Home        What's New?          Products          About Us         Useful Info         Contact

 

 

Useful info

Mold-Making Procedure

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Back to top of this page

 

 

 

 

 

Back to top of this page

 

 

 

Contacts

 

Northstar Polymers, LLC

3444 Dight Avenue So.

Minneapolis, MN 55406

Tel: 612.721.2911

Fax: 612.721.1009

E-Mail:

info@northstarpolymers.com

 

 

Northstar Polymers, LLC is a member of Polyurethane Manufacturers Association.

 

Copy right reserved by Northstar Polymers, LLC 2000 - 2007.  Northstar Polymer prohibits duplication of the contents of this web site for the purposes of public display and/or using on another web site without a written authorization by Northstar Polymers, LLC.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mold-Making Procedures for Concrete Parts

 Using Room-Temperature-Curable Polyurethane Casting Materials

 

 

Introduction

 

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.

 

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.

           

Contents

 

1.       Safety

2.       Tools

3.       Procedures

3-1  General Procedures

3-2. Flat mold without undercuts or with very shallow undercuts

3-3.  Block mold

3-4. Two-part mold

3-5. Skin/glove mold

4.       Materials handling

5.       Source Information

6.       Miscellaneous information

 

 

Section 1                  Safety

 

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.

 

 


 

Section 2                  Tools

 

For safety

 

Rubber gloves

Protective eyeglasses

 

For Mold-Making

 

      Mixing container

      Measuring cups or scale

      Level

      Stir stick

     Hand-held power mixing tool (optional)

      Mold release (For polyurethane) 

      Paint brush

 

For storing and cleaning

 

      Paper towels or dry rag

      Nitrogen gas or - 40 F dewpoint dry air

 

Utility

       Knives

       Other hand tools (screwdriver, caulking, etc.) for frame/box making

 

For de-gassing

 

       Vacuum chamber (optional)

 

  

 

Section 3                  Procedures

 

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. 

 

 

3-1  General Procedures

 

Outline of steps for mold making:

 

1.       Model or original piece to copy

2.       Build a frame or box to retain the liquid polyurethane material

3.       Clean the model and frame/box

4.       Apply mold release

5.       Mix the components of the polyurethane

6.       Pour into the frame or box

7.       Cure polyurethane

8.       Demold

9.       Repair and re-frame or reinforce

10.   Apply release

11.   Pour concrete to make your products/parts

 

Model

 

Here, we use the term "model" as the original piece you are going to copy from.

 

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. 

 

 

Frame/Box

 

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.

 

Estimate the total amount of polyurethane you need

 

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.

 

 

Calculate amount of each material required

 

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))

 

Mixing / Agitating

 

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.

 

De-gassing (Optional)

 

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.

 

Pouring

 

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.

 

Curing

 

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 F 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 F to supplement the heat.

 

Curing pattern

 

The catalyst level in the polyurethane material determines the curing pattern.  In many cases, the users of polyurethane materials want longer pot life and shorter de-molding time.  However, if the catalyst level is high it will yield shorter pot-life and shorter de-molding time.  If the catalyst level is low, it will be longer pot-life and longer de-molding time. 

 

De-molding

 

Usually, you can de-mold the parts overnight or within 16 hours in room temperature (72 to 77 F).  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.

 

Hardening of polyurethane

 

Polyurethane continues to slowly cure and gradually becomes harder for a few days.  Although you can de-mold the polyurethane in 16 hours, and it can be used in concrete molding operation, the evaluation of the hardness and other mechanical properties should be conducted a few days later. 

 

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.  

 

 

 

3-2  Flat mold without undercuts or with very shallow undercuts

 

 

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. 

 

Recommended materials

                                   

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.

 

Building a frame

 

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.

 

Make sure the table is level!

 

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. 

 

Flat Mold Diagram

 
 

Cleaning

 

Clean the model and make sure it is free of debris, dirt, solvent, or water.  Using low water content isopropanol (rubbing alcohol) is a good idea.  Be sure to dry all the cleaning solvent before proceed.

 

High Humidity

 

In summer, high humidity in the air may make dew on the surface of the frame or model. This could cause excessive bubbles in the polyurethane mold material.  If you keep the liquid materials in an air-conditioned place and bring it out to a warm workshop, this will cause a condensation to the container and material.  Keep all tools and the materials in the steady temperature same as your work shop.  Avoid high humidity.  Make sure to dry the surface completely before pouring polyurethane into the frame.  You can use a high-power fan to dry the surface.

