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Making Police-Box Glass -- a resin casting primer

Started by deck5, Sep 14, 2009, 09:23 pm

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Sep 14, 2009, 09:23 pm Last Edit: Feb 09, 2010, 01:27 pm by Scarfwearer
Here's a short document describing how I went about making clear resin castings of a sheet of pebbled glass.  Because the clear resin I  used remain somewhat bendable even when cured -- and especially when warm -- I reinforced the castings with embedded sheets of plexiglass.

I tend to work in lash-up fashion, improvising from materials at hand.  There are fancier ways of doing some of what I have outlined here.


ItemWhat I used
MasterOriginal sheet of pebble glass
Silicone rubberSmooth-On OOMOO 30, a 1:1 mix with no shrinkage
Stir-sticks for the rubber
A large flexible plastic measuring cup
Clear resinCastin' Craft EasyCast Clear -- 1:1 mix low odor water-clear polyurethane
Disposable measuring cups for the resin
Disposable mixing cups and stir-sticks for the resin
Optional: Transparent and/or opaque tints for your clear resinA drop of yellow tint for the front panels
Plastic sheet1/16" plexiglass sheet
Mold boxCookie sheet at least a half-inch larger than the glass sheet all the way around, with a suitably high wall
Non-drying clay
Double-sided foam tape
CA glue ('super glue')
Bench knife
Bubble level


Building the mold box

Your original object -- in this case a sheet of glass -- is called the 'master'.


The box you put it in to pour rubber over it is called a 'mold box'.  I made the mold box for this project from an old cookie sheet and a bit of clay.  This was cheap, easy, and effective.  Other options you might consider for building a mold box include Lego, foam-core sheet seamed with hot-glue, a box custom-made from wood or plastic, etc.  Assuming you want to follow my method --

Use the double-sided foam tape to fasten the original glass sheet, pebble-side up, to the inside of the cookie sheet.  Make sure the tape is very close to the edge of the glass, and press down firmly; you don't want any gaps either on the glass side, or the cookie sheet side, or the silicone rubber will seep underneath -- not a tragedy if it happens (as any excess rubber can be easily removed later) but a waste of silicone rubber.  If you do have gaps, fill them with non-drying clay.

Leave a half-inch (or so) margin all the way around the master.  If the cookie sheet is much larger than your glass, use non-drying clay to build walls at the appropriate distance away on the sides that need it.  Make sure the clay meets the bottom and sides firmly, and is about twice the full height of your taped-down glass sheet.  Don't leave any weak spots where the wall might fall down.  Neatness is not a requirement here; don't worry about lumps.

Some silicone rubbers advocate the use of a mold release that comes in spray cans.  This shouldn't be necessary with glass and metal, and I didn't use any, but your mileage may vary.  If you use it, coat the glass and the mold box evenly.

Make sure the whole setup is level.  A couple of wedges of wood should suffice if any adjustments are required.

Pouring the mold

Calculate the amount of rubber you'll need.  For example:

Sheet of glass: 7" x 8" x 1/4" cubic inches
Thickness of foam tape: 1/8"

Area of mold box containing the glass: 9" x 9"

We want the rubber to have a 1/4" (or so) base.  Google does the calculations for us:

a)  7*8*(1/4+1/8) cubic inches in oz = 11.6 oz
b) 9*9*(1/4+1/8+1/4) cubic inches in oz = 28 oz

Subtract a) from b) = 16.4 oz. = the volume of rubber needed.

This means we need roughly 8 oz. each of parts A and B of the silicone rubber.  After stirring each according to the directions that came with the rubber, I poured both together into a large polyethylene plastic measuring cup and mixed according to the directions.

Once mixed, the rubber is ready to pour.  From a couple of feet above the mold, pour a very thin stream of rubber into one corner of the mold box.  This helps minimize bubbles.  The rubber will spread out from this spot to cover the master.


Once the master is fully covered, let the rubber fully cure according to the directions.

Removing the master

Once the rubber cures, you can peel it away from the master.  Do this somewhat carefully; you don't want to introduce tears.  It should come away cleanly.


Note: In my case, the rubber I was using was old, and cured too fast: the resulting bottom surface of the mold was therefore not flat.  To compensate for this, I made what is called a 'mother mold'.  I first coated a normal opaque fast-setting resin over the outside bottom of the rubber mold, and let that cure; it was still irregular, but now it was rigid.  I then tacked the coated mold to the inside of a cookie sheet with a bit of clay; I built a new wall across the cookie sheet with clay, and then poured opaque resin around and under the rubber mold.  This, when cured, fully supported the rubber mold.  I then glued the rubber mold to the inside of the mother mold with CA glue.  This whole setup, cookie sheet and all, became the new mold.

Pouring a casting

The amount of clear resin you require is easy to calculate.

a)  Volume of glass: 7*8*1/4 cubic inches in oz = 7.75 oz
b)  Volume of plexiglass sheet: 7*8*1/16 cubic  inches in oz = 1.9375 oz

Subtract a) from b) = 5.1825 oz. of resin.

This means roughly 2.5 oz. each of parts A and B of the clear resin.

Cut a plexiglass sheet to fit inside the mold (plexiglass can be scored with a bench knife and snapped).

Temporarily test-fit the plexiglass in the mold, and make sure the whole setup is level.

Mix the clear resin according to the directions.  If you want to add a tint or other filler, it's added during this mixing process as per the directions provided by the manufacturer.  Once mixed, pour the resin into the mold.  Spread it with a stick to reach the corners.  Lay the plexiglass sheet in the resin, starting at one end and lowering it slowly to minimize trapped air.  I find it necessary to tip the mold slightly to get the resin to fully coat the top side of the plexiglass.  If the sheet is prone to floating up, or air bubbles appear under it, press down in the center with a bit of wood to force the bubbles out.

For police-box glass (this project), tiny bubbles just add to the quality of the end-product!

(You can see the mother mold here, it's the yellowish resin surrounding the rubber mold.)

Allow the resin to cure.  If you're unmarried, or wish to work toward becoming unmarried, you may be able to use the oven to warm the mold at about 60C for a couple of hours to speed the cure.

De-molding the casting

After the plastic resin cures (again, see the directions for how long this takes), it can be pried up out of the mold, starting at one corner.  Be somewhat gentle; you don't want it to catch and tear your rubber mold.  (TIP: If you do get tears in the rubber, they can be mended with CA glue.)


That's called a 'pull'.  And that's the whole process.  The mold can be used quite a few times before it finally gives up the ghost.


The EasyCast resin I used takes a few days to become completely rigid, but the embedded plexiglass ensures ther's no permanent deformation at any stage of the game.

To further finish the clear castings, I coated them in Future Floor Polish (which is actually a clear acrylic paint) and once dry, polished the result with Novus plastic polish.  If your master is larger than the window panes you want, individual window panes can be cut from the castings with a suitable plastic-cutting blade on your favourite saw.

Let me know if you have any questions!