Warping Rayguns, OpenSCAD, and the Heated Build Platform

Simple OpenSCAD Raygun prototype

I’ve started working on a simple design for a printable modular raygun. The image to the left shows a 1/5th scale prototype print.

The problem is warping. This is a wide and shallow print. Each half of the raygun is 75mm wide, and only 12mm thick.

PLA warps less than ABS, so up until now I’ve not needed to worry about warping. I’ve noticed a little on objects with flat bases, like the Sim City 2000 factory but it has not been too noticable. This raygun print changes that.

I designed the raygun as simply as I could imagine. The barrel is a cylinder, with a cone at each end. One cone streamlines the back end, and the other is inset as a ‘reflector’.  The grip is a rectangle set at an angle to the barrel.

Then I added some twiddly details. There’s a hollow tube down the centre of the barrel for darts or a light. I put a football shape on the bottom of the handgrip. The finger cutouts are made by subtracting toroids from the grip. All the sharp edges are smoothed with constant radius curves.

3D printed Raygun in two halves, ountside view

The outsides of the raygun's two pieces

Since undercuts and overhangs are hard to print on a ‘fused filament deposition’ printer like my Mendel Prusa, I’ve had to cut the model into two pieces with flat bottoms, to print it.

Just like most plastic toy guns, I cut it down the middle. This is easy to do in OpenSCAD. You can see the parts in the image on the left.

The disadvantage to this method is that the prints have to stay exactly flat on the build surface. Otherwise the two halves won’t fit together perfectly.

Top view of the barrel of the raygun, showing the gap caused by warping as the PLA cools

The gap caused by PLA warping as it cools

Elastic bands hold the flat centre portion of the barrels together, revealing the warping you see in the third photo. This occurs because PLA shrinks slightly as it cools and sets.

Imagine the printer extrudes a long thin noodle of plastic, and assume it’s exactly 100mm long. As it cools down, it shrinks a little, becoming shorter – say 98mm.

The printer then lays another noodle on top of it, exactly 100mm long. There is a 1mm overlap at each end. The two pieces bind together due to the heat in the top strand. Then the top strand cools and wants to shrink to 98mm, trying to make the bottom strand shrink with it. But the bottom strand has done all the shrinking it can. So it bows up a little instead.

This happens with each new layer, and the result is the ends lift up off the print bed.

Really good adhesion of print to printbed helps a little, but you have to be able to get the print off when its finished. Only printing small objects helps, but that’s no fun.  I’m going to have to stop the lowest layers from cooling down so soon, so there is less shrink stress forcing the edges up and away from the bed.

That is why the heated build platform was invented. It warms the plastic from below, without being so hot as to make it melt. Then, when the print is finished and the heated bed turned off, the whole print shrinks slightly as it cools, and pops neatly off the bed without having warped.

That’s the theory. There is less consensus about how to mechanically achieve this holy grail. That is why I haven’t built my heated bed yet. Maybe tomorrow. Which is already today, because it is now 1:30 am.

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7 Responses to Warping Rayguns, OpenSCAD, and the Heated Build Platform

  1. Paul Stevens says:

    Zap!: Ray Gun Classics
    by Leslie Singer
    This has been out of print for maybe 20 years don’t know if you have it, lovely designs from the 1930’s-50’s if I remember correctly, still available through abebooks.com.

    Paul

  2. Mark C says:

    Andrew,
    I was about to embark on my heated bed until I saw your post on the Google Group and realized it may be more complicated than I thought. Does that mean the pictures on Brock’s addendum have the PCB upsidedown since the side with the silkscreen is down?

    Also, why are you printing on blue tape and not the polyamide tape?

    Keep up the great work and excellent blog!

    • BrazenArtifice says:

      Hi Mark,
      I’m glad I’ve got the heated bed PCB working. I had been putting it off because there were so many conflicts between the different things I had read about heated beds. I’ll write a post right now about what I did, and why.

      Thanks for your support – its nice to know you get some value from what I’m writing.

    • BrazenArtifice says:

      Forgot to answer the point about the blue tape.

      I can buy blue tape locally, and I’ve been printing on it for a few weeks now, so I’ve seen how it works. I prefer to change only one thing at a time. That way I’ve more chance of working out what difference the change made.

      Right now, that change is temperature, not build surface.

  3. I’ve heard of two other ways to try to counteract the dreaded shrink (on your ray guns – I tried to get a shrink ray joke in there, but alas my brain isn’t quite up to it today).

    First, is to print on a raft – which is a little wasteful and usually means I get sore hands from having to try to pry it off the raft with a sharp knife afterwards.

    The other method is to add ‘mouse ears’ to the corners of your print – little flat circles that sit on the corners like embedded coins, which help adhere the corners of your model down on the bed. You then cut the ears off (gruesome) once the print is done.

    If your heated bed is not quite sufficient (and my Thingomatic came with one as standard) then you might try one of these as well.

    Lastly, mix and match modular raygun parts would be utterly brilliant to print! It would be like ‘My Robot Nation’ but home printed. I think they’d do particularly well with the individual parts printed in different plastics – especially with the slow introduction of gold, silver and glow in the dark filament. Why yes, I would like a red-barreled raygun, gold handle with yellow lightning bolts along the side!

    • BrazenArtifice says:

      Hi Tinkergirl,
      Wow, level design as a profession – whoo!

      Thanks for the suggestion of rafts and mouse ears. I’d heard of them some time ago as being needed for ABS, but not for PLA. It didn’t even occur to me to try them first, before assembling the heatbed. As you’ll see from my follow-up post, the heated bed cured the warping problem, at least at this size (~75mm). When I up the scale to more like 150 – 200mm we’ll just have to see.

      Yes, I’m really keen to get modular raygun parts going. I have silver PLA as well as clear to play with at the moment. You’re right that glow-in-the-dark PLA pieces would be awesome, as internal components lit by hidden pulsing LEDs, maybe. And plugin tiny arduino boards for sound effects. Though that’s getting away from printability.

      Robot Nation is such a cool idea, but I got lost when I got to selecting surface decoration. Way too many visual choices and too few structural choices.

      I’m going to have to learn CAD design from scratch, so wish me luck!

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