Link to Arul's email detailing the mold making process. I'll continue to edit this page as I progress.
The following to-do list is adapted from Arul's original email (linked above).
Before starting, check that the Epoxacast 670HT is not expired. If it is, order more here.
- The 670HT is located in TLTL, in the cabinet nearest the hoods, top right shelf
- Consider ordering this mixer as well to compare with the current jiffy mixer
Also try to find a sheet of .010" thick fiberglass. If you can't find it, order more from ACPCompsites here.
Creating a support structure
Note: step number 5 of this section contains an overnight cure time. Plan accordingly
Before casting the epoxy, we need to create the support structure that will help keep the epoxy mold rigid, as well as the acrylic walls that will serve as the walls for our mold.
First, create CAD for...
- The walls
- The end plate
- 5x rod holes, (beginning .1” from mold bottom, as oriented while casting)
- Arul says: you may need to scallop the lower edge [of the end plate] (as I did) to clear any runners you've made. (See runners, below).
- My CAD (Onshape again): end plate.
- 5x carbon rods
- Small diameter, 5.75" in length
- Jason says: If you hold one end of the rod with a vise, the dremel can cut these rods very quickly. However, most of the cutting blades have a pretty large kerf. Cutting to about 5.75" and then sanding to fit worked pretty well for me. Make sure to use a mask (at least N95) and safety glasses while using the dremel. Carbon dust can make your soft tissues pretty unhappy.
Note: Casting the mold only requires one set of walls. However, it's highly recommended to laser cut 2 sets because a mistake while gluing can make pieces unusable. The instructions in this document will assume you are cutting 2 sets.
- Walls (4x each)
- End plates (2x)
- Cut one hole first to test fit your rod
- Stick a piece of 0.010" fiberglass to the aluminum foam in one of Arul's drawers in the file cabinet using solder mask, place a weight on top, and let it dry overnight.
- Make sure the fiberglass is larger than the aluminum foam. (We don't want to get silicone into the aluminum foam)
- Also, more fiberglass makes the daughter-mold molding easier.
- Some notes on solder mask:
- Even though the solder mask's bottle says the cure time is one hour, it turns out to be more like 3 or 4.
- Jason says: I used a lot of solder mask in my first attempt, and it took a very long time to rinse out. The next pattern I tried (in the photo below) worked out pretty well.
- It's nicer, but not required, to machine in channels (runners, from here on) so that on the permanent mold excess silicone can be evacuated easily.
- One possible geometry is at D:\bdml\Google Drive\sasuresh\Adhesive Manufacturing\Daughter Mold though you'll have to re-do toolpaths and possibly setup the tool if it's since been removed.
Casting the silicone master mold:
Note: contains a 24 hour cure. Plan accordingly.
- Sand the underside of the fiberglass, then wipe it down with IPA.
- Prime the underside of the fiberglass.
- Tae advises that the primer will permeate nitrile gloves. Therefore, he suggests putting on one pair of gloves, then wrapping the fingers that will come in contact with the can with a KimWipe (or piece of paper towel), and then putting a second pair of gloves.
- Fold a square of KimWipe, and cover the opening of the primer can
- Then, tip the can to apply some primer to the folded KimWipe (you don't need very much)
- Apply to the underside of the fiberglass
- Wipe off any excess primer with a KimWipe
- The primer is the Dow-Corning PR-1200 RTV RED primer located in TLTL's flammables cabinet
- Tape over edges of runners so they fill with silicone
- Cast the mold
- Arul says: When casting, keep a lot of pressure on the back; I usually put the two (approx. 6x2x0.75") rectangular weights in the TLT fumehood on top.
- Allow cast to dry (24 hours)
Preparing for epoxy casting:
Bring from BDML:
- Modeling clay
- Mann EZ release 200
- Double-sided red tape
- Cut rods
- Laser cut walls (4x total) and end plates (2x)
- A tall, disposable plastic beaker from BDML
- These are normally in the cabinet below and to the right of the sink)
- Flush the aluminum foam with water in the sink until the solder mask dissolves.
- Wash out the foam and leave it by the sink to dry before putting it back in the drawer.
- Wash off the back of the fiberglass.
- Carefully peel out the master from the mold, trying not to put too much curvature into the fiberglass.
- Double-stick tape the fiberglass master to a reference flat (smooth-faced wax block ideally, makes later cleanup easier), so that the wedges are up.
- Make sure not to get any bubbles and get it as flat as possible.
- Carefully, with a microtome and/or xacto, scrape away any thin films of silicone that are not the actual adhesive, and not the runners.
- The fiberglass will define the reference surface (lowest surface) in the daughter mold, so we want to make sure there's no silicone where it shouldn't be.
- To get square edges to the daughter mold, build side walls around the master patch of adhesive using acrylic rectangles.
- You can hold these down to the fiberglass and seal the corners using very light application of superglue; we'll just snap them off later.
- Jason says: you can also use acrylic cement on the corners if (like me) you have trouble getting the superglue to keep the walls together. It's in the flammables cabinet in TLTL directly under the granite slab.
- Lightly glue the walls to the fiberglass
- Glue together the stiffener structure with superglue to hold it together while you assemble everything else.
- If you cut runners, if the resulting silicone ridges on the master mold don't reach the acrylic sidewalls, use small pieces of modeling clay to extend them out to the walls
- Mask off the area of the adhesives by covering them (and only them) with red tape.
Casting the epoxy:
- Locate the Epoxacast 670HT
- Note: A previous epoxy mold was made from Epoxacast 650. 670HT was chosen for its high temperature properties
- Use the jiffy mixer (located in the back right corner of the hood in TLTL) to mix part A of the epoxy
- Mix the epoxy you're using in the tall disposable beaker.
- The EpoxACast 670HT requires about 250 grams of part A for the CAD linked in this doc.
- The tall beaker makes sure that the epoxy doesn't overflow its container during de-gassing
- Vacuum-degas the epoxy by itself.
- Degas until the epoxy releases about 1 bubble per second
- Spray Mann EZ release 200 (the yellow and black one) on the mold surfaces, being sure to get the sidewalls.
- Place the assembled rods and end plates (without mold release on it!) into place.
- Put the master mold in the vacuum chamber, and carefully pour in the epoxy.
- Goals while doing this:
- Don't pour directly onto the wedges, let the epoxy flow over it.
- Keep the carbon fiber frame from moving; either pour carefully, or hold it in place with a stick (can't remember exactly which I did).
- Goals while doing this:
- Close the lid and turn on the vacuum
- Vacuum until the bubbling subsides to about 1 bubble per second
- This allows the epoxy to displace any air trapped in the wedges
- Let epoxy cure overnight