Aluminium hydroxide as a filler for PU resin

[udog]

Hi
I´m casting some PU resin pieces in silicone molds.
Sometimes I use marble powder as a filler, to save resin and reduce shrinkage (not totally).
But due to a rainy summer and having the studio near a river the marble powder must have picked moisture, thus, foaming the resin.
So I purchased some aluminium hydroxide I found from a supplier. I had never used it but had heard about it. It´s pre dehydrated, less chance of moisture problems. Works great, none o very little foaming (can have to do with other causes). And it does cut shrinkage a lot.

But I´m having an issue I didn´t expect. Weight does not correspond with the previous castings with marble powder.

For example:

The same casting.
With marble powder: 370gr (135A+135B+100gr marble powder)
With Aluminium hydroxide: 450 gr (150A+150B+150gr Alum hydroxide)
So I´m getting a heavier part and am not saving resin. Moreover, Iḿ using more I´d say.

I assume it has something to do with densities or similar. Maybe I should be adding more filler and cutting down resin?. I think I could go up to 100-150%.

Some info I found (another supplier):

Type - Hydrated Alumina
Cast Volume - Resin + Up to 150%
Particle Size - 0.07mm
Density - True 2.4 / Apparent 1.2

I´m waiting for the supplier to answer but decided to ask here in case someone knows.

Thanks in advance

 

[RacerMachX]

Hi
I´m casting some PU resin pieces in silicone molds.
Sometimes I use marble powder as a filler, to save resin and reduce shrinkage (not totally).
But due to a rainy summer and having the studio near a river the marble powder must have picked moisture, thus, foaming the resin.
So I purchased some aluminium hydroxide I found from a supplier. I had never used it but had heard about it. It´s pre dehydrated, less chance of moisture problems. Works great, none o very little foaming (can have to do with other causes). And it does cut shrinkage a lot.

But I´m having an issue I didn´t expect. Weight does not correspond with the previous castings with marble powder.

For example:

The same casting.
With marble powder: 370gr (135A+135B+100gr marble powder)
With Aluminium hydroxide: 450 gr (150A+150B+150gr Alum hydroxide)
So I´m getting a heavier part and am not saving resin. Moreover, Iḿ using more I´d say.

I assume it has something to do with densities or similar. Maybe I should be adding more filler and cutting down resin?. I think I could go up to 100-150%.

Some info I found (another supplier):

Type - Hydrated Alumina
Cast Volume - Resin + Up to 150%
Particle Size - 0.07mm
Density - True 2.4 / Apparent 1.2

I´m waiting for the supplier to answer but decided to ask here in case someone knows.

Thanks in advance

Hi,
I like to use microballoons fillers. They are made of glass, so no issues with moisture. The parts are lightweight and easy to sand. Typically use as epoxy filler for boat repairs...

 

[udog]

Hi,
I like to use microballoons fillers. They are made of glass, so no issues with moisture. The parts are lightweight and easy to sand. Typically use as epoxy filler for boat repairs...

Thanks for answering.
I know them and in fact have some here. Could run a test. Do you get to save resin with them?. What ratio do you use?

 

[udog]

Didn´t think of microballons because thought they´d thicken the mix too much. Probably wrong.

 

[RacerMachX]

Thanks for answering.
I know them and in fact have some here. Could run a test. Do you get to save resin with them?. What ratio do you use?

The ratio, depends on what type of characteristic you want the finished casting to have- strong, light, medium....
Lots of variables, incl the chemical properties of the PU resin. Some PU resin might not like too much filler ratio. You might want to contact the tech support of the PU manufacture and ask the question. I do save resin with them. It does thicken the resin, depending on the ratio. I do multiple pours with a beauty coat 1/4 filler, other coats 1/2 filler. But this is for rotocast helmets.

 

[udog]

The ratio, depends on what type of characteristic you want the finished casting to have- strong, light, medium....
Lots of variables, incl the chemical properties of the PU resin. Some PU resin might not like too much filler ratio. You might want to contact the tech support of the PU manufacture and ask the question. I do save resin with them. It does thicken the resin, depending on the ratio. I do multiple pours with a beauty coat 1/4 filler, other coats 1/2 filler. But this is for rotocast helmets.

I understand. Yesterday I tried a 150% ratio (the supplier said I could go up to that) with the Aluminium Hydroxide and it thickens too much, unpourable.
These are full pour castings. No detail, all smooth roundish shapes, but need a minimum low viscosity so I can pour. I´ll ask the manufacturer, the resin is very low viscosity though.
I´ll make a test with micro balloons and see what happens.
I suspect the fact I´m having to pour more material has to do with the hydroxide density. Most probably I´ll have to stick to that. Marble powder made the trick to save resin but it picks to much moisture, or the bag I got did.

Thanks.

 

[udog]

What bugs me about this Al. Hydroxide thing is that the more I try to increase the filler ratio (within the viscosity boundaries I need), the less volume I get. It´s like cab-o-sil with polyester resin, less volume.