hybrid RTV/Silicone mold
35 replies to this topic
Posted 24 October 2007 - 07:16 PM
ok todays update. interestingly i experimented with very large blocks of silicone to very small. it seems there might be some ideal(opitmal) cut size. interestingly in the areas that i finely choped and mixed high ratio of cut(well cured) silicone in with rtv those areas are not curing at the same rate. infact they are still liquidish(24 hrs latter). i would mention that i slightly overdosed the catalyst to make extra sure that no soft uncured spots exist. also i mixed very well. i re-skim coated the lure and those areas cured compleatly, so im somewhat mystified what is causing the areas with the cut cubes to cure so much slower! i am also now running an experiment with bonding the rtv to a block of silicone, and the reverse applying liquid silicone rubber to cured block RTv. if the liquid silicone shows good adherance to the rtv then next week i will attempt to cure a block of caulk over a skin coated RTV.
Posted 24 October 2007 - 09:35 PM
The problem is to create a stable, distortion free mould, using as little of the expensive RTV as possible.
My solution is to thick skim the pattern in the lego box, as outlined by Husky. After the RTV has set, top up the mould box with Plaster of Paris. The Plaster of Paris former holds the shape for the casting process. The RTV thick skin can be easily separated from the Plaster of Paris former, for easy de-molding of the casting. The Plaster of Paris former is sealed with 50/50 PVA and water. This will prevent dust contamination and help prevent chipping, although small chips in the former will not affect the function.
A quick, cheap and cheerful solution. Would it work?
Posted 25 October 2007 - 02:01 PM
I'm looking forward to pictures.
Posted 26 October 2007 - 10:46 AM
yes it was! 100% silicone rubber caulk that is. i am now woundering if the silicone ruber caulk does infact contain sulfer in its chemistry??? it seems to be setting up slowly. so that is actualy probably a good thing as the slow cure will make for a longer lasting more durable mold.
Posted 26 October 2007 - 10:52 AM
I am contemplating some version of this. in the past i didnt like the way rtv skins over Plaster of Paris molds cast. it seemed that having a super flexible skin didnt stick inside the mold well, it flaped and sloped around resulting in somewhat distorted results. but i supose like you say, if the skin is layed down thick, that might be enough. especialy for the price!(HA HA, thanks !!!)
Posted 26 October 2007 - 10:56 AM
oh i almost forgot. the update. my small experiment on bonding strength. the liquid silicone caulk(100%) applied over cured RTV did cure and adheare to the RTV. pulling with some pressure was ok, but allot of pressure would separate the two. i think that for very limited number of cast cycles this would work fine. ...the RTV liquid applied to cured Silicone Caulk is still wet and curing(3days now!).
Posted 26 October 2007 - 11:13 AM
I only have a little experience with RTV, but I have quite a bit with epoxy. If some RTV formulations are anything like some epoxy formulations, the two parts MUST be mixed EXACTLY as intended or there will be a cure failure. This could be part of your problem since you said you overdosed on the hardner. Now, you would think that if you OD on hardner (or catalyst) the material will cure faster and maybe harder. Sometimes this is the case, but not always. I would think that those RTV formulations that have to be mixed according to ratio/weight, instead of 1:1 by volume, would be more succeptable to cure failure if not mixed to the appropriate ratio. I'm not sure, but that could be the real culprit to your cure failure. Just a thought.
Posted 26 October 2007 - 01:54 PM
Call it a hunch, but if it hasn't cured yet, it won't.
Can you tell me the ingredients, name and brand, please.
Posted 27 October 2007 - 01:01 AM
Hi!It is very important to follow the mixing ratio for rtv silicon!!!If you dont!It will never cure!and!personely I use all rtv silicon when I make a mold!It cost me a little more but im sure that my mold is perfect!Time is money!and I dont have a lot of time to lose!Cheers and good luck.Dan
Posted 27 October 2007 - 06:05 AM
dow corning 3110. ive used this one in the past before and like it. also the manufacturer has several different mix ratios of catalyst. the different ratios will cure faster or slower, and the end result will be different hardness and streach properties. I think you are right though. the last time i had a mold cure this slow it never did after weeks of waiting. i think i will be forced (like last time) to coat the surface with pure catalyst. and wipe it clean after curing compleately. oh for those interested the last time this happened to me, it was because i didnt mix the heavy setled bottom of the silicone jar well enough, the result when i scraped everything out to pour the mold, was areas that had thick unmixed(with catalyst) sections. adding extra catalyst might alter the cured properties to the point that it is a tad hard but atleast it is cured at that point. now due to my experimentations a mold that took me 14hours to set up, has tons of soft spots due (not caused by lack of catalyst) but to interference with the 100% silicone rubber(which i am pretty sure has sulfer in its chemical structure, the sulfer is interfearing with the polymerization reaction) . well this should get interesting. thanks all again
Posted 05 November 2007 - 07:24 AM
ok updates on the mold. finaly finished it. not completly happy with results. i would deffinitly do it different next time. firstly i recomend large sized chunks of Silicone caulk if you plan on adding it to uncured rtv. as it slightly affects rtv cure rate. rather i would try next curring caulk over the skim RTV coat, as it will adhear almost as well as the rtv over rtv with time. if it is a valuable casting then one pour rtv is the only way
Posted 05 November 2007 - 07:30 AM
also for the record i would like to mention that in spots were the cured caulk chunks were placed close to other chunks or close to the outside wall i had spots that cured slowly or were soft. one spot a caulk chunk was lying against the box wall and all rtv was still soft after over 1 week of curing(well mixed batch of rtv+catalyst).
Posted 05 November 2007 - 07:37 AM
ok, finaly not trying to be anoying. i had problems casting this weekend i used up a bottle of alumilite regular(super plastic) trying to get just one model out of my mold. the stuff set up thick in like 3 seconds. hardly enough time even to compleatly mix the stuff. and not enought time to be able to even pour the stuff. i ended up trying to fill each mold side up with the thick paste like substance and glue two sides together with 2-ton. once cured each side was brittle and would break under almost no pressure. what am i doing wrong! oh and this is a large casting. did also try mixing with smaller batches with no results. and i had the work area a cool dry place.. one person recomended i switch plastic types. im currious what others say as far as ideal urathane and proceddure to cast a large musky bait. thanks
Posted 05 November 2007 - 01:30 PM
ive studied that tutorial several times. it makes perfect sense. what i didnt probably realize is that alumilite regular is not the same as the Rc3. regular has pot(set up time) of 90 sec. it actualy set to the point of not being able to stir at all within 4-10 sec range! im afraid to shovel another $100 in experimenting on another plastic. im tossing it in my head between a 2gal alumilite RC3 and the smooth on feather lite. this time im going to chill the stuff way down to perhaps say 30deg or so. as i need to mix a fair amount and still pour it in through my pour hole.