Planets ala Dollar General

A few months ago I posted how I make planets (and any general hemispherical shells) by pouring water-putty in a silicone mold.  The problem with that method is unless you want to use an entire tube of silicone window-caulk, the size of the planets must remain small.  I have since come up with a cheap and easy method of hemisphere shell construction which also eliminates having to wait for toxic silicone to cure.

First, go to Dollar General (USA) or any other dollar store, and buy a plastic inflatable ball.  The pictures here show a 12.5 cm diameter ball, but this technique should work fine with larger spheres.  The ball I purchased cost US$1.00.  Big picture here: we're going to cut the ball in half and use its inner surface as a reusable mold.

Now that we've brought the ball home, we need to make a rigid sleeve to ensure the mold keeps its spherical shape after we've deflated the ball.  OK, so with the ball inflated, start applying strips of soaked paper across the ball, making a little cap for the ball.  Take a look at picture 3 below and this will make more sense.  I dip strips of shredded paper (1-2 cm wide) in a glass of water and smooth them out on the ball along the meridians.  Make it as smooth as possible as this will define your mold shape.  make sure the sleeve does not go all the way down to the "equator"; make it just large enough to hold the mold, maybe 1/3 the total surface area.  When the strips are all laid down, mix up some Durham's water-putty (3:1 ratio) and slather it on the paper.  As the putty cures, apply strips of paper to the "mud" perpendicular to the meridians.  After about an hour or so the putty should be set enough to handle.

Wait until the putty cures.  Holding the ball by the sleeve you just created, make an incision and deflate the toy.  Remove the ball from the sleeve and set the sleeve aside.  Cut half of the ball away (you can see in the pictures I leave two pull tabs to help peel the mold from the cast later).  You now have a hemispherical mold that, if kept clean, will make hundreds of casts.

Mix up some water-putty (5:2 powder/water ratio) and pour into your mold.  Hold the mold by the sleeve and rotate your wrist for a minute or so and ensure the entire surface of the sphere is coated.  Wait 40 minutes and repeat with another batch of putty.  Wait 40 more minutes and add a third coat of putty.  After another 40 minutes, mix up another coat of putty.  As soon as you've added this 4th coat, start pressing 1-2cm wide strips  of paper into the putty.  The paper will add the tensile strength required to remove the cast from the mold and survive table-top action.  When the entire inner surface is covered with paper strips, mix up more putty and pour over the paper and swirl around.  Immediately begin applying a second layer of paper strips perpendicular to the first layer.  Mix a final batch of putty and ensure the second layer of paper is coated.  You may use plastic window screen vice paper as in picture 2; it adds strength but also adds expense. 

Second coat of putty

Strengthening material added

After waiting at least an hour (but preferably overnight), gently pull the sleeve off the outside of the mold.  Now, gently peel the mold back from the cast.  You should now have a near perfectly smooth hemisphere which you can detail in various ways.  Future posts will detail how to make continents for earth-like planets and craters for moons.

Sleeve, mold being peeled from cast, 80mm spaceship miniature

Size comparison with scratchbuilt moon

Continents are easy