Resin and moulding problem

[sn]

At the moment I'm trying to do some resin castings from a silicone mould. The positive is a plasticard low relief industrial building.

The problem I am encountering is that the positive does not lie flat in the mould(Probably because its a a bit thin and on the light side) and as a result the resin creeps up under it and behind it.

 

I'm presently making the moulds on a foamboard base and surrounds. 

 

I suppose what I need to master/solve  is how to secure the low relief positive down to the base of the moulding container in such a way that it stays flat and stops the resin from getting behind it while at the same time being able to remove the plasticard positive  afterwards without having large chunks of adhesive stuck to the plasticard positive. Hmmnnnn.

 

 

 

Has anyone done anything similar? Encountered the same problem and solved it? 

 

Len

[kandc_au]

Len,

If you want flat castings the master needs to be secured flat, all around before pouring the rubber to make the mould
So YES it does need to be secured !not only around the edges but also in the middle and the rest of the master.

 

Khris

[peter220950]

Although I have used foamcore, on a big casting it too can distort, I tend now to use 5mm Perspex, and make mould boxes from Lego, or clone stuff from Wilko, to get an nice square flat mould.

 

Melamine covered chipboard is also a good base to work from.

 

 

Peter

[macgeordie]

Unless the master is very warped double sided sticky tape around the edges usually works.

 

Ian

[chris p bacon]

I'm similar to Peter and use Lego/Wilko bricks for the mould, I also stick larger objects down with double sided tape at the edges.

 

One issue you can have with clone bricks is they can 'leak' silicone, so what I do is place a square of cling film down on the mould base, stick/place the master down then wrap the film over it. I then build the box around it and unwrap the film so it makes a liner within the box.

 

One tip is not to make the master too thin, I sometimes build up a sacrificial edge that can be cut off afterwards so that it gives the master some rigidity, it's surprising how the silicone can move the master when you pour, as trapped air will fight to rise up.

[sn]

Thanks all. YesI think the master may be to thin and I'll bulk it up thinks I. The double sided tape might be an idea too.

[sn]

Thanks all. YesI think the master may be to thin and I'll bulk it up thinks I. The double sided tape might be an idea too.

 

Double sided tape worked a treat, (and I sellotaped down the edges too, to a glass base this time as suggested ) as did making the positive itself twice the depth (with the aid of somefoam board glued to the back of the plasticard and trimmed accordingly. As a result I produced a very acceptable mould at last. Thanks to all for the advice.

 

 

So...

 

I go to do a resin casting this evening and as I'm pouring it into the mould I somehow drop a tack into it which I quickly retrieve with some tweezers, however this (I think casued some sort of chemical reaction with the metal?) ) perhaps explains why the resin mixture then bubbled up and hissed like a snake and dried complete in about 30 secs (I kid you not!)  The back of it where the air tried to escape now resembles a cross between a crunchie bar and cemented rice cripsies. The front is patchy and pitted..

 

I will try again tomorrow.

 

Is this what caused it do you think?

 

Len. 

[peter220950]

Hi Len,

 

I don't think it's anything to do with metal in the resin, I regularly cast nuts and mdf into it, to make knobs and the like, without any issues.

 

How old is the resin, and how long was it left poured into mixing cups before actual mixing?

 

I have had problems in the past with this happening, and carried out an experiment. I poured resin and hardener into two cups and left them out for 24 hours, then mixed them. This was the result,

 

 

I think the resin or hardener is hygroscopic, and draws moisture from the atmosphere, this then reacts when the two components are mixed. It's just a theory, but I can't find any other explanation at present, and the result is similar to the Gorilla glue that advises one surface to be wetted.

 

Hope this helps.

 

Peter

[sn]

Hi Peter

 

The resin was brand new and poured in almost immediately.

 

I wonder what's going on then?

 

Len

[Steve_Beattie]

Check the use by date on the tin of the resin. You don't always get new stock.

 

For thin open mould casting you need to thicken the flat sheet. I suggest a minimum 2mm. The master can be glued down using a solvent free adhesive which can be peeled off after use. 

[jonhall]

You need to be a lot more specific about what sort of resin you are using - there are lots of types and brands, and just because one does this doesn't mean others will or do.

 

Jon

[sn]

I think, having slept on it, that the problem was that i measured out water into the mould before casting in order to determine the amount of resin I needed. Didn't fully dry the mould  off and I suspect its the excess moisture that has caused this to happen.

[kandc_au]

Hi Len,

 

I don't think it's anything to do with metal in the resin, I regularly cast nuts and mdf into it, to make knobs and the like, without any issues.

 

How old is the resin, and how long was it left poured into mixing cups before actual mixing?

 

I have had problems in the past with this happening, and carried out an experiment. I poured resin and hardener into two cups and left them out for 24 hours, then mixed them. This was the result,

 

20171113_205301.jpg

 

I think the resin or hardener is hygroscopic, and draws moisture from the atmosphere, this then reacts when the two components are mixed. It's just a theory, but I can't find any other explanation at present, and the result is similar to the Gorilla glue that advises one surface to be wetted.

 

Hope this helps.

 

Peter

That resin looks like it has a lot of air trapped in it from when you have mixed it!

 

Khris

[peter220950]

Khris,

 

No air in the mix, it just goes like that if you leave the ingredients out for a day or so, it was done to test my theory on moisture in the mix. It looks like it proved correct, and that water in the mould was proably the cause of the OP's issue. The rest of the days casting with the same resin went perfectly fine,

 

 

I had noticed that the last pours from old resins tended to be like an Aero bar, and it happened once when I left new resin out for a day, the test piece was a deliberate mistreatement of the resin, and boy did it bubble. Air in the mix just reults in a few trapped air bubbles, this stuff foamed up like a volcano!

 

Peter

[unklian]

I think, having slept on it, that the problem was that i measured out water into the mould before casting in order to determine the amount of resin I needed. Didn't fully dry the mould  off and I suspect its the excess moisture that has caused this to happen.

 

You got it in one. Polyurethane resin does not like water. It is also very important to keep tins of unused resin tightly closed when not in use as the resin is hydroscopic. Which means it will draw in moisture from the air. The bubbles are caused by the heat of the resin reaction turning the water to steam .

[sn]

You got it in one. Polyurethane resin does not like water. It is also very important to keep tins of unused resin tightly closed when not in use as the resin is hydroscopic. Which means it will draw in moisture from the air. The bubbles are caused by the heat of the resin reaction turning the water to steam .

Now done a series of very successful castings! Thanks all!

 

...I think I'm getting addicted too!

[sn]

Just for the bedevilment of it and to make a comparison, I'd now like to cast from the same mould using plaster of some sort.So...

 

Two questions arise from this:-

 

1. What plaster/ brands of plaster have people used in the past with success/failure?

 

2. Would I need to use a release agent?

 

Cheers

 

Len