Life is a scratchbuilding project
Build On &
Cab & Chassis Basics
March 19-22, 2016 : The next step in the process is to create a mold to make good, final copies of the wheels we will ultimately use on the model. To do this I needed to make a mold that gave me both front and back details. Most who might do this would engineer a mold where you would pour in the casting material through a filler port and might have some vent ports as well, but even taking those precautions, air bubbles often mar the final product. To address this, I do something a little different. Keeping in mind that bubbles rise the trick is to engineer both the mold and the technique to minimize bubbles getting trapped on the surfaces of the mold and marring the casting. So how do I do this?
The principle is this: A smash or slam mold is a two part mold that works like a peanut butter and jelly sandwich. A great PB&J sandwich gets peanut butter on both pieces of bread, then jelly on both pieces of bread. The two sides are then quickly smashed together making a terrific, loaded PB&J sandwich. A great wheel casting using a smash mold works the same way. You create a two part mold, then smear casting resin into each side as the halves of the mold are laid open like two pieces of bread. Once resin has been applied to the surfaces, working the resin quickly into the details and crevices, the balance of each side is then filled with resin. When the resin starts to flash or cure, the two sides of the mold are slammed together like a PB&J sandwich. When the resin cures and the parts are demolded, the cast parts will have a thin amount of flash around the seam line, but be free of the usual bubble problems since all the bubbles had a chance to rise away from the detail surfaces.
So to create the mold, you also have the think about doing it in a way that makes the bubbles rise away from surface details. The air bubbles are the result of the catalytic reaction of the two-part mold making material or the two-part casting resin, but in either case you want the bubbles moving away from details.
In this case, I first created a standoff for the wheels that would essentially suspend them in the mold making silicone as it dried. I used a few small pieces of Evergreen strip plastic to attach the wheels to a pair of Legos. To insure the super glue would adhere to the side of the Legos, I roughed up the surface first with a few passes of 400 grit sandpaper to remove the gloss from the plastic.
Instead of just building a box and pouring – which would have created bubbles against the underside of the masters – I flipped the masters upside down and applied a small amount of mold making compound on the back surface of the masters and set them aside to dry. Since silicone RTV will fuse to itself unless some kind of mold separating agent is used between the layers, I knew I could create a bubble-free mold facing that could be fused to the rest of the mold in the next step.
Once this first pour cured, I cut away any excess and then built a box that suspended the two masters in a Lego box mold. I then added a sheet plastic bottom to the box and taped it to the box with clear packing tape. This also sealed the Lego box to keep any seepage under control. I then poured enough RTV into the mold to fill the mold half way up the masters. It was then set aside on a level surface and allowed to cure. Note in some of the photos the air bubbles that started to rise to the surface as the RTV cured. Next, we’ll prep the mold for the second pour, building in a couple of alignment keys, and doing the actual second pour.
February 24, 2016: I've been thinking about doing this for a while and then a good friend and modeling buddy, Dan Baker from Cactus Car Modelers and Moonlight Modelers here in Phoenix said something in passing about finishing "epic" projects and I realized I hadn't worked on (or finished) anything relatively simple in a long time (read: "years"). So The Beast seemed like a pretty good idea.
The project got off to a roaring start. I had to extend the chassis, and had to recraft a set of wheels to narrow them and get them closer to what was on the truck. The dump back is made of sheet plastic treadplate and now has removable sides and a working tailgate. Here is The Beast model just roughed out to check for sizing, fit, and other details. I have some details to sand off of the cab (badges on window pillar, marker lights going on the wrong direction, door handle globs...) and I think I'll need to add some spacers under the back to lift the back end a little, but it's coming along. More to follow, but REALLY trying to keep it simple. Honest Dan, I am. No, Seriously... (lol)
Dateline: April 18, 2016 - Over the weekend I was able to do a fair amount of work setting up the hinges so the dump body would tilt. While I was at it I made sure that I had the wheels spaced out from the chassis and had the right amount of lift on the axles. The tailgate is hinged and drops like the real one does, the dump body tilts back and the lift hinge functions, though I have a little refining to do to get it aligned a little better. All in all, the project is coming along and I hope to get the chassis done and detailed as the next step. Stay tuned!
Creating the second generation mold that become the Wheel & Tire first generation masters
The Beast of Goose Pond Scout Reservation
Creating "Final" Masters of the Wheels & Tires
Most of my projects begin with a real vehicle or prototype that I then try to replicate. These are some samples of the real thing and you can click on the link below to take you to other material used to build the model.
February 29, 2016: After completing the rims and mounting them in tires, I glued them to a piece of thick sheet plastic and then built a box around them with Legos. This was sealed with clear packing tape on the outside of the box. I then mixed some Alumilite High-Strength 3 silicone RTV mold making compound and poured it over the tires and rims. As the RTV cures, gas bubbles are created and rise to the top (the bubbles in the mold photo) These rise AWAY from the masters glued to the bottom and the mold should be ready to de-mold in about 12 to 18 hours of good curing.
When I was the Ranger for the Northeastern Pennsylvania Council of the Boy Scouts, we had a light dump truck that was called "The Best". We called it a lot of other things too: "Frankentruck" and "Truckzilla" were just two of the other nicknames, but they all came from the fact that the thing was a real mishmosh of parts taken from all sorts of places. It was originally a military pick-up with a diesel engine. Our volunteers pulled the diesel out and replaced it with a Chevy 350ci V-8 topped with a Holley 4-barrel carb. Out the back end, the exhaust system featured a pair of recycled "Cherry Bomb" Thrush mufflers. And when I first came on board, the truck hadn't been painted yet, so you could clearly see that several parts had come from other trucks. Even the electric dump body which replaced the original pick-up back belonged on something else before being mated to The Beast.
