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Mold and Cast of BJD Knee Joint Ball Part with Magic Sculpt

What am I trying to do here?
I need to make a mold for this part (double balls for knee joints) – see the photo just below. I need a part, consisting of 2 balls, which are  connected, flattened on each side, with the vertical central hole.

Why can’t I just roll some clay in my hands and make those parts? Well, I can and I did on my previous version. Doll’s standing and ability to hold the pose depends on well-fitting tight joints. The fit on the hand-made knee joints on the previous doll was mediocre.
Here I am trying to make a MOLDABLE part with PRECISE SPHERICAL joint working surfaces, which can be reproduced to make the second knee joint and also be reused in the future dolls.

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Tools and Materials:

hollow plastic balls for modeling spherical surfaces in future ball-socket joints
x-acto knife to make holes in the balls
1/4″ thread screw for center hole
lego base and blocks – for mold box
– vaseline petroleum jelly – release agent for magic sculpt
disposable brush – to brush vaseline on the mold and the part
magic sculpt epoxy putty – for molding
digital scale – for measuring equal parts of magic sculpt
8-32 thread screws – for securing 2 parts of the mold
Living Doll clay – for modeling the part
forceps – for supporting the part during baking
metal bead sizer triangle – precision shaping of the ball after molding
sanding mesh – to polish the flat surfaces of the part
ice pick – to pry mold open

 

**********
So:
Made holes in the plastic balls and put the balls on the 1/4″ threaded screw

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Made a box out of lego pieces to accomodate the part. Brushed the vaseline petroleum jelly (releasing agent) on all surfaces which will touch the magic sculpt (the lego box walls, bottom, the screw, the balls)

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Measured equal parts of Magic sculpt, for me it is easier to do by weight. Mixed well.

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Made a “bed” of magic sculpt – bottom part of the future mold.

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Press down the part into the magic sculpt.
Brushed vaseline petroleum jelly on top of the first layer of magic sculpt.

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Measured and mixed second batch of magic sculpt.

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Second layer of magic sculpt (top part of the mold).

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Brushed 8-32 thread screws with vaseline petroleum jelly.

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Pushed the screws down on 4 corners through both layers of magic sculpt.

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Waited until magic sculpt hardened (about 3-4 hours).
Removed the lego pieces (easier with the lego pry tool, because the pieces are are slippery from the vaseline).

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Unscrewed the 4 screws.

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With the help of ice pick pry the mold open.

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Filled the cavities with clay and put the 1/4″ thread screw.

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Closed, pressed and opened the mold a couple of times to remove the excess clay.

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Carefully removed the part from the mold and baked.

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Polished the baked beads with a metal circle sizer, put the beads back on the screw.

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Cut the sides off the beads to make the joint flat. Added clay to attach beads to each other and to form the knee joint.

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Here it is – half-way prototype for knee joint

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Return to I want to make my own BJD – Part 3

One comment on “Mold and Cast of BJD Knee Joint Ball Part with Magic Sculpt

  1. […] Most of these ball and socket bead prototypes were made simply by putting a plastic ball on a screw, adding some clay and baking. The elbow and knee beads were made differently, because they are not really round. The process is described here. […]

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