Silicone Box Mold for Prop Making




Introduction: Silicone Box Mold for Prop Making

Silicone box molds are a fairly simple and very effective way to mold simply shaped objects. I molded a pipe wrench, but you can mold many things (but not everything, which I will get to later) with this style mold. Very soon I will be posting the second half of this process, which is casting a part from the mold. I will be making a flexible foam wrench that looks exactly like the real thing, but that you can hit someone over the head with. Just creating a mold takes many steps, each of which is incredibly important, so just focusing on that should help maintain clarity. This style mold is best used with a simple, fairly uniform shape, with very little or no undercuts. Undercuts are pieces that hang off the part you are molding, that would stick in the mold, not allowing you to remove the piece you cast. Envision trying to mold a flower, each petal would stick into the mold in different directions. A two-part boxed mold is terrible for molding things like that, but is great for molding things like a cell phone, prop weapons, or a cigar (all things I have molded in this style).

Step 1: Materials and Tools


- Platinum Cure Silicone. The cost will be dependent on what you are molding but a gallon kit (silicone and catalyst) usually costs around $100 per gallon. I explain in a later step how to estimate how much silicone you will need

- Sulfur-free plasticine clay. $5 a block at any art supply store. Plasticine clay is an oil based clay. Make sure it is sulfur-free because sulfur inhibits platinum cure silicone, which will ruin your mold. Again, how much you need depends on your mold.

- One 4'x4' sheet of plywood. $20 at any home center

- Petroleum Jelly (Vaseoline)

- Baby Oil

- Wood Glue

- Two large clean mixing containers

- Stir sticks

- Disposable cups

- 2" chip brush

- eight 3/8" nuts

- Two small plastic funnels

- Rubber Gloves

NOTE ON SILICONE: Silicone is very expensive so I will show you ways through-out how to minimize your use. Make sure and purchase platinum cure silicone, not tin cure silicone. Platinum cure does not need to be degassed, which is a process of putting silicone into a vacuum pot to get out any air bubbles, Tin cure does.


- Safety Glasses

- Skill saw or table saw

- Hot glue gun

- Hand drill

- 1" Forstner or paddle bit

- Pencil

- Utility knife

- Tape measure

- Scale which reads to a hundredth of an inch

- Screw gun

- 2 Large Clamps

Step 2: Making the Box

If you do not know how to make a plywood box there are some great Instructables that will show you how. To establish the inside dimensions of your box, set your piece on the corner of your plywood and then add 1/2" around the entire perimeter of the object. This allows 1/2" silicone walls to form around your piece, which is plenty in a box mold. Measure the height of your piece and then add 1 1/8", this allows for 1/2" top and bottom for silicone and an 1/8" for the kerf of the blade when you cut the box in half. These measurements set the inside dimensions of your box, but remember your overall box will be larger because of the material thickness. Cut your box in half and then mark your sides because the box will only line up correctly one way. Because of the shape of the wrench I added some filler pieces to my box to minimize unnecessary silicone (the stuff is expensive). The shape of your piece will dictate if you need to add filler. Just remember to have at least a 1/2" of clearance around your entire piece. Drill two evenly spaced 1" holes in one side of the box.

Step 3: Claying Your Piece

The beauty of silicone is that it flows into every nook and cranny of the part you are trying to replicate. This is why it makes such great molds, but you need to make sure it doesn't get into bad places. Use the plasticine to block off any hole or crevice that is not crucial to the part, or is somewhere that only a very thin layer of silicone will get. Once you have clayed the area, smooth it out to make a nice finish. The silicone will pick up every detail so you need to make the clay on your part blend into the piece the best you can.

Step 4: Claying the Box

Fill the half of the box without holes completely with clay, getting it as flat and level with the sides as you can. Use a wire or piece of straight wood to check level and scrap down high spots. Once your clay is set put your piece on the clay and use a pencil or awl to mark around the piece.

Step 5: Claying the Box and Piece

Scrap out the clay where the piece will sit, keeping in mind that only half the piece will sit in the clay. I used a sculpting tool that you can get at any art store but you do not have to. A pencil and a spoon will work. If possible mark the center of your part with a Sharpee. I had the casting marks on the wrench to follow so I did not have to make a mark. It is important to get very close to the exact center of the piece. Once the piece sits in the clay correctly fill in clay around the piece, making sure to get the clay tightly around the entire perimeter and then smoothing the clay as flat as possible.

Step 6: Keying and Funnel

Correctly keying a mold is one of the most important steps in any mold process. Keying makes sure every time you take your mold apart and put it back together it lines up correctly. The beauty of a box mold is that the box itself is a key around the entire perimeter. Other types of molds require many more keys. 3/8" nuts filled with clay make simple and quick keys. Place them in each corner and then add more as needed. Make a few small indentations 3/8" away from the part where you can using a clay tool, pencil, or screwdriver and then smooth with your hands. Add a half circle of clay to the top of your part making sure it goes from your part to the edge of the box. Once the mold is completely finished the negative of this clay will act as a funnel where you will pour your casting medium. Where to add the fill spout is different on every piece, but here are some rough guidelines for placement.

