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10/10/2017: currently out-of-date, use at your own risk. Will fix soon(ish).
This article describes the process used to manufacture the adhesives used in the human climbing paddles and perching devices. The adhesive manufacturing process has four main stages:
- Micro-machining molds in wax using the Haas OM-2A
- Fabricating fiberglass backing rectangles
- Casting and post-treating adhesives
- Dicing adhesives into tiles and attaching tendons
You will need the following items:
- Wedge machining fixture (in a labeled black carrying case)
- Wax mold block; to prepare a new one, or resurface an old one, see details on preparing wax blocks.
The Haas CNC machine is located in BDML. It is outfitted with a pneumatic pallet chuck which makes it easy to hold a workpiece in a repeatable position. The pneumatic chuck is actuated by a foot pedal. For most applications, we use a wax block in the vise for machining. There is also a pallet with a 12x12" wax surface and a lubricant catching tray if this is used.
- Open the CNC door.
- If the vise pallet is not in the machine, depress the foot pedal and remove the pallet. Place it on the shelf with the other pallets. Put the vise pallet in the machine while depressing the foot pedal, then make sure it is locked in place. Fully open the vise jaw.
- Make an aluminum pan to catch lubricant; Take an approximately square piece of aluminum foil and place it along the inside surfaces of the vise. Insert the wax block, then tighten the vise. Carefully, without tearing the foil, crimp the corners of the foil to form a pan that will catch any lubricant dripping off the sides of the block. Make sure that the top edge of the pan is slightly below the top of the wax block.
Before micro-machining begins, you have to cut the top surface of the mold to remove the old molds. There is a program stored in machine memory to do this, O04443.
- Turn on the machine by pressing the green Power On button. Wait for the machine to boot up.
- Insert a 3/8" tool in the spindle (See here for step-by-step instructions.), and select the "Wax Facing" program, O04443 (See here for step-by-step instructions.).
- Set the coordinate system as noted in the program comments, on the front right corner of the block.
- Turn off the coolant main valve, and adjust the air blast to point at the tool.
- Make sure the wooden plug is in the bottom of the enclosure to keep wax out of the coolant.
- Turn on air blast using the toggle switch mounted to the control pendant.
- Press Cycle Start to run the program.
- Once the program is complete, remove the tool from the spindle, clean it, and return it to the rack.
Next is the actual micromachining process.
- Remove the wedge machining fixture from its carrying case, and wipe off the inside of the collar and the outside of the spindle nose to remove any machining debris.
- Slide the fixture onto the spindle nose, until it is pressed against the collar on the spindle nose. Rotate until the dowel pin is located by the collar on the spindle nose, then tighten the fixture using the two screws on the front.
- Set the work coordinate system to the top of the wax block, centered in Y, and at the right edge of the block in X. Lower the Z height offset by 0.005" (See here for step-by-step instructions.).
- Liberally apply lubricant to the surface to be planed (10:2 by mass Ajax dish soap and water).
- Put the machine in optional stop mode.
- Select and run the setup program, adding more lubricant at each stop between planing operations.
- Apply more lubricant, remove the machine from optional stop mode, and run the experiment program.
At this point, the mold can be washed, and the machine vacuumed to remove as much of the wax as possible. When washing the mold, gently wash with water until the soap is removed; this will be marked by an abrupt transition from hydrophilic to superhydrophobic behavior. Rinse the mold surface with isopropanol, then set aside to dry.
For scheduling purposes: This process involves laser cutting and 3 separate, 8 hour cures for the epoxy.
You will need the following items:
- .047" solid fiberglass sheet (from ACP Composites http://www.acpsales.com/Fiberglass-Sheets.html).
