Is the casting in open cast or closed one? there may be more than one reason of bubble.

If they are generated during mixing of two components, you may choose to use low speed mixing that may not entrain any air.

If there are no bubble when you pour the mixture, and they are generated during curing, it is quite possible that your polyol has some moisture. In this case you should dry the polyol in vacuum oven @ ~ 80 C overnight.

you can annealed in vacume oven at about 100 C for 50 minutes for thin polyurthane film.

To prevent bubbles, 1) all the apparatus used should be dried at 100C° for 30 min

2) if you used polyurethane pellets, should also dried at 90C° for 30 min.3) placed the mixture in the Buchner flask (vacumm flask) and connect to vacumm for removal of air from the solution 4)spun the mixture under vacuum at room temperature until the polyurethane pellets dissolved completely.

Chintankumar Patel, thankyou so much. You've gone to the root of the problem I think. The bubbles are indeed generated after mixing. I will try out the solution you suggested.

Ibtisam Abbas, thankyou for your answer. I am using Part A and Part B both of which are in liquid form, but vacuum degassing may also work in case of bubbles generated after mixing. Would curing under vacuum affect the pot life of the mixture, that is the question. I'll try out all the options and let you guys know what worked.

Thankyou so much for the help.

Provided that all other parameters are same, vacuum alone will not change pot life. Pot life is affected by temperature, catalyst, reactivity of the components and solvents (if they are part of your composition) so vacuum only will not change your pot life that you are getting without vacuum.

It is a very simple easy procedure

Thankyou for your answers everyone. The problem was solved to a large degree by first degassing part A and part B under vacuum, mixing both, and again leaving the mixture to cure under vacuum. Thankyou all for the help.

Try using defoamers (e.g: BYK 110, BYK 066N) , mositure scavengers (e.g: purmol 3ST powder) add them in PART A (polyol) before adding PART B (isocyanate) and mix it uniformly to avoid forming aggregates (which will show impact on your mechanical properties).

Also dry all your samples as suggested above!

Thankyou Banothu Kumar for the suggestion. You are right to point out moisture as the main source for the bubbling problem. Moisture scavengers should be the right solution. The problem was solved temporarily due to vacuum degassing in our case.

However, the actual issue is the shelf life of castable PUs.....once the can is opened, this invariably reduces shelf life, in some cases from one year (originally) to six months (after can is opened). The main problem is the reaction of Part A of PU with moisture in the air. The cans themselves are very large; the solution provided on the company FAQ page is to 'use the polymers as quickly as possible'. This is not viable in our case, when the product cycle is still in the prototype development and research phase; and the PU used for these experiments is not in a large quantity.

Thankyou so much for the answer, I will be exploring your suggestion in the current scenario and in any other cases where there are similar issues.

HI, I am not 100% clear with the previous question but if you are talking about the hardener part (isocyanate) to keep it mositure free, Lets say if you have 20 Litre can.

Simple way could be, taking  2-3 liters at a time and keeping it in oven 30-40 degrees continuously and use it and keep the 20 Liters drum closed all the time, it will extend shelf life a bit.

Another thing people do is, they use freshly distilled isocyanate everytime they make the samples.

We are actually using pre-manufactured Part A (isocyanate) and Part B. The manufacturers instructions are to keep the cans at 23 degrees celsius for storage.

Thankyou so much for letting me know about the freshly distilled isocyanate, we can use that suggestion for future experiments.

I understand that you are using pre-manufactured Part A (isocyanate) and Part B as well as the manufacturer’s instructions is to keep the cans at 23 degrees celsius for storage. However, isocyanate can react with moisture in air, in the mold etc. Therefore, mold surface is also important. The away you introduce the polyol to isocyanate or reverse way also cause the bubbles. In addition, curing temperature, under vacuum or not.

Hope this will help

TD

TiDung, Ngo, thankyou for your answer. You are absolutely right, we were using pre-manufactured Part A and B.

We understood (after some trials) that this product has a shelf life (6 months to 1 year), after which period such issues will arise during casting. Yes (Part A especially) can react with moisture in the air, which will eventually cause bubbling during casting.

In our set-up, casting was being carried out at room temperature, under vacuum. This problem usually does not occur if the shelf life of the pre-manufactured Part A and Part B has not passed; or if the storage conditions are strictly controlled.

I have found with many polyurethanes that a good process is

1.  Mix in plastic cup, scraping sides

2.  Pour mixture into second clean cup

3.  Mix and then Degas second cup 

4.  Vent

5.  Pour into mold

6.  Do not degas mold!  Instead, use a pipette to suck out bubbles you see

7.  Allow to room temperature cure overnight, becoming solid

8.  Heat per manufacturer's instructions to fully cure.  Often 80 C for 16 hours.  

With PDMS, step 6 would be to degas the mold, but it seems that by the time you get to step 6 with polyurethane, the bubbles will not pop.  They just get big under low pressure.  Then, if you cure it with heat while its still liquid, the bubbles expand due to temperature.  

Andrew H. Cannon, Ph.D.

1900engineering.com | microspeckle.com

Hi just wondering. Does those methods discussed applied for coating applications too? I am currently doing some coating related applications with PU mixtures on synthetics slings. It seems like many bubbles have formed on the surface and when I cut open a balance left in the pot after it cured. I saw several signs of porosity which indicates bubbling, I feel that it is most probably due to reaction of humidity with isocyanate resulting in CO2 been trapped in it. But it's a bit hard to bring that idea to the folks.

how can we remove the voids in manufacture scale.

Dear Mir,

Trapped air in a mixed polyurethane resin system can produce bubbles in the surface or wall of a cast part. Cold temperatures. pouring too thick. the piece you're resining is releasing trapped air.

To prevent the formation of voids, care must be taken during the mixing process so that air is not whipped into the polyurethane. Before the material is poured into the mold, it should be vacuum degassed. A butane or propane torch, on the other hand, is easy to use and provides the heat, control and intensity to remove bubbles for a flawless.

Warm your resin before casting

Cast resin in a warm room

Decrease the surface tension of a mold by dusting a powder on the surface.

Ashish