Looking for help on casting small parts

[twilight]

I've been trying to replicate some hopper door latches and they're not turning out so well.

Here is what I'm trying to replicate.



Here's how they've come out. I've painted them so I could see the detail or other imperfections.


If you look closely, you'll see air bubbles. Either molded on or small cavities. Some maybe usable but many are not.

I've been using this to make the mold.


And using this to make the casting.


Any thoughts or suggestions would be appreciated.

Thx,

Mark

[e-paw]

I would go with a two part RTV ( Like they sell at Micro Mark ) to make your molds from. I have used it several times with very good results. As for your casting resin I can't say as I have no experience with that brand. Generally you want a thinner, as far as the viscosity goes, casting resin. A thinner resin will get into more of the fine details. You Tube has tons of videos on casting parts for pointers on technique.

Hope this helps.

[twilight]

I used the Micromark RTV before, but I use it so infrequently that usually on part of it goes bad. Do you use anything to help ensure shelf life? That's why I tried this Composi-mold stuff. You can use what ever you need and then just melt it down and store it. Wasn't that pricey either like the Micromark stuff. Not really sure I'm pleased with it though. Ill probably end up going back with the RTV stuff.

Mark

[doctorwayne]

I've no experience whatsoever on casting small detail parts, but Steve's suggestion of a casting resin which flows more readily sounds reasonable to me.
However, were I casting such small parts, I'd simply mix a small batch of JB QwikWeld, then with an old #11 blade in my X-Acto, dip its tip onto the epoxy and poke that little bit into the cavity in the mould. Quickly do so with all of the mould cavities, then, since it flows quite readily, repeat the process, starting with the one first-done, which should fill or over-fill the cavities. Next, use a #18 chisel-type blade, and drag it over each little mound of not-yet-cured epoxy, and, assuming there's a cover for the mould, quickly put it in place.
This epoxy sets in 5 or 6 minutes and supposedly cures in 40 minutes.

I think that the act of placing the material into the mould cavities, rather than simply pouring it in, is less likely to create air inclusions (bubbles in the finished castings) and the contact of the blade's tip against the mould should help to break the surface tension of the epoxy, allowing it to flow more freely.

Naturally, I can't say for sure that this will work, but it certainly wouldn't be too expensive to try, as you don't need to create another mould.

I have found that the contact factor (using a bit of the casting material on a tool to touch the mould's interior) helped when making large castings using plaster. Also, for large castings, vibrating the just-filled mould often helps to remove included air, though I doubt it would have any effect on such small parts.

Wayne

[dave1905]

When you make the mold, use a toothpick or wire to get the mold compound in all the crevices around the master. A small brush can be used too to work the mold material all in around the master.

Study the castings you made and see where the holes are. Then when you pour the resin in, pour it from a few inches above the mold in a thin stream, that breaks bubbles in the resin coming out of the mixing cup. Then take a toothpick or wire and work the resin into the mold, making sure the areas that commonly had bubbles are covered (you will have to work quickly most resins only have 1-3 minutes from mix to cure.)

[doctorwayne]

Hey, Dave, good to hear from you, again!

The suggestion to use a toothpick may be better than mine of using the tip of an X-Acto #11 blade, especially if the mould could be damaged by the blade.

Wayne

[railbuilderdhd]

I've heard about this "Blue stuff" for making a master and then use the JB Quickweld. You can get the Blue mold stuff here http://www.ebay.com/itm/like/28114164133...noapp=true.

I got some but not tried it yet. I have worked with RTV and found you can get good results with that and some resin castings. That stuff cost more then the blue stuff so if this is all you want to make then try the blue stuff. Will not set you back like the RTV will.

I'll see if I can find the post I read about the stuff before.

Dave

[twilight]

Thanks guys for the suggestions . I did purchase a RTV mold making kit from Allumilite and I'll give that a whirl when it arrives at the house. I'll post the results when I get a chance.

Mark

[Green_Elite_Cab]

Thanks guys for the suggestions . I did purchase a RTV mold making kit from Allumilite and I'll give that a whirl when it arrives at the house. I'll post the results when I get a chance.

