Introduction
This edition of our newsletter describes making molds, liners, and stamps for concrete surface decoration products using polyurethane. These materials are also commonly called “mold-making rubber,” although polyurethane isn’t technically a rubber. Unlike silicone, latex, or fiberglass materials, polyurethane is more affordable, faster to cure, and flexible yet strong—ideal for concrete mold making.
Safety
The materials used are typically MDI-based polyurethanes. While MDI is safer than some other isocyanates, always follow these precautions:
• Work in a well-ventilated area.
• Wear rubber gloves, safety glasses, and long sleeves.
• Read and follow the Safety Data Sheet (SDS).
• Have soap and water or an eye wash station nearby.
• Do not eat or drink in the work area.
• If skin or eye contact occurs, wash immediately with water.
Tools Checklist
Safety Equipment
• Rubber gloves
• Safety glasses Mold-Making Tools
• Mixing container
• Measuring cups or digital scale
• A level
• A stir stick or powered mixer with a proper mixing fin
• Mold release (compatible with polyurethane)
• A paintbrush (for brush-able mold release)
• Vacuum pump/chamber for de-gassing (Optional) Storage & Cleanup
• Paper towels or dry rags • Isopropyl Alcohol (rubbing alcohol)
• Nitrogen gas or argon gas
Utility Tools
• Knives
• Basic hand tools (e.g., screwdriver, caulking gun)
Procedure Overview
Steps may vary by mold type, but the basic process is as follows:
1. Prepare the original model.
2. Build a frame or box to hold liquid polyurethane.
3. Clean both the model and the mold frame.
4. Apply mold release thoroughly.
5. Mix polyurethane parts A and B.
6. Pour the mixture into the mold box.
7. Let the polyurethane cure fully.
8. Remove the cured mold.
9. Repair or reinforce the mold as needed.
You can adjust these steps based on the specific type of mold or application.
More details…
Model
The "model" is the original object you want to replicate. It can be made of plaster, wood, plastic, stone, clay, or other solid materials. Clean the model thoroughly to remove dirt, moisture, and any solvent.
Avoid using water if possible, as polyurethane is moisture sensitive. All surfaces must be completely dry before use. If using moisture-containing materials like wood, dry them thoroughly and seal them with a waterproof coating. For a plaster or concrete model, it must be oven-dried for several hours at about 212 °F.
Porous surfaces like wood or plaster cause bubbles. Apply a sealer (e.g., acrylic, shellac, polyurethane sealer) and let it dry completely.
Frame/Box
Build a box or frame around the model, leaving at least 1/2" space on all sides for the mold material. Use screws or clamps so it can be disassembled after curing. Make sure your workspace/table is level.
Estimate Polyurethane Needed
Estimate the volume needed. Check the Technical Datasheet for the correct component mixing ratio. To avoid running short, calculate the volume of material required and prepare a little extra. Calculate the required amounts of part-A and part-B components based on the Technical Datasheet of the formulation. Note: Because the densities of the components are different, the mixing ratios by volume and weight are different.
Apply mold release
Before mixing the resin components, apply mold release to both the model and the inside of the frame. Use a non-water-based silicone release agent. If solvent-based, let the solvent evaporate fully. 100% solid (no solvent) silicone-based brush-able release agent is recommended for the ease of use and effective release function.
Mixing & Agitation
Pour the calculated amounts of part-A and part-B into a round, flat-bottom container. It's not critical which part goes first. If adding color, pre-mix the colorant in part-B and blend homogeneously before mixing it with part-A.
Mix thoroughly with a stainless steel spatula or hand-held powered mixer with a paint-mixing fin. Avoid high speeds, which can cause bubbles. Scrape the sides and bottom with the spatula as you mix for 1–2 minutes (longer if making a large batch).
Note: Soon after dispensing part-A, seal the container and purge the headspace with nitrogen gas to prevent moisture contamination.
De-Gassing (Optional)
Manual-mixing introduces bubbles into the resin mix. Often bubbles are harmless in concrete molding, but if bubbles becomes an issue, you need to remove them by placing the mixed resin in a vacuum chamber. The vacuum pump/chamber set needs to reach a vacuum of 29 inHg within 2 to 3 minutes to effectively remove the bubbles.
Pot Life and Catalyst
Pot life is the time you have to pour the polyurethane after mixing—before it becomes too thick. Making larger parts usually needs longer pot life since mixing and pouring take more time. The amount of catalyst in the resin controls the pot life.
Addition of catalyst will make the material cure faster, but you will have to deal with a shorter pot life, which may be difficult to make larger parts. Reducing the catalyst content will provide a longer pot life, but you need to let the resin sit in the mold for a longer time before you can handle the part as a solid material.
If you make different-sized molds, we can supply the resin and catalyst separately so you can adjust pot life as needed. We’ll provide a guide for mixing ratios.
Pouring
Pour slowly to avoid trapping air. For flat or large molds, pour from one end to the other without layering. For uneven mold depths, pour from the deepest part to the shallowest to reduce turbulence. Watch for spots where air might get trapped. Tilting or gently shaking the mold box helps. Do this before the material starts setting. Small defects can be repaired with silicone caulking. In some cases, you may need to machine the finished part.
Note: If you pour the last drops of resin stuck on the surfaces of the mixing container by scraping it off, it may get surface defects. The resin staying on the container surface is often not mixed well. Just pour what you can by gravity to avoid this issue.
Curing
After pouring, the resin will go from liquid to gel, then tacky, and finally solid. You can de-mold once it’s no longer tacky and feels firm—usually within 6 hours at room temperature (72–77 °F), depending on the formulation and mold size.
The resin continues hardening gradually over 3 to 5 days at room temperature. The hardness and physical evaluation should be done at least 3 days after the resin is poured.
Polyurethane generates its own heat, but heat-absorbing materials can hinder the process. If your frame/mold/model absorbs heat (e.g., stone or metal), you may need to preheat it to 90 – 110 °F to help the resin cure properly.
We can adjust the curing pattern of the formulations. Please consult Northstar Polymers.
Demolding
You can typically de-mold after 3 to 5 hours at room temperature, but this varies based on the formula, batch size, processing temperature, and mold design. Make sure the mold is firm before removing it. Gently separate the mold from the model to avoid damage. For complex shapes, you may need to disassemble the mold/frame to avoid tearing the molded part. Inspect and repair any defects using silicone caulking after cleaning mold release off the molded part.
Material Handling and Storage
Moisture Sensitivity:
The component materials are highly sensitive to moisture. After dispensing the material, purge the container headspace with nitrogen gas or argon gas, then close the lid immediately. Always keep lids tightly closed.
Disposal:
Do not pour liquid material into drains or dispose of it illegally. Cured (solid) material can usually be discarded as industrial solid waste. Check the Safety Data Sheet and follow local disposal regulations.
Storage:
The component materials should always be kept in air-tight containers stored indoors within the temperature range between 72 and 86 °F. Materials are sensitive to cold temperatures. When part-A component (isocyanate component) is exposed to cold temperatures, you may permanently damage the material. When part-B component (polyol component) is exposed to cold temperatures, the constituents may freeze and/or separate into layers. Heat the part-B component to 90 – 140 °F to thaw and agitate the content.
Frozen Part-A Component
Part-A component can be ruined if it is frozen for a long time. During the cold seasons, part-A component may freeze during transportation. You must thaw the material immediately after the material is delivered. Use a drum/pail heater or an industrial oven to thaw the material.
Supply Sources
