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:
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I have actually sent a kit to the exceptional On3 modeler Dave Adams and you can see more at www.jpstructures.com (http://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.
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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.
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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.
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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 (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.
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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.
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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.
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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.
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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.
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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.
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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
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
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.
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 (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
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
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
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? :o
Dave
Dave,
You could always sandbag it. Lots and lots of sandbags.
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. ;D
John (Post any questions here and I will answer them as I can with my self-taught know-how.)
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/ (http://www.tiranti.co.uk/)
excellent value and quality materials.
James
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
Excellent info and a beautiful bridge kit.
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
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/ (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
John,
QuoteJames, 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
John,
Great insight & tutorial.
Philip
I'm drilling and tapping a bunch of mold boxes now so it won't be too long until I show the RTV process. I think you all will get something out of seeing it done. The basics are not that hard, and it is lots of fun when the first castings out.
John
The next step after finishing the aluminum mold boxes was to use them. As you see above (page 1) my current method is to build master parts on what I call a master plate. Here the plate surface defines the mold part line. I had most of them laser cut with the holes for the alignment pins perfectly in place. I use the holes to drill the mold box flanges for the alignment pins and screws. Also, the outline of the part is lightly etched so it's location is accurate. When stacked in a mold box for RTV application, the mold box halves and the master plates are perfectly aligned.
But sometimes you may have an odd shaped piece that is not conducive to to an easy slice down the center plane. Or, you may have a piece you made complete and wish to make copies. This is where the clay lay-up method comes in.
The clay defines the part line for the mold halves. It also fills one side of the mold. Injecting the RTV will fill the open side to create the first half of the mold.
You need to consider several things when planning the clay lay-up:
--None of the part can be trapped with more than a small amount of extending into the RTV. Large entrapments will stress the rubber so it tears when the part is pulled. 100% entrapped parts need to be cut out of the rubber!
--The entry and exit for the resin needs to be planned. I always use a one way in/one way out method to push most air through. The pressure pot takes care of the rest. Some how-to articles show complex parts with extra vent holes at dead ends. I would try to tie them together to the one exit hole.
--Holes in the parts need to be addressed. Most of my bridge parts are four-sided and fairly flat, so the holes are left open to the master plate. Since the master plate is the part line, there is no entrapped rubber in a through hole.
--Sometimes that doesn't work. Another method is to use pins to hold the space open. One way is to have the pin go through the box at precisely the center line of the hole. See above (page 1) my small mold box for the bridge foot. The pin goes through to hold the bridge foot pin/bolt hole open. I pull it out before removing the part.
For the Phoenix beam, I used a different plan. The center line of the holes are on the center of the part and are in the same plane, so I defined that plane as the mold part line - but at the ends only.
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The above photo shows the location of the pins for the three holes. I used a steel pin for the larger lower hole and cut drill bit ends for the other two. I placed the part in the mold box in the proper orientation for the pins.
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The photo above is a rough attempt to show the internal arrangement of a mold by having the top in a translucent state. You can see I attached a styrene sprue piece to each end of the post castings. I placed them so the upper surface is even with the center line of the post and hence the pins. I also sketched in the locations of the future resin inlet and outlet passages. I place 1/8" styrene rod through the mold box openings until they touch the sprue surface. (The post is propped up at the proper height for the photo using a bit of clay.)
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This photo is of the clay lay-up. I have filled the bottom of the mold box with clay and have very carefully molded it around the column, pins and sprues:
--The clay is non-drying and non-sulphur. Micro-Mark or any mold product supplier will have it.
--The clay is built up around the pins so it is half way up on the pins. When I make a casting later on, all I need to do is place these same pins into the RTV areas, close the mold and inject the resin. The holes will be in the part when the part is pulled and the pins are slid out.
--The part line for the column is different from the end castings. I desired to have the part line along the flange so clean up of flash is simple. I could have just continued the part along the center plane but then the part line flash would be along the smooth surface of the column creating undesirable extra work. There are mild undercuts (flange, roundness of the column) causing a bit of entrapment, but the part will pull out easily if I remove the rubber from the mold box first.
--The clay was tamped, smoothed, touched up and generally caressed to meet very smoothly and neatly with the part. This is a critical area- allow no voids, bumps, jagged lines or smears. Otherwise, the cast part will need lots of clean up, or a thin feather of RTV will tear and extra resin will be deposited on the part. I used custom cut styrene sticks, hobby blades, razor blades and whatever else I needed.
--You only have to lay-up one side. The first pour of RTV will form the part line when the second side is poured. Select the easiest side to lay-up.