 

Some regions inherently have high humidity, and humidity may not be avoided.  We may need to formulate the material specially for this kind of situation.  Please consult a Northstar Polymers representative.

 

Apply Mold Release

 

Apply mold release on the surface of both the model and frame.  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 uses solvent in the release, be sure to evaporate the solvent before pouring polyurethane material.  Excessive amount of mold release may cause water-mark-like defect on the surface. 

 

The mold release that you use in concrete molding may not be suitable to use with polyurethane.  Hydrocarbon based, or water born release materials often cause defect.  We recommend testing the mold release before using it in the mold making.

 

Mix and pour polyurethane material

 

Please refer to the General Procedures section for details.

 

Curing

 

Please refer to the General Procedures section for details

 

Backing

 

Polyurethane molds are flexible.  If you need to keep your mold or liner straight, you can build in a hard backing.  Backing board can be dry/non porous wood, thick hard plastic (poly carbonate, poly ethylene, poly propylene etc.) or any material that is hard, dry, and non-porous.  Drill large holes (1" to 3" diameter depending on the mold size) in every 6" - 12" to allow liquid polyurethane to go through.  When you pour polyurethane into the box/frame, pour it to the half to three-quarters of the way, place the backboard, then pour the rest of the polyurethane liquid material.  (The backboard may need to sit on something to avoid sinking down.)  The polyurethane liquid goes though the holes to encapsulate the backboard.  You can then mechanically attach handles to the backboard after polyurethane is hard.

 

De-molding

 

After the polyurethane is hard and cured you take out the mold from the frame and separate it from the model.  Be sure to do it slowly, avoid tearing and damaging the mold or model.  You may need to disassemble the frame to de-mold. Slowly separate the cured polyurethane mold from the model.  If you did not apply enough mold release, you may have a hard time de-molding. Plan to spend enough time for de-molding. 

 

Repairing

 

After wiping off the mold release on the mold surface, you can inspect the polyurethane mold and repair the imperfections.  You can use RTV silicone  adhesive/caulking for repairing.  The mold can be designed with or without side walls to retain concrete material.  In either case, you may need to build a reinforcement frame around the polyurethane mold to bear the weight of the concrete material for your parts.

 

Now you have a polyurethane mold ready to make your precast concrete parts. Apply mold release on the mold surface.  Generally you can use any kind of common mold release for your concrete parts production.  However, if you see your mold release damaging the polyurethane mold, change the mold release to another non-invasive kind such as silicone mold release.

 

 

3-3.  Block mold

 

Block molding is one of the easiest molding techniques when you are making three-dimensional parts.  The same result can be obtained by glove/shell molding, brush-on mold, and other molding techniques.  However, block-molding is easiest because you would need only one mold-making material and the pouring operation is a simple one-step. 

 

 

Recommended Materials

 

For this block molding, we need to use very resilient polyurethane materials. We carry very soft room-temperature-cure polyurethane materials designed for this type molds.  They have hardness of Durometer range of shore 15 A to 30 A.  This material is soft and resilient, yet resists cut and tear.  Durometer shore 40, and 50 A materials can be used for this type molds.   Generally, harder the materials stronger the mold is, but it may be harder to de-mold as more difficult to stretch. 

 

Model

 

Here, we use the term "model" as the original piece you are going to copy from.

 

The model can be made of plaster, wood, plastics, stone, clay, or any solid material.  If you are using clay to make model, polymer clay is recommended as it does not contain much water.  There is a limit to what you can mold.  For example, it is difficult to mold parts with a hole penetrating from one end to the other.  The model should be designed with consideration of the molding limitations.  Machining may be optional together with molding.  Certain inserts can be embedded to achieve specific functions.

 

The model should be cleaned and made free of debris, moisture, and solvents.  You can use water, soap, solvent or any appropriate cleaning agent.  You need to be careful not to damage the model by the cleaning method. The water, solvent, or other cleaning agent must be removed and dried completely.

 

Predetermine the parting line

 

In most cases, you will need to cut open the mold to take out the model from the mold.  Predetermine the position where machining/finishing is easy, or where you can hide it in the decorative pattern of the parts. If the mold design is easy to de-mold by stretching the mold, you may not need to cut the mold.

