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2 Part RTV silicon molds

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flyonline View Drop Down
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    Posted: 27 Oct 2011 at 3:50pm
I finally got round to doing another RTV silicon mold today after a few false starts (new shape, new bib etc.).

There are many different ways to do this, I find this the easiest and usually the quickest. You can simply place the lure in a container, pour the RTV silicon around it and cut a slit along the back/belly to extract the plug to create a 1 piece mold. Pour your plastic through the open slit onto wire insert and/or bib, lightly clamp closed and wait. Other 2 part methods are pushing the lure into a base of clay/plasticene etc. until it's half covered, form a box around the plasticene and pour the mold plastic/silicon. You can also create a box, fill it with your mold plastic/silicon and push the lure/plug into box until it's half covered. All of these require that the mold half plus lure/plug is removed, cleaned up, returned to the box (or create a new one) and then the second half is poured. Permanent inserts can be wire hangers, bib slots/alignments, mold strengthening such as Aluminium tubing etc.

There are also many different types of plastics etc. you can use for the molds - RTV silicon, car bog, casting epoxies etc. I like RTV silicon because nothing sticks to it (except the same silicon) and you don't need to use any release agents with PU bodies.

I forgot to take a photo of the first part, but I place the lure onto a piece of MDF and trace around it with a pencil. Cut out the shape and carve/sand until the lure sits firmly but flat in place (hook hangers help a lot here) along half of the thickness of the lure. Fill any gaps with plasticene/clay/car bog and use a piece of plastic or square paddle pop stick to cut any excess off.


Some keys should be cut to provide indexing and keep eveything aligned (I forgot for this mold so don't worry if you can't see them). If using a soft parting board (plasticine etc.) simply push a marble, marker, etc. in to create a small divot about 5mm deep. If using a hard parting board (MDF etc.) use a router, dremel etc. to cut a similar size hole. Any key needs to be a) smooth and b) open so that when the 2nd half gets poured, it will fill around the key but not get caught when pulling the two halfs apart. I usually use a 8mm ball cutter on my dremel to sink a hole about half the diametere (4-5mm). 2 to 3 keys is usually enough, just leave enough room to cut the sprue and air exit holes at the end.

Take a small brush/cloth/foam brush and smooth vaseline over all the top surfaces (don't do the sides of the parting board) including the keys. Double check everything is covered well and remove any lumps. (This pic was taken after I'd poured the first half).



Next, form a box around the board using foam, balsa, MDF etc. Insert wire hangers (large nails work well).



Fill in any gaps with plasticene/clay/hot melt glue and double check that everything is sealed. Very small gaps can be filled with vaseline. Hold the box up to the light/sun and see if there are any gaps.

Measure out your silicon as per instructions (a good way to get an accurate volume is to fill the mold box with rice before brushing on the vaseline).



Add together and mix well. I find it very helpful to stand the mix in some hot water for 5min or so, mixing well as it will decrease the viscosity and speed up the cure time. Leave for another 5min in the hot water for the bubbles to rise before pouring - DON'T MIX AGAIN BEFORE POURING.



Pour into the mold box, starting in one corner and letting the silicon flow over the plug. Too much moving around can trap bubbles on the face of the mold, but I've not had that happen yet.



Here you can see the bubbles rising to the top.

Place in a warm place and allow to cure as per instructions. Once cured, remove box and wire hangers if necessary BUT NOT THE PLUG/LURE. Clean up any plasticene/clay etc. and the wire hangers. Trim any scrap silicon hanging off. Re-vaseline everything and replace wire hangers.



Assemble mold box again (or a new one) filling in any gaps with plasticene/hot melt glue/etc as before. Measure and mix silicon and pour second half.



Place in warm place and allow to cure overnight.

Tomorrow we part the mold and cut the sprue and air exit holes (these can be created by adding some tubing to the mold/parting board prior to pouring silicon too).

Steve

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jeff Quote  Post ReplyReply Direct Link To This Post Posted: 27 Oct 2011 at 4:08pm
This is an awesome tutorial so far , I look forward to the rest  Big smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Screamingreel Quote  Post ReplyReply Direct Link To This Post Posted: 27 Oct 2011 at 6:16pm
Very Interesting, looking forward to part 2
regards John
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Post Options Post Options   Thanks (0) Thanks(0)   Quote flyonline Quote  Post ReplyReply Direct Link To This Post Posted: 28 Oct 2011 at 10:00am
Thanks guys!

I knew I forgot something in the rush to get it done before heading off fishing Cry The keys to keep everything aligned correctly - however the wire hangers should be good enough if I take some care.

The keys should be pressed into flat surface (using plasticene etc.) or routed/drilled out if using a hard surface (MDF, car bog etc.). They only need to be about the size of a marble, the second half fills the space and provides the indexing to keep it all aligned.
They will end up looking like this:



Anyway, I cut the sprue and air exit holes using a sharp scalpel, they need to be big enough to pour the mixed PU in, but as small as possible to keep the scrap down when trimming after it's hardened. You can also use a small hole and a syringe to suck up the PU and inject it into a smaller sprue hole.



There, done! Tongue

I've included the full bib here because I want to try a) gluing in the persex bib directly with the PU b) different shapes c) bib towed lures (in which case they will be glued in place with an extended through wire exiting through the bib). Bibs glued in after pouring are done by placing a vaseline'd bib into the slot which will be removed before painting etc. The bib will be aligned, trial run and then glued in permanently.

The first one poured this morning using the same method here: http://www.lurelovers.com/forum/2-part-urethane-lures-the-easy-way_topic6165.html (a few air bubbles formed along the backbone due to a too small a sprue hole and some fiddling to get the PU in before it cured).





I should be going past Barnes in a few weekends to get some foaming PU so I can make some floaters.

Hope this helps someone.

Steve
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Screamingreel Quote  Post ReplyReply Direct Link To This Post Posted: 28 Oct 2011 at 10:15am
Thanks Steve  a fair bit to process, need several visits to take in all the detail.
I believe there was a book and diagrams on the market at boat shows about 5/10 years ago  I bought a kit for around $20-00 but never used it. This seems to be a lot clearer.
Thanks for posting.
 
regards John
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Post Options Post Options   Thanks (0) Thanks(0)   Quote flyonline Quote  Post ReplyReply Direct Link To This Post Posted: 28 Oct 2011 at 11:02am
John

http://makebetterfishinglures.com/plasticlures.html/ has a book on making plastic lures, was this the one? (He's also has a wood shaped one).

It's really not that complicated, I keep reading people saying how difficult it is and have trouble not laughing. If it seems at all difficult or complicated, it's my poor writing, photos or lack of detail. I cut my teeth building composite glass molds for RC planes and this is waaaaaay easier than that - no hand layups, wetting out fabrics, just mix 'n' pour with a bit of preperation beforehand.

There are some good videos out there both on Youtube as well as Larry Dahlbergs http://makelure.com/HowTos.cfm

Sing out if you want any more info/pics and I'll help all I can.

Steve


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Screamingreel Quote  Post ReplyReply Direct Link To This Post Posted: 28 Oct 2011 at 12:54pm
Steve.
Thanks really appreciate your input, as do quite few of the other members.
will have a look at those sites you have posted later today
regards John
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