Operation Guide for room temperature curable polyurethane casting materials

This is for cleaning the tools. You can use MEK, DBE, acetone, n-pyloridone, isopropanol, or other industrial solvents. Mineral spirit or paint thinner does not work well with this material. Please consult your supplier for the safety and handling of the solvent or cleaning agents.

For cleaning. If you use cloth towel or rag, expect you cannot re-use. Cured polyurethane does not come off by washing. Follow your local rules when disposing.

When you are done with mixing, the part-A material needs to be blanketed with nitrogen gas and the lid has to be closed tight for storage. Praxair 800-772-9247 can provide you information on how to obtain nitrogen gas supply.

The part-A has chemical called isocyanate, which is highly reactive to water/moisture. It is so reactive that it reacts to any form of water including the moisture in the air very quickly. If you leave this material out, it will react with the moisture in the air, which will spoil the material. When part-A is reacted with moisture in the air, it will loose isocyanate content and will not cure properly. In a worse case, it hardens itself and will be deposited at the bottom of the container or make a skin on top. Nitrogen gas will not react to isocyanate, so it will keep the quality for longer time.

Minus 40 ºF ( - 40 ºC) dewpoint dry air can also be used instead to blanket the components.

If you are using the entire content of part-A at a one time only or using it up in very short time, you may not have to use nitrogen gas.

All materials and tooling should be in room temperature (about 70-100 ºF), when you do not need to heat the material.

1. Heat the material (100 to 180 ºF), if needed. Consult Northstar Polymers for the appropriate temperature range.

2. Prepare the mold. Apply the mold release lightly. Be sure to dry the solvent (if needed).

4. Open the can of part-A or B, pour the necessary amount into the container as you weigh. It is a good idea to pour thicker (higher viscosity) material first. If the viscosity is the same, pour part-A or B whichever the smaller ratio component first. (The order of part-A and part-B to be poured into the mixing container is not critical. Pour whichever is more convenient first.)

5. Open the can of the other component, pour into the container as you weigh necessary amount.

6. Close the cans. When you close the can of part-A, be sure to blanket the material with nitrogen gas and close the lid tight.

This part-B material is not as much moisture sensitive as part-A, so no need to blanket with nitrogen gas. However, it is a good idea to close the lid tight to avoid moisture and other contamination, when you store.

When mixing by hands, use a stir-stick and agitate it vigorously, but try not to fold in air. Scrape the side and bottom of the mixing container as you agitate. If you do not heat thicker component, it may take a longer time to mix. Mix carefully so the components are mixed homogeneously.

If the material is less than a half gallon, mix at least one to two minutes. Even if the material appears to be mixed, they are often not enough. Thorough mixing is very important.

If you use a hand-held powered mixer, mix at a lower speed. High speed can fold the air into the material. Also, scrape the side and bottom of the container for thorough mixing even with a powered mixer.

Put 29" Hg of vacuum until you see most of the bubbles are gone. If your vacuum does not reach 29" Hg, it can take longer time to degas. Do not leave the material too long in the vacuum as it many start quick reaction.

If the part is small, you can pour in one paddle. If you pour bigger parts, avoid pouring layers; pour straight from one end to the other and try not to come back. If you are pouring a flat part with different depth, pour the deeper end first and move to the shallower end.

For your information, the time between you mix the part-A and part-B and the time the material becomes too thick to pour is called "pot life". The typical pot life of this type material is about 8 to 25 minutes. Curing time pattern can be controlled by catalyst level. Consult us for your specific curing time requirements.

Curing is the process that the components of polyurethane react to each other and harden. First they are both liquid. Then the mixture becomes thicker. It will be too hard to pour as liquid, it will be a gel that is do not move itself, and finally becomes solid.

Observe the change in hardness of the material. It will be useful later to know the shortest time you can demold without damaging the part. For testing, cure over night before de-mold.

The material hardens gradually and it becomes solid. De-molding time of parts differs depending on the formulation as well as the part sizes, curing pattern of the material, and complexity of the part. Heating (up to 180 ºF) will accelerate curing and ensure the part integrity when demolding.

When you are evaluating the mechanical properties of the material for your specific application, please conduct at least for 3 days as it gets the final hardness. This material continuously hardens for several days until it comes to the final hardness. If you need to conduct physical test sooner, you can post cure at 180 ºF for 12 to 16 hours and cool to room temperature.

