Mold-Making Procedures

This document is a guide for concrete parts producers to make a polyurethane mold. The mold making materials, which we discuss in this document, are room-temperature-curable liquid polyurethanes. Please click here for the room-temperature-curable materials recommended for the mold-making applications.

Flexible mold-making material are often called "mold-making-rubber." There are several kinds of mold making materials including silicone rubber, fiber-enforced polyester, latex rubber, and sulfate rubber. (Although, polyurethane is not technically "rubber.") Polyurethane is generally less expensive and faster to cure comparing to latex and silicone rubber, much more flexible than fiberglass/polyester resin, and mold-making procedure is relatively easy. Molds made of polyurethane are flexible and strong.

This page mainly explains the aspect mold-making. Please also refer to Instructions for Elastomer Material Hand Mixing page for the detail processing information for the room-temperature-curable polyurethane casting materials.

The polyurethane materials used here are MDI/Polyether or MDI/Ester polyurethanes. MDI is in part-A material (different by each manufacturer). Although MDI is a safer type of isocyanate (compared to TDI), the users operate a mold-making procedure in a well-ventilated area. Operators should wear rubber gloves, protective eyeglasses, and long sleeves to avoid skin/eye contact. All personnel handling this material should understand the safety procedures in the Material Safety Data Sheet before the operation and follow the instructions by the supervisor who is responsible for the safety. In case of skin/eye contact of the material, quick washing with water is required. Please prepare an eye batch or other washing facility with soap in the operating area. No food/drink should be permitted in the area of the operation.

In this section, we will talk about the mold-making procedures. Certain parts of the procedures differ depending on the type of mold you are making. First, we will discuss the general information on mixing, then the procedures for the three different types of molds. There are other mold types available. You can modify these instructions to best-fit your application.

The model can be made of plaster, wood, plastics, stone, clay, or any solid material. The model should be cleaned and prepared to be free of debris, moisture, and solvent. You can use solvent or any appropriate cleaning agent. Polyurethane is a water-sensitive material, so if possible, avoid using water. Solvent, water, and cleaning agent must be dried completely.

Porous surface of the model may cause bubbles and air trapping. Use a sealer coating (acrylics, shellac, polyurethane, or any other waterproof coating) to seal the surface. The coating must be dried completely before pouring.

Model materials with moisture content such as wood and plaster needs to be dried completely. Kiln-drying or oven-drying is recommended when possible, and it should be coated with a water proof sealer.

Build a frame or a box to fit the model. It needs space for polyurethane mold material, so the box/frame needs to be at least 1/2" to 1" larger than each dimension of the model. The box/frame may need to be disassembled for demolding. Use wood screws or clamps that allow you to assemble and disassemble multiple times. Make sure the table or floor you are building is level. See the later section on each mold type for details.

You should always calculate the amount of the material you will need for your mold-making project to avoid shortage or excessive waist. To prevent a critical last minute shortage, making slightly more than your estimate is a good idea.

How much polyurethane material you need to make your mold

Please skip this if you are already familiar with volume/weight calculations.

Here is how to estimate the amount of material you need. Find out the approximate volume of the model. The minimum thickness of the mold is 1/2" at the thinnest. Subtract the volume of the model from the volume obtained by multiplying 1/2" plus the highest point of the model to the two-dimensional area of the model.

Volume of polyurethane material =

(Model Length x Model Width x (Highest point of the model + 1/2")) - (Volume of the Model)

First, calculate all in inches with decimal digits. For example 2' 8-1/2" is 40.5". You will then get cubic inches. Then you can convert the cubic inch value to the other measures according to the following table. Let us say "V" cubic inch is the volume you got from the above calculation.

"V" cubic inch / 231 = The Volume in Gallons

"V" cubic inch / 1728 = The Volume in Cubic Feet

"V" cubic inch / 874.43 = The Volume in Liters

One gallon of polyurethane is approximately 8.4 LBS. For the estimation, let us use 8.4 LBS per gallon as the volume to weight conversion.