 

Build the box

 

An open-top box to hold mold-making material needs to be build.  Wood will be a good material for the frame.  However, the moisture within the lumber could cause bubbles in polyurethane.  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 and/or clamps can be used to construct the frame so you can unscrew to disassemble.  Do not completely weld steel plates to make the box.

 

Place the model at the bottom-center of the box.  If the model is made of a light material, such as wood or plastic, secure the model to the bottom by two or more screws or glue to prevent it from moving or floating.  If you need to copy the bottom side of the model, use PVC plumbing pipe, cut to 1/2" to 1" length, to elevate the model and secure the pouring hole for the polyurethane mold-making material as well as concrete material. Secure the PVC pipe with glue.

 

Make sure there is clearance of 1/2" minimum from the model to the side wall of the box.  The height of the side wall should have at least 1/2" higher than the highest point of the model.

 

Caulk the seams to prevent leakage of the liquid polyurethane material. Do not over-do, as you will need to disassemble the box later to 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.

 

 

Block Mold Diagram

 

 

 

Apply Mold Release

 

Apply mold release on the surface of both the model and frame.  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 uses solvent in the release, be sure to evaporate the solvent before pouring polyurethane material.  Excessive amount of mold release may cause water-mark-like defect on the surface. 

 

The mold release that you use in concrete molding may not be suitable to use with polyurethane.  Hydrocarbon based, or water born release materials often cause defect.  We recommend testing the mold release before using it in the mold making.

 

Mix Polyurethane

 

Please see the "General Information" section.

 

Pour

 

Be sure that the box is level. 

 

Find out how long the pot life of the material will be, and plan to finish pouring within the pot-life. Slowly pour correctly mixed polyurethane material between the mold and model.  Make sure not to trap air.  Tilt or shake the box to let out bubbles.  If the model has many air-trapping patterns, it may not be possible to eliminate all the air traps. Repair the mold after de-molding or plan to machine-finish the product. 

 

Cure

 

Please see the "General Information" section

 

De-molding

 

With block mold making procedure, this part needs much care.  Plan to take enough time.

 

Take out what is inside from the box, by disassembling the box. Do not destroy the box as you will need it later.

 

If possible, take out the model by stripping the soft mold.  Do it slowly to make sure not to damage the model or tear the mold. 

 

If it is difficult to strip-off, cut the soft mold at the predetermined parting line.  It helps to have two people as one person can hold the mold open with another person maneuvers to take out the model.

 

Be very careful with your knife.  The material is not very easy to cut.  Make your best effort to avoid personal injury.

 

If there is not enough mold release applied, this procedure could be very difficult.  If you see the polyurethane material stuck to the model, slowly pull them apart to minimize the damage to both the model and mold.

 

 

 

Repair the mold

 

After wiping off the mold release on the mold surface, you can inspect the polyurethane mold and repair the imperfections.  You can use RTV silicone adhesive/caulking for repairing.

 

After repairing and caulking/taping the part line to prevent leaking (if necessary), flip the mold open-side-up, and put it back into the box and re-assemble the box. For smaller parts, you may not have to use the box for concrete casting as long as the shape of polyurethane mold is not distorted by the weight of concrete (or other casting compound) material.

 

Now you have polyurethane block-mold ready to make your precast concrete products. Apply mold release on the mold surface.  Generally, you can use any kind of common mold release for your parts production.  However, if you see your mold release damaging the polyurethane mold, change the mold release to another non-invasive kind such as pure silicone.

 

Cast concrete through the pour hole and cure.  The product can be machined/finished to make the pour hole and parting line less conspicuous.

 

3-4.  Two-part mold

 

By repeating the casting process twice, you can make a two-parts mold.  A major benefit of a two-part-mold is demolding rate of the product.  Since you can remove have of the mold easily, de-molding procedure will be match easier than block mold.  Drawbacks may be that it needs to have a parting line straight in the middle of all surface, also it involves little more to make the mold.  Typically, the model is suspended in the middle of the box from one side; a mold making liquid is pour to the half way of the box (to the midway line of the model).  After the half-way-poured material is completely cured, apply mold release on the cured material, then pour the rest of the way to the top of the box/frame.  After it is cured, the model is taken out of the box.  The box is cut in half, a couple of hinges and locks are installed.  Drill a pour hole at the position where the model was attached to the box.  Set the mold in the vertical position with the pour-hole-up, cast concrete from the pour hole. 

