Regular readers know I like to work with a variety of materials. Playing with new materials and methods can be fun.
Subject here is a set of intake ducts for a cooling plenum. They're typically fiberglass/epoxy, fixed solidly to the engine, and coupled to an aluminum ring in the cowl with a silicone hose, neoprene wet suit material, or other soft tubular material to allow relative movement.
My cooling and plenum scheme doesn't follow current RV convention; I've gone the low velocity inlet route (which means large inlets) and relocated them so they no longer center on the cowl split line. The whole cooling scheme is a subject for another thread; this one deals with an experimental method for constructing one of the components. Somewhere in the design process I got thinking "Why can't the whole duct have some flexibility?" and this was the result.
If you look at any of the available silicone hose couplers you'll see that they are constructed with one or more glass fabric reinforcements molded into the rubber. To do something similar I needed a room temperature curing rubber material with low enough viscosity to impregnate glass cloth, and the right physical properties (Shore A hardness, service temperature, tear strength, etc) to handle the application.
The rubber I selected is a two-part urethane primarily intended for concrete molding, although it has lots of other uses.
http://www.reynoldsam.com/index.php?cPath=6_1117_1142
Spec sheet here:
http://www.smooth-on.com/tb/files/Vytaflex_Series_TB.pdf
I used the Shore 40 material. Another builder will soon be trying the Shore 50, mostly because of the longer pot life. Both are 2000cps viscosity, thin enough to soak glass cloth with a little help. Service temperature is not listed, but the tech guy says 180F, above which it drops off in Shore and may lose some strength. In any case, it's not likely to mimic the Wicked Witch of the North at engine compartment temperatures.
Two of the $25 "sample kits" was enough to do two plenum ducts plus a few experiments. This material mixes by weight or volume. They also sell a compatible pigment. I used black, one drop for each 20 grams of mixed rubber.
Fabrication method is straightforward. You'll need a male plug for the desired shape. It can be foam, plaster, wood, or anything else you like to work. In this case I used blue block foam filled with micro, sealed with epoxy, then sprayed with a a cheap high build primer and some ordinary Rustoleum enamel.
The final surface finish must be very good as the rubber will precisely mimic the finish. Unlike a conventional glass layup, there is no opportunity to surface fill or rework the finished part later; what you got is what you'll live with.
Wax the plug with two coats of mold release wax or plain carnuba, then spray it with two coats of PVA or similar.
Pre-cut glass cloth to the general shapes you need to cover your plug. Gather a roll of 4 mil clear plastic sheet, a pizza cutter, a roller, and a box of nitrile gloves.
Mix some rubber and paint the prepared plug with a first coat. Wait until it gels and paint it with another coat. You can do a third and fourth coat if you wish to build more thickness; what you're doing is much like a gel coat in a polyester layup. The painted coats provide a nice surface on the finished part and help hide any little bubbles and flaws in the subsequent fabric layups.
The "until it gels" part is important. This urethane doesn't bond well to cured urethane; you must add layers before the previous layer cures. That means when you start the layup process you must work all the way through to the end without pause, at a rate appropriate to the pot life of the material. It will pay to have everything ready and at hand; for example, that's why you precut the cloth.
Impregnate the cloth by laying down a sheet of 4 mil plastic, then a sheet of cloth. Pour mixed rubber on the cloth. Cover it with another plastic sheet and use the roller to spread and work the rubber into the cloth. It is easy to see how you're doing, in particular with black pigment in the rubber. You can even pick up the sandwich and flip it over to see both sides. It is no harder to work into the cloth than the same process using West epoxy.
Roll excess rubber to the edges of the sheet, use the pizza cutter to trim the excess plastic and rubber. Peel one plastic layer and position the wet sheet on your plug. Peel the other plastic layer, then use your gloved hands to smooth the wet fabric over the plug. Wipe in opposing directions with both hands (it won't slide around if you use opposing force) to push any trapped air bubbles out from under the fabric. I found it was easier to chase air out of the rubber layups than with West.
Proceed like you are doing any multiple ply glass layup; alternating overlaps, trim excess unsupported fabric at the edges, etc. The gel behavior of the rubber has a nice benefit; previous layers don't slide around much if at all.
Heat accelerates cure, and post cure improves properties. As my ducts were black, I just put them out in the sun. Removing them from the plug wasn't too bad. A few blasts of compressed air between plug and part separates them. Like any rubber there is a lot of surface friction between the rubber and a smooth surface, so initially slipping the part off the plug was like pulling a bicycle grip off a handlebar. Although I didn't try it, I'm pretty sure a spritz of water between the plug and part would make the PVA mold release slippery and aid removal.
Here's the finished product. The aluminum ring and black foam rubber seal were both added after trimming; they're germane to the particular cowl sealing scheme I'm using for the large, off-split inlets.
I'd suggest future experiments involving the number of glass plies, ply orientation, and fabric weight. The finished flexability of the rubber duct is mostly governed by those fabric factors, as well as the shape of the part relative to expected movement. With 4 plies of 8.9 oz fabric, this duct is by no means anything close to being described as "floppy". The actual feel is something like the black baffle seal material supplied in Van's baffle kits, maybe a little softer.
