How to Build a Mold Box

How to Make Mold Boxes for Urethane and Silicone RTV Molds

Urethane and silicone RTV rubbers provide a simple way to create molds by pouring liquid RTV over a part and allowing it to cure. These types of molds are commonly referred to as block molds. However, in order to properly contain the mold material and minimize waste, a suitable mold box or container must be used. This guide outlines various methods for constructing mold boxes based on the shape and size of the original part.

Choosing a Base Material

The base material for your mold box should be non-porous to prevent mold rubber from seeping underneath the part. Recommended base materials include:

• Acrylic – Transparent, allowing you to inspect the mold seams from below.

• Polyethylene – Ideal because nothing sticks to it.

• Melamine Board – Readily available from lumber suppliers and easy to work with.

Attaching the Part to the Base

Before constructing the mold container, securely attach the part to the base to prevent it from floating during curing. A hot glue gun provides a strong but temporary bond. Ensure the edges are sealed to prevent mold rubber from seeping under the part and encapsulating it.

Creating the Mold Container

The shape and size of the part determine the best type of mold container. Below are various container solutions based on different part geometries:

1. Circular-Shaped Parts

For circular objects, a paper cup makes an excellent mold container. Simply cut the cup about 1/2 inch deeper than the part and glue it securely to the base. Be sure to create a watertight seal to prevent leaks.

2. Rectangular or Square Parts

For rectangular or square parts, a custom acrylic box works well. Follow these steps:

1. Attach the part to the base with hot glue.

2. Cut four acrylic strips to create a border at least 1/2 inch away from the part.

3. Use a glue gun to secure each strip, forming a tight seal.

4. Check for pinhole gaps by inspecting the glue joints from the sides and bottom (clear acrylic helps with this).

3. Irregular or Complex Shapes

For uniquely shaped objects, a standard rectangular box may waste excessive material. Instead, a flexible vinyl strip (1/8 inch thick) provides a custom solution:

1. Cut the vinyl strip to the required width (about 1/2 inch taller than the part).

2. Wrap the vinyl around the part, maintaining a consistent border.

3. Secure it to the base using a glue gun.

Alternative Mold Containers

Many everyday containers can be repurposed for mold-making, including:

• Plastic food containers

• Storage boxes

• Disposable cups

• Household items found in the kitchen or garage

By thinking creatively, you can find cost-effective and practical solutions for making mold boxes. Choosing the right container will help minimize material waste and ensure a successful mold-making process.