Procedure
SUMMARY OF NOSE CONE MOLD MANUFACTURING
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- Make sure you account for the thickness of ablative/whatever mold prep you're using when you make the mold (for example, if you said you'd use gel coat and increased the size of the mold, you either have to use gel coat or make a new mold because using epoxy to fill it in is not the best/most accurate solution)
- Don't use Fibrelease if you're using wax
- Don't fill cracks in with Bondo- it's difficult to work with and surface of the mold won't sand as easily/epoxy won't permeate the Bondo
- Heat guns are effective in speeding up curing
- Sanding wax is difficult. Make sure you only add wax when you're sure you're ready
- Alignment dowels work well but ball bearings are probably better
- Epoxy sides of mold, not just inside the mold, and apply wax to ease mold release
- Use smaller squeegees for applying the epoxy because larger ones won't fit the curvature of the mold as well
- Make sure to account for the fact that the mold will have to be clamped down before you glue the MDF layers together
- Make sure your step size is no larger than 1/2 the diameter of the tool you're using (in our case, a step size of 0.25" was fine)
- Don't skimp on glue when gluing the layers of MDF together or you might get cracks in the mold where the layers come together
Below is the recommended procedure for making a nose cone mold, learned through trial and error and thanks to the advice of Prof. Mark Drela, an leading expert in aerodynamics and composite layups.
- Obtain necessary supplies
- Medium-density fiberboard (MDF)
- Layup supplies (squeegees, acetone + shop towels for cleaning)
- Clamps
- Sharpie
- End mill for router
- Sandpaper + spray bottle of water
- Mold release wax
- PVA film (from Fibreglast)
- Fine-haired brush
- Bearings or dowels for alignment holes
- Get access to necessary technology
- Router: like a giant mill, drills and moves laterally to route out mold. There's one in the Hobby Shop (which you have to become a member to access) and in Gelb.
- Horizontal bandsaw or tablesaw to cut MDF
Notes about Gelb router:
- CAM program is called "Cut3D" and is a dedicated toolpath for CNC machining
- Make sure to orient part correctly, paying attention to the axis along which you want to cut it. There is usually more than one way to orient the part, just make you set it up on the router according to how you defined it in the software
- Cut3D gives you the ability to scale parts if you need
- The "roughing cut" removes most of the material. For this mold Cut3D says it takes 23 minutes (will vary depending on what you're routing)
- The "smoothing cut" finishes the mold by smoothing the surface. Cut3D says it takes 56 minutes for this mold.
- Make sure you select the proper "feed rate" and "plunge rate." This depends on the material you're using.
- Make sure that the toolpath is correct and save it. Give the file time to download onto the USB
- When routing, make sure that the tool does NOT intersect the table
- Put a foam block under the tool when you turn the power off or it will crash into the table.
- You can either use the vacuum attachment while routing or vacuum periodically while it's routing.
- CAD mold using CAD of nose cone
- Check out Solidworks mold-making tutorials for help
- If you want a linear section at the base, determine how long (you want at least 2" for enough engagement with the coupler or things will be bendy/not rigid)
- Include holes for alignment pins (around three on each side of the mold). Half-spheres work even better because you can put a bearing inside and alignment will be easier/more accurate.
- Prepare Mold Material (MDF)
- Measure: The amount of MDF you need depends on the thickness of MDF you're using, and the dimensions of the nose cone you want to make a mold for.
- Mark the cut lines in Sharpie, accounting for the thickness of the blade that you're using to cut the material.
- Cut: Using a bandsaw works, but a table saw might be better. Basically you want the lines to be as straight as possible and the pieces you're cutting might be pretty large, so plan on how you're cutting in advance.
- Have more than one person helping you, because cutting large pieces of MDF by yourself is difficult.
- Either cut slots in the side so that you have something that you can use to clamp, or attach brackets after the layers are glued together (described below)
- Epoxy layers together
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- Route mold
- Prepare CAM
- Set finishing passes (the step size for the roughing cut should be no greater than 1/2 the diameter of the tool you're using)
- Verify that the tool head would not run into the wall of the nose cone while routing (this can happen if you're cutting too steep of an angle: see images below)
- Attach mold to router bed. You want this to be secure because if the mold moves around while you're routing, your mold will be ruined. (if you didn't cut slots into the MDF layers before epoxying them together, add brackets so you can attach securely to the routing table)
- Route!
- Don't wear clothes you care about. Have vacuum ready because lots of sawdust will be generated.
- Ground yourself by touching the router (but make sure not to be in the way of any moving parts) or you'll probably get shocked.
- Zero the router (zero for router and CAM file will be different so make sure you're zeroing correctly)
- Do a test pass offset above the mold
- MAKE SURE THE ROUTING TOOL DOES NOT CUT INTO THE ROUTER BED
- If anything goes wrong, pause if necessary (instead of stopping the router altogether, set the speed to 1% so you don't have to re-zero everything after. It won't technically be "stopped" but it will be moving very slowly so you're effectively pausing it). Or if you're panicking and don't have time to set the speed to 1%, you can also just press the E-stop button.
- Prepare CAM
Mold Preparation
- Sand mold to 500-grit. Sanding takes a long time so prepare for this!
- When sanding, make sure to protect the edges of the mold with something straight like a piece of aluminum. If you don't do this, the edges will be curved and when your part comes out of the mold it will have an annoying seam that you later have to sand off.
- Wet sand mold 800-2000 grit
- Add one layer of epoxy
- Mix epoxy (slow-cure)
- Apply thick layer and let to the inside of the mold, the top, and sides of the mold. Let sit for 20 minutes so that it soaks into the mold. Make sure you get epoxy on the top too, but not in the alignment holes if you have them.
- Wipe excess epoxy away. If you don't wipe it well, sanding will be more difficult later.
- At least 24 hours later, sand back up to 2000 grit. Shouldn't take too long because the mold should have been very smooth before epoxy was applied in the first place.
- Wax the mold
- **Note: wax is difficult to sand off, so make sure you're ready before you apply it. (i.e. you still need to sand more epoxy, etc.)
- Using a shop towel, get a dab of Meguiar's mold release wax (or other suitable mold release wax) and apply a moderate amount to the mold (wherever you applied epoxy, so the inside, top, and sides)
- Rub in circles until there are no more streaks
- Get a fresh shop towel
- Continue to rub in small circles (this is called "buffing") until the mold is really smooth. You'll feel the resistance decrease a lot suddenly, and you'll know it's ready when it's super easy to glide the towel along the mold.
- Repeat this 4 times (so you want 5 layers of wax)
- Apply PVA film
- Using a fine-haired brush, apply PVA film to the inside, top and sides of the mold (everywhere there's epoxy and wax).
- Let sit for 20 minutes.
- **Note: be careful not to wipe any of the surface with acetone as this will dissolve the PVA film and make mold release more difficult.
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