 

Recommended Materials

 

For this kind of molds, you can choose the hardness depending on the level of undercuts you have in your part design.  Durometer Shore 45 to 50 A materials can be used for designs with high level of under cuts.  Between Shore 65 to 80 can be used with designs with less undercuts.   Generally, harder the materials stronger the mold is, but it may be harder to de-mold as more difficult to stretch. 

 

Model

 

Here, we use the term "model" as the original piece you are going to copy from.

 

The model can be made of plaster, wood, plastics, stone, clay, or any solid material.  If you are using clay to make model, polymer clay is recommended as it does not contain much water.  There is a limit to what you can mold.  For example, it is difficult to mold parts with a hole penetrating from one end to the other.  The model should be designed with consideration of the molding limitations.  Machining may be optional together with molding.  Certain inserts can be embedded to achieve specific functions.

 

The model should be cleaned and made free of debris, moisture, and solvents. You need to be careful not to damage the model by the cleaning method. The water, solvent, or other cleaning agent must be removed and dried completely.

 

Predetermine the parting line

 

Predetermine the position of the model so the parting line will be at an inconspicuous position. The parting line may become obvious.  So, you may want to choose the parting line to be at an easy-to-machine place.

 

 

Build the box

 

An all sided box needs to be build.  Cut the box in the mid way line and put a couple of hinges. This will become the outer casing of the mold.  Clamp the two parts of the box together when you are casting polyurethane to make a mold.  Top board can be loose so you can cast polyurethane or you can attach the top board and drill a pour hole (2 - 4 inches) and a few vent holes (1 - 2 inches). 

 


 

Plastic-covered or sealed wood will be a good material for the box.  The moisture within the lumber could cause bubbles in polyurethane.  Dry the wood completely. Besides wood, you can use other materials, such as plastics, aluminum, steel, or other materials as long as they are clean and dry from moisture/water or any solvent.

 

Two-part Mold Diagram

 

Attach the model to one side of the box suspending it in the middle.  You may want to use more than one screw to attach so it will not move during the mold-making operation.  Make sure there are clearances of 1/4" to 1/2" minimum from the model to the side-walls of the box. 

 

Caulk the seams to prevent leakage of the liquid polyurethane material.  Air leak from the bottom will cause a critical problem.  If you need to have seams at the bottom, make sure seams are completely sealed.

 

Apply Mold Release

 

Apply mold release on the surface of the model.  If you want the elastomer mold part to come off the box, apply mold release to the surface of the box, too. Other wise, the box and the mold will become an integrated piece.  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 uses solvent in the release, be sure to evaporate the solvent before pouring polyurethane material.  Excessive amount of mold release may cause water-mark-like defect on the surface. 

 

The mold release that you use in concrete molding may not be suitable to use with polyurethane.  Hydrocarbon based, or water born release materials often cause defect.  We recommend testing the mold release before using it in the mold making.

 

Mix Polyurethane

 

Please see the "General Information" section.

 

Pour to the mid-way line

 

Be sure that the box is level. 

 

Find out how long the pot life of the material will be, and plan to finish pouring within the pot-life. Slowly pour correctly mixed polyurethane material between the mold and model to the midway line.  Do not pour on top of the model.  To make sure not trapping air, tilt or shake the box to let out bubbles.  If the model has many air-trapping patterns, it may not be possible to eliminate all the air traps. Repair the mold after de-molding or plan to machine-finish the product. 

 

Cure

 

Cure the material completely.  Please see the "General Information" section

 

Apply mold release

 

Apply mold release on the cure surface of the polyurethane.  Be sure to apply generously.  If there is any wet spot from the first casting, wipe it off with a dry cloth and isopropanol (or other solvent) and dry, then apply mold release. 

 

Pour the other half

 

Follow the same procedure and pour the rest to the top of the box. 

 

Cure

 

Cure the material completely.  Please see the "General Information" section

 

 

De-molding

 

De-molding of the model should be as easy as when you are demolding your cast products.  Choice of the polyurethane's hardness is critical for de-molding.  Unscrew the model and open the box to demold.  If it is difficult, to demold, disassemble the box and use a knife to cut open the mold.

