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Duplicating solid parts with resin

This method uses a silicone rubber mold and resin.

Materials

Hints and Tips


Making The Silicone Rubber Mold:

  1. Shown below is me starting to make the first half of the mold the wrong way. Follow along and I'll show you later why it goes so badly. You can see the original Fokker D-VIII cowl from the WW9 kit, a mold box, and a balsa jig to hold the cowl suspended in the box.

  2. Here you can see the cowl suspended and ready for me to pour in the rubber. This is where we can first start to imagine what might go wrong. As the level of the liquid rubber rises in the mold box any cavities in the face of the cowl will have air trapped in them, and any air bubbles trapped in the rubber will rise up and get trapped against the cowl face.

  3. The liquid rubber is mixed 1:1 and poured into the box. Pour slowly so that a thin stream of rubber exits the mixing cup, busting any air pockets too large to fit into the thin stream. As the level of the rubber rose I carefully adjusted the cowl to be level with the rubber. Keep in mind, this is not a good idea and this rubber mold will turn out poorly.

  4. After 4 hours remove the rig and cut a few triangle shaped holes into the rubber to aid in alignment of the mold halves. Now coat the rubber mold surface with a rubber-to-rubber release agent. Try not to get any on the plastic as the bottle cautions that it could damage it. Don't worry about a mold release on the plastic part, the rubber pretty much only sticks to its self, it won't even stick well to the porous balsa box.

  5. Not much to see here. Pour in enough rubber to cover the part and have a good 1/4" or more of rubber thickness. Firmly tap the box on the table to help free air bubbles trapped in the rubber. At this point I have used about 6 fl. oz. of rubber which comes out to about $6. Your mold is the most expensive part of the process but can be used over and over.

  6. Shown below are the complete mold halves and the original cowl that was used as a pattern. The mold half on the right is perfect but the one of the left has three issues relating to air pockets. See the next picture where I try to patch up the mold.

  7. First off, notice the little pieces of dowel. As the rubber level came up the air in these recesses in the cowl had no way to escape. Also notice the dark blue patch where an air bubble got trapped, the hole is filled with Klean Klay. The last problem cannot be fixed, the bottom of the mold is lined with dozens of tiny air bubbles that actually stick out into the mold cavity and result in pock marks all over the resin part.

Casting The Resin Part:

  1. Look at the picture below and you'll see what happened. The Klean Klay did an ok job of filling the hole but the dowels came loose and made a big mess of everything. Also, you can't see them, but there are lots of little divots all over the face from the tiny air bubbles I talked about earlier.

  2. To remake the bad mold half I made a new balsa box around the good mold half. Then with the pattern sitting on the good mold half I poured a new half for the front of the cowl. I don't have a picture for this so the previous description will have to suffice. So how do we make the front half of the mold correctly if we don't already have half of the mold? Since I haven't done this yet I can only surmise that I would have to place the cowl face up in the bottom of a balsa box and pour in rubber. When cured I would pretty much have to rebuild the box and put the cowl and first half of the mold into the bottom of the box and pour the other half. If and when I duplicate another part using a rubber mold then I will update this guide.
  3. Up until this point I have been using what Micro-Mark calls the "squash" method. This is a simple process of pouring the resin into the concave mold half then pressing the convex mold half onto the other half. The excess resin runs out the sides along with the air. In theory this works, but I found that I was still trapping air bubbles. After this I changed to a more traditional method as shown below. First I cut two channels from the cavity in the mold to the outside edge of the mold. One at the top of the part to let out the air, and another off to the side a little for pouring in the resin. Now reinforce the side of the mold halves with wood and secure with rubber bands. Slowly trickle in the resin, allowing air to escape out the other channel. When the resin has filled that part and backed up into the channels gently tap the mold to help dislodge any trapped air. Now wait for one hour then demold.

  4. The best way to learn something is to make mistakes, the second best way (or is it the best way?) is to learn from someone else's mistakes. In the picture below the two cowls on the right were made with the "squash" method. Notice air pockets around the engine cylinders or even in the face of the cowl near the hole for the prop (I cut these out, they are not a casting error). The cowls on the left were made by pouring the resin in. Notice that these cowls are air free except for the recessed circles near the hole for the prop thrust bushing. It is not as noticeable on the of two, but all four cowls have some sort of air in one of these recesses.

  5. To remove the air from those circular recesses I drilled a hole from the top of the recess and out the back of the mold. You don't want to go out the front of the mold or you'd have a fun time cleaning off the sprue. In the picture below you see the original part, the rubber mold with added air vents, and two cowls made since adding these vents. The cowl on the left came out perfect, but the one on the right managed to get an air pocket right at the top. I think it got there when some resin leaked out of the bottom of the mold and I failed to maintain a properly level of resin in the sprue lines to compensate. The cowl on the right is not perfect, but a dab of putty and you'd never know it was there. But for the purposes of perfection, only the cowl on the left passes muster.

  6. Trim off the sprues and any seams from the mold and install on your airplane. I found the the original cowl was 5.9 grams and the resin one is 6.2 grams, an increase of 5%. The resin parts are not quite as stiff as the original plastic part (though they are less brittle), but should still be stiff enough even for rubber powered flight. If you need serious hardness then CR-900 resin may fit the bill.
  7. Shown below is the original cowl and an exact duplicate.

  8. I have not described the use of a rubber-to-resin mold release as it didn't seem to be necessary. However, use it if you have detail that could be easily damaged (like the cylinder cooling fins) or to help extend the life of the mold by easing the demolding process.

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