--The dimples will help the mold half alignment close to the part. The dimple will create a nub on the other mold half and they will interlock. They were made using the end of a paintbrush handle.
That's it for now until I pour the rubber. I may not get to it until next week, but ask any questions you may have about mold design, part lines and sprues.
John
This is just too cool. Your mold boxes are incredible. I expect outstanding results. -- Russ
John,
thanks for posting this- very interesting to see your Step by Step process. One question- do the locations of the pouring holes make a difference in terms of successfully removing air bubbles. I assume there are 2 or more pouring holes for each mold or do you stick to 2?- one to pour in the resin- another to allow air out at the other end as well as supporting the master in the mold box? If the holes are not at opposite ends of the piece do you find air can get trapped in the blind end?
I'm guessing that with some pieces one needs to experiment with sprue locations to get a reliable mold.
Look forward to the next installment.
James
James- I plan a one-way trip for the resin. All my molds have only two holes. Even on the molds with three different parts they are laid out parallel and the end sprues are tied together. I hold the molds vertically to inject the resin. When the resin is pushed through I know it has gone into all the part channels when it comes out the top hole. I also keep pushing the resin in until the bubbles have stopped appearing in the outflow. You'll see that here in about a week.
Russ- Thank you. I have been working in a vacuum (hah) with this project so it is nice to see people interested. It keeps me going. I'll send a grab bag of parts once I get some column part cast.
John
A great tutorial!
It might very well make me have a go at resin casting myself.
-Håvard
OK boys and girls, here's another installment in A Reason for Resin or How a Spare Bedroom Can Be Used For Exciting Chemistry Thanks To An Understanding Spouse. Remember, this all happens in a small bedroom upstairs -the basement gets too cold for winter casting. I ran an air hose through the wall so the compressor keeps its noise in a basement closet.
At this point I also want to say these important and serious points-
--These techniques require pressurized air and vacuum. Any pressurized object must be treated with respect.
--Read all instructions that come with equipment. Do not modify or misuse your equipment.
--Use two methods of controlling the pressure to your pressure equipment, such as a regulator and a safety valve.
--Always wear safety glasses when working with pressure and chemicals. Even when injecting resin with a syringe, enough pressure is there to shoot the stuff across the room.
--Wear chemical resistant gloves and old clothes.
--This is not for kids. Nor pets.
I last left you with a layed-up mold ready for RTV. Here is a pretty detailed explanation of how to pour the RTV.
As you read, remember it seems the biggest problem people have with RTV is with a tacky partial cure.
--Mix according to instructions.
--Mix by mass or by volume as indicated. 1:1 or any ratio is not the same for volume vs. mass.
--Use plastic utensils.
--Measure everything, no guessing.
--Keep all contaminants away, such as all liquid cleaners, liquid latex, and especially sulphur modeling clay.
--The room temp must be 75F to 85F.
--Mix like you mean it.
I use a high-quality platinum-cure RTV that has a 60 hardness (Brinnell scale). For accuracy in casting harder is better. The rubbers are of a much higher viscosity so they do have to be de-aired. And, they are generally not 1:1 ratio so they need to be weighed out to mix. For starting out, short runs or small amounts the thinner 1:1 rubbers will carry the same details into the castings. However, they will be fragile, have a short life and possibly distort more.
Most RTV stays fluid for a long time so you do not have to hurry these steps. Read the instructions.
Before handling the rubber I performed calculations to determine how much rubber to make.
--I used an Excel spreadsheet where I entered the inside dimensions, calculated the volumes and then converted to grams of RTV.
--All RTV data sheets will have a density figure that tells you the mass per volume.
--I mix about 20% more to account for loss, the extra puddles I use and variations in how much volume the master takes.
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I use an RTV that is a 10:1 ratio by mass. That means I have to weigh it.
--The plastic container is put on the scale and the scale zeroed.
--I first pour in the small bottle material part. I added drops until it turned over to the figure I needed.
--I zero again.
--Then I pour in the thick RTV stuff.
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I zero the scale so I do not need to make calculation in my head during the heat of the pour. Enough can already go wrong. By starting with the smaller portion, I can weigh that very accurately. Then, if I am off a hair with the larger part it doesn't matter much. The rubber needs to be weighed out within 2% or so I found. Otherwise, the RTV may not have the correct characteristics. Don't try to make small amounts of a 10:1 RTV.
Next, stir the material very, very thoroughly. The parts need to be completely in contact for the reaction to occur. I use a styrene paddle. Do not use wood because it can carry moisture into the mix (especially critical with resins).
