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Release Agent & Removal


dr_gn

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Guys,

If I'm using an existing plastic part as a mould, what is the best release agent to use (for silicone moulds) and how do I remove it completely so that I can still paint the original part?

Thanks.

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Silicone rubber only sticks to itself, so if you're making a one-part mould you shouldn't need any removal agent. If you're making a two-part mould, brush vaseline on the first part of the rubber mould before pouring the second. Ask for it in pharmacies. Don't cough, wink, or mumble, or heaven knows what they'll provide to you.

Edited by Bonehammer
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Thanks guys. I'm wanting to replicate this cowl:

SU23cqHn.jpg

WQuTWvPL.jpg

leB4lCgc.jpg

I'm thinking of moulding it in two parts as per the middle image above. i've come up with this scheme for making a two piece mould for the cowl (presumably I can't do this in one-shot, since the overhang of the collecter ring is too much to pop the original/castings out?

YwiRkZON.jpg

First I'd build a cylindrical or square box (A), then make a spigot which is a tight fit in the end hole, and would rest on the bottom of the box. I'd then pour silicone around the outside of this assembly level with the top of the cowl and leave to set. Once set, I'd remove the spigot, and pour more silicone into the centre until it rached the top edge of the cowl, this would form a silicome core (D). The bottom of the box and the sides would have features that form clocking marks to allow the two parts of the mould to go back in the correct orientation after the original part was removed.

Does this method sound OK?

Will the bump on the collector ring prevent the part from being removed?

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As you asked for opinions, I'll offer mine,FWIW.....take it or leave it as you see fit.

I think that if you want to cast the thing as you have shown,hollow as it is,it would be a challenge even to a professional caster. I am not one,well not in resin any more, but think it would be a real challenge.

Might I offer an alternative approach? If you want the cowl to be hollow,or atleast open on the inside after casting, you could try mounting the back end of the cowl assy on a sacrificial round,or better,square block. Fill the front end of the cowl with soft wax or modelling clay,so that it has just a lip around the edge. Make your now once piece mould with the whole lot in the middle,naturally. Then cast it in one lump.When set,mount the back (now you see why I said square) block in a vice,under a pillar drill,and drill out the middle of the cowl from the front in progressive steps until you have a hollowed cowl.Then carefully saw the block of the back and sand it square.Voila....I don't know what scale you are working in,so cannot guess the inner hole size,but if in 1/72 I am sure you could get the fight size finished size drill. Resin (polyurethane)is pretty easy to drill and there won't be much dust,just shavings and swarf.

This way would only involve a single piece mould (much easier,and cheaper) and a far higher chance of casting a solid cowl rather than a thin walled hollow one,which could then be machined.

Good luck with your project.

mtd

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Sounds legit to me. You might have trouble with not being able to displace the air at the bottom of the mould, resulting in a bubbly front. I know pressure pots and/or degassing are meant to solve the problem but I've never even understood how they work, much less used them. (Myself, I'd try a centrifuge thing).

Not only the bump will be a pain in the neck when removing the part, it will also trap air bubbles. See if you can live with adding an extra part to smooth the transition so that bubbles can rise. Of course this part will need to be cleaned on the copy.

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Thanks guys. I think the issue with the cast-solid-and-drill-out method may be that to get the wall thickness right by drilling, it would be just as difficult to do without breaking the part as trying to cast it. Plus it would be difficult to get the inside of the compound curved collector ring to be the same thickness as the rest of the cowl. Not saying it wouldn't work, and I may well end up doing this or similar.

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If you use the right grade of silicon for the mould the sticky out bit is no problem. The silicon will be flexible enough to allow it out.

Pack the cowling with plasticene. Level off perfectly. Make 1st part of mould over this solid item. Dig plasticene out of cowling, leaving it in the 1st mould [or put it back in]. Make second part of mould by pouring silicon into the now hollow cowling in 1st part of mould, and some on top, totally enclosing cowling.

