some notes on Resin Casting

[Philip Smith]

John,

Great insight & tutorial.     

Philip     

[JohnP]

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

[JohnP]

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.



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.



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.)



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

[finescalerr]

This is just too cool. Your mold boxes are incredible. I expect outstanding results. -- Russ

[james_coldicott]

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

[JohnP]

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

[Hauk]

A great tutorial!

It might very well make me have a go at resin casting myself.

-Håvard

[JohnP]

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.


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.


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.


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.
  --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.)


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.


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.


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.


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.


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.


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.


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

[eTraxx]

Holy Moses!!! I need a emoticon that nothing but THUMBS UP!! Excellent!

[JohnP]

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

[Philip Smith]

Wow John!

Your taking this to a new level! 


Philip

[finescalerr]

Most satisfactory. In fact outstanding. -- Russ

[JohnP]

Actually Russ, by making the statement

group 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

[RoughboyModelworks]

This continues to be an excellent thread John... thanks for posting your SBS... saving it for future reference.

Paul

[JohnP]

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!


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.


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!


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.


Here I am pouring into my mixing container. I have ten molds lined up and ready to "squirt" (my term ). 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.


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.


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.


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.


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.


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.

 
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:


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?