resin casting

Dave Stevens (Lumberyard)

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there is a team of guys doing a custom made limited edition kit project of a bomb vessel cross section you can go here and see the prototype build.

http://modelshipbuilder.com/e107_plugins/forum/forum_viewtopic.php?2097

notice the name it is Mike41 who is doing the Blandford cross section also did a prototype of the bomb vessel.


the problem we ran into is producing the mortar and cannon. We did do a 3D print file and had a high end master print made. Our original idea was to have the guns printed but the cost for high end prints were far to expensive. so we thought ok lets make a mold and cast the guns in resin, that is way easier said than done.

So far we did make a master silicone mold and tried a few castings the problem is no matter what was tried we get air trapped in the mold.
is there anyone on the forum who can do resin casting or know of someone who can?

some say cast the mold and place it in a vacuum chamber to suck out the air, ok but if you don't have a vacuum set up that is kind of hard to do.
We tried to job the casting out to someone set up to do it BUT they all will not work with the mold they want to start from scratch and make their own mold at $350.00 plus set up fee.

the model is in 3/8 scale and quite impressive I am just stuck on the casting part
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and by the way yes I did cut air breather passages in the mold and tried injecting the resin with a syringe to force the resin in the mold results were less than poor

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before a mold of the mortar can be made there is the drawings that have to be done first as a CAD drawing then that 2D cad drawing is converted to a 3D drawing and then into a 3D printing file.
There are a lot of types of 3D printing from low resolution where plastic is melted and a print is built up. Then you have high end resolution prints which are done in a vat of liquid resin. This type of 3D print is 10 times more expensive, but when you need only one for a master the cost is worth it.
you can see the ultra fine detail of the cypher on the guns.

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the credit for this type of high end work is that of a team of experts. Joe who we all see his CAD work on the forum with the Roebuck and the bomb vessel. Printing is done by a fine 3d artist Stacy and I am struggling with making a master mold. This is only part of the team working on the bomb vessel cross section model (link is posted in the first post)

the idea behind this team and the goal is to produce quality building projects for members of this forum to learn this fine art.
It is not a problem in design or producing the pieces and parts the talent in this team surpasses any obstacle encountered except one.

The one big problem is the high cost of production of a limited amount. Take the guns for the bomb vessel cross section as an example. The many hours into the design and cad work, the high end 3D printing of masters, production molds and casting. You end up with hundreds of dollars invested in making 10 guns which ends up a set of guns is $150.00 way to expensive for a kit.
 
the first attempt at casting without a vacuum chamber resulted in air getting trapped in the mold cavity and without putting the resin in a vacuum before pouring results in hundreds of tiny bubbles on the surface.

to make a home made vacuum chamber to draw 60 to 80 PSI is like making a bomb, not recommended for your DIY hobbyist
so can a casting be made with no trapped air in the mold cavity and a smooth surface?

check out try number 3 and see for yourself the bubbles at the back resulted from a poor mixing and not de gassing the resin. what happened I ran short of resin so I quickly mixed more and just poured it into the mold. This is just a test pour so I did not matter. The good news this method of not using a vacuum pot is repeatable time and again with perfect results every time. notice the sharp detail of the cypher and rings.

the guns were a big block to produce a kit of a bomb vessel cross section. now that the problem has been solved and guns can be cast in black resin a hand crafted limited edition kit is possible.

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I don't know if you are still working on this or not but trapped air can be handled by planning for the release of trapped air, using a different resin and of course vacuumming the pour. Seam line problems can be reduced by using cut moulds.
 
When I was producing kits, I was always faced with this same dilemma. I turned to resin casting as well and did a lot of research and testing before I had success. The tricks I'm going to share with you should result in clean molds without the bubbles and clean resin castings. The equipment you need is not as expensive as you might think. I'll give you links where you can buy it. Once you've made that initial investment for the equipment, it will pay for itself over time because you'll use it time and time again to make parts.

I can tell you from experience that first you need a vacuum chamber with pump which you can find on eBay pretty cheap. Here's one that costs $130.00.

The type of silicone I used was here from Smooth-on. You want a silicone that is mixed at a 1 to 1 ratio. That way you don't need a scale. I got to the point that I could tell by the color of the silicone when I poured the part 1B into the part 1A when I had added a sufficient amount but small glass measuring cups work great for this part which you can find at any department store like Walmart or Target.

When you mix the silicone for the mold, it needs to be in an oversize bowl. I used to use large 4 quart glass kitchen measuring cups for this. The reason is that when you mix it, you put it in the vacuum chamber to let the pump suck all of the air out. The stuff will start to bubble big time and it will rise. It will rise to a certain point and then seem to explode dropping back down to its normal level. That's when you can shut the pump off and remove the mixing bowl.

It will seem like it still has bubbles in it but that's normal. They'll escape and rise to the surface once you pour the mold. But before you even mix the silicon you have to get the mold setup for pouring the first half.

