Resin Casting

[Angus Shops]

I have no problem filling bubbles  with the same resin as used for the casting (Smooth Cast 300). I open the bubble from the inside of the casting, or back side of a flat casting, so that the 'finished' side of the casting is not punctured and blob in a drop of fresh resin. The finished side needs to be intact so that the fresh resin does not run onto the finished surface of the model. That said, if the finished side does not have any detail that will be lost, a drop of resin can be applied to the finished side and then filed/sanded smooth. The former bubbles are usually filled perfectly and are secure in that the 'filling' does not fall out. Too many bubbles in a casting makes the process a bit time consuming, but then you can toss that casting a try again.

As for those problem boilers, it may be possible to try this: Mix up a little resin and let it begin to set to the point that it gets a little thicker before working it into the opening. With Smooth Cast it seems to be useable up until the point that it starts to turn white and will be pretty pasty and thick just before turning white. You will need to work fast, and probably repeat the process until the problem is properly fixed. You will need to be able to file and sand it so this probably won't work if there is a lot of detail in the area that needs repair. You can also use a sharp hobby knife the carve off some of the excess material fairly easily before it fully cures to lessen the sanding chore. You might also try using some tape on the inside of the casting as backing for the resin patch, but the tape will probably become a permanent part of the casting.

I find resin casting a very versatile material. I don't use any equipment such as vacuum chambers and the like so bubbles are a fact of life, but they are easily fixed, most of the time. It is important that the resin is 'fresh'; once opened I like to use it up pretty quickly so I usually try to schedule a batch of 'small finicky' items with some that require larger amounts of resin so that I don't waste it unnecessarily.

Geoff

[pnolan48]

Is this method theoretical or actually practiced?

[glakedylan]

Pete
thank you for the ideas and hopefully soon to be found solutions.
the epoxy is not something I had thought about.
sounds promising!
thanks, again...

[superturbine]

NOT trying to hijack this thread
BUT I have a question...
I received two resin cast boiler shells for a steam locomotive
each shell has gaps in it as if the casting was not done well
--do not want to name names, so let's just keep it to the information provided above--
NOW the question:
is there a remedy other than a load of putty?
can the current shells serve as a master for casting new ones that would be complete and not with gaping holes?
I put some good money into this purchase and would like to see something useable come out of it.
OK...back to OP purpose of this thread with hopefully a few answers to my questions thrown in.
sincerely--

Gary,
Show a picture of the hole.  They should be easy to fill.   Keep in mind casting are cast with as much detail as possible with the understanding a few parts will have flaws and need to be replace or filled.   Also, I've always liked gap filling superglue with accelerate to fill any interior flaws.

[Doc Frankenfield]

essential for a good quality casting is:  pressure pot for the resin stage, vacuum chamber for the RTV (silicone phase)
 proper prep and cleaning  / polishing of the master
  while smooth on is great for consumer use, there are far better, more durable materials that cost far more.

 drop me a line I can probably help you out with doing the work for you.
 here and also can be reached at raven@ravendesigngroup.com

[peteski]

  while smooth on is great for consumer use, there are far better, more durable materials that cost far more.

Really?
Their website (including video demonstrations) and the quantities they sell seems to indicate that they are geared towards professionals (with consumers being secondary).  resin casting in general is a low-volume manufacturing (as compared to other manufacturing methods like injection-molded polystyrene).

But you are also correct that there are many other urethane resin manufacturers.

[sirenwerks]

I find it very hard to mold and cast any complex surface less than 0.030" (0.75mm) thick. I suggest you use a longer setting casting formulation (eg. SmoothCast 310 rather than 300) to allow air to escape and resin to flow, along with lots of vibration.

What do you use for vibration? (Restrain yourself, peanut gallery)

[randgust]

I have a couple friends that use the smooth-on route and really have to design for shrinkage.   I've used CR600 and it's never been much of an issue, even checking dimensions with a micrometer.   But I've certainly seen it on other materials.