- Epilog laser cutter (in TLTLab)
- EZ-Lam 30 epoxy
- Glass plates and 25lb + 2x10lb weight (in BDML)
- Reynolds commercial plastic wrap (in TLTLab)
- Granite surface plate, leveled (in TLTLab)
- Dremel tool
- 180 grit sandpaper
- Acetone and isopropyl alcohol
- Two toothbrushes (one kept in a clean container)
- Dow Corning PR-1200
One batch of adhesive requires at least two backing rectangles. For each rectangle, follow the following procedure:
- Use the laser cutter to cut two 3"x6" rectangular pieces out of .047" solid fiberglass sheet. This takes about 5-7 passes at 10% speed, 100% power.
- Laminate the two pieces into a 3"x6"x.1" rectangle using EZ-Lam 30 epoxy.
- Place the rectangle on top of a teflon sheet on a glass plate.
- Plastic wrap also works
- Place another sheet of glass on top of the rectangle and place a large weight on the glass.
- Wait 8 hours for the epoxy to cure.
The resulting laminated rectangle is not flat enough, so both sides must be "floated" in epoxy:
- Place a piece of plastic wrap on the granite surface plate and tape the edges to the granite, creating a near uniform biaxial tension in the plastic (avoiding wrinkles).
- Carefully squeeze the air bubbles out from under the plastic wrap using a finger.
- Pour EZ-Lam epoxy onto the plastic wrap, making a puddle large enough for the rectangle.
- Carefully place the rectangle into the epoxy puddle, avoiding trapping a bubble underneath or getting epoxy on the top of the rectangle. Touch one of the long sides to the epoxy first, and then roll the other side down into the epoxy while an assistant pushes on the center.
- Let the rectangle float freely as the epoxy cures for 8 hours. Do not wait longer than necessary or the plastic wrap will not come off (at around 24-30hrs).
- Detach the plastic wrap from the granite surface plate, and then peel the plastic wrap from the rectangle.
- Trim the excess epoxy using a Dremel bit.
- Repeat these steps to float the other side of the rectangle as well.
Next, the rectangles must be prepared for casting:
- Carefully sand one side of the rectangle with fine (180 grit) sandpaper. Do not remove too much material. Sand evenly and stop when the glossy finish disappears.
- Leave the other side of the rectangle glossy and smooth.
- Clean both sides of the rectangle thoroughly using acetone and a toothbrush.
- Also brush the thin edges with acetone.
- Rinse both sides with isopropyl alcohol, then wipe with kimwipes.
- lay down 3 layers Scotch Packaging Heavy Duty Tape on the sanded side to make tape pillars.
- Put on gloves.
- Repeat the cleaning process with the second, cleaner toothbrush.
- Clean both sides and all edges with acetone, then rinse with isopropyl and wipe dry.
- Be careful not to contaminate the surface from this point on (or you might have uncured areas of silicone).
- Apply Dow Corning PR-1200 primer to the sanded surface of the rectangle using a kimwipe that is damp with PR-1200. Once the entire area is covered with wet primer, immediately wipe off excess with a dry kimwipe.
- When the correct amount of primer is applied to the rectangle, the color is very faint. If it's bright pink then there's too much primer (this causes the silicone to delaminate from the rectangle).
- Wait 2 hours for the primer to dry completely.
- The primer should be applied to the rectangles no more than a few days before casting.
- Bring the wax mold and fiberglass rectangles to the TLT lab.
- Use several pieces of masking tape to mark the edge of the mold.
- Put the wax mold on the covered granite (not to scratch the granite surface) to make sure the mold is horizontal in every direction.
- Mix liquid PDMS PartA and PartB with 1:1 ratio (each 30g and together 60g).
- Use the vacuum chamber to degas the liquid PDMS.
- Pour the PDMS carefully on the wax mold and distribute the liquid PDMS evenly on the mold.
- Gradually lay down the fiberglass rectangles on the mold so that there is no bubble in the molding area.
- Press hard on the taped area of the fiberglass until one can see the wax color instead of the PDMS black color. Repeat this with all the taped area.
- Wait for a day.
After casting and demolding, the samples must be cleaned up for post-treating.
