Some PU casting questions .....

[jas46]

I've just done my first test pour today in the pressure pot and things didn't go well lol
I know these are basic questions but I want to double check to make sure I'm not being an idiot..
I mixed by weight my water clear PU resin 100 a to 120 b

Now I take that as 100g of a to 120g of b it should be 20% extra for b is that right ?

I poured 20g of a and 22g of b mixed for a few min's added color and left for about 10 min's and then poured into a small pot and swirled the colors.
Then into the pot, presured upto about 50psi and left it for about 3hrs
Here's the bad bit
It's really tacky and soft I know I poured around the 20 degree mark but as soon as I realised I got the shed upto about 25 degrees
Does anyone know what went wrong ?

Thanks
Jas

 

[Buckeye]

Just check that the measurements are supposed to be by weight and not volume. It should still go off and harden up, but it may take a few days, put it in the airing cupboard and check it in a fews days.

10% of 20grams is 2grams 20% would be 4grams

Peter

 

[jas46]

Cheers Peter
Yes my auto type changed it to 22 doh
On the data sheet it's says p.b.w which I'm assuming is parts by weight, but my logic of the calculation is it a correct one ?
The odd thing is the spare I left in the cup is still tacky 2hrs later as well
I am wrong in thinking that if the specs say demould time of 60 min's it should be hard after at least 2hrs

Thanks
Jas

 

[Buckeye]

Cheers Peter
Yes my auto type changed it to 22 doh
On the data sheet it's says p.b.w which I'm assuming is parts by weight, but my logic of the calculation is it a correct one ?
The odd thing is the spare I left in the cup is still tacky 2hrs later as well
I am wrong in thinking that if the specs say demould time of 60 min's it should be hard after at least 2hrs

Thanks
Jas

You should be able to demould after 1 hour, but I have found that when it is under pressure it always takes much longer, but if your cup is still tacky after two hours then maybe it's just too cold or the mix was wrong. I would keep it warm, not hot, for a few days.

Peter

 

[jas46]

Cheers mate it was very cold I the shed when I was mixing, I will try again tomorrow but with a higher temp I the shed
I've written of the 1kg of resin for trials so it doesn't bother me too much lol
Jas

 

[Buckeye]

I always put the cups in water that is about 30c, if you are doing 2 colours for example you can put the cup of resin for 1 colour in a water bath of about 35c and the other colour at about 25c the 35c one will set quicker so when you do pour the 2nd. colour they should give a better separation, but you still need to judge when to pour the second colour without the first one setting up too far.

Remember to wipe the water off the cups before you pour as PU will foam if you get water in it.

Peter

 

[jas46]

Cheers Peter I will try that tomorrow
I've been watching some vids and figured out that it seems to be my fault
My scales only do Inc of 1g and can mess up the ratio badly, so off to buy some new scales tomorrow
And I didn't mix the two anywhere near enough lol
But live and learn
Thanks for the tips mate
Jas

 

[Buckeye]

If you use the wide tongue depressors to stir then cut off one of the rounded ends so that it is square across, that way you gat scrape the bottom of the cup to help mix it properly, make sure you also scrape the sides as well.

Peter

 

[jas46]

I've got plastic flat bottomed ones as I didn't think you could use wooden ones with pu
Just hope it all goes good tomorrow lol
Thanks for the tips mate

 

[ArtyCrafty]

Do let us all know how things worked out..we can all learn from mistakes as they are just as important as the successes.

 

[jas46]

Well I tried again today ....
I mixed up 44g total in 25 degrees and made sure I mixed the living daylights out of the mix...and it did exactly the same...45 mins later and the drabs in the cup are still liquid.
The pink one I turned yesterday just melts when I try and lathe it dispite being hard ....
Back to the drawing board for me ...
Jas

 

[jas46]

Right this is getting weirder and weirder, i just took the sample piece out of the pot after 3hrs under 50psi and its perfect well except i poured way to early again lol....but the drabs in the cup are still tacky and soft...