Cold tooling (molds, frame, or model) could hinder the curing process of polyurethane. If you are using steel or other metal for mold or tooling, especially in wintertime, you may need to warm up the tooling at around 100 to 120 ºF. Do not heat it too hot (exceeding 180 ºF) as it could affect the quality of urethane. When you are using a natural stone for mold, the same situation may be.

Heating up the material would help curing; however, it also shortens the pot life.

At overnight de-molding, the material may be solid but still soft. Pull away from the mold slowly and carefully. It may be difficult to demold, if you have not put enough mold release.

The time between when you mix and the material becomes hard enough to demold is called "demolding Time". Demolding time can also be controlled by level of catalyst.

The curing pattern varies depending on the formulation, catalyst level, and temperature. Consult Northstar Polymers for information on curing pattern of the particular formulation.

Problems	Suggestions
The material does not cure.	· Temperature may be too cold? Heat the mold, model, inserts, or frame and cure for longer hours. Cold tooling hinders the curing process. Heat your tooling to 100 - 120 ºF. If you are using natural stone or any heat-absorbing material for your model, it has a greater effect and it requires heat. · Check the mixing ratio. Make sure you are using the right ratio of the right combination. · Contamination of material? If part-A is badly contaminated with moisture, material may not cure. · Catalyst level may be too low? Contact Northstar Polymers · Separation of the contents of the container for the components? Agitate the contents before dispensing.
Bubble Problems (If you hand mix, there will be some bubbles. Use a vacuum chamber to degas. However, if there are excessive amount of bubbles, it could be a problem.)	· Wet tooling? If you are using wood tooling, dry it very well by kiln or some other dynamic methods. Plaster tooling must be coated after baked and completely dried. Wood surface may need to be sealed or coated with plastic coating or paint. · Avoid using water to clean the tooling. · Condensation on the material could cause bubbles as well. Bring the components to warm room and leave it there for overnight before opening the container to avoid condensation problem. · Wrong mold release? Do not use water-base mold release. Hydrocarbon-base mold release could cause bubbles sometimes as well. If you are using solvent base wax or other type mold release, dry the solvent completely before pouring polyurethane. · Material contamination? If part-B material is contaminated with moisture, often a bubble problem occurs. To de-contaminate, Put the material in a vacuum chamber, and run the vacuum at 29 Hg or above for more than a half-hour to dry the material. When storing components in drums, cans, or pails, make sure the cover is tightly shut. · Whipping in air when mixing? Use slow speed mixer or mix carefully with a spatula.
Parts sticks to the mold	· Use right type of mold release. Silicone mold release is recommended. If you are using wax mold release, do not heat it too hot as it could loose effectiveness of the mold release. · You might have missed some the spot when you are applying the mold release. Ensure the mold release is applied to the entire surface that polyurethane material is in contact. · Not enough mold release? Try increasing the amount of mold release agent. · Mold has a pattern, which may be difficult to demold? It may be that the urethane is too hard to demold. If possible, choose softer polyurethane material so it flexes when demold. · Using porous surface mold material? Seal the surface and make it smooth. · Damage in the mold? If the mold or frame is leaking, the material could get into the leak holes and solidify. Examine the mold for leakage.
Wet spots on surface	· The components may not be mixed well. Even if it appears to be mixed, the material may not be mixed enough. If you are mixing less than one gallon, agitate for at least one minute. Increase the mixing time according to the amount for homogeneous mixture. Scrape the bottom and side of the mixing container as you agitate the components. If you do not scrape the sides, a last few drops out of the mixing container may not be mixed and make wet spots. · Wrong mixing ratio? Check the ratio again. · Cold mold, inserts, or model? Operating temperature too cold? Warm the tooling to 100 - 120 ºF. · Contamination of the material? The mixing container or tooling may not be clean. It could be contaminated with incompatible substance, which appeared to the surface?
Pot-life too short	· Processing temperature too high? Mix at room temperature. · Catalyst level too high? Contact Northstar Polymers.
Does not cure fast enough	· Heat the material in the mold after it is poured. The maximum temperature is 180 ºF. · Catalyst level too low? Contact Northstar Polymers
The parts not as hard as expected.	· It may not have been cured completely yet. Often it takes longer time for the material to cure when the room is cool. You may need to move the part to warmer area. Some material can take 7 days to get to the final hardness. Use auxiliary heat if you need it to cure faster. · It may need to post cure. Thinner sections of the molded part may need auxiliary heat to cure. Put the demolded parts into an oven at 180 ºF and post cure for overnight. · Inaccurate mixing ratio? Often, if the part-B is put in more than the suggested ratio, the outcome becomes softer. Try to be more accurate on the ratio. Slightly higher ratio of part-A can turn the material harder.
Voids in parts	· The material may be loosing the flow too soon? Specially for larger part, the material needs time to flow throughout the mold. If the material gels too soon, it cannot flow to fill the mold. Consult Northstar Polymers for a material with lower reactivity. · Too much mold release? If you are using silicone mold release, a very thin layer of mold release should be efficient. · If you are making a very small part or need to cast into a very narrow section, the liquid consistency of the mixed component material may be too thick to pass. Heat the component materials to about 120 ºF may lower the viscosity and give a better flow. Or consult Northstar Polymers for an alternative formulation. · Dirty mold? Clean the mold. · Employing the right casting technique? Air pockets made from the mold pattern may cause voids. Tilt the mold when you pour halfway, or vibrate the mold to avoid air pockets. Avoid the turbulence of the polyurethane mixture in the mold. Pour in one paddle and try not make layers. If you are pouring a large part, pour from one side to the other without coming back. Pour from the deeper end to the shallow end if applicable. · Exothermic reaction may be too fast or material is out-gassing? Contact Northstar Polymers.
High shrinkage	· Exothermic reaction may be too fast. Use lower processing temperature. Contact Northstar Polymers.

