Jun 08

Constructing the fin mold

For this first set of wakesurf fins we are going to be making a mold that is very similar to what would be used in injection molding. Here is a picture so that you can refer back to it as we go through this process. We should tell you that this process is easily within the grasp of anyone with basic hand tool skills. If you can use an electric saw you can make ourself a set of fins.


Now injection molding is a rather complex process, where the mold is heated and clamped and then small resin pellets of PE are melted and injected into the mold cavity. Those pellets are typically High Density PolyEthelene, or HDPE. You’re familiar with that material it comes in everything from plastic kitchen cutting boards to plastic toys. A similar material is a LOW density PE and it is referred to as LDPE. Both of those materials are sturdy enough to be injected and then used without reinforcement.

We won’t have that luxury, our fins will need some type of reinforcement. As we showed and discussed yesterday we’ll be making our fins with a core of Soric and then external to that we throw on a layer of fiberglass. The fiberglass gives the resin we will use strength and stiffness and the core of Soric, lightens up the whole finished unit and also gives us the stiffness by creating thickness.

Fortunately for us, we have some options with resin because we aren’t using ACTUAL injection molding. We’ll need a resin that can flow easily, such as epoxy, polyester or vinyl ester. All of those will flow into intricate shapes, are readily available, are water proof and can be made strong enough for our intended useage of wakesurf board fins. What we’ll need for that to work, is for our fins to be oriented vertically during the molding process. The only part that can have a somewhat unfinished or less than perfect surface is at the bottom of the base. That section of the fin is hidden from view and presses up against the fin box in the wakesurf board, so as long as it’s not sharp, we really don’t care too much what it looks like. Gravity will then allow us to fill the mold with our resin. Now that we have our molding orientation, we can start work!

We are going to use silicone for our mold material. It will allow us to pour our mold, which is a gazillion times easier than laminating one or cnc’ing it. It has some drawbacks, but for our very limited production of a few fins, it will work just fine.

What we will do is suspend a set of fins inside an open box, such that there is adequate space all around each fin and then fill up the empty space in the box with our liquid silicone. Once the silicone cures, we’ll have a flexible mold! You saw the finished mold being made, the fins were suspended inside the open box. For that purpose we drilled small holes in the base of each fin and then through a small section of wood, to attach the fins too. We don’t want to deform the sides of the base of the fin, so we use a very small wood screw and tap into the fin just a small amount, just enough to hold them in place but also to allow us to adjust them a little in case we needed some movement inside the box. Here is what the fins look like attached to the scrap of wood that will suspended them inside the box.


It’s a little hard to see, but each fin has 1 wood screw that is through the scrap wood and then into the base of the fin. We chose to cinch them tight to the scrap wood so that they could still be moved, but wouldn’t flop around during the mold making process. We use a spray on mold release, make sure that everything has a generous coating of mold release and then don’t touch it!

We won’t go into significant detail here, but build a box that is open ONLY on the top, that is larger than your final fin size by about 1/2 inch all the way around. The silicone will remain pliable, later, but the thickness will prevent the walls of the mold from deforming when we are making out fins. Any thinner than 1/2″ can cause deformation of the mold walls from the weight of your resin. For your mold box, you will be taking it apart and putting it back together with each mold cycle. We used wood and wood screws so that we could easily take the box apart. Hot glue and plastic would work, as would metal and threaded holes. Whatever you have available and are most comfortable working with. Just remember, you’ll be taking the silicone out of the box each time you make a set of fins.  Also during the mold making process, we need the box sealed up at all of the seams so that when we pour the liquid silicon into it, it doesn’t run out all over the floor.


Once you have the mold box made, we dusted the inside of ours with mold release, you could coat it with some car wax or even line it with plastic wrap. We just want to prevent the silicone from sticking to the porus surface of the wood box. For the most part, it’s not a real problem, the silicon, once cure won’t stick to the box. But we’ll be safe rather than sorry. Then, we attach the fin holder so that the fins are suspended inside the box and not touching anything, plus leaving a 1/2″ gap all around. Mix up an adequate supply of material, use the manufactures directions, and pour it into the box.


A few pointers. Level the box so that the liquid material will cover the fins totally. We completely covered the fin base so that when we were complete, we’d have a slight ridge above the base. When we go to use the mold, we want a little excess material at the top of the base that we can sand off flush. Affix the fin suspender to the box during the cure cycle. If it shifts you very possibly may ruin the mold and make it unuseable. Don’t scrimp on the mold making material. Nothing is worse than pouring into the mold box and being 2 ounces short. Don’t worry about any excess running over the edge, it’ll cut or peel off easily, plus it’s a production tool, no one will see it but YOU!

Leave that to cure, most room temperature cure silicones take a few days to reach full cure. THEN we’ll be ready to start molding fins.

Thanks so much for following along, we really appreciate it.

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