For the bubbles issue.... At least for CR600 that stuff is incredibly sensitive to both temperature and humidity.   The process is exothermic and that reaction tends to form bubbles in the process that moisture of any kind exacerbates.  I simply cannot cast above 75 degrees and 65 percent humidity and I closely monitor it.  A dehumidifier raises the temperature as it dries the air so it's a tradeoff; I simply don't mold when it goes over that threshold.   I've tried various methods and finally it was just easier to put the stuff away rather than waste more time and material and frustration trying to fight something that simply doesn't want to work.   I can make glassy-smooth and perfect parts, or I can make parts that look like swiss cheese internally, all based on that environment issue.

I also highly recommend designing molds for being injected with big plastic syringes like from West Marine rather than actually pouring.   If you are bottom-injecting and driving the air up and out your interior bubble problem pretty much goes away, and you can put enough pressure on it to completely blow the mold apart if you are not designing for it.    Working with syringes it's not unusual for me to make 30-50 parts in one casting session, molds all lined up and ready to go, as fast as I can inject them.   Whole different approach.

I've successfully filled holes with both the resin itself, and also with the white Tamiya putty - whatever is in that stuff is semi-solvent to resin, it really grabs onto it and doesn't let go.   It's also grainy enough that it will absorb ACC and that make a really, really solid structural repair when necessary.

[eric220]

What do you use for vibration? (Restrain yourself, peanut gallery)

Well, there is this tool that's been used to settle ballast... Oh never mind. 

[pnolan48]

What do you use for vibration? (Restrain yourself, peanut gallery)

Depends on setting time. For fast setting resins, I sometimes use an orbital sander, and sometimes I do a drum and dance solo with my hands and feet) on the table and floor. I have one bouncy floor, and the solo is good exercise for a few minutes.

With the long setting materials, I try to make sure the mold is manipulated a bit (perhaps flipped over a few times) and vibrated a little before just leaving it be. If the trapped air doesn't get out in the first 30 seconds or so, it will likely never escape.

I do use Randy's technique of injecting from the bottom. My real bear is cowl ventilators. I inject the ten-piece mold from the top one at a time. Each piece has a bottom vent, and I just keep refilling until the resin starts to set. Makes a mess and wastes a little resin, but the cowls are very thin, and this way they do get filled. These cowls, come to think of it, are much less than .030".

[Angus Shops]

I use Smooth On products because they work well for me and they are available in a retail situation close to me. A joyful coincidence. I buy their resins in the smallest quantities available, 0.9 lb. for each of the two parts of the resin. I understand that the resin absorbs water from the atmosphere and that this is what make old resin unusable. Older resin not only bubbles but it actually foams! When I'm making castings I decant smaller quantities (.2 lb. maybe?) into plastic squeeze bottles both for ease of use and to maintain the quality of the material. The Smooth Cast 300 (short  curing time) works well for me but on the one occasion I tried 310 (longer curing time) I found it quite frustrating. It seemed to 'foam' more readily and produced inconsistent castings. I was left wondering if the 300 cures so fast that 'foam' does not have enough time to form...
Shrinkage of moulds is a concern but I've found that over time some of my moulds actually expand! I know, I know, it's not supposed to be possible but I've checked and some of my older moulds are noticeably longer than my masters. For an 85 foot passenger car side, this can be up to a scale foot! I've learned to keep an eye on this and make new mould from my original master when required. Moulds also loose detail over time and need to be replaced fairly regularly so moulds are replaced regularly.
I make one master and use it for all subsequent moulds, so in that sense all my moulds are 'first generation'. I've done one piece cars bodies (with two part or male/female moulds) (with separate floors), but only for lightweight passenger cars with relatively little surface detail. I also use two part moulds for roofs for hip roofed structures, but I shy away from two part moulds in general because the 'squeeze out' wastes a lot of resin.
For more detail intensive cars such as heavyweight passenger cars I use 'flat castings' (one piece moulds) for each of the two car sides, the car ends, roof, and floor, as well as details for the underbody and internal body braces. Car sides for example are 0.020 thick and are assembled from white styrene glued to a 0.060 or thicker black styrene base (for contrast). I build up the cars side on the black base so the car side perfectly flat. When complete I form a box around the side and make a mould.
When casting I fill the mould with resin, over filling just bit and use a toothpick or similar to ensure the resin has entered details such as cast on grab irons, handrails, the up and lower drip and belt rails, and etc. I place a second flat piece of mould material (usually the flat side of a decommissioned former mould) over the resin filled mould and apply even pressure. If dome right you should end with a bubble free casting that has a perfectly flat backside, that is consistently thick at 0.020, and has flash in the windows that can easily be removed with hobby knife. After about 30 minutes I peel of the top mould piece and check for quality. If bubble or 3 or 4 is present I use a hobby knife to open the bubble from the exposed backside of the casting BEFORE THE CASTING IS REMOVED FROM THE MOULD, and fill the bubbles with a bit more resin.
Sorry for the long winded post, but I hope others can use my low tech techniques and expand their modelling skills. Casting has allowed me to model the CPR to levels of accuracy that would not be possible otherwise, with accurate models of unique CPR and Canadian models that will never be available otherwise, including freight cars, heavyweight and lightweight passenger equipment, and structures.