- cut excess off edges
- scrape sides
- acetone back
- remove pillars
- cut along the edge of the flat
- scrape pillar region to remove all pdms and marks
- use tape to remove debris
- lay down Scotch Matte Finish Magic Tape
- lay down 2 layers Scotch Packaging Heavy Duty Tape
- repeat on other side
- cut tape at an angle leaving 1/4" sticking out
- make polyimide spacers 0.3"x2.8"
- mark rectangle number, direction, and boundary of features on back with sharpie
Go down to the TLTLab for the spinner and granite plate for post treating.
- Get two wafers and clean their backs
- clean the surface plate and the spinner
- put a wafer on the surface plate and slide it to the back out of the way (this wafer should be clean but not brand new) and put a cover over it
- mount a wafer on the spinner as close to center as possible
- wet the kapton strips with isopropyl and lay them down over the tape pillars
Next steps have to be done quickly, so practice is crucial. Need two people for this.
- mix unfilled silicone 3g A 3g B 60g hexane
- quickly pour some onto the wafer in the spinner and spin 4000 RPM 60sec
- while the wafer is spinning, clean any dust off the adhesive rectangle using a piece of masking tape (but be careful not to lift up the kapton films)
- Put wet wafer onto granite with flat to the side
- place microtome blades with tape handles onto edges of wafer
- hold 4 corners of sample and lower it over the blades, then drop it from a few mm up
- adjust position so that the boundary of the features is inside the wafer
- Person A holds bottom of the sample down on the microtome blade and is very careful not to shear it the wrong way. Person B slides the top microtome blade out.
- Person A holds the wafer so it doesn't slide on the granite. Person B slowly pulls out the bottom microtome blade.
- wipe the back side of the rectangle to make sure there are no particles. Likewise for the epoxy face of the loading plate.
- place the loading plate on the rectangle.
- Check the interference fringes.
- place special brick with tape cushion down on the loading plate. Then add 3 more bricks.
- remove bricks and slide loading plate downwards to remove it from the rectangle.
- turn the sample+wafer over and separate them by pressing the bottom of the rectangle away from the wafer.
- Person A cleans the wet wafer while person B removes kapton films from rectangle.
- Person B places the rectangle adhesive side up on a kimwipe, slides the dry wafer back to the front of the surface plate, makes sure it is still clean
- repeat process for dry wafer, but leave bricks in place.
- The adhesive can be demolded after 24-72 hours but will remain extra sticky for a few days.
- Don't leave it on the wafer for more than 3 days or it might be difficult to remove.
- Create a DXF file with a bunch of 1 inch^2 squares.
- Scratch a piece of acrylic sheet with the outer boundary of the squares.
- Align the adhesives with the boundary. Use double side sticky tape to fix the rectangle adhesives on the acrylic sheet with the PDMS side facing down.
- Use the TLT lab 70W laser cutter with 30% speed, 100% power, 5000Hz, to cut 6~10 passes depending on the when the one can barely see cutted lines from the adhesive side. Be sure to lift the working plate of the laser cutter with 3 presses on the up arrow after every 3 passes. Don't use auto focus after the first pass.
- Take the rectangle out and break it into 1 inch^2 square tiles with hand along the cutting lines. This should not be too hard, otherwise one needs more cutting passes.
- Trim the edge of every tile so that nothing sticks out of the PDMS plane. Use scotch tape to clean the PDMS surface.
- Take the drilling jig out of Dry Adhesive Fab drawer. Clamp the tile and use an approximately 0.03" drill bit to drill holes on the edge of the tile. This will lead to a 6~10 degree loading angle.
- Sand the small vicinity area near the hole on the back of the tile.
- Lead two threads of Spectra string tendon through the hole.
- Use tweezers to wear the string to create more glueing area.
- Use super glue to glue the worn part of the string on the back of the tile.
- Use pull off test to check the tendon attachment.