casting on30 boxcars

Dave

Thank you for your informative tutorials. I have a couple of questions if you don't mind answering them. I was planning on buying my materials from Hobby Lobby since I usually can get a 40% off coupon but if I understood you correctly, their rubber is to stiff to make a good 3D mold and Micro Marks seemed to have a pretty short pot life. Who do I buy good soft rubber from that has at least a 30 minute pot life? The last question I have is that I read someplace that I might have problems with a 3D casting as large as an on30 32 foot boxcar warping, Is that true and is Alumilite a good resin to use for this? I really didn't want to do flats and have to deal with the corner joints just as you eluded to in you tutorial but will if i have to.

Thanks Sam.. 

RTV

Alumilite (and Hobby Lobby) sell the Dow High Strength RTV (HS2 or HS3) in addition to the Alumilite regular (Quick Set) RTV.  The Dow RTV comes in a white "jar" about 4" in diameter and about 4" high.

If you want to do an On30 boxcar you will probably need two molds, one for the shell and a flat mold for the underframe.  An On30 boxcar will only be a bit larger than the H O boxcars I made.

The key is keeping the undercuts small and having a good mold frame that supports the mold so it doesn't warp, and allows the "male" part of the mold to register with the "female" part of the mold.  You can make a frame out of plywood or heavy plastic.  Make sure the frame is built so it comes apart and the mold can be removed from it, you will need to do that to get the casting out of the mold.

Thank You

Dave

I appreciate the answers. Now to start working on my masters and mold frames.

Thanks again, Sam

Solid vs. hollow

My assumptions were that you didn't want a solid model.

If you did make it a solid boxcar, a block of resin, you could do that, breaking the halves of the mold along the center of the roof.  I would have separate roof walks, and hide the vents along the roof center line and make a filling sprue on each end that an under the model and came up under the car near the bolster.

While simpler to make (hardest part is getting good mold break lines for the halves) the down side is it would be expensive.  A 28 ft boxcar would be about 2" wide, 2" high and 7" long, which is 28 cu in or about 15 fl oz.  The resin is about $30 for 32 fl oz, so it would take about $15 in resin to make one car.  If you get a Hobby Lobby 40% off coupon that drops that to about $9 a car. If you can live with the expense that might be simpler.

Also check the quantity of RTV in the package and calculate the volume of the RTV needed to fill the mold.  For a larger car, you might need two packages of RTV to make one mold.

Many thanks Dave! Haven't

Many thanks Dave!

Haven't checked it out in all detail yet but it looks like an excellent clinic!!! Much appreciated info.

Yannis

@Dave

Haven't watched your videos yet, but will when time permits.

I've been casting resin ties for TT scale tracks and other assorted items. I've used Alumilite in the beginning from Micro Mark. And as you say it has a short pot time. So I started searching for something better. I came across Smooth-On and have been using that with great success. What is your opinion on Smooth-On other than it's more expensive than Alumilite?

Bernd

Edited: Dave I just watched the videos. Question was answered.

Supplies

I have used Alumilte Quick Set, High Strength 1 and High Strength 2, Smooth On OOMOO-30  and Hobby Silicones RTV.  I have used Smooth On, Alumilite White and RC3, Hobby Silicones and Bragdon resins.

I liked all the RTV's except for Alumilite Quick Set, too rigid.  The OOMOO and HS RTV's were the most flexible.  I spoke with Alumilite and they told me the Quick Set was actually intended as an "encapsulating" rubber, used to coat electronics in rubber to insulate and protect it.  Might be another use on a model railroad.

I actually had issues with the Smooth On resin, I ordered some from Smooth On and must have gotten a bad batch.  The silicone oil separated and would ooze out of the casting, it would "bleed" oil.  Then the casting would warp and curl.  I called Smooth On and their only suggestion was "shake it up to mix it well."  That didn't work at all.  I have never gone back to Smooth On for resin, never had to.

I like the Alumilte RC3 and the Hobby Silicones resins.  I've had really good success with those.

Bragdon is a nice resin, but Its not really designed for what I am doing.  It is used for making rock castings and has to be flexible for a while and a bit remoldable. Makes wonderful rockwork, a local modeler used it and had spectacular scenery. Not as good for casting cars in my opinion.

Dave out of curiosity, Since

Dave out of curiosity,

Since you mention if i am not mistaken accuracy of scales at some point. I was wondering of the effects on the RTV or the resin if the proportions are a bit off. In other words, instead of getting 50-50 you get 45-55 etc... small deviations. Any ideas?

Mix

Haven't tried varying the mix, I'm sure that a couple percent either way isn't a big deal.  Its just that with really small batches, being off a couple grams can be a big swing.  I'm sure at some point it will affect the cure or cure time.  Probably it would also depend on whether you had more part A or part B.

If I am using 5 grams of each part and I go 6 grams of one and 4 grams of the other, it goes from a 50-50 mix to a 60-40 mix, a 20% difference in quantities.  With large quantities, the difference is smaller and not as critical.  If I am mixing 50 grams of each and I am off one gram, 51-49, that's probably not going to be noticeable.

You could probably eyeball the quantities and be pretty close, but for less than $20 bucks more you can get a better scale and be sure.

Thanks Dave! Using a digital

Thanks Dave!

Using a digital scale here by the way & measuring cups. I wish at some point to find a 50-50 mix RTV instead of the 5% one i am using.

Great workshop!

Really enjoyed your videos on mold making and casting. It gave me a lot of ideas that I will try when I do some casting next time.