Biomimetics and Dextrous Manipulation Lab

CreatingADaughterMoldForGeckoAdhesives

Category: AdhesiveManufacturing, Manufacturing

(WIP)

1.  Machine Master Mold in Wax

Time Estimate:

1.5 hr.
3-4 hours to dry mold, or can be vacuumed when casting.
  1. Do some stuff

2.  Prepare Fiberglass Backing for Master Adhesive

Time Estimate:

1 hr followed by 24 hour cure.
15 min followed by 2 hr cure.
  1. Locate a flat, clean glass 12x12" plate, Chemask water soluble solder mask, and the aluminum foam backing plate.
  2. Cut a piece of 0.025" thick fiberglass which is approximately 8x6". The piece should fully cover the aluminum foam when placed onto it. This is to prevent silicone from seeping into the aluminum foam when casting later.
  3. Wipe the glass and fiberglass clean, and then place the fiberglass clean-side down onto the glass plate.
  4. Trace the edges of the foam backing plate, and liberally fill the interior with the solder mask.
  5. Place the aluminum foam backing plate onto the solder mask, apply weights to the back, and allow to fully dry. This typically takes 24 hours at room temperature.
  6. Once cured, lightly sand the exposed surface of the fiberglass with approx. 600 grit paper, wipe clean, and then prime with PR-1200 primer. This will need to dry for a further 2 hours.

3.  Cast Master Adhesive with Silicone

Time Estimate:

1 hr followed by overnight cure.
30 min.
  1. Take the machined wax mold and fiberglass backing to the TLTL.
  2. Seal the edges of the mold with tape to hold silicone in the runners while casting the master adhesive.
  3. Mix approximately 25g total of Sylgard 170, degas, and pour over the mold, taking care to fill all of the runners with silicone.
  4. Briefly pull a vacuum to ensure there are no bubbles in the runners.
  5. From one end, slowly roll down the fiberglass backing, and then press firmly until the backing seats on the reference flats in the mold.
  6. Place the two approx. 6x2x1" weights on the back to apply pressure while the mold cures. Leave overnight at room temperature.
  7. Before attempting to demold, make sure to peel off the tape used to seal the runners, as well as any silicone adhered to the sides of the wax block.
  8. Clamp the wax block in a vise, and using a piece of acrylic to spread the pressure, gently pry up one edge of the backing until the master adhesive releases from the wax mold.

4.  Build Support Structure

Time Estimate:

30 min.

The first epoxy mold we made was found to warp over time, losing flatness of the reference surface. To mitigate this, later revisions have incorporated an internal stiffener structure, most recently using waterjet carbon fiber parts from Big Blue Saw.

  1. Collect three spars and two end plates from the bag containing the waterjet carbon fiber parts.
  2. Set up a clean 12x12" glass plate to work on, super glue, and at least two small binder clips.
  3. Using the binder clips to hold the carbon fiber parts perpendicular to the glass plate, glue a spar to one of the end plates. When gluing, there is extra lateral wiggle room; bias the parts to either side, just make sure the joints are perpendicular and the spars are pushed in all the way along their length.
  4. Glue the remaining spars to the same end plate.
  5. Glue all three spars to the final end plate.

5.  Create Mold from Master Adhesive

Time Estimate:

1 hr
Add time for laser cutting if acrylic mold walls are not already prepared.
  1. Each mold needs a set of acrylic sidewalls, two long ones measuring 150mm in length, and two short ones measuring 103mm in length. All walls should measure 0.625" in height, and be cut from 1/4" acrylic sheet.
  2. Begin mold preparation by carefully trimming away excess silicone from the molding process. To preserve flatness and backing thickness defined by the wax mold, any silicone which cured in between the fiberglass and the wax mold reference flats should be left in place, and not scraped away.
  3. Start with the long edges of the silicone on the fiberglass+carrier backing , trimming them in straight, parallel lines, 100mm apart. All silicone outside these parallel edges should be scraped away, leaving a clean fiberglass surface.
  4. Repeat the trim and scraping process for the short edges, keeping the edges 150mm apart.
  5. Ensure that the silicone ridges created by the runners in the mold reach the trimmed edges; if they do not, use a small amount of modeling clay to extend the runners to the trimmed edges.
  6. Tack down one of the long acrylic sidewalls with superglue. To allow ease of removal later, glue these by initially holding them firmly in place, and using a toothpick, wetting the outside corner where the wall meets the fiberglass with superglue. The result should look like this.
  7. Repeat with the opposite side, ensuring the acrylic wall is parallel to the first, and that the ends are aligned. Use the carbon fiber support structure as a guide, as it should eventually be a slip fit into the mold.
  8. Glue the end walls on, using the ends of the long walls as a guide.
  9. Seal any gaps at the corners with superglue.
  10. Using red tape, carefully apply pieces of tape to cover just the wedge features in the mold. This will protect them from mold release.
  11. Apply Mann EZ-Release 200 or equivalent mold release agent to the mold, taking care to cover all exposed acrylic surfaces.

