some notes on Resin Casting

[JohnP]

I have put together a series of photos of how I got to my present level of casting thin resin bridge parts. There are a bunch of good-size photos plugged in here so I hope it loads right for you.

Some of you might already know I have been trying to create an 1:48 model kit of a D&RGW truss bridge:

I have actually sent a kit to the exceptional On3 modeler Dave Adams and you can see more at www.jpstructures.com.

I started about four years ago with the idea of making the beam pieces as one piece and the lacing another.
The castings were to be very thin, and I wanted no flash and no voids. I also wanted to try to use no pressure or vacuum. Here are the first points:
--Internet research lead me to use a solid box to contain and control the rubber. No floppy jerry-rigged cardboard with rubber bands would work as often demonstrated in other how-to articles.
--My thoughts told me that if I had one way in and one way out for the resin I would push the entrapped air out. I designed the parts in an almost streamlined way and the molds so there were no traps.

The first boxes were of basswood with machine screws and threaded inserts. Like all my future boxes, they are solid strip sides with a sheet attached top and bottom.
--The master parts were constructed as a single piece. I used clay at first for the layup. Dimples aligned the halves; the box was not indexed.
--I started with the Micro-Mark basic casting kit with low-viscosity rubber and resin.

The above photo is of a main beam (three sides) and another mold for the lacing. You can see the cast parts and a finished beam in front.
--This worked well. I had no voids and flash even without a vacuum chamber or pressure pot.

This photo is of a floor cross beam. The master is below and the casting above. These came out pretty well at first.
--Note the odd "sprue" I designed in to the ends. I wanted the resin to flow into and out of the extremes of the beam ends to avoid bubbles.
--Also note all the molds will have a small inlet and outlet hole. I use 1" lengths of 1/8" dia drink mix straws as disposable filler tubes. I inject with a syringe with a tapered nozzle.

I started to have trouble with the molds closing down and the parts changing dimensions, mostly too thin. I felt the wood was changing dimensions even with a sealant coating. To get better control I went to styrene.
--For more than a couple uses, the threaded inserts are needed.
--The design concept remained the same with a few refinements.
--I found the three sided beam castings warped over time due to a greater mass of resin on one side curing at a different rate. Or something like that. So I redesigned the bridge to use four sided (four part) beams. I also chose a slightly different prototype at this time.

The photo above is for a top main chord beam: sides and upper solid plate. The lacing is another mold. I also completely changed my mastering method. This technique I believe is uniquely my idea that I could find.
--I had a laser cutter http://www.kingmill.com/shop/index.php cut plastic "mold plates" for me.
--The plates have accurate holes for alignment pins.
--The plates had a light burn of the part outlines according to the drawings I sent. This method allowed me to build fragile, thin masters on a sturdy backing plate.
--The features on the back side would be perfectly aligned because of the laser-cut alignment pin holes and part outlines.

The photo above is of the portal brace (right) and a single-side mold with gussets, pin nuts and brace brackets.
--You can see on the master plate of the portal brace the dark edges of the part. I also had Chris at KingMill cut master parts for me. Many of the tedious parts were done by machine. Yeah!
--On the left, the single side master only needed a solid cover for the mold box.
--Note that the example parts are as they appear after demolding- very little flash.


Here is a family of mold boxes needed for the bridge, except for the floor beams.

I now made another major change. The styrene was too soft and distorted and cracked. So I went to using metal. Now, I'm just a poor country boy in SW Virginia and had no Bridgeport mill in the garage, so I built up the mold boxes out of aluminum strips.

This photo is of the first two aluminum boxes.
--I used flat head screws to hold the parts together in addition to the box screws and alignment pins. That was a lot of drilling and tapping.
--Those mold plates I did myself as a a trial. I also made them two-sided. I never got the sides aligned to laser cutter precision so I have to tweak their placement every time.
--The box on the right is after casting and before demolding. Note the straws sticking up and the excess resin around.

More changes came too. I started using a pressure chamber with the plastic molds and use it every time I cast. It is safe- I never need to use a very high pressure. I never get voids provided I filled the molds completely. At a later time I will outline the actual process for mixing and injecting the resin.


This mold box is for beam lacing. It is a very successful design.
--The one resin path method was important for the lacing. I get no voids.
--The pins hold the alignment well. The mold box constrains the rubber location.
--All I need are four screws for most boxes in aluminum. Styrene needed twice that and still had gaps.

This little mold box (1" x 1.5") is for the shoe master (which needs repair...).
--It is my only open mold- I just pour in the resin.
--The metal pin holds the hole open in the casting.
--This mold gets stressed more than the flat ones so it need re-rubbering more frequently.


