About the author

Rusty White has been an avid modeler for over 30 years. In that time he has written many articles in Fine Scale Modeler magazine, The IPMS/USA Journal, and other model oriented magazines published and distributed all over the world as well as two ship modeling web sites. He is a past contributor to The IPMS/USA Journal, as a regular author of the “Ships and Tips” ship-modeling column. This series of modeling “how to” CDs is an extension of his enjoyment of writing modeling articles.

Rusty is a professional home designer/illustrator, and model builder in Edmond Oklahoma, designing homes and building ship models for clients all over the world. He has also designed and built model master patterns, which led to Flagship Models first model kit.

His passion for the hobby led him to join IPMS/USA in 1984 and has been a continuous member since. Rusty has risen in the ranks to IPMS/USA Head Ship Judge, where his duties include organizing and supervising the ship judges at the National Convention contest every year. He also sits on the IPMS/USA Contest Rules Committee working with the head judges to supervise all aspects of the contest. Rusty served as Chairman of the 2003 IPMS/USA National Convention that has been touted as one the best organized and run IPMS/USA conventions ever held.

Flagship Models was born from Rusty’s frustration years ago when he couldn’t find photo etched details for the

modern naval vessels he enjoys to build. At the time there was almost nothing available for modern naval vessels, while the market was filled with photo etch for WW-2 era ship models.

Flagship Models was created with artwork Rusty “traded” in exchange from then “Model Technologies” for photo etch services. Flagship’s first detail set was the 1/350 scale Ticonderoga/Spruance class, which to this day is the best selling set of all Flagship Model’s creations.

“I don’t necessarily produce what will sell the best. I produce what I want for the kits I am building. I started Flagship for that very reason. We concentrate our efforts on the modern naval era which is my favorite.”

Flagship Models was the first ship photo etch company in the world to design detail sets using CAD computer technology. Everything from detail set design to their comprehensive instructions is produced using 3D CAD technology. The use of 3D CAD allows Rusty to literally test fit all the parts in the computer insuring a perfect fit.

Join IPMS/USA today and enjoy plastic modeling as you never have! Check out IPMS/USA on the World Wide Web at:

http://ipmsusa.org/

Or snail mail at: IPMS/USA National Office

PO Box 2475 N. Canton, OH 44720-0475

Making RTV Molds Techniques and considerations for making RTV rubber molds for the Novice and Advanced modeler

By Rusty White Copyright 2007 Flagship Models Inc. All rights reserved

Expanding your horizons: Sooner or later we as modelers must improve our abilities and skills. It’s just human nature to strive for excellence.

I started modeling pretty much like everyone else; building kits out of the box and painting them to the best of my ability. As my abilities and skills improved, I naturally wanted to try new techniques to make my models look even more realistic and detailed. Anyone who has attended an IPMS event has been humbled by what can only be classified as works of art on display. My first trip to the IPMS/USA Nationals was a wake up call I will never forget. After seeing the incredible models on display built by the finest modelers in the world I had to try and emulate the techniques I saw on those wonderful models. Once I felt I had out of the box plastic models conquered, I moved up to multi-media kits which provided better detail, accuracy and photo-etched parts to even further detail the models.

Once again when I felt I more or less conquered multi media models I wanted to convert some models to allow me a one of a kind model. Converting models requires scratch building parts; many of which I needed multiple copies. The only way to make a duplicate is to create a mold, then cast as many copies as needed from resin. There are several reasons to create a mold from a “master pattern”. One is so that you will always have the ability to create a duplicate whenever it is needed. This is particularly useful for parts that are used often such as automotive and aircraft wheels and tires, ejection seats, naval guns and turrets, figures, bases (cobble stone, tarmac, streets, etc. There are literally endless uses for parts made from RTV molds. All you need is a master to make the mold from.

Copyright infringement: Copyright infringement is a real problem for those of us in the cottage industries. When someone, makes a copy of

one our parts (without our permission) that we have created with our ingenuity, skill, and hard work, this is an infringement on our intellectual copyright to our parts. While copying any part made by anyone other than you, technically it is still copyright infringement and theft. So do the right thing and do NOT copy anyone else’s parts without permission, because you are literally stealing from them and they have the right to litigate violators. As long as you build the master patterns, you can sell, trade, or give any and as many copies you like.