Cheers, Geoff

[Doc Frankenfield]

this conversation came back up in my feed.
 smooth on does make great produts for hobbyists. I just don't care much for the company
 I use alumilite, they offer a quart kit in a tan or a slower setting material in white
 The solution to bubbles is not vibration, that can cause even more agitation. the best solution is pressure, pacing the mold filled with resin in a pressure pot and increasing the pressure inside to about 60 psi will develop a very nice mold. for this you can use an old pressure cooker and a small air tank, set it up with a valve put the mold in clamp it down and hit the valve releasing the pressure into the pot.
 if you are using pressure it is very important that you have molds that are decoid of air bubbles, to insure that your molds shoud be cured under a vacuum, for simplicity you can place your silicone i a shop vac, close off the intake and let it run for about fifteen minutes. then simply turn it off. that will remove most of the trapped air.
if you are doing short runs, and are having bubble issues, it may simply be static. you can solve this by dusting the mold with talc and blowing it out. that stops the static and allows the resin to flow more easily.

[peteski]

Yeah, if one has the equipment, pressure casting is a way to go for bubble-free casting.  I also agree that both Alumilite and Smooth-On provide quality resins.  Personally, I had some bad experiences with Alumilite.    Overall, Smooth-On to me seems to have a wider selection of various casting/molding products, but Alumilite is not far behind.

I didn't realize that a ShopVac would provide enough vacuum to degas the RTV.  I also was under the impression that talcum in the mold was used to prevent bubbles by reducing the surface tension in the liquid resin. I never heard of static electricity causing bubbles, or talcum dissipating static.

I learn something new every day....

[Spades]

This reply was in response to glakedylan hole in the shell post,

I built two SP C series 2-8-0 for a friend.  Had to remove a dome and fill the hole in the shell.  On the first one put clear packing tape on the inside of the shell to match the radius (miniscule)  Placed 3M fine line tape around the edges of the opening on top of the shell.  Filled with JB Weld until flushed then scraped away the excess.  Removed the top tape let dry. Once dried taped the top hole edges again and applied 3M Red Acrylic Body filler. Once dried, I sanded and remove tape.  Ran out of JB Weld, for the second shell tacked a small thin sheet of styrene over the top of the shell opening.  Used the shell as a mask, shot some paint on the inside of the shell, I now had a pattern for the plug.  Cut the plug, glued in place.  Placed tape on the inside again to hold the plug.  Masked the edges of the opening  on the shell to cover excess. Filled with 3M Red scraped excess let dry and sand. 

I think I could have just gotten away with just taping the hole on top of the shell with the clear packing tape and filling from with the shell JB Weld or even just the 3M putty. That will be for the next conversion.

G

[pnolan48]

Vacuum will degas RTV, but to me the main benefit is the removal of moisture. Yes, degassed RTV looks better--clearer and deeper in color--but I'm not sure about the process removing visible air bubbles. My biggest problem has been bubbles in the casting resin. Here, pressure will certainly reduce them. And degassing the B part of the resin will at least prolong the shelf life--I use a 1:1 formulation. But the majority of the improvement comes from letting the A and B parts settle for a while before mixing, and using longer setting resins that give trapped gas a longer time to escape.