This final photo is of the stack of fresh mold boxes I recently made for the rest of the bridge, and one for the Phoenix beam column (right deeper one).
--I now use JB Weld instead of all those darn flathead screws to hold the parts together.
--I have researched rubber extensively and find only a very hard rubber has a chance to create consistent results.
--Hard rubber is thicker and requires vacuum de-airing. I created a system that works with a compressor for under $75. More on that at a later date.
--I also find that even the best rubber expands a bit as it is used. The rubber absorbs chemical from the resin and changes a bit, plus I believe the rubber expands over time anyway. I am still searching for a fix.
--I find using a minimum of rubber thickness is a key contributor to a stable mold. I now use 3/16" side strips instead of 1/4" if I can. If I had a mill I would mill out pockets for the rubber just larger than the part, and maybe mill in the sprues. The mold is cheaper that way too, because good platinum based rubber is expensive.

So there you have a few tidbits about resin casting for accuracy and thin section. I have lots more to say and will provide more info on clay layups, de-airing rubber, pressure casting and so on as I get a chance. I hope you have some questions too.

John

[SandiaPaul]

This is great stuff! I wish I had this info about 20 years ago when I had to invent similar methods! I am eager to see more installments!

Paul

[eTraxx]

John, what's the length of the longer molds? Curious. I don't have a Bridgeport (wish wish) but bought a Proxxon Mini Mill the other day .. haven't measured how much table travel I have on it yet.

Great post. I joined a couple of Yahoo groups that discuss resin casting so this fits right in with that.

[JohnP]

Thanks Paul, I'll be moving slowly but steadily onward so I will share more.

Ed, the longest is 9" by 2". I meant to post that. 9" x 2.5" is the max size to fit flat into an average paint pressure pot. Because the fill and vent straws are open ended I have to lay the molds flat to cure.

I was thinking of getting a Taig mill http://www.taigtools.com/mmill.html someday, the 12" version should do all I need.

One thing I learned from a Yahoo forum is don't use Harbor Freight pressure pots- they blow up. I use a Sears confidently. I go up to 50 psi and have no bubbles ever with the resin I use. Others recommend 60 - 80 psi but it is not needed; less stress on the pressure pot.

Casting fantasy figures etc takes different techniques. Technical model castings tend to be somewhat flat so my techniques will be useful for building parts, rolling stock parts and so on.

John

[finescalerr]

Beautiful work and excellent information. Three friends (Jerry Kitts, Doug Heitkamp, and Bob Uniack) have created terrific cast resin products over the years and, if you need a few tips, I could put you in touch with them. -- Russ

[JohnP]

Russ, thanks for the comments. I would be very pleased to contact others with resin expertise.

Jerry has a note on his Foothill Model Works site about problems with rubber consistency. He would be great to contact. Maybe he has similar troubles as I do.
Doug I am not familiar with but a quick search indicates he created resin car masters that others such as Foothill use.
I have seen Bob's work over the years and he, like the other two, works to a very high standard. This opportunity is way cool.

Information on making molds for repeatable, precise results with resin is hard to find. I have read that much of it is closely held. I don't mind posting the info in a quality forum like this. It is yet another tool to be developed for the high-level and/or quasi-business oriented modeler. I am trying to fill a void in railroad modeling. Good bridges are simply not available save for one or two examples. Plus, it helps with my therapy for my infatuation with old riveted structures.

Thanks, John

[DaKra]

I only recently started casting, and find myself at the bottom of yet another learning curve so this thread is a very interesting development.    I found, as you did, that information on precision casting is scarce.  So thanks for your generosity with this.   

By the way, I've been using a Harbor Freight pressure pot.   Should I be afraid?  Should I be very afraid? 

Dave

   

[eTraxx]

Dave,
You could always sandbag it. Lots and lots of sandbags.

[JohnP]

Dave,

I read a few years ago that more than one user had "unintended acceleration" of the lid when a clamp or two broke.

What I recommend is to order a quality relief valve from McMaster-Carr or Grainger at 50 psi. Attach that right on the lid input. Then, feed the tank via a dedicated regulator that is set to just below the relief. Set up the line to the tank so it goes: compressor--regulator w/gauge--valve--gauge--six-foot line to lid.  When you are ready to pressurize it, keep your head and body away by putting it under a stout workbench. Now you can open the valve and watch the pressure rise. Shut the valve when it is full. You can monitor the system for leaks by watching the extra gauge. There is no need to have a gauge on the tank.

