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
