Moonliner Molding info - The TECHNIQUE of Model Building - All - ATOMIC CITY - Message Board - Yuku

RSS
Email

0 Kudo

Moonliner Molding info

<< Previous Topic Next Topic >>

Author	Comment
CaptCBoard	Moonliner Molding info	Lead	[-]
Posts: 1250 (07/30/07 23:30:53) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	Tags : None You guys have been very patient and your patience is about to be rewarded. Yes, its ALMOST true... Your kits will be coming soon! I thought I'd be posting much better news than this, but this really isn't so bad-- its just frustrating... What you see here are two fuselages. The one on the left is from the old molds, the one on the right is from the new molds. We had some alignment problems with the old parts. The reason I still have the parts seen on the left is that they were too bad to be sent out. As those molds were used, their weaknesses were exposed and I had to plan out how to replace them and get better parts. The solution was to reverse how they were poured. Originally, they were poured with the open end 'up'. Now they are poured with the open end 'down'. The reason this wasn't done in the first place is the other way was the simplest, from a pouring standpoint, and I could not forsee any real problems. But, all is not well with the new mold of the bottom half of the fuselage. What you see on the right is the way this detail should look, but the image on the left shows the way it comes out. Why does it do this? It took me about 15 minutes to figure it out and it boils down to something I neglected to do. Can I explain it here? Sort of. Have a look at the next shot... This is the problem area. As you can see it is really screwed up, but the master is in fine shape. That is, the outside of the master is in good shape. Now follow along carefully... Here is the master for the lower half of the fuselage and the mold that was made from it. At this point, all we have is a mold of the outside of the part. Unfortunately, this master is not hollow, so we have to resort to mechanical trickery to generate a hollow part-- but I don't have the photos to describe how that is done. Just know that it was done and I'm going to use one of the parts from the first molding to help us here... Here you can see an important new feature of these molds (its the same on the mold for the upper fuselage.) The angled area is not a part of the master, as such, since it is not a part of the finished part. It is a shape that will keep the two halves of the mold from over-compressing, but at the same time providing a good seal, so the resin doesn't leak out. Because this end of the mold makes the open end of the part, maintaining this shape is very important. Another way to look at the elements in this photo is to know that the mold (pink rubber) was poured around the master and its mount. The gray part of the master is the part we want to wind up with, while the brown part of the master is used to create an accurate mating surface. This is what we are after-- the inside of this casting. This casting has a good wall thickness and was centered very well when it was cast. That means one side of the casting is not much thicker than the other, so the wall thickness is very even over the entire part. The hole at the end is part of the new mold arrangement. Just remember it is there and when we get to why, you'll get it... This is the outside of the same part. I have worked this casting over with a sander so that when I put it into the mold, none of the details in the mold will hang up on the part. Because the details are reversed in the mold, they will just float in these spaces and not become distorted or guide this part in ways that would ruin its registration. To describe the use of this part in different terms-- What I want to be able to cast is a hollow part with an even wall thickness. To do that, I need to create that part of the mold that will take up almost all the space inside the mold, leaving only empty space between it and the surface of the outer part of the mold, where the resin will go to make the new part. Since I already have a mold of the outside of this part, when I put this modified casting in place, it takes up the space the resin needs, leaving only the space I need to fill with rubber to create the new interior 'core' part of the mold. And if I had a photo of this part IN the mold, you'd see what I mean! So, here I've cut the mold in half and seated it into its jacket. Since the mold is no good to me, doing this won't hurt anything and it makes a great visual aid! Note that at the bottom, there is a lip. This lip and the larger, similar feature at the top assures the mold will stay in registration to the jacket-- and hopefully defeat any shrinkage that might try to creep into the mold. In this shot, you can see how the core part of the mold fits. You can also see the space where the resin will flow to make the part. At the bottom, you can see an aluminum rod in place. This keeps the core centered at that end, so the wall thickness of the part is as even as possible. This close-up of the mating surfaces shows how the seal works. The conical shape assures proper centering, while the blocks that interrupt the conical shape insure the mating conical surfaces don't squirm past 'zero'. In other words, its possible that without the blocks, the mold could be over-compressed, flairing the end outward and distorting the part. Remember, rubber being flexible-- anything can happen! This shot shows how the part is poured. Note that there is a hole in the exact center of both halves of the mold. This is for the alignment peg and that is why there was a hole in the end of the part used to form the core. When the rubber was poured, the alignment peg was in place to create the hole you see here. The other two holes are the gate and the vent. The hole below the center hole is where we stick a funnel and pour the resin in. The other hole, above the center hole, is the vent, which allows the air inside to escape when the resin is poured. This last photo shows this in a different way. You can see the centering peg and behind it is the gate. I've stuck a pipe-cleaner in the hole so you can see how this works in cross-section. And you can also see the reason why the exterior part of this mold was ruined... See that nice, crisp line near the end of the core? To create the uniform shape of the end of the core, I had to saw the end of the resin casting off so I could dress the surfaces. Having done that, I glued the end back on. My intention was to put a very small amount of resin into the casting and swirl it around, sealing this joint. I forgot to do that. I stuck the casting in place and poured the rubber in. It seeped through this incredibly small joint and stuck to the inside of the exterior part of the mold. Most of what seeped through pulled away when I opened the mold, but I never knew about the big blob that stuck right on one of those strut-plates until I made the first casting! A new kit of rubber arrives tomorrow (July 31) and I'll make the NEW exterior half of this mold the next day. And THEN we'll have a GOOD mold for the lower half of the fuselage and we can start getting these kits out! I know this was probably way too much information, but I needed to get some use out of this $100 worth of rubber! Scott CaptCBoard@AOL.com