I am paranoid about pressurized metal things. I may take it more seriously than others. But a few extra parts and a secure location means you never have to worry.

Sandbags are good- for a workout though.

John (Post any questions here and I will answer them as I can with my self-taught know-how.)

[james_coldicott]

John,

thanks for posting this here- fantastic results with the bridge and clearly great determination to get to this stage.

I'm really interested in the processes from a semi- commercial viewpoint- my plaster walls are all done in-house but just started getting prices in from resin companies for figures and wagon components and some of them have blown my mind! I can virtually set myself up to do centrifugal casting with all the augmentables, RTV-101 rubber and 1KG of high grade pewter for the cost of a molds worth of figures (50) in resin from one supplier and that includes the casting machine and a regulated melting pot for the white metal!

If anyone wants to share information on pressure resin casting either here or privately then I'd love to hear it. FWIW anyone wanting to try moldmaking and casting in the UK would be well advised to look here...

http://www.tiranti.co.uk/

excellent value and quality materials.

James

[finescalerr]

For advice on resin casting I first would consult the Guru:

Jerry Kitts
FOOTHILL MODEL WORKS
P.O. Box 470, Willits, CA 95490-0470, USA
(707) 459-5587, 9am to 5pm (Pacific Time, GMT-8), Mon. - Sat.
info@foothillmodelworks.com

Jerry mostly works with injection molding now but he raised resin casting to an art form.

Russ

[Chuck Doan]

Excellent info and a beautiful bridge kit.

[Carlo]

John, all -
Has anyone else had experience with spin-casting of resin parts. I had a commercial outfit (in the fantasy mini-figure business) do some 1:24 figure parts for me, and they came out very well... very high detail, and no flash. They were much better quality than any of my earlier attempts at traditional resin casting. One problem is that there is some shrinkage during the mold-vulcanizing process (about 8-12%) so masters have to be made a bit oversize. Another problem is finding a casting source who will do small batches (runs of 100 or less). I know one if anyone is interested.
Carlo

[JohnP]

James, you gotta learn it for yourself. You know that saying, if you give a hungry man a bunch of fish he'll be hungry again but if you give him a fishing pole he'll trade it for a ham sandwich and a beer right away. Or something like that. Maybe Carlo's source can help you with figures. But a variety of people make figures so look on the web for how to approach casting them. Explore the http://www.smooth-on.com/ website for techniques. The wagon parts, depending on how much of a kit (# of parts) you want, could be done like my bridge parts in a fairly flat method. I can help answer questions on that.

Carlo, I believe the shrinkage indicates a rubber made for high-temp metals but they used resin in it. A platinum-cure RTV will have negligible shrinkage. I haven't seen much spin casting for resin when vacuum and/or air takes care of all voids if done properly. Vacuum won't help metal but the spinning force pushes it down into the mold pockets as you can imagine.

And, thank you Chuck, and thanks Russ for the contact.

It may be also time for a word on quality. I have seen in reviews and in person a variety of quality levels in resin castings for all modeling hobbies. I am endeavoring to supply parts that are as close to injection molded as possible. That is why it has taken these four years (and surgeries and layoffs and other distractions but I digress...) for me to get almost there. Other suppliers seem to think that since you can't get it elsewhere then you can take what you can get. I'd say the high road of clean, well-shaped, void-free castings will give back more in the long run. My woes come from the desire to offer .015" thick castings. Heavier parts are a lot easier; a -10% change in size affects my stuff greatly but for other parts it is not that bad. Naturally, parts for your own use can be anyway that works for you.

John

[james_coldicott]

John,

James, you gotta learn it for yourself

isn't that just true of everything? I have already experimented with every type of silicone rubber for mould making I can lay my hands on and have a home made vacuum chamber with a vibrating base that gives me pretty much 100% success with my plaster walls. I'm going to set up to do the figures in pewter but there will obviously be a bit of a learning curve with that- advantage is that I can do loco components and tools too. The wagon bodies are going to be one piece so not straightforward- I may have to get them done professionally in order to buy some time to learn how to do them in house.

Definitely agree with you that any shrinkage in a mould is down to vulcanization needed when using high temp silicone- if the company was casting in a centrifuge their core business will be casting in metal so their expertise will be in using high temp rubber. When I get my 'spinner' working I may do some resin in standard silicone rubber to compare percentages with vacuum and pressure moulding.

You mentioned the possibility of posting some more about your pressure vessel and injection process- if you could do that would be great- don't have the same access in the UK to those mentioned from Harbor Freight or Sears.

Cheers

James