Manual Casting of PU 3

[Speedy]

Manual Casting of PU

I have a problem and it goes like this;

We are casting a component in PU. The PU consists of the 2 components Polyol and Isocyanate. We also add a pigment. We mix the 3 components by hand in a beaker, vac and then pour into the mould. The problem is that the potlife is way too short and we end up trapping air bubbles in the casting. If we leave any longer than 3 mins in the vac chamber then the mix starts to cure.

The obvious solution is to purchase a suitable dispensing machine but our numbers are too small to justify the investment. If we are careful when hand mixing and minimise the amount of air we stir in, we run the risk of insufficient mixing and poor curing. We could drop the temp of the components pre-mixing but crystals form in the Isocyanate if dropped below 0 Celsius.
Any suggestions?
A small mixing vac unit would be ideal. Are they any out there?
Is it possible to inhibit the cure rate? Our PU supplier says no.

Appreciate any help.

 

[GregLocock]

BTDT.

Try using an ultrasonic shaker to get the bubbles out, in the vac chamber. This worked very well for us, although a 3 minute gel time is pushing things.

Why not build your own dispensing system, then ultrasonic the unmixed components in the dispensers to eliminate entrained air, and then mix in vacuo? It doesn't sound very hard to build.

Our epoxy supplier would give us preloaded catridges that were pretty much de-aerated. The mixer nozzle was also continuous flow, so our epoxy mixes were OK.

Cheers

Greg Locock

 

[RobWard]

Hi.
You can try degassing the resin first as well.
Ensure that there is no moisture present in your processing facility or resins or pigment.
You can get inhibitors for some PU systems, but usually the supplier would be aware of suitable ones.
Alternatively ask the supplier if they have an alternative system with similar properties but a longer pot life.
Also look at the way you are casting:
make sure the mould is oriented correctly and that any necessary bleeds are machined in.
Try to mould the component so that any critical surfaces are at the bottom, and any trapped voids are not detrimental to the part and thus are acceptable.
You might need to over-cast the part and machine it back to the correct dimentions to achieve the finish you need.
Make sure that your processing temperatures are the ones specified by your supplier. The material might become viscous through being too cold.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[Speedy]

Guys,
Thanks for your help.

We have since discovered that it was the pigment we were using that was reacting with the polyol and causing it to foam. We had mistaken this for trapped air. Without the pigment everything worked fine.

 

[GregLocock]

Well, that made me laugh anyway. Glad you got it sorted.

Cheers

Greg Locock

 

[patsurf]

hi
im trying to set up a production for molds p.u block for surfboards,i try to hand mixing but with problem of air traps ,so i dont know how invest ,to buy a mixing machine or just a blender vaacum ,any of you have info where to buy a vaacum blender or what is the best mixing machine low pressure but doesnt produce air bibbles in the mixture.
i hear also about ultrasonic ,but there system already working for mixing p.u.the cost is not so important ,i need a good quality machinery.

patricksurf@iol.it

thanks
patsurf

 

[Speedy]

patsurf,

Try these guys in the UK;

http://www.island-scientific.co.uk/SquareCustomBuild.asp

I progressed from my earlier posting. We found that the temperature of all the PU components and the temp of the mould are critical. We had to keep everything at around 15C. This allowed enough time for degassing of the mixture in the mould, i.e mix in atmosphere, do not degass the mixing container. Even a few degrees C was enough to cure the mix before it finished degassing.

Ideally though I would recommend you get a custom built unit.