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Watching the progress of the Coach 64 build and other's projects, I realized that resin casting just might solve one of my scratchbuilding dilemas--the need for a number of archbar trucks.  Yes, I could buy them but what's the fun in that, eh?

The trucks will be loosely based on a Bachmann truck but with some of my own designs changes.  No undercuts in the prototype but there will be a few round parts.  I'm expecting to have to build a two-part mold.

But after reading various posts and watching countless videos on YouTube, I have some questions.  Hopefully someone can point me in the right direction.

1) There are a boat-load of products out there in terms of types of resin as well as mold making materials.  How did YOU decide on what you are using?  What characteristics should I look for in the resin and the mold-making?

2) Seems the best material for what I'm going to be casting, in terms of a mold will be urethane.  How does it hold up to mutiple parts/uses?  If not urethane, then silicone?  If silicone, what hardness?

3) I know that most plastic parts these days are injected.  what resin(s) approach the hardness characteristcs of injected plastic without being too brittle?

4) How do you decide how much resin to mix?  Use water and then measure the volume of water? 

Thanks in advance.

Bill W
 

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Bill,
If you read through all the posts in my coach 64 thread, I list the products you will need. Jerry is right about the brearings. I'm using brass bearings in mine.
A two part mold is a must. In addition, you will want to pour some kind of shell around it. I just use a ready mix product I get from the local hard ware store whose main ingredient is hydracol. It sets fast, and hard.
I'm not sure how and arched pair of trucks will turn out if you just pour them. Those are some mighty thin pieces to be pouring. Those types of parts are best using the injection molding method.
If you do decide to pour, at the minimum you will need a compressor and a vacuum pump. You can pickup a good pump off ebay for about 150, here is a link to the one I recommend RECOMMENDED PUMP
This is a good pump because if you read the specs, it reads that the guy has flow tested to 1.8 cfm and hg 28. Thats all you need to remove air from silicon and to aide in successful casting.
Most resins you buy from hobby sites are 1 to 2 minute gel times. That can be an issue when you are pouring a complicated piece like a truck.
1 - you want to have enough time to pour slowly enough that you don't introduce any air bubbles
2- you want a long enough gel time so that you can vacuum any trapped bubbles (about 20 seconds) and then compress the resin to at least 60 psi.
Most of the RTV (mold making materials) sold at hobby stores or sites is too soft. Even the hardest alumlite I've bought is much softer than the circle kay RTV I use now. You do not want a really soft RTV if you plan on having some orphaned positve spaces on one side of your mold. They will bend just from boyancy when the casting resin is poured in.
Last, don't start your first project, with something as complicated as trucks. Do something without so many negative and positive space (less detail and cut outs or undercuts), and something small (so you don't waste a lot of material during the pouring process).
DON"T try this with latex. Latex does not set evenly, or quickly. If you want it to cure fast, you need to brush it on in layers, and even this could take days. Then after you remove your piece from the latex the mold will shrink considerably during the valcanization process.
Hope that was helpful.
 

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Discussion Starter #4
No worries about the axles and bearings.  I've already got that figured out and plan on using some brass/bronze for a bearing surface.  If this works out, I may well switch to real ball-bearings.

As for the details and small-ness of the mold:  I have been playing with some alumilite and casting small parts of various degree of detail.  Been happy so far with the results.  I know I may need to vaccuum/compress and have both a compressor and vaccumpump--three types of pumps as a matter of fact.  One of my other hobbies is woodworking and I have some done some vaccum veneering (even though I can't spell vacuum, I know what it is :D )

Is this going to be a challenge?  You bet, that is a big part of the fun to me.  Yeah, I could go and buy a bunch of ready-made cars, locos, etc. but that's not what attracts me to this hobby.  I like the creativity needed to build it all myself.  In fact, if I had a metal-lathe, I'd probably be turning my own wheels....maybe next year.

So, keep those suggestions and comments coming.  /DesktopModules/NTForums/themes/mls/emoticons/w00t.gif


Bill W
 

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Perfect timing for this thread, Bill. I just received my shipment from Vagabond yesterday.

I hope you don't feel like I'm intruding on your thread, but I was wondering if the nice folks out there in internet land have a suggestion for an instructional book. There are so many details that I don't know about, I'm not sure what questions to ask. Like, when Richard says to pour a shell of Hydracol, is he talking about pouring a shell to support the outside of the mold?

Thanks,
Matt
 

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Thats correct Matt, the shell supports the mold. Some folks use plaster of paris strips, I use to dip strips of news paper, but now I like to make a second form around each half and pour. Its faster ;)
As far as books, I didn't read any, I googled "making silicon molds" and read about 12 of the results that were step by step. Then found that they didn't apply to what I was doing and made up my own method for figures. For 2 part molds, and use a little of all the techniques I learned with some of my own island flavor added to make the process easy to follow for me.
Start small and work your way up.
 

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Oh,,, make sure that the second half of the shell is casted right on the first half. Some folks seal the plaster and pour on top of it. I do it a little different, I pour a lip of the RTV for the first half of the support to rest on like a gasket. I flip the mold over, and pour the second half.
I'll try to take some images of the trucks when I lay them up, and that might be more helpful than me trying to explain it with words.
 

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Did you view the Videos on the TAP Plastics website?? The videos will answer a lot of your questions, such as How do you measure the volume of a mold cavity? Answer...Use Dry Rice!
I've been selling mold making materials for over 27 years, in fact, I was the one who convinced the powers in charge at TAP Plastics to first carry Silicone RTV. We first brought in Circle K because that is what they used at Industrial Light & Magic and they were big customers of mine. TAP Plastics now carries a high end silicone manufactured by Rhodia.
Click on the banner above the BUILDING forum. Look for the link to videos. There are also sections on the mold making materials, including the urethane casting resin.
As for books, right under the heading for Fiberglass, there is a link for Books & DVDs. Look at the Prop Builder's Molding & Casting book. This is the best book we've ever found dealing with mold making.
Russ Miller
 

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Thanks Russell, but a warning about the rice. It is small, and it gets stuck in a lot of cavities that aren't easy to remove on all molds, especially my figure molds, so I use math.
for a cylindrical mold, I use the following
V= volume
P=3.14625
R=radius
H=height
V=(P*(R*R))*H
If you are using inches to measure height and radius then you take the result and divide it by 1.826, which will give you ounces.
Prior to this, I am assuming that you already measured the volume of your prototype and its setup, and can subtract that from the sum.
For a rectangular form you do basically the same, but the formula is height*width*length=V
It seems like a lot of work, but when you are working with small forms and figures where a grain for rice is a pain in the yoohoo to get out of an arm pit or crotch or a crook in the elbow without tearing the form apart, its worth the trouble.
 

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Thank you from me, too, Russell. I have been looking at TAP's videos, and they are very helpful. I also downloaded simple instructions so my feeble memory doesn't have to blow a breaker trying to remember what was in the video.
I 'll have to take a look at the book.

Best Regards,
Matt
 

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Discussion Starter #12
Second the Tap Plastic videos.  Very nice.  There are quite a few other videos on YouTube about resin casting as well.

Bill W
 
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