I took modeling clay and flattened it out making it about 1/2" thick. I built walls all around the clay using Lego's and made the walls about an inch or more taller than the clay. Then I pushed the object into the clay about half of it's total thickness. Here's the tricky part. You need to get some small dowels about 1/4" in diameter and use a pencil sharpener to put a blunt point on them. Then you sink those into the clay at points around the part so that the blunt point is pointing towards the bottom of the mold and the thick part touching the side that will become the top of the mold. These will create air vents.

Next, you use styrene rod that is the same diameter of the blunt end of the pencil dowels. You can just warm the styrene in hot water so that it can be bent into 90 degree angles. What you need to do is to create a tube from the pencil dowels to the actual part to form a complete channel from the part to the dowel for venting air. It's important to get these air vents at points near the bottom of the part - the bottom is that area of the part when the mold is standing up ready for resin pouring.

The final step in making the mold is to create registration points. I would take a brand new pencil and press the eraser into the clay at various points to create dents in the clay. That way when you pour the silicone you'll get small bumps which will fit into small dents created when the second half of the mold is poured.

Once you get your mold set up with these air vents and the part, you're ready to mix the silicon, take the air out of it with the vacuum chamber, and do the pour. Let the mold sit over night to harden. Then you need a special mold release compound that is designed for rubber against rubber. I used one I bought from Smooth-on.

After the mold has set for 24 hours, remove the mold and the clay from the lego walls by tearing down the walls first, and then peeling the mold from the rubber. Now turn the mold over but leave the part in the mold. Rebuild the walls around the mold again making them an inch or more taller than the top of the mold and part. Then spray the mold with the mold release agent. Spray it good so that after you pour the second half, you'll be able to pry it apart.

Now mix up a second batch of the rubber, vacuum the air out, and pour it over the first mold to create the second half. Let it set overnight. When you remove the second half of the mold, you tear down the lego wall and handle it in your hands. It will be a little difficult to find the junction where the first half of the mold ends and the second half begins. Once you find it, you can use an Xacto knife to cut around this junction to pry the two halves apart. Just work around the entire mold prying a little bit at a time. It will eventually come apart cleanly thanks to the mold release you sprayed before you poured the second half.

Remove your part and the other stuff you used to create the air vents. You will probably have to clean out these air vents so that they create a clean path from the top of the mold to the part. I used tweezers to grab any fragments of rubber that was blocking the tube created, especially where the tubes meet the part.

For the resin casting, you'll need a pressure pot also on eBay. You probably already have an air compressor but if not, you can get this one on eBay for less than $60.00. The investment in the vacuum chamber with pump and the pressure pot and compressor will be about the same as your quote for $350.00 to make the mold, but now you'll have the equipment to make lots of molds.

Now you are ready to pour the resin. I used this resin from Alumilite for my resin casting. It's a 1:1 mixed resin that sets rapidly and is very strong. Spray both halves of the mold with the same release agent you used before. Put the two mold halves together and wrap thick rubber bands around the mold so it doesn't come apart. Mix the resin, pour it into your mold, put the mold in the pressure pot and close it up, turn on the air compressor and adjust the pressure for about 50 psi. Let it sit for 10 or 15 minutes. You'll see the resin turn beige which tells you it's almost hard. Wait a couple of minutes at that stage to be sure the resin is hardening. Then turn the pump off and remove the mold. Wait a few more minutes before removing the rubber bands and opening the mold up. You should have a part with no bubbles in it. Clean it up by removing the vent parts.

I've used this procedure hundreds of times in the years that I produced kits and always got good parts. You can get the resin in black even so that the parts don't have to be painted or you can add a special coloring agent which is liquid based. Just a few drops into the resin when you're mixing it works great.

That's the best way I know for casting good resin parts.

Hope that helps,

Bob
 
thanks bob for the information. I did realize after a few pours there was no way to get bubble free castings without a vaccuum chamber.
 
