Picture 1: The completed fin

Two-part Fin Mold

Rick recently decided to build a 2.5x upscale of the Estes Big Bertha using a fiberglassed 4.125-inch diameter body tube, a home-turned basswood nose cone, 38mm motor mount, and fins. Hmm. What to do about fins? Rick and I started conspiring on a method to make molded composite fins. Our requirements were few simple, fast, cheap, and easy criteria we hadnt applied this strictly since the days we were evaluating co-eds back in college.

Below are instructions for building a two-part (left and right) fin mold. With this mold you will make strong, lightweight, 1/4-inch thick fiberglass composite fins with a foam core.

Mold Materials:

  • 1/8-inch melamine coated fiberboard (whiteboard)
  • 3/4-inch Medium Density Fiberboard (MDF)
  • Masking tape
  • Non-Drying modeling clay
  • Carnauba wax
  • 1/4-inch hex bolts with nuts and washers

Melamine laminate (sold commercially as Formica®) has a smooth non-stick surface perfect for forming flat fiberglass parts. It can be purchased at most building supply stores laminated to 1/8-inch fiberboard, marketed as whiteboard for use with dry-erase markers.

MDF is made from fine wood dust mixed with a binder and heat-pressed into panels. The sheets are very stable and warp-resistant, and can be sold as-is or with a veneer skin like oak, maple or melamine. You can usually find melamine covered MDF at building supply stores sold as shelving material, but if you use this make sure the melamine is smooth and not textured. Rick bought his MDF without the veneer (paint grade) and glued on his own layer of smooth melamine. He also doubled the thickness of his MDF to 1.5 inches by gluing two pieces together it made his mold pretty heavy, but he doesnt worry about distortion or breakage when he clamps the halves together.


Picture 2: Mold Components and Assembly

Mold Construction:

  1. Cut two rectangles each of MDF and 1/8-inch whiteboard, roughly 125% the height and breadth of the fin shape. (Cut two additional sheets of whiteboard and glue them to the MDF if it didnt come with a melamine veneer.)
  2. Stack the two pieces of whiteboard together with the melamine sides facing each other. Tape the edges together with masking tape.
  3. Sandwich the MDF pieces around the whiteboard stack with the melamine sides facing inward.
  4. Clamp together and drill a 1/4-inch hole in each corner through the entire stack, about an inch from each edge.
  5. Remove the whiteboard pieces from the center of the stack, keeping them taped together. Trace the fin pattern onto the non-melamine side of the whiteboard, with the root or tenon edge flush against one edge of the sheet.
  6. Cut out the outline. You will discard the fin-shaped cutout and retain the rectangular piece with the fin-shaped cavity keep this in mind as you cut. (see Picture 2)
  7. Separate the whiteboard pieces and position them on the MDF, melamine sides up. Align the corner holes using the 1/4-inch bolts and securely tape the edges.
  8. Make a smooth fillet of modeling clay around the inside edges of each fin cavity.
  9. Coat all exposed surfaces with six layers of carnauba wax (found in auto supply stores), especially the unlaminated edges.

Fin Materials:

  • Laminating epoxy resin and hardener (30-minute pot life minimum)
  • 6-ounce fiberglass cloth
  • Microballoons
  • 1/4-inch foam core art board

Fin Construction:

  1. Cut four pieces of fiberglass cloth, each slightly larger than the fin cavity (about 1/2-inch on each edge). Cut two pieces of cloth with a vertical bias (weave running north-south and east-west) and two with a diagonal bias.
  2. Cut one piece of foam core art board slightly smaller than the fin cavity (the root edges should be even, but the other edges should have a 1/4-inch gap).
  3. Mix resin, hardener, and microballoons to a thick, honey-like consistency to create a sandable topcoat. Spread a thin, even coat inside all surfaces of the fin cavities and allow to cure (See Picture 3)
  4. Steps 4 through 10 must be done quickly, without allowing the resin to cure before all steps are completed:

  5. Lay one layer of fiberglass cloth (vertical bias) inside the left mold cavity. Completely soak with mixed resin/hardener, removing all bubbles from under the cloth. Repeat in right mold cavity. (See Picture 4)
  6. Apply second layer, same as step 4 but with a diagonal bias to the cloth.
  7. Position the trimmed art board in the left mold.
  8. Mix resin, hardener, and microballoons to a thicker, peanut butter consistency. Fill the space between the edge of the art board and the edge of the fin cavity with this mixture. Also apply a similar amount of this mixture to the edges of the right mold cavity. (see Picture 5)
  9. Mate the two mold halves together using the 1/4-inch bolts extending through the alignment holes. (see Picture 6)
  10. Apply pressure to the mold halves using clamps or weight. Add nuts and washers to the alignment bolts and tighten. The space between the two molds should virtually disappear.
  11. Wipe away the excess resin from around the root/tenon opening.
  12. Allow resin to cure to green (set but not completely hardened). Carefully trim visible excess fiberglass. Remove any resin that seeped onto the external edges of the mold.
  13. Remove clamps, weights, and bolts. Carefully separate the mold pieces and remove fin. Trim extrusions and sand edges smooth. (See Picture 7)

The surfaces of our fins came out absolutely perfect. The opposing bias of the glass layers improves rigidity and the foam core minimizes weight and actually increases fin strength. Additional layers of glass can be added to the casting if increased toughness is desired.

The biggest headache with fiberglass molds comes when parts stick together. The fin cavity pieces can be replaced if damaged, or even if you want to change the mold and cast a set of fins with a different shape. The modeling clay fillets can be easily repaired and can also be easily cleaned from your part if the fillets should happen to stick to your fin.


Picture 3: Apply topcoat first

Picture 4: Lay in glass and permate with resin

Picture 5: Lay in foam core

Picture 6: Mate the mold halves

Picture 7: Complete fin before flashing is removed.
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