Next, it is ready for the vacuum chamber. RTV needs to be de-aired to greater than 26 in/hg to get the bubbles out. Some recommend more towards 29 in/hg. However, that would take a real vacuum pump like a belt-driven Welch. The set-up below works for me. Surprisingly well I might add. I found it in a desperate web search.
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I use a vacuum generator made for HVAC service. It is from Harbor Freight. It was cheap of course. Plus, it has no moving parts. That is the only reason I got it there. Details:
--It uses moving high-pressure air to create a vacuum. I feed it the compressor max, about 120psi.
--I use a portable 10 gal air tank tied into the 10 gal compressor tank. This gives me a whoosh of high pressure air, enough to get good vacuum for a short time.
--Per the gauge, I can get to 26.5 in/hg. That is enough to create a usable vacuum.
--The chamber itself is a FoodSaver vacuum food storage container http://www.foodsaver.com/Product.aspx?id=c&cid=1181&pid=262 (http://www.foodsaver.com/Product.aspx?id=c&cid=1181&pid=262).
--I put a fitting in the top to connect into a regular hose.
--I have chosen a plastic mixing container that just barely fits into the vacuum container.
--I wrapped the container with heavy packing tape so in case it implodes the parts may be contained. (Although I read it is the lid that will likely go first.)
(https://www.finescalerr.com/smf/proxy.php?request=http%3A%2F%2Fwww.jpstructures.com%2Fsitebuilder%2Fimages%2FRTV_expanding-476x600.jpg&hash=ee58d660d7ef07db498976b179041eaa843bbcb9)
Here you can actually see the RTV has foamed up. And it has gone over the edge and made a mess. RTV will expand 2x to 3x as it de-airs.
--I put the container in and placed the lid on top.
--With a fully charged air supply I fully open a valve to the chamber. As it reaches 26 in/hg, the RTV foams up.
--With my system, the peak vacuum goes fairly quickly, so I shut off the air supply and let the compressor catch up.
--I do that generally about three times. Usually the RTV will not foam the second time even if I got a good vacuum on the first try.
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Here I am injecting a mold.
--I made clay dams around the mold openings. I leave a puddle over the openings so, if there is any trapped air inside, there will be a small supply of RTV to fill in when the air is driven out in the pressure pot.
--The syringe is a common large size, 60ml. It fits comfortable into the mold hole.
--I hold the mold box on a slight tilt to help drive air out.
--This mold is filled. The RTV has come out the exit holes. I know the mold is completely filled.
--The mold volume spreadsheet is on the bench so I know how much each mold takes. I do not start injecting without enough RTV to complete the pour.
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This gadget is my mold carousel. It holds nine molds plus a few stacked if needed. I'll probably add another layer. I poured eight molds, first half, last night.
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This is the pressure pot set-up. The carousel is inside. When all the molds were poured, I placed them into the pressure pot and turned on the air.
--I did nothing to it except remove the paint spray regulator and add the valve. My regulator is on the wall below the bench.
--I also added a 60psi blow-off relief valve. I have had great success working at 50psi pressure for the RTV and the resin.
--The valve is to cut off the air supply, and then vent the tank. I do not like to leave a pressurized line on the tank more than needed. It only lost 3psi overnight.
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Here is a mold box post-cure (24 hours). It is the Phoenix post with those PAP castings I had made. You can see there was a bit of draw back after pressurization. That signifies I had an air pocket inside.
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This is the exciting part. I have opened the mold. Everything looks very optimal. The impression of the part in the RTV is very clear. There is a little Dull-cote sticking to it which will come off on the first casting. You can also see where the hole-keeping pins fit into the rubber. They will be placed back in at every casting.
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Now, I have prepped the mold for the next side pour.
--I pulled the master out of the clay, removed the clay and cleaned up the mold and the master.
--IMPORTANT! I next applied a rubber-to-rubber release agent. RTV loves itself. It sticks like it never had a seam. You will be doomed if you do not use a specific RTV release agent- you'll have to cut the mold apart after the second side pour.
--The master with keeper pins has been pushed into its RTV impression.
--Then the cover went on and the clay dams applied.
I poured the second half of the molds tonight. After clean-up, the molds need to be conditioned at low heat per the manufacturer's directions. That makes the rubber more stable and last longer. Otherwise the next step will be resin casting, maybe this weekend.
I know I forgot some stuff so I will add more later. I posted a bunch of pics so y'all can see what it looks like. I am hoping if I take a little of the fear of the unknown out of it some of you might like to try Casting For Fun And Profit.
John
Holy Moses!!! I need a emoticon that nothing but THUMBS UP!! Excellent!
