Large Scale Central

Wax casting

Modeling Technical Modeling
#1

As many of you know, a couple years ago, I started making couplers in brass. The process was to design it in Sketchup, have one (a pattern) made by Shapeways in brass, and send that to a foundry. There, a rubber mold was made, which was used to generate wax copies for the investment casting process.

Recently, I started clandestinely researching wax casting on my own. I figured with the experience I had in RTV mold making & resin casting, I should be able to make the jump to wax casting.

I experimented by melting jewelry wax in a crock pot, and injecting it with a syringe. It almost worked, but I couldn’t get consistent results. The more I experimented, the more I saw that heat & pressure had to be finely controlled. That led me to buying a wax injection machine:

I decided to try it out on a coupler/draft gear pattern that was more prototypical, so I designed these parts & had them printed in FUD (Frosted Ultra Detail):

I found the wax injection process to be a new learning curve. Unlike resin, wax has to be injected under pressure. The tin-cure RTV (MoldMax series) I normally use, was too soft. Platinum-cure RTV offered a wider range of durometer ratings, and after some experimentation, I settled on Smooth-sil 950.

Learning to cut the molds was also a new experience to me. In wax casting, all molds are glove molds. That means the pattern is mounted on a sprue, and completely enveloped in RTV. After the RTV sets, the pattern is carefully cut out, making a two part mold (sometimes more than two parts). I quickly realized this was an art. I watched video after video on YouTube on cutting rubber jewelry molds, and butchered a couple of my own, before I got the hang of it.

Anyway, here are the wax patterns I was able to make:

These will go off to the foundry next week, and I’ll find out in a couple weeks if he can cast them or not.

In case you’re wondering why I’m doing this, this puts the burden of inventory control on me. Slight revisions are easier for me to keep up with than the foundry. There is also an up-front cost to having them make the rubber molds (starts at $50 for the smallest molds), so I’m hoping this will let me lower my end costs.

#2

Very Nice Burl. I have always admired your work, but alas I am in 1:20.3 so your products have had no direct interest to me. what are you using for shrink factors to get the proper size in the finish casting?

Al P.

#3

Thank you.

I make the parts 103% over-sized. I may adjust that after the first run, since that was the foundry’s spec with him doing the waxes.

#4

Vulcanized rubber molds take hot wax a lot better.

Your sprues should always attach to the thickest part, as it solidifies metal still feeds from the sprue to the part because the metal shrinks as it cools. If there is a thin section in between, it will solidify first and starve the object creating porosity. Multiple sprues can feed several areas of an object, the sprue should be thicker than the part also.

You can get away with more shooting the wax, than casting the metal. I’ve cut over a 200 molds on the job…

Your parts look clean and should cast nicely, from what I can see.

John

#5
Actually, one of my goals was to avoid vulcanized rubber.  The foundry wanted patterns in brass, regardless of whether he made a vulcanized mold, or RTV.  Worked great for him, but I can get it printed in FUD for 1/6 of what brass costs me.  It gets delivered a week earlier too.
 
People keep telling me that FUD will burn out like wax in investment casting, but I don’t think any of those people have tried drawing a coupler in Sketchup.  All the compound curves are a nightmare.  Printing it in FUD also allowed me to finish some of the difficult curves by hand with putty & sandpaper.
 
The first mold I made was for coupler trip pins (not pictured here).  I figured that would be the simplest mold I’d make, so start learning in baby steps.  The yoke mold took me a while to figure out how to cut, until I realized I should treat it like an elongated ring with a hole where the stone setting would be.  I also tried not to stray from the foundry’s sprue placement (observed from previous projects). 
#6

Very interesting. I love your stuff, but like Al not my scale. If you ever get the bug to try anything in Fn3 I’ll be watching !

#7

Keep it going Burl, keep advancing our hobby!

The pictures shown is that the EOC couplers from your PS 5077 cu ft boxcar?

#8

No, this will be a standard draft gear arrangement.

I already paid for molds to be made for the EOC couplers. This is what they look like:

#9

The ultimate in body mounts. Great looking work Burl. I’ve been following just not commenting. (http://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-cool.gif)

#10

Very nice work! Where are you having your molds and castings done?

#11

Wow, that draft gear box is sharp!

#12

Got my stuff back from the foundry today. Of course, I had to stop everything I was doing to put one together:

(http://www.burlrice.com/_LS_PS5077/brass1.jpg)

(http://www.burlrice.com/_LS_PS5077/brass2.jpg)

(http://www.burlrice.com/_LS_PS5077/brass3.jpg)

(http://www.burlrice.com/_LS_PS5077/brass4.jpg)

#13

Awesome looking … how is the fit , does it work as well as you hoped ?

#14

I will have to put everything together before I can say for sure. As you can see, there is no knuckle in the coupler. I’m also still considering how to do the draft gear itself (the part that goes inside the yoke, and absorbs shock). I’m leaning towards a plastic lug with springs on each end. But so far, all the tolerances look good.

#15

Wow looking good. Any chance we could use a Kadee #1 coupler with that draft gear box? Get away from resin casting from your Shapeways design.

#16

Hadn’t considered it until you asked. I was trying to get away from the Kadee design by doing this.

#17

Very interesting process Burl! How have you done the knuckles and locking pin? Can you do them to scale ie prototype practice? I have been thinking about 3/4 size couplers In 7/8th scale, works out to about the same as standard couplers in 1-20 scale. The knuckle on some of the MAINE two footers were slotted and drilled to catch a link and pin

#18

The knuckle is sprung, and the trip pin falls in (by gravity) when the knuckle is pushed into the closed position.

The knuckle length is exaggerated by a couple scale inches to allow enough play for it to couple with a Kadee coupler.

#19

Was just curious. Have so many Kadee #1 couplers waiting to be installed. But I like the idea of your brass couplers, more prototypical and stronger.

#20

It would take some carving to make a Kadee fit in this. The striker face is one piece, and is soldered to the center sill.