Fig.1 Smooth-On RTVR and resin (shown here) comes in trial sizes so you can decide if it is right for your application. It can also be ordered in 1 quart, 1 gallon, and 5 gallon buckets.

RTV rubber: The compound that I and my counterparts use to create molds is

known as RTV (Room Temperature Vulcanizing) rubber. There’s nothing new about RTV rubber. Hollywood and the architectural industry have been using it for many years now to create a variety of special effects. Only recently has specific types of RTV rubber been designed for scale model makers. These new compounds are designed to migrate into almost microscopic nooks and crannies giving the model maker the ability to cast highly detailed parts that are actually more detailed than their plastic counterparts. The detail that RTVR can reproduce is so fine that the grain of very fine wood can be captured and looks just like the real thing. In short, no detail is too fine or small for RTVR to capture. I have cast a human hair that accidentally fell onto a master pattern I was casting. I didn’t notice it until I made a test shot of the part and sure enough; I saw a hair actually molded on top of the part!

RTV rubber MUST be mixed PRECISELY with the hardener according to the directions to get predictable results. Too much hardener will cause it to set too fast. Sometimes this is good and sometimes not. Too much hardener and it set prematurely. I.e. if you use a rubber with a 20 minute pot life and it starts to set in 15 minutes you can have some real problems. If you mix too little hardener, the RTV will take longer to set up if at all. Too little hardener and you run the real and present danger of casting a box of liquid rubber that won’t set up. If you have never had a bad experience mixing RTV (if you haven’t, you will eventually) it is an awful mess to clean up from your pattern and there is no way to make the RTVR work at that point so it must be discarded. Also, you must thoroughly mix the rubber and hardener or you will end up with spots in your mold that do, while others don’t set at all. SO READ AND FOLLOW THE MIXING INSTRUCTIONS WITH YOUR RTV OR RESIN TO THE LETTER!

Mistakes can be costly if you are casting a large mold. Unfortunately, RTV rubber and resin isn’t cheap, which is one reason resin cast models cost so darn much. The average life of an RTV mold (if properly taken care of) is about 40 casts (40 times). After that, the mold begins to deteriorate (tear, crack, and chip), so the mold must be replaced.

I purchased some plastic containers to mix the rubber in. They can be found at various hardware and home center stores around the country as well as from the companies that sell the rubber and resin. You also need some stir sticks. Not the Popsicle sized, but the larger, wider ones sold at arts and craft stores like Hobby Lobby or Michael’s. If you plan on making a large mold you will need some wooden paint stirrers found where paint is sold. The easiest way to properly mix the mold rubber is to first weigh the bucket and make a note on the weight on the bottom. You can then add the mold rubber and subtract the weight of the bucket for an accurate weight. Do the same for the hardener. The brand of mold rubber I use is Smooth-On Smooth Cast 321. It has a pot life of 20 minutes before it begins to set up. The 20 minute time frame allows me plenty of time to mix, de-gas, and pour the mold. Smooth-On makes many varieties of rubber with different shear strengths and set up times, so do some research before you purchase. Smooth-On has a web site that has all the information you will need to make an informed decision on the mold rubber and resin right for you (http://smooth-on.com/default.htm).

Mold types and terminology: There is no standard way to make molds. The type of mold you decide to make is determined by what type of part

you want to make. The Smooth-On web site can be a great help to help you decide the best type of mold for your application. They also have a toll free number you can call for advice. If you have any questions, don’t hesitate to call them before ordering.

There are many different types of RTVR (Room Temperature Vulcanizing Rubber) molds and ways to make them. However, there are three basic types that are most often used by Flagship Models Inc. to cast our line of American Civil War warship models (check them out at www.flagshipmodels.com). This CD will cover the various techniques I use to make these molds so you can cast your own parts with success and vastly improve your models.