 

Be very careful with your knife.  The material is not very easy to cut.  Make your best effort to avoid personal injury.

 

If there is not enough mold release applied, this procedure could be very difficult.  If you see the polyurethane material stuck to the model, slowly pull them apart to minimize the damage to both the model and mold. 

 

Repair the mold

 

After wiping off the mold release on the mold surface, you can inspect the polyurethane mold and repair the imperfections.  You can use RTV silicone adhesive/caulking for repairing.

 

Drill a pour hole

 

Drill a pour hold at through the side board which the model was suspended from.  You can make big enough hole for your cast material (concrete) to be poured in.  Put the mold in the vertical way so that the pour hole is on top to cast your concrete (or other casting material) into the mold to make your products. 

 

Apply mold release on the mold surface.  Generally, you can use any kind of common mold release for your parts production.  However, if you see your mold release damaging the polyurethane mold, change the mold release to another non-invasive kind such as pure silicone.

 

Cast concrete through the pour hole and cure.  Demolding should be as easy as just opening a box.  The product can be machined/finished to make the pour hole and parting line less conspicuous.

 

 


 

3-4.  Skin/glove mold

 

This mold making method employs brush-on or spray on method.  The polyurethane is mixed and applied on the surface of the model and cured.  Often, an outer shell made of harder material is added to support the shape of the mold when casting compound (concrete, plaster, etc.) is poured into the mold.  This method is popular because of its economical way of making a large mold.  The amount of mold making materials used in skin/glove mold is much smaller. 

 

Skin/Glove Mold Diagram 

 

 

To spray polyurethane material, you need special material and special equipment.  However, brush-on can be done without special equipment, but this require modification of the polyurethane material you have.  If you use the same material as it is, the material sag down soon after it is applied on the surface.  The material needs to be thickened.  You can obtain a thickening additive.  Fumed silica is widely used in this application.  Fumed silica is more widely known as its product name "Cabo-Sil".  It is a fine powder and adding it to any liquid will increase the kinetic thickness (viscosity) of the liquid.  Please see below for the source information.  Also, you need to increase reactivity of the material by adding catalyst.  The additional catalysts are available through Northstar Polymers.  

 

First, you need to make a thin skin of polyurethane on the surface of the mold.  This is needed because if you put the glove part right on the model, it will create many voids and you cannot achieve fine resolution of the surface.  The regular mold making materials are set to cast, not for brush-on.  So, you want to add catalyst to increase the reactivity of the polyurethane.  Other wise, the skin part may not cure at all.  Ask Northstar Polymers for additional catalyst and recommended dosage. 

 

For the skin layer, mix the polyurethane material at the correct ratio.  Brush-on the material to the surface of the model after mold release is applied on the surface of the model.  The skin should be so thin that the air bubbles in the material get mostly popped.  Let it cure to form skin.

 

Mix thickening agent (Cabo-Sil) into part-B material.  Be sure to measure the part-B before the thickening agent is added.  Control the amount to get the desired thickness of part-B. Add part-A at the correct ratio based on the part-B before the thickening agent is added.  Apply the flexible "glove" part right on top of the skin.  Be sure to cure the skin before applying glove part.  Do not apply mold release on the skin as the skin and the glove will the one part.  Apply between 1/8" and 1/4" thickness.  Cure the glove. 

 

After it is cured, you can make a hard outer shell with fiberglass/polyester etc. to keep dimensions well.  This is optional. 

 

When all the mold parts are cured, you can de-mold by stripping it off the model.  Mold then can be buried in a sand box to retain the shape when your casting compound (concrete, plaster, etc.) is cast into the mold. 

 

Source Information:  Cabo-Sil (Fumed Silica)   CABOT Corporation  1-800-222-6745

(Please follow the safety instructions from your supplier when you use this material.) 

 

 

Other Kinds of Rubber (elastomer) Molds

 

Spray on mold

 

Same as the above "Glove and Shell" mold, it consists of two mold layers, inner mold is soft the outer layer is hard.  It is often used in large decorative structure parts such as decorative pillars for large buildings.  One-component silicone rubber materials (again more expensive and generally slower curing pattern than urethane) can be used in this together with fiberglass enforced polyester.  Polyurethane can be used for spray on mold application; however, it requires a high cost (at least $3,000 - $4,000 minimum ) equipment. 