Gee thanks Ed.
Other people keep this info so close like it is a state secret. I was only able to find out a few things by researching, mostly on-line. I have experimented enough now to figure out a lot of key process parts. Plus, I had worked in six-sigma problem solving for a while, and work as a process engineer now. So I got most of it down to a controlled methodology.
I am showing the processes now because I have found the right group of intelligent and skilled modelers. I am glad you are enjoying it.
John
Wow John!
Your taking this to a new level! :o
Philip
Most satisfactory. In fact outstanding. -- Russ
Actually Russ, by making the statement
Quotegroup of intelligent...modelers
I figured I would end up in the corner with the cool kids.
It has been fun doing this and making the photos to go with it. Soon there will be castings in hand!
John
This continues to be an excellent thread John... thanks for posting your SBS... saving it for future reference.
Paul
We last left our heroine with blue rubber on her hands but no resin to live. ::)
After pouring both sides of the molds, I placed the molds with masters in a just warm oven for a few hours. The rubber I use actually doesn't specify what post cure it needs but many rubbers do mention a low heat cure. It brings the rubber to maximum hardness and tear strength. Tin cure RTV exudes alcohol as it cures so a post cure helps remove that- it can interfere with some resin curing. Just follow carefully the data sheet!
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The Phoenix post mold now looks like this. I picked off extraneous rubber bits that went into a couple of thin spaces in the clay. I have inserted the three keeper pins that create the through holes in the post. RTV does not need a release agent when casting urethane resins, but I did spray some Mann Ease Release 200 on the pins. Mold release will extend the life of a mold but it is heck to remove without destroying fragile model parts.
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Here is the mold ready for the resin. I have inserted my fill/vent tubes. I could inject the resin directly into the mold hole However, I have found that if there is an air bubble that gets pushed out by the pressure pot, there is no resin to fill it and a void will form in the end of the part. I add small sprues to the parts, and I suppose I could make big deep sprues, but they would be more difficult to demold and cut off. Also, the tubes direct the vent resin away from the mold as you will soon see.
Also note the mold box screws come in from the bottom. I had too many screw heads filled with resin so a tool couldn't get into them. This works better.
Oh, and what are the fill/vent tubes? I drill a 1/8" hole. Into that fits "Sip-Stirs - A Party's Best Friend". They are drink stirrers from the grocery store. I cut them about 1" long. Since they are one use only I needed cheap and plentiful. And no, they are unused!
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Here is the casting resin kit and kaboodle.
--The two big containers are the two-part urethane resin.
--Because they are awkward to pour into a small mixing container, I use a couple of small milk bottles as temporary storage. If you buy resin from Micro-Mark or other sample kits, it will come in reasonably sized bottles.
--The syringe is a 60ml, same as I used on the RTV. It has a Luer-Lok nozzle that fits the 1/8" fill tubes.
--The mixing container is a plastic lab wear beaker. It has the markings molded in the side.
--Be aware that some beakers with printed markings are inaccurate- just plain wrong- and will mess up your mix.
--My styrene mixing paddle in the photo. Never use wood! Wood holds moisture, which is not good for resin.
--A spray can of "Xtend-It". It is an inert gas (not computer spray air etc) that you spray into the resin containers after the session is over. It makes opened resin last much longer.
Here are points about resin:
--We are using urethane resins in model making. This is not epoxy resin.
--There are very many types of resin from many companies. Their characteristics vary widely. Try a few if you will be doing much casting.
--For detailed scale modeling a resin needs the following characteristics:
--Ultra-low viscosity (75-200 cps)
--Very low shrinkage (nil-.005 in/in or so) Shrinkage is specified by the dimensional change in inches per inch of material.
--High hardness (70+ Shore D scale)
--Good tensile strength(3000+ psi)
--Somewhat flexible, no brittleness
--1:1 mix by volume
--In my opinion, avoid the fast cure resins. A moderately fast cure is sufficient.
--Fast cure is anything about 5 minute cure
--Fast cure resins will have greater shrinkage
--Fast cure resins put out greater heat. This will deteriorate the mold at a greater rate.
--Fast cure resins have a pot life of about 30 sec. That means it will start to gel in half a minute. That is just too quick for calm mixing and pouring.
--If it starts to gel when you finally get it into a pressure pot, it may not squeeze the trapped air into solution.
--Select a resin with a 30 minute to two hour cure. This gives you a 5 to 10 minute working time.
--Always keep resin at room temperature. Use resin at moderate room temperature.
--Cold will separate out the chemicals in resin. Vigorous shaking in a warm room may- or may not- recover the resin.