The one characteristic that makes the RTVR invaluable to modelers is “shear strength”. Shear strength simply put, means the RTVR is highly resistant to tearing by bending, twisting or stretching, making it perfect for making model parts with its longevity. While near impossible to tear, it cuts very easily using a Xacto blade. All RTVR also has a pre-determined “pot life” designed into it by the manufacturer. Pot life is the time the rubber can be worked with before it begins to set up. Another characteristic that I considered before purchasing the RTVR is its flexibility. This is important

Fig.2 A base plate is required for open pour molds. This is for the CSS Chicora.

Fig.3 A typical open pour mold box was made for the CSS Chicora. You can now see why this type of mold is perfect for this type of model. Note the plastic support brace to keep the sides from flexing out once filled with RTVR. Don’t forget RTVR is heavy.

Fig.4 Look carefully and you can see the clay sealing the seams for the CSS Tennessee mold. The bottom of the hull was cast the same way.

because in order to remove more complicated patterns from the mold we will need to flex or twist it to remove the part. Obviously, a stiffer rubber isn’t going to work well for this, so a very flexible rubber is required. The Smooth-On web site has all this information and they will be happy to send you a catalogue with this included. The combination of these factors will determine which RTVR is right for your particular application.

Open pour molds: Before we can proceed to casting a mold, you must first

know how to properly build a mold box to contain the RTVR. Most of Flagship Models’ larger parts (like hulls and superstructures) are produced using “open pour” molds. The reason being, that the design of American Civil War ships make using this type of mold the most economical and practical. This may not be the best mold for your particular casting. That’s why we will be covering the big three of molds. An open pour mold is just that. The top of the mold is left completely open. The resin is poured into the mold literally to the point of overflowing. These are the simplest molds to make. Basically all that is required is some heavy sheet plastic. For a small mold box thinner plastic will suffice.

A base plate must be made to attach the master pattern (part to be copied) from heavy sheet plastic. This will act as a reservoir to hold the overflow for my ACW warship of the CSS Chicora this was pretty easy and quick. This reservoir is essential if you are pressure casting. I just traced around the upper hull and made the plate about ¼” larger than the master. The model was then well attached using super glue or rubber cement. The part must be well glued because the RTVR is designed to migrate into the smallest nooks and crannies. Failure to seal the part properly will result in the RTVR running into these areas and the resin will do the same.

The box is made from sheet plastic cut to the proper length (see Fig.3). Be sure to allow about ¾” to 1” (less for smaller parts) all the way around the part to be cast and cut the plastic accordingly. I always use plenty of liquid cement to bond these parts together. This is important because if there are ANY holes at the attachment points the RTVR will seep out. The RTVR has to set 24 hrs before it can be de-molded depending on what type you use, so a substantial amount of RTVR will leak out before it sets. An easy way to ensure all the seams are sealed is to line them all with modeling clay. Not Play Doe, but sculpting clay found at arts and crafts stores.

Before you add the rubber to the mold box, always be sure to spray on a release agent to keep the rubber from sticking to the parts and the walls of the box. I would suggest purchasing the release agent from the company who makes the RTVR to ensure that there won’t be compatibility problems. Once the rubber has set, just break away the walls and carefully remove the part from the mold.

Making a reusable mold box: If you plan on molding large parts, I would suggest building a mold box

from Formica lined shelf material. I had one of these built for me with some generic sized inserts that fit inside the box so I could make various sized molds (see Fig.4). In Fig.4 you can see the partitions I inserted to make a slightly smaller mold. All the outside walls are removable with the inside ones held in place by the clay sealer. This box is for making larger open pour

FRONT VIEW Heavy Sheet Plastic FRONT VIEW Base Plate RTVR Base Plate

RTVR Super Structure RTVR Hull

Fig.5 TOP VIEW

Base Plate SIDE VIEW Hull

Heavy Sheet PlasticBase Plate

Superstructure (Top)

Fig.6 Most manufacturers who make full hulled ship models use this type of mold. The mold is then peeled apart to pop out the cast. Note the Basswood gate on the bottom of the CSS Neuse master pattern.

molds. The inserts allow you make any size mold as long as it fits in the outside walls of the box. The outside walls and bottom are all removable, held in place with dry wall screws with large threads. The Formica lined shelving can be purchased at any home center or hardware store. The particle board won’t hold to too many repeated unscrewing of the screws, so use bar soap as a lubricant to fill the threads to make inserting and removal of the screws less stressful on the particle board. Granted this is a more expensive route to go, but with care, the mold box will last indefinitely.

