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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.
- Open the CNC door.
- If there is a pallet in the machine, depress the foot pedal and remove the pallet. Place it on the shelf with the other pallets.
- Place the lubricant catching tray on top of the chuck, lined up so that the reference ground flats are not blocked. The longer side of the tray should be to the left.
- Put the adhesive mold pallet onto the chuck and depress the foot pedal. The adhesive mold pallet should move down slightly and be clamped in place. Make sure that the side marked FRONT is in front.
Before micro-machining begins, you have to cut the top surface of the mold with a fly-cutter to remove the old molds. It is recommended that the fly cutter be run at 550 RPM and 7 IPM feed.
- Turn on the machine by pressing the green Power On button. Wait for the machine to boot up.
- Run the warmup program to warm up the spindle. The program is named "O02024" and it is located on the internal memory. (See here? for step-by-step instructions).
- While the warmup is running, find the fly cutter in the drawer labeled "ER20" in the tool chest. Remove any wax that is adhered to the cutter.
- Place the fly cutter in the spindle. It doesn't matter which tool slot you use. (See here? for step-by-step instructions.)
- Check that the machine is in Inch mode. (See here? for step-by-step instructions.)
- Decrease the Z offset in work offset G154 P50 by .012 so that .012" of material will be removed from the surface of the mold by the fly cutter. (See here? for step-by-step instructions.)
- Insert the USB drive with the micromachining toolpaths into the USB port on the side of the control panel, and load the fly cutting program into memory (See here? for step-by-step instructions.)
- 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 fly cutter from the spindle, clean it, and return it to its drawer.
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.
- 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.
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.