6.  Cast Daughter Mold with Epoxy

Time Estimate:

1.5 hr.
24 hours to cure mold.
  1. Locate the Epoxacast 670HT
  2. Use the jiffy mixer (located in the back right corner of the hood in TLTL) to mix part A of the epoxy.
  3. Mix the epoxy you're using in the tall disposable beaker.
    1. The daughter mold requires about 300 g of EpoxACast 670HT.
    2. The tall beaker makes sure that the epoxy doesn't overflow its container during de-gassing
  4. Vacuum-degas the epoxy in the beaker.
    1. Degas until the epoxy rises, then falls, then give it about 10 more seconds. Unlike many other epoxies, this epoxy has key components that are volatile, and so cannot be vacuumed indefinitely.
  5. Remove the red tape protecting the wedges.
  6. Place the assembled stiffener structure into the mold.
  7. Put the master mold in the vacuum chamber, and carefully pour in enough epoxy to cover the wedges to about 3/16" depth (approximate).
  8. Degas the beaker and the mold, to ensure any air trapped in the wedges is released.
    1. This one is more of a judgment call on when to stop; wit about as long as for the first degassing, but not several times as long.
  9. Pour the remaining epoxy from the beaker gently into the mold.
    1. Ensure that eventually, the entire support structure is covered.
    2. The epoxy is moderately viscous, so you may need to pour at different locations in the mold. THis is fine, just make sure to pour gently and smoothly enough to avoid entraining air.
  10. Allow the epoxy to cure overnight.

7.  Finish Processing Daughter Mold

Time Estimate:

3-4 hr to release backing
2+3 hours for mold post-cure schedule.
  1. Do some stuff

8.  Materials and Vendors

PNVendorItem
FS-10ACP Composites12x12x0.015" solid fiberglass sheet

8.1  Revision 1 of Instructions Follows

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.

9.  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...

  1. The walls
    1. .625x4
    2. .625x5.75
    • My CAD (Onshape link): width, length.
    • Edit: Onshape doesn't seem to be cooperating. I've put the files in a dropbox folder instead.
  2. The end plate
    1. 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.

Cut:

  1. 5x carbon rods
    1. 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.

Laser cut...

  1. Walls (4x each)
  2. End plates (2x)
    1. Cut one hole first to test fit your rod
  1. 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.

Runners:

  • 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.

10.  Casting the silicone master mold:

Note: contains a 24 hour cure. Plan accordingly.

  1. Sand the underside of the fiberglass, then wipe it down with IPA.
  2. Prime the underside of the fiberglass.
    1. 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.
    2. Fold a square of KimWipe, and cover the opening of the primer can
    3. Then, tip the can to apply some primer to the folded KimWipe (you don't need very much)
    4. Apply to the underside of the fiberglass
    5. Wipe off any excess primer with a KimWipe
    6. The primer is the Dow-Corning PR-1200 RTV RED primer located in TLTL's flammables cabinet
  3. Tape over edges of runners so they fill with silicone
  4. 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.
  5. Allow cast to dry (24 hours)

11.  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)
  1. Flush the aluminum foam with water in the sink until the solder mask dissolves.
  2. Wash out the foam and leave it by the sink to dry before putting it back in the drawer.
  3. Wash off the back of the fiberglass.
  4. Carefully peel out the master from the mold, trying not to put too much curvature into the fiberglass.
  5. 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.
  6. 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.
  7. 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
  1. 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
  2. Mask off the area of the adhesives by covering them (and only them) with red tape.

12.  Casting the epoxy:

  1. Locate the Epoxacast 670HT
    • Note: A previous epoxy mold was made from Epoxacast 650. 670HT was chosen for its high temperature properties
  2. Use the jiffy mixer (located in the back right corner of the hood in TLTL) to mix part A of the epoxy
  3. 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
  4. Vacuum-degas the epoxy by itself.
    • Degas until the epoxy releases about 1 bubble per second
  5. Spray Mann EZ release 200 (the yellow and black one) on the mold surfaces, being sure to get the sidewalls.
  6. Place the assembled rods and end plates (without mold release on it!) into place.
  7. Put the master mold in the vacuum chamber, and carefully pour in the epoxy.
    • Goals while doing this:
      1. Don't pour directly onto the wedges, let the epoxy flow over it.
      2. 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).
  8. 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
  9. Let epoxy cure overnight
  10. Post cure in overn at 80C for 2 hrs, 150C for 3 Hrs.
Page last modified on March 29, 2018, at 04:15 PM