Mark

Allumilite is what I use (I've also tried the Micromark stuff), and I probably would have recommended it if I had known you were working on these things. I'll probably be making additional molds of small parts for my MU car kits soon.

It looks like you are making a one-piece casting. In that case, I'd temporarily glue the masters to a sheet or something and pour the mold material over top of it. This way, the bubbles will generally rise up and away from the part.

I think heating the mold also helps with casting the part, as the heat will let the resin "run" into the details better. Their are probably more specific instructions for whichever version of Alumilite you picked up.

There is also a technique to mixing and pouring to help reduce bubbles. I don't know that you could totally remove the air bubbles, but for parts like this you should be able to get consistent results.

Chris

[dave1905]

Alumilite is good, the regular RTV is fairly stiff. the HS2 or HS3 RTV is a lot softer and good for castings with undercuts.

Micromark products are repackaged Smooth-On resins and RTV. If you like them and want other options, check out Smooth-On.

I have also used Hobby Silicones products, both resins and RTV, and would also recommend them.

The key things for the resins are set and cure time. I do fairly complex castings to make carbody shells, so I like a resin with a 3 min + working time.

The key thing for RTV is how stiff it is, the "Shore" number. I prefer softer RTV's for complex castings, flat molds can often use a stiffer RTV. Softer RTV's need a mold box to make sure they retain their shape.

[twilight]

dave1905, thanks for the advice. I did order the Allumilite HS3. Should be to my house Monday or Tuesday. I checked out that Smooth on stuff you suggested and I like that you can purchase sample sizes. Anytime I've purchased resin or mold making material it has gone bad because I don't make molds that often. I really get bugged about paying Micromark's shipping cost too. Did they have permission to copy Smooth on's product?

Mark

[dave1905]

Yes, Smooth-on sells them the stuff for them to rebrand. HS RTV is actually DOW RTV.

Its a fairly common situation. A friend of mine works in a pet food plant. The difference between some brands is the label, the actual food is exactly the same. Mercuries were rebranded Fords, a Lexus is an upscale Toyota, Fords and Mazada share some components and design.

[twilight]

Ok, here's an update on how I progressed with making a mold and casting parts.

Here is what I ended up purchasing and like dave1905 it is very flexible.


Here are the hopper latches in the mold box. Yes they are LEGO's, I read about using them years ago and have used them since. But my daughter gets mad at me when I use them(she thinks they're hers, but they are from my private stash).


And set to cure for 18 hours.


But, what happened is two of the masters floated when I poured the mold material in(unknown to me ), making two of the molds unusable. But making the most of a bad situation I was able to use the good molds to make 2 castings. And I added them to the original 4. I need 6 latches for each hopper I'm working on. I have four hoppers total. Another 18 hours of cure time, here's where I got to. The grayer looking castings are the castings I made.



When I laid the masters out again I CA'd them to a piece of stage glass from an old overhead projector instead of using double sided tape like I did on the first mold.

Here's how they came out right off the mold. When I poured the casting resin on the mold I used a popsicle stick to push resin into the mold, breaking surface tension and releasing air bubbles. Also mentioned in the directions of the mold material is to brush the mold surface with talc or baby powder to release surface tension which I did. I also used mold release. Also, when the castings came off the mold they were a little soft. So, I stuck them in the toaster oven at 200 deg for like 3 minutes. Which really harden them up.


And finally with a coat of primer.


Thanks everyone for your suggestions. I'm very happy how these turned out. I've made 24 of them already, enough to do 4 hoppers.

[e-paw]

Glad they worked out for you....Try ,try, try. Never give up till you get a result that you like. Then sit back , have a beer and enjoy what you accomplished.

[Mr Fixit]

I remember from reading MR particularly when using resin for water features that the trick is to pour the resin down a stick of some description so that you can direct the flow but not introduce air bubbles.
Another trick particularly for plaster is to attach the formwork to your electric sander and let the vibrations float the air bubbles up and out of the mix. This may also work for resin.
Hope these tricks help
I had to break out some polished concrete paving last week [oh my aching back] and it was interesting to see trapped air bubbles in the concrete where the vibrator had not floated them out. So bubbles can be a problem in anything you pour into a mold for casting.
Mark