--Hot temps will accelerate the cure. Your pot life will be shorter and the cure may be different (greater shrinkage etc)
--Once resin is open, use it up. Xtend-It helps quite a bit, but it has a limited life until one day the cure does not fully harden.
--The cure time for resin will vary by the mass of the material. A big chunk will cure normally. A thin piece, like my castings, take up to 3X longer.
--If resin never cures, there was a problem with the mix or material:
--The resin was old
--The resin had too much air or moisture exposure
--The resin was not mixed in the individual bottles or had settled out in the cold
--The mix was not thorough (more of a problem with a big pour volume)
--The ratio was more than 5% off.
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Here I am pouring into my mixing container. I have ten molds lined up and ready to "squirt" (my term ;D). I arrange them in order of channel size so the small parts get the thinner resin and the heavier parts get filled as the resin starts to get a bit more viscous.
--I dare to use the same container to pour in both parts. A smarter way is to measure the correct amount into two separate containers, then mix into a third.
--I use the styrene paddle smoothly, reversing directions to get a complete mix.
--Scrape the sides and bottom so all the chemical chains buddy up and all areas cure.
--Don't stir like a maniac, this will cause the resin to start combining and before you realize it is hot and gelled in the cup.
--Shake both parts like crazy to make sure the raw resin is completely mixed. Shake, then pour quickly. The air will come out in the pressure pot.
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Now I am quickly pouring the resin into the syringe. After it is all in, I turn it over to push up the plunger and purge the air. Hold a paper towel over the top so you don't squirt your face!
--Even syringes deserve a word- you can find them at lab supply places on-line and some hobby suppliers.
--My latest batch has a ring around the top inside that acts as a stop so the plunger doesn't get pulled out. I scrape them off, they're in the way.
--The Luer-Lok nozzles can also come with rings inside that won't let you push the old resin slug out. These you may not be able to re-use.
--I spray the Mann release on the plunger rubber so it slides easily.
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Here I am injecting a mold. I have inserted the nozzle into the lower tube and started to push. As the resin reaches the top, I keep pushing until a stream comes out the vent tube as seen in the photo. I watch for any bubbles to stop, then keep a little pressure on as I lower it to the horizontal. Once I pull away the syringe, there will be resin visible in the tops of both the tubes. The mold is completely filled.
--Some molds need to be rocked and spun so the resin displaces the air in various deeper pockets.
--Don't push too hard or the resin will squeeze between the RTV halves and make a big mess. The really thin lacing molds take a bit of time for the resin to come out the vent tube, but I never get voids.
As I pour, I load the molds onto the carousel as you saw above. I quickly place the carousel into the pressure pot, secure the lid, and gradually let the pressure in.
For clean-up, first I squirt the rest of the resin into the work tray paper towel, if it hasn't jelled yet. This puddle becomes my test sample. If it is cures enough so it snaps in half, I know the cure of the parts will be good, and that I can demold the parts. I also wipe out the mix container before it cures because it is easier than removing a resin slug from the bottom. I wait until the resin is cured to pull out the slug in the syringe.
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Here is the post after curing. I removed the rubber from the mold box so I could flex it and pop it off the part. Not too bad on the flash.
--I first remove the fill/vent tubes with pliers. Grab and twist.
--When making the mold, the styrene plugs I use to create the fill/vent passages in the RTV have a taper on the end where it contacts the sprue. Most of the time the fill/vent tube will break off conveniently at the sprue.
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To fully demold this part I had to flex the mold apart by hand and ease it out from behind. Note the pins will come out with it. After removal they get used again.
--Frequently a very thin flash will form that breaks away from the part but sticks to the RTV. After much frustration with tweezers, air and whatever, I now use packing tape to remove the flash. It pulls it off nicely in one stroke.
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After all that work, you finally get to see some results.
--I needed to spend a few minutes cleaning up the parts. I had messed up one flange edge in the rubber when cleaning RTV flash. Plus, the clay-up was complex at the top end and created a little flash.
--The first casting had a void in two upper corners. I rotated the mold when injecting resin and that seemed to help.
I also had an unexpected problem with a very thin layer of RTV getting on areas of the first pour. I believe this was because the master did not stay pressed fully into the first half rubber when I set up to pour the second. That is what created the void spot in the top of the right casting near the hole. Subsequent casting should be even better.
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Now I can make the most difficult part of this bridge model easily. The rest can be scratched with styrene and brass rod, and maybe I'll get the eyebars laser cut. I'll show that someday soon. Plus I was hoping to make it into an article for one of The Books.
