Biomimetics and Dextrous Manipulation Lab

PreparingMolds

Categories: AdhesiveManufacturing, Manufacturing

This page describes the process of applying the softer mold wax on top of a machinable wax carrier to prepare for micromachining molds.

If preparing a new wax block:

  1. Make sure the piece is cut to 6.0" in length.
  2. The vise in the Haas OM-2A can only clamp on blocks a little under 4" in width; if your block is nominally 4", machine the block to just fit in the vise by:
    1. If at least one of the large faces is not machined flat, machine it flat with a few light skim cuts.
    2. With this flat face against the fixed jaw of the vise, use a metal rod to clamp the block in place with one of the sides facing up. Take skim cuts to flatten this face. The moving jaw will press against an as-yet-unmachined surface, so the metal rod is to prevent the movable vise jaw from applying moments and twisting the block out of alignment with the fixed jaw.
    3. Flip the block 180° so the base remains against the fixed jaw, and the just-faced side is flat against the bottom of the vise, using the metal rod to clamp again. Take skim cuts until the block measures about 3 7/8" in width.

Resurfacing

Note: melting the wax can take up to 40 minutes (depending on how many blocks you're making). Plan accordingly.

Melting the wax

  1. Cut the yellow wax
    • The yellow wax is located in TLTL, in the bottom row of the acrylic sheet storage in a cardboard sleeve
    • While you're there, also retrieve the large beaker and aluminum tape, located in the drawer labeled 'BDML', directly to the left of where the wax is stored
      • Note: if the beaker still has wax in it, melt down the wax at 100 C.
      • Pour out the wax, and make sure there are no particles in the beaker that will get into your new batch
    • Since the sheet is 1/8" thick, cut about two times the area of the machinable wax block(s) you will be surfacing to add 1/4" to each.
    • As of Fall 2017, we still have stock of the old "Master Sheet Wax," which has since been discontinued. If restocking becomes necessary, purchase the replacement formulation from Freeman Manufacturing and Supply Co., PN 203125.
  1. If you get the stuff that comes with adhesive attached, you should heat it to 55 C and peel the adhesive off at that temperature.
    • Jason says: This 55 C number is not arbitrary. I played around with a few different temperatures, and the adhesive was tough to remove at all of them. It peels right off at 55 C.
    • Leave the wax in the oven for about 15 mins for easiest peeling.
  2. Wipe both sides of your new wax down to remove any particles
  3. Completely melt the yellow wax in the TLTL oven at 100 C.

Preparing the dam

We use the aluminum tape to create a dam that contains the poured wax.

Before starting this step, make sure the old surface of the yellow wax is completely clean of soap, old silicone, dust, etc. This can be as simple as an isopropanol rinse, or may require planing off the top layer and then thoroughly washing.

  1. Cut a strip of the aluminum tape that is the perimeter of your block plus one inch.
  2. Before peeling the backing, fold down the top third of the tape onto itself and press flat. You want it quite smooth, so press the folded-over section and crease flat with an implement (e.g. the handle of an x-acto knife). Then, peel out the protective film, and press the seam closed..
    • Jason says: If you peel before folding, the tape has a tendency to stick to itself.
    • The result should be fully aluminum on one side, and half exposed adhesive and half aluminum on the other.
  1. Beginning midway on a long edge, press the tape onto the sides of the wax block, so that the top of the adhesive portion is about 1/8" below the top of the blue wax, and the folded-over portion extends up forming a wall.
    • Jason says: sticking the adhesive portion below the top of the blue wax is really important. Otherwise, the wax you pour can peel up from the blue wax once it has cooled, compromising the rigidity of the top wax layer.
  2. Proceed around the block by rolling the block along the tape; at each corner, you may need to adjust angle slightly to keep the above-wax part of the tape the same height. You may also find it helpful to use a popsicle stick or similar to crease the corners as you roll the block, to keep the corners sharp.
  3. Once the tape has been initially applied, firmly press the tape into contact along the block to ensure the adhesive is fully adhered.

Pouring the wax

For your planning convenience: The wax takes about 2 hours to cool.

  1. Find a flat base for your wax block that can also fit on the granite slab.
  2. Find or cut another piece of plywood that can cover your wax block (so, a little larger than 4"x6").
    • Wipe this piece down so that it does not introduce particles to your wax
  3. Cover the plywood with a piece of aluminum foil large enough to generously wrap your wax block
  4. Place the wax block with dam on the aluminum foil
  5. Immediately before pouring the molten wax onto the block, heat the top of the wax block with a heat gun. If the surface is blue wax, you want to get to the point where it "sweats" and gets shiny just as it is starting to melt. If the old surface is the soft wax, heat with a heat gun until that layer is completely liquid.
    • Along with cooling the block slowly, this is the most important aspect to preventing yellow wax delamination due to shrinkage. Make sure you nearly melt the blue wax, particularly around the edges of the block. You may need to bring the heat gun quite close.
    • The heat gun can be found in the "Solder Wick" drawer on the right hand side of TLT's electronics bench.
  6. With one hand still applying the heat gun, carefully pour the liquid soft wax onto the block.
    • Jason says: This is the trickiest part of preparing the molds. It might be a good idea to see if someone can help you out with either heating or pouring.
  7. Quickly cover the cooling wax with your plywood cover and wrap the whole assembly in the aluminum foil. At this point, we want to cool it slowly to avoid introducing unneeded residual stresses.

Milling the block flat

  1. Clamp the wax block in the machine, and put a 3/8" tool in the spindle.
  2. Select the wax facing program, #04443, from machine memory
  3. Because of shrinkage as the wax solidifies, the middle will likely be the lowest point. Jog by eye to the center, and then slowly jog down until contacting the wax. Step down about another 0.01" to ensure a fresh wax surface, and set this point as Z zero.
  4. Set the XY zero at the front-right corner of the wax block
  5. Aim the coolant hose at the bottom of the tool
  6. Turn on the air blast.
  7. Run the job to get a flat surface on the wax mold ready for micromachining.
Page last modified on November 01, 2017, at 03:53 PM