How to Make a Two-Part Rubber Mold of a Baluster

This tutorial shows the process of creating a two-part polyurethane rubber mold of a wooden baluster. In this case, concrete will later be cast in the mold.

This mold making technique can be used on a variety of models, and a wide range of casting materials could be used in place of concrete (e.g., polyurethane foam, polyurethane plastic, rubber).

 

Supplies Needed:

 

The Process:

The Model: A Wooden Baluster [painted] ↓

wooden baluster mold making

 Approximate Dimensions of Baluster:

Dimensions of Baluster for Mold

Create a “mold box” around the baluster. Melamine-laminated particle board is used for this project:

Creating a Mold Box

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Making a mold box

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Constructing mold box

Screw (or adhere) all particle board into place:

Securing mold box

The baluster is situated on its corner (as opposed to flat) so the parting lines of the mold lie on the corners of the base of the baluster and not on the flat side. Parting lines can show on castings in some cases, so they should be placed in inconspicuous locations when possible.

Mold box preparation

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mold box construction

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mold box mold making

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mold box mold making

Seal all edges of the mold box to prevent mold rubber from seeping out. Plasticine clay is used here, but hot glue or caulking are other options:

Seal mold box edges in mold making

Embed one half of the model in clay (do not use water-based clay – it will negatively affect the cure of the polyurethane rubber):

two part mold making

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smooth clay with tongue depressor

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embedding models for mold making

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model embedded in clay mold making

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 This half-cylinder piece of clay is added to later accommodate a structural metal rod (for installation of the final cast piece):

Structural support in molds

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constructing mold box

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After the top particle boards have been fitted, add “keys” to the clay. These keys will help to register the two halves of the mold together when casting. Marbles are used here, but a variety of objects, or even a pattern in the clay, can be used to achieve this registration:

Create keys for registering mold

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rubber mold registration

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how to register two part molds

Apply and brush Pol-Ease® 2300 Release Agent onto the model, clay, and particle board:

release agent for polyurethane mold

Secure the top particle board pieces:

Ensure that the work space is level:

Seal all edges to prevent rubber from leaking:

Mix and pour Poly 75-65 Polyurethane Rubber into the mold box.

Poly 75-65 has a mix ratio of 1A:1B, a pour time of 35 minutes, and a Shore Hardness of A65 (this hardness is comparable to the hardness of a car tire).

This rubber naturally cures to an amber color, but red PolyColor dye was added in this case:

Poly 75-65 hard polyurethane rubber

 Allow Poly 75-65 to cure before demolding. This will take at least 24 hours.

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Remove clay and marbles then prepare to pour the second part of the mold:

Molds 156

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two part block mold

Trim excess rubber:

Insert metal pipe to create the second half of the structural metal rod:

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Before constructing the mold box, apply and brush Pol-Ease® 2300 Release Agent onto the model, first half of the mold, and particle board:

Release Agent for polyurethane rubber

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Construct the mold box and seal the edges with clay:

Mix and pour Poly 75-65 to create the second half of the mold:

Polyurethane Mold Rubber

Allow the second part of the mold to cure before demolding (at least 24 hours).

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two-part polyurethane mold

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Casting material, such as concrete, polyurethane plastic, foam, or rubber can be poured into the top of the mold as pictured below:

Two-Part Poly 75-65 Mold with Supportive Mold Box

When casting, surround the mold with a mold box for added stability:

The structural metal rod discussed earlier is inserted and stabilized prior to casting. One end is placed into the space created for it at the bottom of mold while the other end is stabilized by a hole in the mold box (pictured below):

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