When you remove cowling you'll have a void, and a part mould with a plug for the centre. The 2nd part of the mould would need to be a stiffer grade of silicon or re-enforced.

You'll need to cut pouring & air release holes in the mould. Use a slow setting resin. Put it on one of these cheap 'sonic' cleaners, which vibrate, they'll help air bubbles rise to the top.

Thats the quick/short description.

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After several beers last night I had an idea but waited until the morning to post (beer goggle crazy idea filter)

This is similar to the way I'm making the canopy mould on my Meteor build.

If you have lego make a quick mould to make flat RTV base from something like this, If you dont have lego the bottom could be made from plasticine with dimples pressed in to form an interlocking key and a flat bit in the middle big enough for the cowl.

18535268832_b5a42ba0c4_z.jpg

Also get some masking tape an wrap it around the aft end of the cowl extending it by 5-10mm and seal off the front end with another piece.

Fill both with RTV. The cowl should be filled to a few mm above the end of the plastic part as shown on the first line of this diagram and let it set.

18372748489_bffea06e39_b.jpg

Once set remove the masking tape from the cowl but don't separate the cowl from the RTV not even once,(it currently has a perfect seal that you will never get back)

Remove the RTV base from its mould and mix up a small drop of RTV and use this to glue the 2 parts together (the perfect glue for sticking RTV to RTV is RTV) use it liberally then leave it to cure overnight. you should have something looking like the middle diagram.

Spray it all with plenty of mould release, the release can be carefully wiped from the cowl with IPA on a cotton bud if you want to but make sure any exposed RTV is completely covered with release.

build up some walls around from lego or other as in the bottom diagram and pour over RTV and let it set.

When set prise the two halves apart, and hay presto a 2 part mould.

To cast, fill the female half part way with resin (trial and error to find the right amount) and slowly lower the male part on top as far as it will go, maybe stack a couple of jars of paint on top as weights while it sets. dont worry if excess resin oozes out of the top.

The resulting cast will have a thin resin membrane across the front and around the rear, easily cleaned up.

Hope this helps

CT

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CT - Thanks for that. My biggest concern is using mould release agent on a part I will need to subsequently paint. Instead of spraying with release agent, couldn't I paint the RTV with vaseline or whatever, and leave the original plastic pattern totally untouched?

Another Idea I had was to spray the part with acrylic, then once the castings are done, strip the paint off with brake fluid or modelstrip hopefully taking the release agent with it?

Comments appreciated...

Ta.

Edited by dr_gn
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I've never had mould making silicon, RTV or not, sticking to plastic originals. If you paint it with anything any brush strokes will be faithfully copied. If you feel you really must use anything use olive oil.

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So since Cheshiretaurus' method seemed like the least work I went with that. I made a couple of additions though - I've added a steel spigot into the the top of the cowl opening, so that it'll have some positive radial location, and I also added a couple of plastic risers, which I'll somehow route to form vents for the excess resin. If the above doesn't work I can easily blank them off and try again I guess. I also thought I'd try to cast an engine and the gills, these will need some post-cast fettling, but better than nothing hopefully.

3aZoraUE.jpg

8ehERgnm.jpg

It's at this point I had a crisis of confidence regarding whether the silicone would release from the parts - it seemed far more viscous than I remembered. So I've used it on some scrap test pieces, and tomorrow all being well I'll pour "part 1" of the proper moulds.

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You shouldnt have a problem releasing the new bits from the mould G

I used the single mould method to get some Adour jetpipes for the Jaguars

As I had two decent sets to choose from I made two pairs to see which looked closest to the real thing

here is the mould I ended up with

18428102100_cc042e259d_z.jpg

Excuse airbubbles (these are a couple of my fails through air in the resin) but this shows the undercut from the mould

18615778575_61390213aa_z.jpg

And here a view from the outside, it isnt easy to get a look at the detail that the mould captured but I have been very happy with the resultant pieces in all three Jaguars

18589416346_32443ec282_z.jpg

Hope things go well with your casts, I dont expect you to have any glitches ;)

Edited by perdu
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