TYPICAL OPEN POUR STYLE MOLD Envelope molds: As the name implies, this mold is made the same way as open

pour molds with a noticeable difference. The mold must be split about ¾ ways down to allow the parts to be removed due to the small opening or “gate”. The parts are removed by prying the mold apart exposing the front and back of the parts for easy removal, just like an “envelope”. The parts are attached to the gate by a small section where the resin is introduced. Envelope molds are just a bit more of a challenge to make, but the advantage to them is 360 degrees or about 95% of the detail is cast as opposed to the open pour (50% cast) mold that leaves the entire top (or bottom) of the part void of any detail.

Envelope molds are distinguished as mentioned by their characteristic gate. I usually will use a Basswood strip for this but most any material will do. Besides being used as an introduction point for the resin, the gate is also used as a reservoir for the over

RTVR Master Pattern Mold Box Heavy Sheet Plastic Gate (Basswood or Plastic) Cut down the center leaving plenty of RTVR at the top for a hinge

Fig. 7 Gate (glued to keel) Master Pattern Heavy Plastic Sheet

FRONT

SIDE

RTVR

pour necessary on these types of molds. The over pour is needed because most cottage industries pressure cast their resin. That is after the resin is poured in the mold, it is then put under pressure to force the resin into the tiny details of the mold, otherwise bubbles and pits form. So the bubbles and pits found in some manufactured parts are the result of not using pressure casting. So in order to make an efficient well cast part, it must be fairly simple in design.

Once the mold has set 24 hrs and removed from the mold box, the master pattern must be removed. This can be tricky as the pattern must be carefully removed by cutting the mold using a new Xacto #11 blade to free the part from the RTVR. As you will see, RTVR cuts very easily (but will not tear at the cut) so make a cut from the end of the gate (see Fig.6) about ¾ the way down the mold. By gently prying the edges of the cut, you will see where to make the next. Try to leave a fairly thick end on the mold to act as a hinge. Repeat the cutting until the blade reaches the pattern. Cut VERY carefully until all the RTVR is free from the master pattern, and then repeat the process on the other end of the mold.

More detailed parts require a more detailed and more difficult to build mold known as the two piece mold which we cover next.

TYPICAL ENVELOPE STYLE MOLD

Two piece molds: The more detail and cast area you want to show, it seems the more difficult the design of the mold gets. The final

style of mold we will discuss is the “two piece mold”. As the name implies, the mold is made up of two pieces or halves instead of solid chunk of RTVR we have discussed previously. Detail isn’t the only reason two piece molds are used. Model design is one the considerations.

The USS Cairo wouldn’t lend itself to any of the molds previously discussed due to its flat bottom and paddlewheel well. The only way the open mold would work (I found out later it wouldn’t) was if the full hulled model was split at the waterline and cast as two parts. I tried this and found that I couldn’t make the halves at the waterline. It would have just been too difficult and time consuming, not to mention that the mold halves would not have cats the same size due to weight difference. Resin shrinks up to 2% as it cures. If the upper and lower halves of the model would have fairly equal in size it wouldn’t have presented a problem. However, this was not the case so the open mold idea was discarded after an unsuccessful mold was made.

The envelope mold seemed workable at the time, but due to the razor sharp edge at the widest point of the model, making a precise cut right at that point was impossible because I couldn’t see where the edge was since it was encased in RTVR. I considered making an envelope mold with the gate along the keel of the ship, but this idea was discarded

Fig.8 The bottom of the USS Cairo made a two piece mold the only option because of the paddlewheel well in the center of the flat bottomed hull.

Fig.9 The finished flask. The process was repeated for an identical twin to form both top and bottom of the mold.

Fig.10 Both flasks are bolted together for a test fit. We are now ready to prepare the lower half for RTVR casting. Note the 11” Cairo master pattern in front. This is the flasks were designed for.

because of the paddlewheel well in the center. Also, the resulting gate there would have destroyed the scribed in line work on the center of the bottom of the hull. Another consideration against the gate-on-keel design was that since the Cairo had a flat bottom, it would be ripe for air bubble to form over such a large flat area, so the idea was discarded.

