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Archive for February 14th, 2014

Chocolate Mold: Image Preparation Checklist

Start with a monochrome image:

Tux-Shirt

Tux-Shirt

A bit of tinkering produce a height map image:

Tux Mold - Height Map - large

Tux Mold – Height Map – large

I picked a 3.0 pixel/mm scale factor, so a 33 mm mold covers only 100 pixels. That image is 1100 mm tall and will be reduced by a factor of 10 to the final image size: this is not the place for fine detail and fancy lettering!

The conversion process assumes you’ll handle the Z axis scaling yourself, so the script no longer normalizes the gray levels. If you select gray levels using HSV, the V slider gives you a direct reading in percent-of-maximum thickness; Tux varies from V = 80 to 100, so he’s pretty much bas relief.

The border around the image must be 0 = black and will be stripped from the final mold. That’s why Tux doesn’t turn into a bird served on a rectangular platter.

Because this is a mold, its edges must have some draft, which means the outline must shade from black to whatever gray represents the interior of the mold. Do this:

  • Trace the outline using the Scissors Select tool = snap to high-contrast outer edge
  • Create / go to a new layer filled with whatever gray you want for the interior (V = 80 here)
  • Select → Grow the selection by 60 pixels (on a 1000×1100 image)
  • Select → Invert to select the exterior of the outline
  • Bucket fill the exterior with 0 = black
  • Select  → Invert to select the interior of the outline again
  • Select → Border: Add a 30 pixel border to the selection with the “Feather border” option
  • Bucket fill the border with 0 = black
  • Unselect and you have a layer with a nice graduation around the mold

Which looks like this with V=80 gray inside:

Tux Mold - Height Map - outline gradient

Tux Mold – Height Map – outline gradient

The 30 pixel feathered border, scaled by the 10× reduction, means the edge of the mold goes from 0 = black to the interior in about 3 pixel / (3 pixel/mm) = 1 mm. If the interior is 255 = white at 7 mm, the draft angle is arctan 1/7 = 8°, which is probably about right for the deepest part of the mold. The edge of the Tux mold is V = 80 (or about 200 gray), so it’s at 0.8 × 7 mm = 5.6 mm and the draft angle is arctan 1/5.6 = 10°.

Inside the mold, anything goes, but you should avoid 0 = black levels so that the alignment pins don’t poke through the mold. Any 255 = 100 V = white levels will be the maximum mold thickness, which is 7 mm for the molds you see here and that may be somewhat too thick for a chocolate treat. It is really hard to maintain draft on small features, but I think if you don’t get carried away it’ll be all good.

There’s also a 1 mm backing plate below the mold that ensures the deepest mold parts have some substance behind them and the alignment pin sockets have enough depth to be useful.

Scaling the image down by 10× to about 110 pixels tall (including the black border) will make the final Tux mold about 37 mm tall:

Tux

Tux

This image enlarges it by 10× with no smoothing to show the gritty nature of the image. This is why you can’t have delicate detail or fine lettering:

Tux - enlarged to show texture

Tux – enlarged to show texture

Notice the nearly complete lack of draft on the interior features. Each level differs by about V = 5 over the range V = 80(the border) to V = 100 (beak and flipper), so they amount to only 0.05 × 7 mm =  0.35 mm = one or two thread layers at 0.20 mm/layer. I think if you were doing this right, you’d pick an overall thickness so that V = 5 increments corresponded to one layer or use whatever V increments corresponded to a single layer.

Running that image through the Bash script & OpenSCAD programs (more on those later) produces a reasonable result:

Tux positive mold - solid model - oblique

Tux positive mold – solid model – oblique

When it’s converted into plastic, you can count the layers in each V = 5 level (clicky for more dots):

Tux positive mold - plastic - oblique

Tux positive mold – plastic – oblique

It may be a bit less rounded in the tummy than the real Tux, but seems good enough for the purpose.

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