It was brutal to plow snow with the thing. Goose Pond Scout Reservation (GPSR) is a topographic punch bowl with very narrow roads, so one mistake and you were literally into the woods. Still, it was a good old-fashioned truck, and that is the reason I decided to try building it.
Using first generation copies to to begin creating new masters
CREATING THE FINAL MASTER MOLD - PART 2
CREATING THE FINAL MASTER MOLD - PART 1
March 23, 2016 - Once the first pour cured, I cut three keyways out of the cured RTV to help align the two halves when they would be slammed together. These alignment keys were nothing more than two corners cut out of RTV and another piece cut out of the center of the other end. I then applied a good coat of Vaseline Petroleum Jelly to the surfaces and keyways. The Petroleum Jelly will keep the two pours of RTV from bonding to each other and I prefer to use it instead of mold release agent. Mold release agent has to be applied in several coats and I never seem to get enough of it applied while I’ve never missed with the Petroleum Jelly.
Once the mold was prepared I then poured in the second batch of RTV and set it aside on a flat surface to cure. About 20 hours later the RTV was fully cured and I started to demold the masters by first removing the plastic packing tape, then the bottom plate, and finally all the Legos. The two halves of the mold separated easily and with a little coaxing, the masters popped right out of the mold halves. The molds are now ready for casting a few duplicate parts.
After some prompting from Dale Mickley and Dan Baker, I decided to try a "simple" scratchbuild, so the idea was to build this truck without going too crazy. We'll see how that works out (LOL!).
March 12, 2016: The next stage in obtaining wheels and tires that look more like the ones on the real truck were to take the copies I made from the earlier mold and thin them. The width of the original rubber tire was too wide - would that I had actually had tires that wide and aggressive when plowing snow with the real thing!! So I had to thin them and then, er, ah, "tone down" the tread - all to create a new set of final masters so I can create another mold. From THAT mold I'll have tires and wheels I can use on the model.
I used a Benchtop belt sander to thin the tires (THAT was exciting!), then cut the wheel centers out of the back halves of the pairs. The cutting bit in my Dremel made quick work of that and was easy to control. I thin glued the halves together with Thick Gap Filling Super Glue and sanded the tread and tire edges to get just the right nearly bald look. The last photos show how the 2nd generation masters now differ from the first generation originals.
February 25, 2016: The real challenge with The Beast was finding wheels and tires that had the right look, diameter, width, and lack of tread. It took a while to find rims that were close, but they were too wide and the faces too deep. Tires were also a problem, but that was the easier fix as long as I made both rim and tire out of something machinable. I started by creating a mold of suitable originals that, when copies were made, would act as donors that could be made into something closer to what I really wanted. Why didn't I just modify the originals? Well, because I've found it a safer practice to screw up easy to make copies than to ruin rare originals. So the photos below show the process from laying out originals, to casting copies, to machining and thinning the rims, to mounting them in donor tires which would then be turned into another mold.
Dateline: March 6, 2016: Over the weekend I received a bottle of green lacquer paint from Model Car World and was able to get the first coats onto the cab to check the color and look. The color looks spot-on to the real truck and went on beautifully. I'll give it through the day today to cure and wet sand it tonight. In the meantime I've spliced together the chassis and have shaved off the rear end so I can lift the back of the truck by several millimeters. I also painted the interior tub and dash with their base colors. I'll add a "cloth" seat cover and the instruments before completing the interior tub. It will have to be complete and have to be installed before the final coats of paint can go on the cab - just the way the cab is with the undercut, the back wall of the cab is a separate piece, so this will take a little finagling.
Dateline: March 26-27, 2016 - Casting in a slam mold is easy, but it is also a little messy, so avoid doing this on good furniture, counters, or anything precious to your significant other or Landlord. You start by mixing up a batch of casting resin – I use Alumilite – and pour the material into each half of the mold, slightly overfilling the mold. Make sure you start by working a quick surface layer of resin into the mold with a popsicle stick or some other tool to work out any small bubbles and then pour in the rest.
Alumilite has a fair amount of surface tension, so pouring it so you end up with just a bit over the top edge of the mold is easy. Let the two halves of the mold lay side by side with the casting resin beginning to cure and watch for it to “flash” or seriously set-up. With Alumilite this doesn’t take very long and the color change it goes through as it cures couldn’t be more obvious. It will go from a nice, clear amber or honey color to a solid light cream and it happens very quickly once it starts to flash. As this begins to happen, I take the two sides of the mold and smash them together. This will cause excess resin to ooze out between the mold halves and out the openings where the stand-offs attached the masters to the Lego box, but all of that is OK as long as your work area and you are appropriately protected. Set the mold aside on a flat, level place and allow the resin to fully cure.
If you did it right, the finished casting will be free of surface bubbles and have few or no voids in between the two halves which will be fused together. There may be some flash (excess resin) to clear away from the mold seams, but that is easy enough to do with a model knife or razor saw. A few passes with some sandpaper and you should be all set.
I prepped a set for the truck and quickly discovered that the axels were really too narrow for the truck. I wasn’t too keen on them anyway – they were only partials to start with, so I scrounged around for some other axles and some stand-offs and found exactly what I needed. The new tires also helped me set the height needed for the rear axles, so the next step will be to get the width right and begin detailing the chassis.