I made two unsuccessful attempts to make a workable envelope mold and in the end I did succeed. However, the mold would not have lasted but eight, maybe ten casts before it would have had to be replaced, so it wasn’t economically feasible to use that type of mold for a production model. In the end (and after about $300.00 of RTVR in failed attempts) I enlisted the help of some friends in the business that had been making two piece molds for years. With their help I was able to create a very workable, durable mold of the USS Cairo. Both halves of the mold (it’s the big one) can be seen in the headline photo at the beginning of this title.

Terminology: For some reason, the two mold boxes used to create the two

piece mold are called flasks. Don’t ask me why; I don’t make the rules; they just are. Since the Cairo is 11 inches long, a sturdy top and bottom flask that would be sandwiched together would need to be constructed. Once the mold has been made, the flasks will be clamped together around the mold to keep the mold sides from bulging and deforming the parts from the weight of the resin filling the mold. Smaller ones for a figure or other similarly small parts could be made from heavy sheet plastic and not be needed to cast the parts as the RTVR is adequate to support the liquid resin. The principles and techniques are the same. Just substitute plastic for the wood I use in the following techniques.

We will be using the same type gate as described in the envelope mold as it performs the same function. Due their design, two piece molds require using vents to prevent air being trapped in the lower areas of the mold. Vents can be made from any material from bell wire to wood dowels.

Making the mold flasks: A trip to my local home center equipped me with everything I

would need to build the flasks. I bought the same Formica covered shelving material as I use to build the mold box earlier to make the bottoms of both flasks. I also purchased some 2x2 lumber for the sides and some shellac to seal the grain. As it turned out, 2x2 lumber was the perfect thickness for this project. The resulting 4 inch thick flask would easily contain the Cairo pattern. Remember what I said about RTVR and wood grain? The shellac would prevent the RTVR from penetrating into the wood grain to allow the flask to be removed easier. Drywall screws with heavy threads would used to attach the base to the 2x2 frame (see Fig.10). The two finished flasks must be held together tightly to prevent leakage of the RTVR, so I purchased five 5 inch long bolts with large washers for this.

It is imperative that the flasks be identical in size. I positioned the model to be cast on the shelving material and measure off the inside of the flask allowing about one inch on every side so the RTVR will provide a sturdy edge. This is important because the finished flasks will be clamped to the mold every time a part was cast.

Once the bottoms of the flasks had been cut, it was then a matter of cutting the 2x2 material to fit the sides. I attached the 2x2 sides to the bottom by lining up the outside edges of the bottoms with the 2x2 material (see Fig.10). They were attached with the dry wall screws using a countersinking bit to put them flush with the surface. After all this I

Fig.11 The flask with the clay struck off and ready for the next step. The working surface of the clay needs to be pretty smooth.

Fig.12 This is an extreme close up showing the clay sculpted up to the widest point on the hull.

Alumilite synthetic modeling clay is made specifically for casting molds.

didn’t want it to scratch up my counter tops. In retrospect, I would suggest adding a bead of modeling clay along the inside bottom of each side to seal the sides and bottom to prevent leaking of the RTVR. The process was repeated, for the last flask. Now that I had two identical flasks, I clamped them together and drilled holes all the way through with a drill press for the clamping bolts. I test fitted everything to be sure all the bolts and holes lined up. All the exposed wood was coated using the shellac and left to dry for a few hours before continuing. Be sure to label the flasks “top” and “bottom” as it will prove essential later. All in all, the entire flask build took only a couple of hours and cost about $10.00. Well worth it for the quality of the mold and parts they would produce over the months.

Casting the mold: Two piece molds require clay to make. Not just any clay will do.