Meanwhile, the D&RGW 1:48 truss bridge factory is back to full blast. Here are some of the latest and freshest parts- as I pulled them out of the mold without any clean up:
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So now, finally, those of you interested have been offered a good look at one path through the casting process. It took me four years to get to this point. If it wasn't for my slightly expanding rubber syndrome, I would be at 100% control of the process and results. As it is, I can still manipulate things to get a useful number of parts before I re-pour a mold.
In the end, I get to make things that no other modeler has tried that I know of. The Phoenix beam is quite exciting- there were many beautiful old bridge (some still standing) that used this fascinating design feature, including some really large spans. I now know I can make models of them via PAP and resin casting. Plus, I can offer model kits like the D&RGW truss to folks who are smitten by bridges too but didn't know how to approach building one.
Class dismissed, any questions? :)
Fantastic series of posts! Thanks for providing so much useful information!
Quote from: JohnP on August 22, 2010, 07:28:57 PM
Class dismissed, any questions? :)
Yes, one rethorical question.
Is this the best tutorial on resin casting ever posted on the net? I would say so.
Thanks a lot for releasing such an amount of knowledge into the public domain.
And a question for Russ as well:
What Annual do you plan to run the article in?
-Haavard
Haarvard,
I was very frustrated with the info available for precision resin casting. So I started this thread and decided to share pretty much everything. There are still many fine points that come with personal experience as the modeler tries things their way.
Such as, I have found that when plastic-type parts are laser cut they can get a bit of an uneven edge. When that is made into the mold, it can create a tiny undercut in the rubber. So, for example, when thin parts like my lacing strips are pulled out, they catch and break. I now sand the edges of master parts to break the corner.
More later, and thanks,
John
Håvard, John has to tell me when he is ready and send me the photos. When the material is here I will schedule the article.
Guys, I am interested in publishing many of the things you work on but I don't "assign" articles. As I have said before, I need good photos and drawings; nothing gets done without them. There is never a deadline; when I receive the graphics I schedule the article. If you can't or don't like to write, we'll do it by phone: I'll call and take notes.
If you want to get published, please let me know. If I already have asked you about an article and you have finished the model, please let me know. If you need help with photography, please refer to the sticky on that subject.
I think Ken Hamilton owes me a lumber carrier article and, when he finishes the new lumber loader diorama, we'll publish that , too. Chuck's rapid prototype parts will appear in the upcoming Modelers' Annual and the whole diorama will appear next year if it's finished. Jacq will put together a very comprehensive article on his lumber mill layout. Nick owes me articles on a couple of things but he's standing in the corner so probably has been unable to work on them. Marc will do some articles. And there are others.
Just get them to me before I retire, okay?
Russ
I need to make the rest of the bridge now, finish it and take beautiful photos of it. I want the resin casting to have a focus. Dave DaKra will be laser cutting some parts for his forum SBS. Once I have all the material in hand, I can edit the info into an article form and send it to Sir Editor and Publisher.
In the late '90s and later I wrote a number of articles for Hundman's N-Scale magazine. That was always satisfying, but if this project article is successful, hoo-wee I'm getting top shelf bourbon and anoint the book, the model and mostly myself. I'll post a You-Tube of the dance.
Russ, what ever happened to The Railroad Man and his ritual dancing, cheesepuffs, etc?
John
The Railroad Man is now 72 and has slowed down a little. Worse, he and his wife moved up to Tacoma so I rarely get to hang out with him although we talk regularly on the phone. At the moment his big enthusiasm is 1:20.3 scale live steam ... as a dilettante, of course. I don't know whether his wife lets him eat cheese puffs very often. I suspect he sneaks in the secret Railroad Man dance now and then. -- Russ
Guys, I've just got some of John's parts in-hand, and I have to tell you, as good as they look in the photos, the photos don't do them justice! :o Everything looks exactly as it should, the rivets are all perfectly formed and spaced. Some of the parts are paper-thin, but perfectly cast, no warps, no bubbles, and really strong and flexible. Even the post office couldn't destroy them. You can see on the thread the thought and effort John put into developing his techniques, and it really shows in the finished product. Amazing work!
Dave
Imagine the quality of the model(s) we could produce as a group were we to harness the talents of about a dozen of our contributors! -- Russ
Dave is being nice. Of course, he shares my views, and the views of others here, that things in the model railroad hobby world have become stale. So we have gathered together to display our techniques and results.
Meanwhile, I am sure Russ has in mind a reality show with the 12 modelers living in an old Southern Pacific station in the deserts of Arizona. What could possibly go wrong?
