need it.

 

Next is layers and perimeters:  first prove that your print will slice with the 'vase' mode on.  And then turn it off.  I found that two base layers works well.  I went to 3 layers at the top because the first might be a bit flakey - the fan might not react quick enough.  Make sure bridging is checked.

 

The skirt and brim.  A 3mm brim width helps tie tall stuff like wings stay in place.

 

Speed settings:  LW-PLA likes consistent speeds.  I find that 40mm/s is a good one for my filament.  Others might use 50mm/s.  The other thing is that when vase turns off there will be a BIT of stringing inside.  It slices a bit differently and setting travel speed to 250-300mm/s zips it across better so there is less cruft.

 

What does it look like?  Well I stopped this print part way along so that you can see the end as well as the cruft.  E.g. the CF sleeve stops part way and it decided to add in some filler above that before the shear web continued above it.  You

 

And with this you can print several wing sections at the same time!  With two items it will pick a close point to jump across and back so there will be a bit of cruft to rub-sand off.

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Areomodellers would be a good place for those using Cura or other printers to put in their fav settings and hints!

 

Interesting test - most just slump except for the one I use with expands in all directions and seems to hold it shape.

I am currently testing gluing some LW-PLA test boxes with various CA and epoxy in preparation for gluing the wing sections.

I have gotten to a point of putting together the LW-PLA bits for fuselage and tapered wing for a second iteration of the Dragonfly.

Here are the main stats:  
Span 40.5", 8" root chord, 3.43" tip chord, taper of 0.43 root thickness 1.15" tip thickness 0.5", wing area 1.666mt2 (240"^2) aspect ratio of 7.
For comparison the big hybrid (PLA ribs and foam skin) wing with 36" span and 6 5/8" chord and 240"^2 came in at 226g (7.96oz).

The wing was 150g (5.29oz) LW-PLA (uncoated) including servos and partial wiring.  There is a main 6mm CF spar extending 400mm each side and a trailing 4mm CF spar extending 170mm each side.  

All sections of the wing use shear webs (mostly at 20mm spacing) and are printed with 'Almost Vase Mode' so they have closed top and bottom.  Almost no stringing! Nice and stiff even without the CF spar. I used the freeCAD curved shapes tool with three sketches (plan sketch, front elevation sketch (to get root to tip thickness change), and a Clarke-Y sketch thickened to 14%. The centre section is 40mm LW-PLA with two small PLA spar holders. The next two sections are 178mm (the tallest I can manage in the Prusa Mini) with the tip section 145mm to get the overall span of 40.5".  

I wanted to protect the LW-PLA and looked for ideas and found a site which stiff hair spray and another that showed a light sanding and then brushing resin to add abrasion and moisture resistance.  With the resin (and 24 hours of curing), and linkages and a stripes of yellow and red tape that brought the wing up to 180g (6.35oz).  The resin is 30g of additional weight. The LW-PLA is stronger with the resin but it is worth a test to just spray 3 coats of stiff hair spray to limit grass/mud staining.  

The fuselage is essentially the same as the prior Dragonfly slightly tweaked thickness and dimensions to take the ESC and receiver. I also did the resin coating to the fuselage.

The ready to fly weight with a 650mAhr battery was 415g (14.6 oz) with a wing loading of 8.76 oz/ft2 and a cube loading of 6.79.

Here are some images.  Here is the overall looking from the top. I am getting used to the taper. I might decide I like it.

 

Here is the front of fuselage.  The motor is a 2206 1450kVA as used in prior Dragonfly with a 6.3" prop. I used a PLA engine mount which is epoxy'd to the LW-PLA fuselage. Some scraps of 'scrubby pad' to absorb 650 mAhr battery for landing-shock. 20A ESC with a Futaba receiver behind.  I printed a PLA band hold-down strap to limit stresses on the LW-PLA fuselage and a centre-of-wing PLA leading edge protector.

 

A view underside shows the servo-in-fuselage, the taped over notch for the servo wires and the forward band-strap. To accommodate wings of different chords I decided to print a trailing edge PLA stand to fit the 8mm CF boom with a notch to take the band. Whether this works better than a redesign of the fuselage to fit a specific root chord has yet to be seen.  Note seen is a length of bicycle inner-tube to act as an anti-slip for the wing.  I put finger marks for a 27% COG and with the battery it balanced with just a 5mm shift in the battery position. 

 

I used PLA printed servo pocket epoxy'd to the wing webs with the cable running in an exterior slot along the base of the wing.  The linkage includes a horn offset to give a mechanical differential to go along with the transmitter differential.

 

The tail uses a push-pull to keep the weight down and is a hybrid LW-PLA platform/CF sleeve with the rest 3mm foam with tape covering.
The underside includes a printed skid to protect (hopefully) the push-pull horn.

 

What next - well wait for the ground to dry out.  My usual park has been taken over for an outdoor concert and might take some time to recover. I will try some un-powered test flights.

If this goes well - well I can probably get a 1.2m span with the printer. I would probably go for an aspect ratio of 8 rather than 7 and perhaps a 7.5" root chord.