“Play Doe” is certainly not the clay to use. It really isn’t clay anyway. Most people I have talked to who make these molds for their own one time personal use standard sculpting clay. After consulting with my friends in the business, they swear by a product from the Alumilite Corp. called “Alumilite Synthetic Modeling Clay” (part #40000). This clay has certain properties that make it work better than any other type of clay making two part molds. Specifically, it holds the parts firmly when embedded into it so they won’t come loose while the RTVR sets up which would prove disastrous. This clay is a unique blue color and will never dry out and can be used over and over again. Hobby Lobby lists it on their web site but I couldn’t find it at my local store so you’ll probably need to order it direct from Alumilite (see credits).

My mold being fairly good size, required that I have about five pounds of clay on hand. While a little much I ordered the extra pound just in case as it is not expensive. This step is easy in that you basically need to literally fill the bottom half of mold to the top of the sides with clay. Spend a little time forcing the clay into every crack and crevice. It helps to knead the clay little to soften it so you force it in tight everywhere. Be sure not to leave any air pockets because the RTVR could migrate into one of those pockets you do not see. Once the mold is filled above the top of the sides with clay you must “strike off” the top of the clay so that it produces a near pristine surface on the top of the mold (see Fig. 11). I used a metal straight edge with a sawing motion side to side to make a smooth surface. It was then just a matter of smoothing out small pits and holes in the surface.

The reason the envelope mold didn’t work as I mentioned earlier was there was no way to find the widest point of the model since it was submersed in the RTVR. The two piece mold allows us to control the separation point of the mold halves. This characteristic allows us make the separation point anywhere we wish. Since the ideal separation point for any mold is at the widest point on the part, I had to dig out a depression into the clay surface so place the top half of the part below the surface with the widest point of the model about ¼ inch above.

I would suggest buying a few simple clay sculpting tools before you begin this task. They come in very handy getting into places and moving clay in restricted parts of the

RTVR Clay Buildup to Widest Point of Hull

Fig.13 Clamping Bolts Drywall Screws Master Pattern

Formica Covered Shelving Clay SIDE VIEW Master Pattern Vent Gate

model your fingers won’t reach. I bought mine at a local Hobby Lobby, but they can be found at Michaels and other arts and crafts centers. They come in many different shapes and sizes so it’s difficult to describe what tools to purchase. Suffice it to say given the models part you will be working with you will know what tools will work for you once you see them. Don’t scrimp when it comes to buying tools! Buying cheap sculpting tools will result in the cheapos falling apart or splitting (if you buy wood ones). This will not be the last time you use these tools, so bite the bullet and buy some good quality ones.

Fill every depression on the bottom of the part to be cast with clay to prevent the RTVR from migrating into those voids. For me, this meant the paddlewheel well on the Cairo. Then press the part into the depression you cut from the clay being careful to break it. Once the part is pressed securely into the clay with the midpoint just above the surface, the clay must be built up to the midway point.

TYPICAL TWO PIECE STYLE MOLD

2x2 Wood Vent Gate Vent

2x2 Wood Formica Covered Shelving FRONT VIEW Clay RTVR

Clay Buildup to Widest Point of Hull Clay Seal Between Flasks

Fig.14 Ready to make the mold! Note the clay rope creating a seal for both halves and the clay seal in the seams of the top flask. This will effectively keep the RTVR inside the flask where it belongs.

Fig.15 The gate and vents were positioned just above the part so they could be easily joined after the mold was completed.

Fig.16 A used bell jar, base (vacuum chamber), and vacuum pump are a must when working with RTVR.

The Cairo had a line that I used to sculpt to, so I spent some time carefully sculpting at that point so I would have a clean separation for the mold halves (see Fig.13). This was the secret for my getting a mold that would make 40 plus copies before replacing the mold. If the clay was sculpted above the midway point on the Cairo, the resulting “lip” caused by this would soon tear making the mold unusable and unprofitable. Not to mention an inferior reproduction. It is important to keep the part clean with no clay above the mid point (see Figs.12 & 13).

The sides must now be sealed, or the RTVR will surely run through the seams like a sieve no matter how tightly they are clamped down. Roll out a long thin tube of clay and position it down the center of the sides and around the holes for the clamping bolts (see Fig.14).

