Temporay fixative to RTV to Tweak mold..

[Eddie]

Hi all I have just completed a very complicated mold which has gone well except for one part that I need to hopefully correct.

I need to trim back a part and add a temporay piece to correct a small aread with a little pour of extra rtv.

Its should seal anyway but I am conearned about leaks. SO is there a temporay fixtive I can use to hold my piece in place while I pour the extra RTV. Would plasticine do it..

Cheers Eddie

[Blindhorizon]

Can you post pictures of what your trying to get at?

Blind-

[Eddie]

Here you go




there was a hole on the inside of the model which cause air to do this to the parts you see hanging.

I have had to cut them off or the resin casting would get locked into the mold.

SO there is enough at the bottom for me to fit parts over and then top up the RTV inside to become part of the existing mold

I am just concearned about it running out at the bottom so wondered as to some kind of tempoary seal to all allow the part to be fitted, FIlled with RTV and to replace the parts hanging here but complete rather than missing a chunk.

Cheers Eddie

Also any tips on pouring verysmall amounts of RTV into small areas welcome ?

[Eddie]

I think I have sussed it guys, I will packout the part with clay which is pretty sollid to keep the part true while I mold the area again.

How can I pour RTV into very small areas with control ?

[mangozac]

I still don't quite understand what you're trying to achieve, but I'm sure that using clay to fill areas will be fine. Pouring the RTV as you suggest can be quite difficult - normally we would rely on pressure or vacuum to help force it into the areas. I would suggest using a pieces of wire and repeatedly poking it down into the cavity after the silicone has been poured. That will help distribute it and get the air out.

[Eddie]

After Much thought I have sorted it now and thank god it worked..

I mounted the rubber in lego and taped it to a wall to keep it true,
Then I slipped my resin parts over the area that needed re pouring,.

Then I packed it out with clay the air dry brown stuff which stuck to it just fine and made a spill tought to catch the and stray rubber from cocking up the mold and stopp leaks to protect the good parts.

I then wondered how the hell I was to pour RTV into two 2mm diameter tubes.

I had it after a coffee I used a trimmed pipett and this allowed me to gentley injection mold it and mostly eliminate the air bubbles and the good news is it worked a treat and saved my other wise good mold (5 LITRES In total eeekkkkk) No you know why I was worried.

I have managed to perfectly restor the 4 extentions with not a problem at all except a few tiny bubbles at the end but this can be trimmed and extended in the same way and this time with greater ease than before as there is now a significant area to work with safely away from the other parts of the mold.

I forgot to take photos in my anxiety to get it fixed which has been 3 days of worry. after 5 months work on the project still on going.

I will take some pictures soon as I can find a phone charger.


My next question is how do I keep my mold together to stop leaks. I guess making a solid wooden form keeper box is the only way.. Its a huge mold about 1ft tall and I have a pour spout which feeds a huge cavity from the bottom i am concearned it wont work,. The science says it will as resin seeks a level but the area to be filled for my model is about 20cm x 25 cm x 3.5 cm

Does the size of the channel feed for the resin make a difference ?

[mangozac]

Good work! The pipette was a good idea!

To hold the mould halves together during casting you are right that the best technique is a box shape, however do not fasten all four sides together. You want four boards (one of each side) and some kind of tensioning system. For smaller moulds rubber bands to the trick. You might need to track down some large elastic pieces for this. You want them tight enough to apply firm pressure to the mould but not enough to cause warping.

Obviously the bigger the feed channel the quicker the resin can flow. For larger parts it's best to ensure that the feed channel is large enough that you can pour the resin in without it touching the sides. You'll also want to make sure that air can escape through areas other than the pour channels...

[Eddie]

Hmm, Getting over one hurdle there is always another to get over..

My pour spout is the same height as the exit hole the with of the tube which runs vertiacly down to a curve which runs accross the bottom then curves up and then goes up a little before opening out into the huge casting area starts at about 5 or 6 mm, tapering down to about 3 or 2mm. There is no run areas for air so I will add a few if I can.

I used kit sprues initially till it occured to me I could use playdoh to make the channels.

Well I am getting there now I just hope so much I can get a good casting from this mold as It will be a dream come true if I can but a 4 or 5 litre mold is ambitious for a beginer but you don't learn if you don't throw yourself in the deep end!

[mangozac]

Nothing wrong with being ambitious - throwing yourself in the deep end is the best way to learn!