 

Brush on mold

 

Same as spray on, but uses brushable materials.  Latex is often used in this method.  Latex needs longer time to cure and you need to apply many times to achieve a certain thickness for the strength.  It is known to be a time consuming procedure.  It is often used in molds for ornamental statues and garden decorations. 

 

 

  

Section 4                  Material handling

 

Part-A of polyurethane material is very sensitive to moisture.  After removing the required amount, send in nitrogen gas or - 40 F dew point air for a few seconds, quickly close the cover of the container to store.

 

Part-B is not as sensitive as part-A.  However, if you are not finishing up the material in a drum (or pail), we recommend also blanketing with nitrogen gas.  Always keep the cover shut.

 

Do not dispose of any liquid material to the public drain system or an unlawful area.  Usually, the solid material (after being mixed and cured) is safe to dispose of as solid industrial waste.  Read the Material Safety Data Sheet and consult your local authority for disposal information.

 

Average shelf life of the material is 6 months to one year with proper storage conditions.

 

When you store drums of materials, do not put too many drums on one skid.  We recommend four drums maximum.  Organize the drums so that you can read the label on each drum.  Make space in between the drums so an inspector can walk through and read the labels.  Do not pile the drums or skid with the drums higher than two layers.

 

Seasonal Concerns

 

In summer:

 

Polyurethane is sensitive to moisture.  In summer, where you have high moisture in the air, containers and tools left in cool area could have dew (water) on surface.  Please be sure to dry your tools before using.  The frame/box and model could have precipitation.  You could use high power fan to dry.  If the tools are wet, you will see excessive bubbles in polyurethane.

 

Our polyurethane materials do not have solvent.  It is not flammable and you do not need to release drum pressure.   However, for safety and quality of polyurethane material, please store your polyurethane at room temperature (around 70 to 80 F) indoor storage with ventilation and/or temperature control. 

 

In winter:

 

Polyurethane is sensitive to extreme low temperature.  The material should be kept at room temperature.  If in case it has been in very low temperature (for example during shipping), some materials can be effected by low temperature.  The low temperature effect can be seen as lumps, crystallization, or gelation of the materials.  If the material has been in cold temperature, leave it at room temperature for at least 48 hours before using.  If you see gel-like lumps or separation within the material, heat the drum (or pail) to between 100 and 140 F, and agitate before use.  You can use a drum heater or put the drum in an industrial oven to heat.

 

The temperature of the frame/box and models should be at room temperature when you are making a polyurethane mold.  Store the frame/box and models at room temperature at least for 8 hours before a mold making operation to make sure they are not too cold.

 

 

 

 

 

Section 5                  Supply Source

 

 

Polyurethane mold-making materials

 

                        Northstar Polymers        612-721-2911

 

Silicone Mold Release for Casting Polyurethanes

 

                        Northstar Polymers        612-721-2011

 

Nitrogen Gas

Praxair                         800-772-9247

 

Mixing fins

 

                        Grainger (Catalogue)      www.grainger.com

 

 

 
 

Casting Machines

           

            Please consult Northstar Polymers 612-721-2911

  

 

Miscellaneous Information :

 

Concrete Admixtures

 

Currently, we do not have any report on damages caused by concrete admixture.  Acidic materials may effect longevity of the mold.  Surface enhancement agents with strong solvent could also effect the polyurethane mold.  We recommend testing of performance enhancement agents for concrete before making molds.  Contact Northstar Polymers for requirement on special formulations if the application cannot avoid using invasive chemicals.

 

Catalyst and Additives

 

As discussed in "Procedure" section, catalyst can be provided separately to allow versatility in your production.  Various kinds of additives and fillers can be also added to polyurethane material to enhance certain properties of the material.  Examples of additives are colorant (dye), UV absorbent, internal release agent, and thixotropic agent. 

 

When you add those additives in the polyurethane materials, be sure to follow the instructions for safety and the correct usage.  Most of the time, additives are put into part-B before mixing with part-A.  Homogeneous mixing is very important when you add additives.

 

Filler material also can be used with polyurethane materials.  Filler materials, however, weaken the mechanical strength of the polyurethane mold.  Filler materials should also be added to the part-B before mixing with part-A.