Gate and vents: Before bolting the halves and casting RTVR, is the

placement of the gate and vents. The gate and its function we have discussed. The vents serve the purpose of letting

the mold “breathe” while the resin is being introduced. Without the vents, it would take longer to fill the mold as air would be constantly “burping” from the gate as the resin is poured creating a mess. As the resin is introduced to the mold, it forces the air out the vents and allows the resin to be poured much quicker because the resin begins setting within 10 minute after mixing. Just about any material can be used for a gate or vent. I prefer to use dowels for the vents and Basswood or plastic for the gate. Smaller molds can use large diameter wire for the vents.

The vents and gate do not actually need to be attached to the part. I positioned the gate and vents just above the place on the part that were to be attached to then depressed them about midway into the clay. The RTVR can be cut away after the mold is finished to attach the vents and gate to the mold itself (see Fig.15). The last task before clamping everything together is the addition of registration marks. These are the many round depressions you see in the clay. They act just like registration lines on drawings in that they guarantee accurate and precise placement of the top mold half to the bottom so the halves will mate perfectly every time. The registration marks (see Fig.15) are made with a paint brush end. However, in retrospect I would have used a larger diameter dowel and not press in the clay so deep. There’s no need to make deep marks because they will tend to fold over and possibly ruin the cast. I trimmed off the rubber cast registration marks after I finished the mold and the mold worked just fine.

After both flasks were treated with release agent, the bolts and washers were inserted and clamped down with a wrench (see Fig.10). We are now ready to cast the upper half of the mold. There’s no need to tighten the halve too much as it may squash the clay out and cause a leak in the seal, so use moderate pressure when tightening everything down.

Casting the rubber mold: In order to get the best mold, the one thing that must be eliminated is air from rubber. Air bubbles are the number

one cause of inferior cast parts (it causes bubbles, pits, etc.). It should be said that smaller, simpler molds may not require de-gassing, but I would always recommend it to ensure the best quality mold. Degassing is also strongly suggested by

Fig.17 During degassing the rubber will appear to boil as the air is removed.

Fig.18 The flask was opened carefully revealing the bottom of the part. All the excess clay and rubber has yet to be removed and the flask cleaned up.

Fig.19 The excess rubber and clay cleaned up and a new clay seal added to the bottom flask. Note how the excess rubber was removed from the gate.

the RTVR manufacturers. The only way to effectively degas RTVR is to purchase a vacuum pump. They can be found at various compressor supply and on-line stores. Mine will vacuum up to 30 atmospheres which more than enough to do the job. Consult your manufacturer for advice for your requirements. There are also some home made versions I have read about on the Internet sites, so choose what best suits your budget.

You will also need a vacuum chamber to hook the vacuum pump to. This needed to enclose the RTVR in so the air can be degassed. I bought a used Bell jar and base from a business associate and it works very well (see Fig.16) for my business. The bucket and the rubber are placed under the jar, the pump is switched on and the vacuum pump does its thing. Unfortunately, none of this stuff is cheap, but if maintained properly, they will give the average modeler a lifetime of service.

We have already discussed the importance of properly mixing the rubber and hardener to the letter as recommended by the manufacturer. The RTVR I use has a mixing ratio of 10% hardener to weight of rubber. I.e. if you have 16 oz. of rubber in the bucket (don’t forget to deduct the weight of the bucket), it will require 1.6 oz. of hardener. I found out early on that it’s a good idea to mix the rubber in a plastic container about three times the size as the rubber needed for the project. This is important because during the degassing process the rubber expands to almost three times its size, so you need a container large enough to prevent over flow. So you need to keep a close eye on the rubber during degassing, otherwise a real mess can develop (been there done that) before you know it.

Have a garbage can nearby and lots of paper towels to clean up drips and messes. This may sound obvious, but you will be surprised what you forget to do at the critical moment when you can’t stop what you are doing. Mix the rubber and RTV in the container with a disposable wooden stick. It’s best to work with disposable equipment because it’s just easier to toss the stuff as you use it. Mix the rubber slowly to discourage mixing in air as much as possible. From time to time, scrape the walls of the container to thoroughly mix the hardener and RTVR.

When the rubber has turned a consistent color, place the container in the pressure chamber and turn the pump on. The rubber will quickly begin to swell. Once the rubber has “peaked”, it will quickly drop down to its original size. The rubber will be a boiling cauldron of activity. Once the activity slows down, the degassing is complete and you are ready to pour. Turn off the pump and relieve the vacuum.