1: The most important point of filler placement is to not create an air pocket. This is an area that when you are filling your mold with your casting medium air could be trapped. If I placed the filler hole on the small end of my wrench as the mold filled with resin air would become trapped above the adjuster nut. Take a good look at your part and try to envision spots where air could be trapped when placing the filler hole.

2: If your piece tappers or has a very narrow area, like my wrench, put the funnel on the widest area. Unless it will cause an air pocket. If you are molding a sword put the funnel on the butt of the sword.

3: If your piece is fairly symmetrical you can place it on any side as long as it doesn't cause an air pocket.

4: Sometimes your are going to get air pockets in your mold, I will show you on the next Instructable how to fix this.

Step 7: Release and Clamping

Release (or grease) keeps things from sticking to other things. Silicone doesn't like to stick to things (except to other silicone) but it is good to grease everything on you mold (except the box). There are many off-the-shelf releases but the best I have ever used is petroleum jelly (Vaseline) thinned with mineral oil (baby oil). The petroleum jelly is the release, use only enough mineral oil to make it easily brushable. Brush your part and the clay with the grease. Silicone picks up every detail and any brush mark will transfer, so wipe down your part with a paper towel. The brush marks will be gone but the part will still have plenty of release, it doesn't take much. Do not brush the side of your box that the part and clay is not in. The silicone needs to stick to that box so it stays in place. Securely clamp you box together to the edge of a table. Make sure to clamp along the edges. If you clamp down hard in the middle the box will deform and once released the silicone will not be properly backed.

Step 8: Funnels

Set the plastic funnels in the fill holes and mark where they sit. Cut them to that height and then hot glue them to the fill holes. These make pouring the silicone much easier and keep any overflow from going all over your box

Step 9: Calculating and Mixing SIlicone

SAFETY NOTE: When working with silicone make sure and wear gloves and safety glasses. The catalyst is very toxic.

To know how much silicone you will need to pour one side calculate the volume of your box in cubic inches and convert that to gallons. Divide the number of gallons in half and that is how much silicone you will need to pour your first side. Obviously your part will displace some silicone so try and estimate and subtract it. Silicone is usually a 10:1 ratio of silicone to catalyst by weight, but check whatever product you are using. It also needs to be very thoroughly mixed. Using the "two bucket" method, measure out your silicone and catalyst in one bucket and mix. Then transfer to another container to do the final mix. This guarantees you won't have any unmixed. Mix the silicone and catalyst until there are no white streaks.

Step 10: Pouring

Slowly pour the silicone into one of the funnels. Only pour into one hole so that the other hole can function as an air vent. If you pour into both holes there is a chance that an air bubble will form, ruining the mold. Pour until the silicone reaches the bottom of the funnel of the air vent. If you did not mix enough silicone to fill the mold don't panic, just mix some more, you have time. Except for some specialty silicone, most silicone has a very long working time (pot life). Let the silicone fully cure over night.

Step 11: Creating the Second Half

After your first pour has fully cured cut the funnels, and the silicone within them, off and open your mold. Put your part in the silicone side and grease the part and more importantly the silicone. Add the other half circle of clay to finish the future fill spout. Remove all of the clay and then drill two filler holes in the first side of your box, but do not grease it. Clamp up your box, add the funnels, mix and pour the silicone.

Step 12: Finishing

Once your second half has cured open up the box and pull out your original. You mold should look great. All that is left to do is cut out the wood above where you added the clay filler spout (which I had not done yet in this photo) and then start making parts. Very soon I will be posting a video of how to make very realistic soft foam props with your mold.

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    Brilliantly executed and described.


    Why not just use a real wrench, which is cheaper than a $100 prop?


    Reply 8 years ago on Introduction

    I imagine because it really hurts to get hit with a real wrench, instead of a foam prop wrench.


    Reply 8 years ago on Introduction

    Ever try to clean up a mess like that??? Totally not worth it. Plus you have to buy a new wrench each time. Real wrenches can get expencive fast. You could buy a cheapy wrench at Harbor Freight but who knows if it will break before you get the job done? Then you have to make a long trip back to the store to get a replacement, and you have to either take the person with you in the trunk, or hope they dont get away while you get a refund. And even then you have to explain all the blood and hair on the wrench to the store clerk. (Zombie invasion only goes so far)

    Well detailed and well written instructable. I wish more authors would include as many pictures and individual step details as you have. I can't wait for the second part!


    8 years ago on Introduction

    Nice mold,and the best mold making instructable I have seen - especially covering how to use the plasticine to cover your positive spaces you don't want molded.

    You said the box is also a key. Have you ever considered using biscuits or dowels along the edge to really make it a key? I don't think you would need keys in the mold silicone if you did that.


    8 years ago on Introduction

    very nice molding instructable. Thanks! I really like these.


    8 years ago on Introduction

    Neat, was downloaded, and went to this Blog:


    8 years ago on Step 2

    Couldn't you get by with a lot less than 1/2" rubber above and below object?

    Eh Lie Us!
    Eh Lie Us!

    8 years ago on Introduction

    the part of this that blows me away, is that you made a box then cut it in half (2/3s). Genius! I would have spend a better part of a day making two pieces that fit.

    I feel enlightened yet strangely dumb at the same time.


    8 years ago on Introduction

    Very nicely done! Thanks for tackling this sometimes-very-challenging subject....