ignatzprime	Re: July Update	#1	[-]
Posts: 186 (08/01/07 06:26:43) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	Ummm. WOW! I hope you keep all of your mold-making expositions somewhere accessable Scott. They are the best problem-solving tutorials I've ever seen on the subject! How did you determine the shape and size of the interior plug? Looking forward to the Moonliner kit too. It's great to see it coming along. Steven Lee Forbidden Plastic
CaptCBoard	Re: July Update	#2	[-]
Posts: 1250 (08/01/07 09:42:19) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	Interior plugs are relatively simple to achieve, though they involve more steps than one would like. So, to describe this process as simply as possible-- 1. Cast a part completely solid. This would be a complete duplicate of whatever was molded. 2. Using 'something' (table saw, band saw, router) you need to cut a criss-cross pattern of lines. These lines should be no more than 1/4 inch apart and completely cover the casting in something that resembles a waffle pattern. The lines need to be completely uniform in depth and as deep as you intend the wall thickness of the finished casting. In the case of the Moonliner, the wall thickness is almost 1/4 of an inch, so that's how deep the lines were. What you wind up with is the original surface represented by a bunch of tiny blocks and the new surface as the bottom of the lines (grooves). 3. Using a chisel, remove all the tiny blocks. Now you have a part that is uniformly a bit smaller than it originally was. Sand and fill this surface until it is completely smooth. You want it to shine. You do NOT want to put primer on it. Once you've got this done, apply a coat of car wax. This thing should be really slick and shiny. 4. The exact way this step occurs will depend on the shape of the part, but what you want to do is put this 'core' into the mold and pour resin into the gap that is left. You'll need to figure out a way to keep the core centered and that method may or may not be something you need to include when you made the mold. For instance, when the Moonliner mold was made, I had to include the centering peg at the closed end of the mold. If the part being made is like half of an aircraft fuselage, then the core can be suspended using a couple of sticks. These would be glued to the backside of the core and would bridge across the cavity, resting on the material outside of the cavity. 5. When the resin cures, you now have a solid part again, but with some work, you can get the core to separate from the casting. In the case of the Moonliner, I had to cut the part from top to bottom on opposite sides. The result was a part that was in two halves once I pried them loose from the core. I carefully put these two halves back together, replacing the material the saw removed with plastic strips. In the case of something like the airplane fuselage, you now have the finished part and can make the other half of the mold. Just be sure to put release on the rubber, even in the mold cavity. Reseating a part into a mold so you can pour the second half can not be done without risk of rubber from the new pour creeping under the part. Without the release, this rubber would ruin the original cavity. For things like the Moonliner, we move on-- 6. Sand off all the detail on the outside of this part. The reason for this is we don't want the part to engage anywhere because reseating it into this long, deep mold is going to be difficult enough! Once this is done, this master is finished! This part is now the master for the inside only. Any new mold made in the future will now be made by first molding the outside from the lathe-turned master and then doing the next step-- 7. Seat the part back into the mold. Since we don't need to worry about rubber seeping much past the open rim of the part, applying the release over the entire mold is not necessary and would be a hassle to clean out. However, using talcum powder to make the mold slippery is a good way to go here. Once the part is seated, the rim is sealed with vaseline (that's what I use for releasing rubber). So now we have the mold with a hollow part in it, just like what we are going to wind up with in the future. But, to be able to cast these hollow parts, we now need to pour the core half of the mold. In the case of the Moonliner, you have seen the result. Of course, part of planning all of this is also planning how you're going to get the castings out of the mold. In the case of an aircraft fuselage, that's easy, it just pops out. In the case of the Moonliner, each half of the fuselage is buried in rubber and has a huge amount of rubber contained within it. Neither is easy to remove from the casting. So, I made a small vertical slit in the side of the mold, at the closed end. Through this slit I poke an air nozzle and blow air into the space between the mold and the part. The mold inflates a bit, like a balloon, pulling away from the part and actually blowing the part a bit out of the mold. From there I can wrestle the part out. The core is removed with air, but in this case I can blow the air through the centering hole because the peg has been removed. The core blows back enough I can then wrestle it out of the casting. Maybe I'll shoot video of this some day. Its pretty cool to see! Scott
Unregistered(d)	Update News	#3	[-]
Posts: 0 (08/02/07 14:18:11) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	I knew you weren't just sitting around ignoring the Moonliner! You've been working hard for us! Thanks, and looking forward to seeing the kit at my door!
Unregistered(d)	Please?	#4	[-]
Posts: 0 (09/10/07 17:47:36) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	Hey Scott, how about an update? Please?
blackfire2015	Re: Please?	#5	[-]
Posts: 35 (10/13/07 20:36:49) Reply Quote More My Recent Posts Message Me Connection Blocking Invite Ignore User's Posts	Scott, Any movement on the Moonliner?

<< Previous Topic Next Topic >>

Quick Reply