The Volume in Gallons x 8.4 = Weight in LBS

"V" cubic inch x 0.036 = Weight in LBS

Weight in LBS x 453.6 = Weight in grams

The Volume in Liters x (*specific gravity) = Weight in Kilograms

Specific gravity is the ratio between the weight of water and the material. So, if specific gravity is 1.03, it means the material is 1.03 times heavier than the same volume of water. If you want to calculate very closely, we can use this to estimate the weight. However, the weight per volume changes by temperature.

This is estimate only. With liquid casting, leaking, residue on the mixing container surface, and small errors in measuring could cause shortage of the material. Always allow extra.

Please refer to the mixing ratio of the polyurethane material you are about to use. The mixing ratio is given both "by volume" and "by weight." When you are using measuring cups to measure, you can use the "ratio by volume." If you are using a scale, use "ratio by weight." The ratio information is provided by Northstar Polymers.

Just a little more help in math

Please skip this part if you are already familiar with the ratio calculation.

Let us do an example to figure out the amount of part-A and part-B you need for practice. This works for both weight and volume. Let us say you need total of 30 LBS (3-1/2 gallons) of polyurethane. The material has 1: 1.5 ratio by volume and 1: 1.3 ratio by weight. It is always A:B, which means the left side of ":" mark always represents the ratio of part-A, the right side number represents the ratio of part-B.

By Volume

Part-A = 3.5 gallons x (1/ (1+1.5)) = 1.4 gallons

Part-B = 3.5 gallons x (1.5 /(1+1.5)) = 2.1 gallons

By Weight

Part-A = 30 LBS x (1/ (1+1.3)) = 13 LBS

Part-B = 30 LBS x (1.3 /(1+1.3)) = 17 LBS

(The decimal numbers are rounded to approximate.)

In general:

Part-A = Total Amount x (Left number of the ratio / (Addition of the left and right number))

Part-B = Total Amount x (Right number of the ratio / (Addition of the left and right number))

Before mixing polyurethane components (part-A and B), be sure to prepare the frame/mold and apply mold release on the surface of both the model and frame/box. Insufficient application of mold release could cause damage to the mold as well as the model. Non-water-base silicone release agent is recommended. If it is solvent base, be sure to evaporate the solvent before pouring the polyurethane material. Northstar Polymers carry pure silicone mold release (brushable), which is effective at a very small amount; you do not have to wait for the solvent to dry.

Pour the correct amount of both part-A and part-B into a mixing container. The mixing container should have a flat bottom, and should have a round side such as a regular five-gallon pail or paint can. The convention is to pour part-B first and then part-A on top, but it is not very important which one goes first.

Part-A material is very air-sensitive. After the appropriate amount is taken from the pail or drum, you need to blanket the part-A with nitrogen gas and air-tight cover the container. Please read "Storage and Handling" section for details.

If you are adding something into the polyurethane (colorant, extra catalyst, internal mold release, or filler), please refer to the "Miscellaneous Information" for details.

Agitate the mixture with a stir stick or hand-held power-mixing tool. If you are using a power-mixing tool, use it at very slow speed. High speed agitation could damage the polyurethane material and fold in bubbles. You can use mixing heads used in paint application, motor mixer, drum mixer, or any kind that would not cause excessive bubbles. Do not use emulsifying mixers.

As you mix, use your stir stick to scrape the side and bottom of the container to ensure thoroughly homogeneous mixing. The time length of agitation varies depending on the amount and the pot-life* (see the following enclosure) of the material, but mix for at least one to two minutes for less than 5 gallons of material.

*Pot-Life and Catalyst

Pot-life is the length of time between the beginning of agitation and the point that the polyurethane material becomes too thick to pour. In other words, you need to finish pouring the polyurethane material within the pot-life. Generally, if you are pouring for larger molds, you need longer pot-life as you need to agitate longer and it takes longer to pour.

The catalyst level in the polyurethane material determines the pot-life. If you only make similar size molds, you can choose to have catalyst in the polyurethane formulation for a certain cure pattern. If you need to make parts of different sizes and need a variety of pot-life lengths, we can provide you with the polyurethane and catalyst separately so you can control the pot-life. If you choose to do that, we will give you the information on the catalyst percentages for different lengths of pot-life.

