MakerBot Industries

Remember magic shell?

I had the opportunity to talk to Nick Starno of MakerBot yesterday regarding something we are both passionate about – getting the best Skeinforge settings to print sweet awesome things. ¹  Nick believes that the “extra shells” setting is the most underlooked and underappreciated settings in all of Skeinforge. ²

Assuming a typical Skeinforge setup, the extruder will first draw the outline of a layer in a part before filling any of it in with more plastic.  That outside trace is the “first shell.”  The “extra shells” setting will add additional interior traces of the outline of the layer for each additional specified.  This picture should explain it better:

2 extra shells, 0 extra shells

Pretend the lines are the paths of the extruder as it lays down plastic.  The figure on the left has the extruder drawing the outline, then draws two extra shells, and then fills the center of the object with plastic.  The figure on the right has the extruder drawing the outline and then filling the object with plastic.³

The “extra shells” setting is probably just as important to part strength and structural integrity as plastic “infill” or the amount of plastic printed inside the object.  It is probably pretty intuitive that an object that is 100% filled with plastic is going to be stronger than an object with 0% filled with plastic.  But what if you don’t need the strongest part possible?  What if you just need an object that is purely decorative, doesn’t need to be strong at all, that just needs to be only just strong enough for a particular application, or prints quickly?

It depends.  Generally speaking, a higher infill ratio will lead to a stronger and sturdier object that will use more plastic and time to print.  Whereas, a lower infill ratio will lead to a lighter, less sturdy object that uses less plastic and time to print.  When I don’t need a part that is super-strong, I typically print with about a 20% fill ratio.  I find this makes for parts that are very strong and durable while still being quick to print without using a ton of plastic.

However, infill isn’t the only concern.  Laying down extra shells can result in an object that is strong on the outside, while still being sparse on the inside.  However, more shells isn’t always better!

• Thin Parts. When you have extra shells on a thin part, a current bug in Skeinforge will cause your thin parts to be hollow.  Basically what’s happening is Skeinforge looks at the thin section of your object, figures that it cannot fit the required number of extra shells in there, and then skips the shells and the infill. ⁴  So, if you’ve got a small or thin part or a part that has small or thin features, you will want to turn extra shells down to 0.
• High Infill. When you have a high infill ratio with insufficient extra shells, the shrinkage that occurs inside the part with the high infill as the plastic cools causes a lot of stress in the printed part.  If the number of extra shells is too low, that stress from too-high of an infill could cause larger volume objects to crack.  Nick has noticed that this effect seem to be worse with smaller diameter nozzles and small layer heights.  I suspect this is a bigger problem for ABS than it is for PLA since PLA has almost no shrinkage, but I haven’t done enough testing to confirm this.

What Skeinforge setting would you like to learn more about?  Leave a comment and let me know!

1. Photo courtesy of *Micky [↩]
2. You can find the “extra shells” settings in Skeinforge here:
   • Fill -> Extra Shells on Alternating Solid Layer (layers)
   • Fill -> Extra Shells on Base (layers)
   • Fill -> Extra Shells on Sparse Layer (layers)

   [↩]

3. Oh, and those sweet sweet awesome drawzwing skillz?  All mine, baby! [↩]
4. Say, for instance, you have a 5mm thick wall and your extruder is laying down 0.5mm wide strands of plastic.  The most extra shells you could fit in there would be 4.  With zero shells, the 5mm thick wall would be composed of a 0.5mm outer perimeter with whatever infill.  With 1 extra shell, you would have the 0.5mm thick perimeter and one 0.5mm shell, with the rest being infill.  With 4 extra shells, you would have a 0.5mm perimeter and four 0.5mm shells, leaving no room for infill.  If you had extra shells at 5 or higher, this bug in Skeinforge would determine that it could not fit in all the shells required, and then just not add shells or infill. [↩]