Actually, what is amazing here is that the models and processes are accomplished with fairly simple tooling and hand-done techniques, or like with Dave using sophisticated tools in new creative ways (show them the pies yet???). It has been proven that the Chinese are capable of making very detailed models based on engineering drawings from US companies. But that is all done with automatic micro-precision EDM machines and whatever else they use to make molds. Precise, cold, lifeless models anyone can buy. We do it with imagination and personal determination, and the unique results speak for themselves.
John
Wow! This thread and tutorial has been simply outstanding. Thank You!
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Marc
You are all welcome. I will get the entire bridge done soon. It should be a sweet, simple model. And if I think of any other casting stuff to write I will do it here.
John
This thread has been extremely informative and I really appreciate you taking the time to share it with us (and me personally!) I have a question regarding thin castings. How thin have you been able to get them with consistent results? I am designing a master who's wall thickness I am trying to keep between .020" - .030". Would you consider this unrealistic?
Thanks,
Donnell
SD80MAC- you from Mass? I saw those Conrail beauties when they were brand new on the B&A up the mountain. What a sound they made. Software sucked though at first- a train going by me stalled, the engineer was panicking on the radio, the dispatch told him to reboot the computer. After a restart away they went, starting on the hill and accelerating to track speed in less than one train length. Must have used Windows Vista.
.020" - .030" is pretty thin. I have many parts in the .015" - .020" range that cast very well when the mold is new. But, sometimes within 8-10 castings, the resin byproducts have already swelled up the rubber so I get thin spots. Your best bet is to design parts with edge areas that thin but beef up the body of the part so you can tolerate dimensional changes. The art of deception. Another thing to try is to apply a barrier coat first to the rubber before every casting. I am planning on running some experiments to see how DullCote works. The resin byproducts won't penetrate the rubber with the spray, and the spray creates the outer layer of the casting. I have read that mold release is useless for that, and all it does is grease up your fragile parts so paint won't stick. So if you are going commercial work it out first. For a handful of repetitive parts for you and friends, make the parts thin.
Meanwhile, for everyone keeping track of this, I had a nice customer post some good words about my bridge on a Yahoo On3 group forum. I have been inundated with orders and am booked through January. This volume, and the problem with the thinner parts, has lead me to try brass etching for the thin lacing parts on the next bridge.
And to think- I had just bought paint, salt and hairspray to take a break and try rust weathering. Someday, maybe, I'll finish that little Phoenix bridge so Unc Russ has an article....
John
Hi John,
I'm actually in Southern California. I had some minor issues like that with early GEVOs back in '06 (I'm a furloughed UP engineer).
The project I'm working on is a TT scale International wide-vision caboose. It's a farily small project at just over 3.5" long. The plan was to get an RP master printed and then make resin copies. Model was drawn in subsections that would be assembled into the final piece. The parts included the main body, roof, cupola, cupola roof, and underframe. However, I'm thinking of breaking the main body and cupola down into a series flat parts, about .040" thick with some sort of tabbed indexing to help align the many pieces.
I really like your idea of a two-part injected resin mold, and the results of your system yield professional quality parts with the ability to add crisp detail on both sides. Plus, it utilizes much less rubber than a more conventional, deep cavity mold.
Thanks again,
Donnell
Quote from: JohnP on November 10, 2010, 04:31:22 PM
SD80MAC- you from Mass? I saw those Conrail beauties when they were brand new on the B&A up the mountain. What a sound they made. Software sucked though at first- a train going by me stalled, the engineer was panicking on the radio, the dispatch told him to reboot the computer. After a restart away they went, starting on the hill and accelerating to track speed in less than one train length. Must have used Windows Vista.
.020" - .030" is pretty thin. I have many parts in the .015" - .020" range that cast very well when the mold is new. But, sometimes within 8-10 castings, the resin byproducts have already swelled up the rubber so I get thin spots. Your best bet is to design parts with edge areas that thin but beef up the body of the part so you can tolerate dimensional changes. The art of deception. Another thing to try is to apply a barrier coat first to the rubber before every casting. I am planning on running some experiments to see how DullCote works. The resin byproducts won't penetrate the rubber with the spray, and the spray creates the outer layer of the casting. I have read that mold release is useless for that, and all it does is grease up your fragile parts so paint won't stick. So if you are going commercial work it out first. For a handful of repetitive parts for you and friends, make the parts thin.
Meanwhile, for everyone keeping track of this, I had a nice customer post some good words about my bridge on a Yahoo On3 group forum. I have been inundated with orders and am booked through January. This volume, and the problem with the thinner parts, has lead me to try brass etching for the thin lacing parts on the next bridge.