With hand-mixing operation, you will have some bubbles in the polyurethane material. In most of the cases, you can ignore these small bubbles in concrete molding operation. However, if the amount of bubbles is not acceptable to your application, you can de-gas the polyurethane by putting the material in a vacuum chamber. 29 Hg of pressure should be enough to de-gas most of the air/moisture within the mixed polyurethane material.

Our polyurethane has an anti-bubble agent within the formulation to minimize the bubbles.

When you pour a liquid mixture of polyurethane material into a frame/box, pour it slowly and try not to fold in bubbles. If you are pouring large flat parts, slowly move as pouring from one end to the other and avoid pouring in layers. If the mold has different depths, pour from the deep end to the shallow end to avoid fast turbulence of the material. You need to pay attention if the model has an air-trapping pattern. You can tilt or shake the frame/box to avoid the air trap. You need to do this before the material starts to solidify. For small bubbles and air-traps, you can repair the mold with silicone caulking. However, you may need to finish your products (not the mold) by machining if the air trapping is unavoidable.

After pouring, the polyurethane material will become solid. First, it will become gel-like, then it will be tacky solid, and then it becomes non-tacky solid. You can de-mold when the material is not tacky and hard enough to de-mold. De-molding time is also determined by the catalyst level. The material is hard enough to demold at that point, however this type of polyurethane cures slowly over days. The it cures to the final hardness in about 4 to 7 days at room temperature. Curing it at elevated temperature between 140 to 180 ºF will complete the cure in about 16 hours.

If your model or frame/box are made of materials that absorb heat, it effects curing of polyurethane. Polyurethane need heat to cure. Usually, polyurethane material create heat by itself when it is curing. However, if it is in contact with a heat-absorbing material, the self generated heat may not be enough for polyurethane to cure. If you are using natural stone, solid steel or other metal, or any other heat absorbing material, we recommend that the model and/or box/frame to be pre-heated to between 100 to 140 ºF to supplement the heat.

Usually, you can de-mold the parts overnight or within 16 hours in room temperature (72 to 77 ºF). However, it differs depending on the catalyst level as well as the curing temperature. It also depends of the complexity of the part's design. Make sure the mold material is strong enough when you de-mold.

You may need to disassemble the frame/box to demold. See the following sections for demolding information on two different types of molds. Slowly separate the cured polyurethane mold from the model. Be careful not to damage the mold or the model.

Inspect the polyurethane mold and repair imperfections, if any. After wiping off the mold release on the mold surface, you can use silicone caulking to repair the surface.

This type of mold is often used to make flat concrete parts such as walls with surface dressing. The parts usually have no undercuts or a few shallow undercuts. The molds can be as large as 40 to 50 feet in length.

Northstar Polymers room-temperature-cure mold-making materials with hardness Durometer between Shore A 60 and 80. Choose appropriate hardness depending on amount and depth of undercuts and size of the mold. Softer materials are used for parts with more undercuts. Harder materials are used for larger parts with shallow undercuts.

Our mold-making materials stand very good against water and UV light. They have low viscosity, so it is easier to achieve high-resolution replication of the details of the model surface. Material cures in room temperature. The cure time can be controlled by catalysts.

A frame needs to be built to retain the liquid polyurethane material. Wood will be a good material for the frame. However, the moisture within the lumber could cause bubbles in polyurethane. Dried and varnished wood can be used to minimize the bubbles. Besides wood, you can use other materials, such as plastics, aluminum, steel, or clay as long as they are clean and dry from moisture/water or any solvent. You may need to disassemble the frame when de-molding. Wood screws can be used to construct the frame so you can unscrew to disassemble. If you are using metal bars, do not weld; use clamps to clamp the bars to the table. You can then reuse the bars multiple times.

Place the model on the table and build the frame around it. The thickness of the mold depends on the height of the frame. Generally, you need to go at least 1/2" above the highest point of the model.

If the model is made of a light material, such as wood or plastic, secure the model to the bottom by screws to prevent it from moving.

Caulk between the model and the frame to prevent the liquid polyurethane from leaking. Any type of caulking material can be used. Be sure to dry the caulking before going to the next step. Resilient caulking materials are easy to cut open when you de-mold.

Air leak from the bottom will cause a critical bubble problem. If you need to have seams at the bottom, make sure seams are completely sealed.