The cure pattern can be modified. We can change pot life, demolding time, and complete cure time; however they are all related. Shorter one element gets, also shorter the other elements to be. We can control it by modifying the formulation to accommodate your production requirement.

You can use polyurethane grade colorants/pigments to add colors to the material. Particulate dispersion type colorants are commonly used with this type of polyurethane casting materials. They are typically dispersed into polypropylene glycol ether. We recommend using colorant paste dispersed in polypropylene glycol ether of above 3000 molecular weigh. You can add this type of colorant at concentration of 0.1% to 2 % of the total weight without having major concern of the effect to the properties of polyurethane materials.

You can obtain those colorants from the distributors of the following manufacturer:

Fillers can be added to obtain certain properties. However, addition of solid filler would deplete strength of the polyurethane material. Also, microscopic surfaces of the filler would introduce a large amount of air into the polyurethane, and will cause bubbles to form. If the bubbles are a concern, degassing by vacuum is needed. In general, fillers cannot contain moisture as polyurethane components react with moisture and will have adverse effects. High moisture filler materials such as wood powder, pulp, cannot be used. Commonly used fillers are dry clay, silica granule, wollastnite, milled glass, and microsphere glass bubbles. Low moisture calcium carbonate (lime stone powder) may be used at lower concentration with vacuum. Fumed silica is often used to develop thixotropic effect to the liquid material.

Other modifications including adding colorant, UV absorbing agent, anti-bacteria agent, and other additives as well as change in mixing ratio are also available. Northstar Polymers customizes the formulations for each customer.

Some materials are sensitive to cold weather. If the material has been in a cold temperature during the shipping etc., separation within the material could occur. If you see crystallization or gelatin of the component, use a drum heater, or put the container into an oven and heat it to 160-180 ºF, then agitate the material before use to ensure homogeneous re-mixing. Generally, part-A materials are more sensitive to cold weather.

In cold weather, cold toolings (molds, frame, or model) could hinder the curing process of polyurethane.

In hot weather, condensation of moisture could cause a problem. If you store the material in cooler place then bring it to hotter/humid place just before the operation, the material itself and the container could create dew on the surface, which causes bubbling problem. Be sure to leave the materials and toolings in the temperature that is what you are going to operate. If the work place is very humid, it could also cause a problem related to moisture contamination.

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 80 ºF.

Part-B component is 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. 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; however most of the cases, keeping in an airtight container will be sufficient. Store it in a dry indoor storage at a room temperature between 65 and 80 ºF. The moisture contamination of part-B material is reversible. By heating material to 160 - 180 ºF and vacuuming it at about 29" Hg negative pressure for several hours will reduce the moisture level.

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 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/or using an appropriate type of respirator 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 and follow the other procedures of the Standard Industrial Hygiene Practices. Please refer to the MSDS for each component for the detailed health information.

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.