And to think- I had just bought paint, salt and hairspray to take a break and try rust weathering. Someday, maybe, I'll finish that little Phoenix bridge so Unc Russ has an article....
John
GOOD NEWS!!!
I have found a mold release that protects my RTV from the urethane secondary products.
Somewhere in this thread I explained that my casting process is great except for a mysterious expansion of the RTV. The rubber swells until it will not fit in the mold box and all the features are squeezed shut. With my thin designs, this was happening within 6-8 castings. That was intolerable at best and made the business doubtful. I knew a mold release would help as a protection but they all needed to be cleaned off for bonding and painting. My thin casting would not withstand solvent scrubbing.
But this product works like a charm: Ultra 4 Urethane Parfilm Paintable Mold Release by Price-Driscoll. http://www.price-driscoll.com/formulations.html (http://www.price-driscoll.com/formulations.html)
I have used it on a new set of molds and have multiple castings from each mold without any sign of the dreaded swelling. I sprayed it an all rubber surfaces the first time, then on the pattern surface every casting. The parts drop out of the molds within spec. It has also allowed me to use molds that were getting marginal, and I got about 10 more shots out of them so far.
I also sprayed parts with PollyScale acrylic paint. Water based paint performs the worst on a contaminated surface. The paint went on smoothly with no fish-eyes or creep-back. It adhered as well as any other PollyScale paint. It seems the Ultra 4 becomes part of the top surface of the urethane. If it is sprayed on lightly per the instructions, it is not noticeable to the touch like regular mold release is.
I am quite pleased with the product. I recommend it if you are doing even a few hobby castings. I bought it at The Complete Sculptor http://www.sculpt.com/ (http://www.sculpt.com/). They have a good selection of casting supplies including the Ultra 4.
John
Great stuff John! I've only used foam-core to make the mold box. Vaseline is a great silicons to silicone mold release.
I use talcum powder for mold release when I cast up the resin parts.
HERE's (http://tinyurl.com/276rlvh) some progress shots of a mold I made.
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It's really nice that you're thinking of your customers. But it seems to me that anybody who spends over a hundred bucks on a resin kit of an obscure bridge is playing under big-boy rules and should know to clean the castings before working with them. Removing mold release agents is a basic step when dealing with parts, be they fancy cast metal or resin, or just a six-dollar injection-molded styrene kit. The less you as a manufacturer have to dick around with remaking molds with expensive time and rubber, the lower your cost will be, which should lower the selling price, which should mean more people can afford a really cool model kit.
I typically re-wash any parts or subassemblies I've so much as handled with bare skin with detergent and a toothbrush before I paint them, and I've been doing it since I was a kid.
Trouble is my parts are thin and fragile. Any kind of scrubbing will break them. My goal was to make the kit accessible, not a frustrating struggle. So the mold release was a big issue. I had hoped to ultrasonically clean them, but the material manufacturers stated that mechanical action was needed. Plus, I would have needed a pretty big cleaner for the long parts. The spray needs to be applied very sparingly according to the instructions, then it is undetectable on the part. It will work for most; for others they may wish to clean a bit if they deem it necessary.
And boy, I tell you what, don't suggest to any D&RGW modeler that the bridge is obscure- I have had plenty of interest and decent sales for my very part time project. Made it all worth while, enough so that I am moving on to my next bridge.
Thanks
John
Good deal John. Price Driscoll will supply samples if you ask and there product is rotated frequently. I purchased some PD from another company (old stock) that contained ozone depleting materials from way back. I'm not saying your supplier sold you some bad stuff just a FYI and you may call PD with the lot number if necessary or request a sample.
It is odd that the molds are swelling. It must be a combination of force, casting resin and a reaction to the aluminum frame. Never worked with the platinum stuff Weird stuff, glad your getting more pulls now.
I noticed a x-brace on page 2 of this thread. Are you casting the turnbuckle and rod?
Philip
Philip, the swelling occurred with the first styrene frames, so we can eliminate the aluminum factor. I was aware of the contact areas becoming hardened but that was to be expected. Apparently the resin byproducts permeate the entire body of RTV. That may be enhanced by the pressure casting. I could try different resins that are less harsh but the material I use always hardens in thin sections whereas some don't.
The PD spray is the latest material. I am very happy with it indeed.
I really haven't decided on much of the rest of that little bridge. I would hope to buy correct size turnbuckles to save me the trouble. I have been working on filling orders for the D&RGW truss. I have little time for hobby projects in my hobby business!
John