How to Make Your Own Concrete Molds With Latex & Polyurethane

Use Liquid Latex for Concave Molds

• 1

  Build or prepare a structure or frame for a concave mold; these are used for pavers, steps and tiles, where the concrete is poured, scraped, set and flipped onto a drying surface. Liquid latex works best for this application because it can be partially dried and worked with the hands. The frame becomes the mother mold (the structure that holds and supports the mold during pouring and setting). Use the cut-off bottom of plastic buckets for pavers or steps. Build the frame for square steps or tiles from 2-by-4 or 1-by-4 lumber on a heavy plywood base.

• 2

  Spray or paint liquid latex release agent into the frame and allow it to dry for 30 minutes. Spray a second coat.

• 3

  Pour the liquid latex into the frame and work it into the corners and crevices. Allow the latex to begin setting up and test it to determine when it can be molded without running back into the base of the frame. Mold a consistent thickness around the inside of the frame and smooth out the sides so that the cast object slides easily from the mold. For a cleaner surface on the concrete, use a second bucket or target tile and press it into the frame, forcing some rubber out and capturing its exact shape. Be sure to spray these objects with the release agent before using them in this manner.

• 4

  Allow the mold to set for 24 hours. Leave it in the frame to support the poured concrete, keeping the original dimensions of the mold. Spray the interior of the mold with liquid cooking oil and pour the concrete. The cast paver or tile can be flipped out of the mold in 24 hours.

Use Polyurethane Rubber for Convex Molds

• 5

  Select the sculpture or piece that is to be modeled or copied. This can be any sculpture, bust or 3-dimensional object to be reproduced in concrete. Spray the surface of the model with two applications of a mold sealer and allow it to dry for several hours.

• 6

  Spray the entire exterior surface of the object with liquid polyurethane molding release agent. Allow it to dry for 30 minutes and spray on another coat.

• 7

  Mix the polyurethane molding material per the manufacturers instructions in a plastic bucket. This will vary by product and manufacturer, but most are either 1:1 or 1:2 mix ratio. Casting instructions on the label will provide approximate coverage instructions.

• 8

  Paint the first coat of polyurethane molding rubber over the model sculpture with a small paintbrush. Allow it to dry enough to be tacky and paint a second coat of rubber over the first. Be sure to work the second coat into crevices with a dabbing motion. Large models will require a third coat of rubber. Allow the rubber to dry and cure for 48 hours.

• 9

  Build a mother mold or frame around the molded piece or sculpture. Do this by mixing and pouring a liquid polyurethane dense foam mix and forming it into a cubic shape as the foam expands. The goal is to produce a resulting block of hardened foam that contains the rubber mold. This mother mold then houses the rubber mold during casting of the concrete. Follow the manufacturers directions and pour the mixed foam around the base of the mold, forming and shaping it as it expands. Finish off the foam mother mold by leveling its upper surface so it can be inverted, supporting a leveled amount of concrete in the mold.

• 10

  Allow the mother mold to dry for 48 hours. Carefully pull the mother mold from the modeled sculpture. The rubber mold will come with it. Carefully peel the rubber mold out of the foam mother mold and inspect it to assure that it reproduces the original model. Apply spray oil on the exterior of the rubber mold and work it back into the mother mold for pouring of the concrete.