I think the method has possibilities for cooler ducts and a bunch of other applications. Have fun.
Subject here is a set of intake ducts for a cooling plenum. They're typically fiberglass/epoxy, fixed solidly to the engine, and coupled to an aluminum ring in the cowl with a silicone hose, neoprene wet suit material, or other soft tubular material to allow relative movement.
My cooling and plenum scheme doesn't follow current RV convention; I've gone the low velocity inlet route (which means large inlets) and relocated them so they no longer center on the cowl split line. The whole cooling scheme is a subject for another thread; this one deals with an experimental method for constructing one of the components. Somewhere in the design process I got thinking "Why can't the whole duct have some flexibility?" and this was the result.
If you look at any of the available silicone hose couplers you'll see that they are constructed with one or more glass fabric reinforcements molded into the rubber. To do something similar I needed a room temperature curing rubber material with low enough viscosity to impregnate glass cloth, and the right physical properties (Shore A hardness, service temperature, tear strength, etc) to handle the application.
The rubber I selected is a two-part urethane primarily intended for concrete molding, although it has lots of other uses.
http://www.reynoldsam.com/index.php?cPath=6_1117_1142
Spec sheet here:
http://www.smooth-on.com/tb/files/Vytaflex_Series_TB.pdf
I used the Shore 40 material. Another builder will soon be trying the Shore 50, mostly because of the longer pot life. Both are 2000cps viscosity, thin enough to soak glass cloth with a little help. Service temperature is not listed, but the tech guy says 180F, above which it drops off in Shore and may lose some strength. In any case, it's not likely to mimic the Wicked Witch of the North at engine compartment temperatures.
Two of the $25 "sample kits" was enough to do two plenum ducts plus a few experiments. This material mixes by weight or volume. They also sell a compatible pigment. I used black, one drop for each 20 grams of mixed rubber.
Fabrication method is straightforward. You'll need a male plug for the desired shape. It can be foam, plaster, wood, or anything else you like to work. In this case I used blue block foam filled with micro, sealed with epoxy, then sprayed with a a cheap high build primer and some ordinary Rustoleum enamel.
The final surface finish must be very good as the rubber will precisely mimic the finish. Unlike a conventional glass layup, there is no opportunity to surface fill or rework the finished part later; what you got is what you'll live with.
Wax the plug with two coats of mold release wax or plain carnuba, then spray it with two coats of PVA or similar.
Pre-cut glass cloth to the general shapes you need to cover your plug. Gather a roll of 4 mil clear plastic sheet, a pizza cutter, a roller, and a box of nitrile gloves.
Mix some rubber and paint the prepared plug with a first coat. Wait until it gels and paint it with another coat. You can do a third and fourth coat if you wish to build more thickness; what you're doing is much like a gel coat in a polyester layup. The painted coats provide a nice surface on the finished part and help hide any little bubbles and flaws in the subsequent fabric layups.
The "until it gels" part is important. This urethane doesn't bond well to cured urethane; you must add layers before the previous layer cures. That means when you start the layup process you must work all the way through to the end without pause, at a rate appropriate to the pot life of the material. It will pay to have everything ready and at hand; for example, that's why you precut the cloth.
Impregnate the cloth by laying down a sheet of 4 mil plastic, then a sheet of cloth. Pour mixed rubber on the cloth. Cover it with another plastic sheet and use the roller to spread and work the rubber into the cloth. It is easy to see how you're doing, in particular with black pigment in the rubber. You can even pick up the sandwich and flip it over to see both sides. It is no harder to work into the cloth than the same process using West epoxy.
Roll excess rubber to the edges of the sheet, use the pizza cutter to trim the excess plastic and rubber. Peel one plastic layer and position the wet sheet on your plug. Peel the other plastic layer, then use your gloved hands to smooth the wet fabric over the plug. Wipe in opposing directions with both hands (it won't slide around if you use opposing force) to push any trapped air bubbles out from under the fabric. I found it was easier to chase air out of the rubber layups than with West.
Proceed like you are doing any multiple ply glass layup; alternating overlaps, trim excess unsupported fabric at the edges, etc. The gel behavior of the rubber has a nice benefit; previous layers don't slide around much if at all.
Heat accelerates cure, and post cure improves properties. As my ducts were black, I just put them out in the sun. Removing them from the plug wasn't too bad. A few blasts of compressed air between plug and part separates them. Like any rubber there is a lot of surface friction between the rubber and a smooth surface, so initially slipping the part off the plug was like pulling a bicycle grip off a handlebar. Although I didn't try it, I'm pretty sure a spritz of water between the plug and part would make the PVA mold release slippery and aid removal.
Here's the finished product. The aluminum ring and black foam rubber seal were both added after trimming; they're germane to the particular cowl sealing scheme I'm using for the large, off-split inlets.
I'd suggest future experiments involving the number of glass plies, ply orientation, and fabric weight. The finished flexability of the rubber duct is mostly governed by those fabric factors, as well as the shape of the part relative to expected movement. With 4 plies of 8.9 oz fabric, this duct is by no means anything close to being described as "floppy". The actual feel is something like the black baffle seal material supplied in Van's baffle kits, maybe a little softer.
I think the method has possibilities for cooler ducts and a bunch of other applications. Have fun.
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