There are lots of ways to make molds for sword props.  Some, like Smooth-On, recommend a large mold box and a nearly-solid block of poured silicone around the master, as shown here or here.  These techniques work great, but can get expensive when you’re making a sword over 3′ long.

My technique is slightly different.  It requires more work post-casting, but saves quite a bit of money, and works well if you’re aiming for limited runs of large sword props.  The pictures shown here are from the Great Fairy’s Sword I created for Linksliltri4ce.  I have used it for a variety of projects, from lightsaber hilts to shields.

 

Step 1: Setting up a master

My sword masters are always set up in halves.  If the sword is identical on top and bottom, I only need the one half, while if it is curved or has different details between the halves, two molds are required.

 

Step 2: Mounting the master

Once the master is complete, I use tacky glue to attach it to a sheet of insulation foam.  Hot glue can melt the foam or the master, so I wouldn’t suggest it.  This can take a few hours to set, and I recommend setting zip-lock bags of flour or other soft items with weight on the master to ensure it attaches to the foam base.  I often leave this overnight.

 

Step 3: Silicone (layer 1)

Once the master is attached to the foam, I mix up a batch of silicone, either Smooth-On Mold Max Stroke or Mold Max 10-T, and include some Silicone Thinner to ensure the silicone will capture the details of the master.  The Stroke silicone sets faster than the 10-T, so I would recommend it in most cases, but the 10-T is thinner by default and will settle into details such as thin grooves better.  It’s difficult to estimate how much silicone you will need, but I find that a 3.5′ sword needs about 200 grams of silicone per coat.

 

Step 4: Silicone (layer 2)

After the first layer of silicone sets to the point where it is still slightly tacky (press a finger into it, and if it feels sticky, but doesn’t leave residue behind on your finger), mix up another batch of Mold Max Stroke, again slightly thinned.  I also dye the silicone with SO-Strong Color Tint so as I brush it on, I can tell if I’ve missed any areas.  Be sure to add extra silicone to corners for strength.

 

Step 5: Silicone (layer 3) and registration keys

After this layer is set, I add a non-thinned layer of silicone, again colored to see when I have completely coated the last layer.  I then add in registration keys – lumps of silicone, either cast in small cups, or cut up pieces of old or failed molds about 1/2″ thick cubes – which are pressed into the silicone.

 

Step 6: Silicone (layer 4, final)

Once that layer of silicone is set to tacky, I add one more layer of Mold Max Stroke silicone to cover the registration keys and strengthen the mold.  At this point, I let the mold set completely, which can take a couple of hours.

 

Step 7: Mold jacket

At this point, the silicone mold isn’t very strong.  To keep the mold in the proper shape, I add a mold jacket made from plaster cloth.  I cut the strips of plaster into 6″ long sections, which I dip into water, run my fingers over to drain excess liquid, then shape onto the outside of the silicone mold.  Use at least 3 layers of cloth in any area, especially over thin sections of the master.  Make sure that the cloth is tightly pressed against the mold in all areas, including the registration keys.  This will take several hours to dry.

To ensure extra strength, I also paint fiberglass resin or Smooth Cast 321 plastic onto the mold jacket once dry, and sometimes glue wooden dowels to it depending on the length.  Use at least 3 layers of the fiberglass or 4 of the plastic on the jacket, otherwise it may still flex as you work with it, and you want it to last.

 

Step 8: Drying

Once the mold jacket is set, carefully remove it from the silicone mold.  It will likely still be moist on the inside, so at this point set it aside to completely dry overnight.

 

Step 9: Casting

If you haven’t yet, carefully remove the silicone mold from your mold master.  You can then insert it into the mold jacket – be sure to check all the registration keys so they are fully seated, otherwise your cast may come out lumpy or dented.  If desired, you can use Cast Magic powder on the mold now.  Then mix up a batch of Smooth Cast 320 plastic (sets faster than the 321 used in step 7), dying with SO-Strong tints for a base color.  It’s difficult to say how much plastic you’ll need with the first casting, so you may need to make several smaller batches of plastic to cast it – just pour each new batch after the previous has dried to off-white to make sure that the new batch doesn’t negatively impact the last.

The plastic will set in a couple of minutes, but after the mold is completely filled, wait about 1/2 an hour before pulling the mold out of the mold jacket, then removing the cast from the mold.  Reset the mold in the jacket and cast the second half of the sword.  For this half, carefully pour in plastic until the mold is almost full and let it set.  Then, pour a thin layer of plastic and press the first half of the sword into the new casting.  The thin layer ensures that the old half lines up properly and doesn’t displace plastic in the new half, and will bond the two together.

If the sword is especially long or thin, you may want to insert a metal rod or bar into the sword.  To do so, put a layer of plastic into the mold leaving enough space for the bar or rod and let it set, then place the bar or rod on top of the plastic, and add additional plastic to fill the mold.

 

Step 10: Cleanup

Once you pull the completed cast from the mold, you will likely need to do a bit of cleanup – excess plastic squeezed out from the mold along the edges which can be trimmed with a hobby knife and sanded smooth, or dents/gaps which can be filled with Bond-o, then sanded smooth.

If the sword ends up with hollows on the inside, you can put tape along all the edges of the sword, drill into the blade, then inject additional plastic into it using a pipette.  Once the plastic is injected, place a finger or tape over the drilled hole and rotate the sword so as the plastic is setting on the inside, it will fill any gaps and provide additional bonding.  The tape will ensure that the plastic doesn’t escape any holes in the casting.

 

Step 11: Paint/Finish

Finally, paint the resulting sword, and don’t forget some kind of clear coat to make sure the paint stays on even with use and transport.