Method #1 for duplicating the white plastic parts
This method uses a rubber mold and liquid plastic.
- I am using liquid mold rubber and liquid brush-on plastics available from Smooth-On but I suspect that some of the products sold by Micro-Mark may also work. The reason I chose Smooth-On is that they offer a brush-on plastic that is stiff enough to apply to vertical surfaces. The resins sold by Micro-Mark all appear to be too thin and are intended to be poured into the mold, which will work fine if a solid plastic part is acceptable. Despite the high viscosity of the liquid plastic from Smooth-On, it is still difficult to keep it on vertical surfaces.
- Since I am not using the same tools and materials as Guillow's there will be differences. The plastic material that I am using now requires around 3/8" (or 1 cm) in thickness. This makes for heavy parts, and smaller parts and thin edges of parts are more prone to breaking off. Making the part solid is easiest and strongest, but even a hollow part will be around 3/8" thick, this could cause the part to not fit properly.
Making the Rubber Mold:
- The part to be molded may be a simple cowl as shown below, or even an entire sheet. Regardless of the size, we need to make a rubber mold of the outside face of the plastic (the side the faces outward on the model). All of the materials used to make the rubber mold are shown below.
- First, coat the part in a thin layer of liquid mask like is used on canopies. Make sure to coat all areas, ensuring no air bubbles are trapped. Do not worry about brush strokes as the mask actually becomes part of the rubber mold. The surface that really matters is between the liquid mask and the plastic part. The rubber will probably peel off without the mask, but the liquid mask acts like a mold release agent and I know it wont harm the part (especially the clear parts).
- Mix up just enough rubber mold material for a thin coat over the part. In the case of this Bird Dog cowl a 1 fluid ounce (30 mL) mixing cup is more than adequate. For the product I am using the pot life is 20 minutes but the recoat time is 40 minutes...go figure. Apply a thin coat, making sure not to trap any air bubbles. In this example I mixed up about a thimble sized amount of rubber. Make sure to use a disposable brush as you can't really clean it. The brush shown below is rock hard but I still use it to apply the first coat.
- After about 40 minutes mix up enough rubber for a second coat. The second coat can go on much thicker as it sticks to the first coat, even on vertical surfaces. For this example about 1/3 fluid ounce (10 mL) is adequate. I apply the second coat with the popsicle stick, kind of like frosting a cake. Just make sure not to trap any pockets of air, tiny bubbles are not a problem beyond the first coat.
- Just like in the previous step, after about 40 minutes mix up about 1/3 fluid ounce (10 mL) of rubber and smear it on like before. After the third coat you want to have around 1/4" (0.64 cm) of rubber mold around all areas of the part. Too thin and it will break when you demold the part later on, too thick and it could be tough to handle too. But too thick is better than too thin.
- After a few hours the rubber should be hard enough to be handled (but not hard enough for use). At this point I like to carefully peel it loose from the plastic part, then lay it back down on the part to make sure it retains the correct shape while it cures. Allow the rubber mold to cure over night, 16 hours is required for the product that I am using.
Casting The New Part
- The next step is to coat the inside of the mold with layers of liquid plastic. Like the rubber mold, I apply an initial thin coat then smear on another two coats till I am satisfied with the material thickness. The rubber mold easily releases from the original plastic parts, but I find that even a coat of liquid mask does not help as a release agent from the liquid plastic parts. When I buy/find a suitable mold release agent I will update these directions.
- Mix up enough plastic for one thin coat, for this example a thimble sized amount is just about right. This product starts out thinner than the rubber but over a few minutes it becomes thicker and after an hour or two is like taffy. There is no suggested recoat time, but about an hour between coats seems to work for me.
- After about an hour or so mix up another batch of plastic, about 1/3 fluid ounce (10 mL) should be enough. I still use the brush for the second coat as the part is concave and it is hard to get down into the detail with a popsickle stick. If you just want to make a solid part then at this point you could just fill up the mold with the plastic.
- After another hour you can apply a third coat, repeat as needed until you have a good 1/8" to 1/4" (0.32 to 0.64 cm) of plastic in all areas. Anything below 1/8", especially near the edges, risks breakage when demolding. As seen below, I opted to fill the mold to make a solid part.
- A few hours after the last coat I check the newly molded part until it is hard enough that it will not distort when being removed from the mold. If I had a proper release agent I could just let the plastic cure over night then peel off the mold the next day. To remove the mold pry loose a corner of the rubber mold and peel it back on its self, being careful not to break any of the plastic part. If all went well you have a new part!