I sold my resin casting business in 2010 after heart surgery. Prior to that I had a line of figures and some ordnance and did casting work for other companies for 15 years. It took some experimentation but my process is outlined below. I am not saying I invented it. It was the result of talking with a lot of helpful folks in the same business.
I vacuumed both the mould and the casting
Making moulds.
I rarely bed the master part in clay or any material. I found it to be a tedious process and did not have the skills that would create a clean mould seam. I would spru the master piece by attaching the gate and a “well” to the part. This is positioned to give the best “deairing direction” when the well is oriented up. I would then glue the well to a sturdy plastic or cardboard sheet and build four walls around that. All mould box edges were taped but the top was left open (five sided box). Then I would pour the rubber mix over the master and to about 1” higher that the master. This might be a good time to mention that depending on the size of the mould the rubber contained by the box can put a lot of pressure on the box and it needs to be sturdy enough and seams taped well enough to contain it. It is also very viscous and the weight of it hitting the master during the pour can knock the part off the spru. Pour the mould carefully and as far away from the master as you can. I would drill a hole into the master and use a brass rod inserted in the master and the gate to give strength to the set up. I vacuumed the rubber in the mould box so the box walls had to be tall enough to hold the expanding rubber in the box. The rubber mix will expand and then collapse and I would keep it under vacuum for a minute or so after collapse. Then I let it cure for a day. This process pretty much ensures there will be no air trapped in the mould itself. Air in the mould can cause distortion when the casting is put under pressure.
I should have mentioned earlier that I would draw a seam line on the master before boxing it up and pouring the rubber. This would be a guide for cutting the master out of the mould when cured. Later I started taking digital pictures of the sprued up parts prior to boxing so I could see where I had to cut and what was ahead. I had a lot of different mould to cut and still have a poor memory. Pictures were very helpful.
Using scalpels I would cut the master out of the mould following the marked seam line and guided by the pictures. I started using my fingers as spreaders but blood make silicone rubber slippery and difficult to keep the spreading pressure going so after a while I clued into the fact that there were tools that would make the job easier. Probably 95% of the moulds were cut on only one side and the short cutting action created the keys. Rarely did I have to cut completely around the master.
Casting.
For me, it was a two part process. I would pour the resin into the mould and vacuum it. The resin boils up and is captured by the well that is attached to the gate and drains back into the mould. I would then top up and re vacuum. It is important that resin fills the part and overflows into the well/gate. It is also important that the well is big enough to capture the expanding resin or you will spend precious time topping up. Remember the resin is curing. Then the moulds were put under pressure and left until they were set.
I found the best materials for my processes were RenCast 205-3 (Huntsman?) and Silicones GI-1000. If I recall correctly the resin has a 7 minute kick time which was enough for the vacuuming and pressure.
 
I used paint pot as a pressure chambers and made my own vacuum chambers out of PVC fittings. I had them in different sizes. I think the largest was a 15" diameter. If you won't be making large castings you could use a 2.5 gallon paint pot as both. Just make a lid that has a gasket and an outtake valve and a release valve. That way you can vacuum and pressure in the same pot without having to shift molds. I used 1" Lexan for the lids .
 
this is how i made the mold for the mortar. I set it up in clay then built a box around it.
Nest i use plastic wrap and placed that in the mold and filled it with water, i did this to get the volum of silicone i needed for the mold. The large clear piece is used for the 3rd section of the mold. Once the 2 halves are poured in silicone then i will rubber band them together and pour the 3rd part. This will pour all the way down the mortar giving me the bore.

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after casting i could either drill out the bore or cast it into the mortar. i figure casting it into the mortar is faster and easier.

This was a test run and it was decided to eliminate the stand from the casting because it was actually made of wood.

i did try the idea of casting without a vaccuum chamber and you might get 1 out of 10 that is any good. The idea was to use a slow setting resin you paint the inside of the mold and let it setup, now you have a bubble free shell close the mold and pour in the resin. This works BUT it is slow and works one at a time, but when you want to do production casting it just does not work out.

casting will be done for the bomb vessel cross section as well as for the Blandford and battle station cross sections.

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With 3D design software and 3D printing it is possible to create realistic dioramas of battle stations with the crew that will have to be painted and everything finished.

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I wish was better at explaining things. I have demonstrated my failings in my earlier post.

If I had neither pressure or vacuum I would still use a one piece mold. I wouldn't need a deep well to allow for the expansion of the resin during the vacuum. I would spru from the rear (Pivot Point) and leave a large enough hole/slot to get the resin in quickly. I wouldn't have the wood struts unless they are part of the bbl.

Before mixing the resin I would paint the inside of the mold with talcum powder paying attention to any under cuts. Then I would blow out the excess powder. Rubber bands on the mold making sure they cause no distortion. For the first pours I would eyeball the amount of resin needed. You can fine tune this as you develop your routine. Mix the resin and half fill the mold Cover the gate and slowly turn the mold so that it will release bubbles and coat the surface. Don't over agitate and add bubbles. Set upright and tap the sides of the mold and slowly finish the pour not adding any bubbles. Maybe pour down a stick. I had a friend that did not use pressure or vacuum to make his filled castings . Once the casting was poured he would blow across the top surface (gate). He believed that it reduced pressure and cause bubbles to rise. I am not so sure. He would still get a bubble or two but had the luxury of being able to patch and polish so a bubble did not set him back.
 
step 1 is to turn the masters i use Delrin which is a machinable plastic, very tough and machines smooth. Once the cannons are turned they get drilled for the trunnions.

setting up the mold some like to use the single mold method of pouring silicone as a solid block and then cut the mold in 2 pieces. This method works great, i have done it a number of times. This method is to place the cannon into molding clay. Years ago i use to make plaster molds for casting ceramics so i am quite fast at setting up a mold.
Looking at this setup first there is a little block at the bottom of the trunnion this is because as the resin pours into the mold it trapps air in the round trunion, the cavity at the end is where air will be trapped as i pour. Next is the pouring chamber at the top this sits against the end of the trunnion. The resin does not "flow" into the mold i can inject it or dribble it in until the mold seems full then continue to fill the pouring chamber about half full. finally is a Teflon plug this extends down into the cannon , When the cannon is cast the resin does not stick to the Teflon so when i remove the cast cannon i can plull out the plug and the bore will be cast into the cannon.

step 2
i now have to make sides for the mold



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