 

 

Casting Machines

 

Our room-temperature-cure polyurethanes are designed for hand mixing/pouring and do not require a casting machine.  However, if you have larger volume to pour, you may want to consider obtaining a two-component casting machine.  Usually, these machines have metered pumps and a static mixing tube.  The two metered pumps suck the materials from two separate tanks and send them to the mixing gun.  The mixing gun has a plastic tube (static mixing tube), which mixes the two components of polyurethane materials right before casting. The casting machine often improves the quality of the polyurethane product (or polyurethane mold) and improves production efficiency when you are producing larger quantity.

 

The prices vary between several thousand dollars to a few hundred-thousand dollars.  The different capabilities come with different models.  It is nice to have a heating capability, variable ratio, and vacuum capability.  We can recommend several machine manufacturers for your specific operation.

 

 

Storage:

 

Part-A component (prepolymer) contains isocyanate component, which is very much sensitive to moisture.  If it is left in air, part-A will react with atmospheric moisture and will be ruined.  This reaction is non-reversible.  Soon after opening a can and dispensing the content, nitrogen gas or negative-40-degree-due-point dry air needs to be injected to the can to blanket the material.  Silica gel or calcium chloride desiccant filter should be installed to 55 gallon drum-vent for your drum feeding system.  The storage temperature should be at a room temperature between 65 and 90 F.

 

Part-B component may be hygroscopic.  If the material is exposed to ambient air, it may absorb moisture. Moisture contaminated part-B material may become source of degradation or excessive bubbles in the product.  The moisture contamination of part-B material is reversible.  By heating material to 160 - 180 F and vacuuming it at about 29" Hg negative pressure for several hours will reduce the moisture level.  Avoid exposure of the material to air.  Purging the empty space in the container with nitrogen gas or negative-40-degree-due-point dry air is also recommended to prevent moisture contamination of part-B as well. The storage temperature should be at a room temperature between 65 and 90 F.

 

Safety:

 

The component materials are industrial-grade chemicals.  Please keep them in a secure place and prevent access from any unauthorized individual.  The personnel who handles these materials needs to read the Material Safety Data Sheet  (MSDS) for detail information on safety and handling of the material.  The MSDS for each component is sent with the shipment of the material. 

 

When conducting a test or producing your parts using this material, be sure to operate in a wide-open area with good air movement, or in a well-ventilated area. Wear rubber gloves, long sleeves, and protective eyeglasses to prevent skin/eye contact of the material.  When your operation involves heating or spraying of the material, we recommend, in addition to the above, installation of a proper ventilation system and using a half-face respirator recommended for the use to prevent inhalation of the fume.  

 

Direct contact of polyurethane raw materials to skin/eye, as well as ingestion may lead to health problems.  No eating or smoking should be permitted at the working area.  The operator should wash hands well with soap and water after handling the materials.  Please refer to the MSDS for each component for the detailed health information.

 

Person with isocyanate hypersensitivity   In normal operations, this material should have MDI in vapor under the threshold limit value of 0.02 ppm.  However, an individual who has been sensitized by an isocyanate (TDI/MDI/IPDI etc.) may have hypersensitivity reactions such as irritation of the eyes and respiratory tract (similar to asthma-like responses) even under this threshold value.  To minimize the risk due to the exposure to the materials containing an isocyanate, operate under well-ventilated condition (dynamic ventilation), wear protective cloths and gloves should be worn.  A half-faced respirator can also be used for extra protection.

 

For any questions, please contact Northstar Polymers.

 

Tel:          612-721-2911.

Fax:          612-721-1009

Web Site:     http://www.northstarpolymers.com

E-Mail:       info@northstarpolymers.com

 

 

 

 

Disclaimer

 

Notice: All of the statements, recommendations, suggestions, and data concerning the subject material are based on our laboratory results, and although we believe the same to be reliable, we expressly do not represent, warrant, or guarantee the accuracy, completeness, or reliability of same, or the material or the results to be obtained from the use thereof, neither do we warrant that any such use, either alone or in combination with other materials, shall be free of the rightful claim of any third party by way of INFRINGEMENT or the like, and NORTHSTAR POLYMERS DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, OF MERCHANTABILITY and FITNESS FOR A PARTICULAR PURPOSE.