The rubber was then poured into the mold. Do NOT scrape the sides of the container regardless of the temptation to do so! Any unmixed hardener or RTVR may be there. If this is mixed in with your mixed RTVR you may have spots in your mold that will not set up properly ruining the mold. As wasteful as it sounds, it’s all a necessary part of the process. Hold the container over the mold as long as you wish to pour the RTVR, but remember the rubber has a set

Fig.20 Smooth-On Universal Mold release was recommended by the manufacturer as a release agent for resin, while Ease Release 200 is recommended for rubber to rubber molds.

Fig.21 Look closely. The flasks and molds have their own unique shape that allows the molds to fit very snug in the flasks. Look closely at the bottom of the photo and you can see the flask is labeled “Cairo Top”.

pot life you must always be aware of. It is very helpful to have a timer set when you begin degassing the rubber so you can keep track of the time you have left.

Removing the mold: After the rubber had set overnight the flasks had to be broken

apart. Because of the size of the Cairo mold it needed a little more leverage to break the flasks apart, so I used a small crowbar. I tapped the bar into the seam and gently worked my way around the mold prying just a bit as I went along. I took my time as I didn’t want the flasks damaged since they would be reused again and again. For smaller molds made from plastic sheet, gently pry the

plastic away from the RTVR and carefully remove the rubber. Now that I had the flasks apart, cleanup must begin for the

next step (see Fig.18). Using a new #11 Xacto blade I carefully trimmed away the excess RTVR from the mold using the sides to guide the blade. The clay was removed from the master pattern and the flask using the wood sculpting tools. The entire mold, flask, and master pattern were kept in position and washed down in the sink using warm soapy water and a stiff brush to remove the clay from the scribed in line work and the mold sides. The cleaning process was repeated on the bottom mold flask to prepare for the mold cast and everything was left to dry well (see Fig.19). Now the whole molding process has to be repeated for the lower flask. There is a notable twist everyone must be aware when casting rubber to rubber molds. Rubber will stick to rubber creating one sold part if a release agent isn’t used. Moreover, a different release agent is required for rubber to rubber applications. Consult with your RTVR manufacturer for the proper release agents as it is always best to use the products they make for their rubber.

Spray on the rubber to rubber release agent, clamp the flasks halves together and repeat the mold process. The next day the halves came apart with no problem and I had a beautiful 2 piece mold! There was one final step to prepare the mold for casting parts. I carefully cut away the rubber to attach the gate and vents to the mold. The first test shot was perfect. So good in fact, I used it for my first built up model of the USS Cairo. You can see the completed mold and as well as the first test shot still in the mold in the headline photo of this article.

Remember when it would be obvious later on why we label the mold flaks top and bottom? Like finger prints no two flasks are identical in shape and configuration. When the RTVR was poured for each mold the RTVR took on that unique shape. Now the mold fits in the flasks like a glove. The advantage to this the flasks will provide extremely strong support not allowing the molds to move ensuring a perfect cast every time. Now every time I cast a Cairo hull, I’ll assemble the flasks just as you see here.

Well, that’s about it race fans. While I am by no means an expert on making molds, this CD should get you well on your way to making your own. Believe it or not, you have just taken a bold step beyond average modeling. With the techniques displayed here, there is no limit to your modeling. You now know enough to build simple or very sophisticated model conversions allowing you build truly one-of-a-kind models. I hope to see some of your efforts in the coming months at IPMS events.

Rusty White Flagship Models Inc. www.flagshipmodels.com

CREDITS The invaluable help both of these gentlemen have provided with assistance in the production of this CD is

incalculable. Without their help and kindness, this CD would not have been possible. Please visit their web site and spend some money.

Loose Cannon Productions

Specializing in 1/700 ship models and accessories http://home.earthlink.net/~loosecannonproductions/index.html

Hugh Letterly

Loose Cannon West 12384 East Cedar Circle

Aurora, CO 80012 EMAIL: hugh1lottie2@earthlink.net

David Angelo Loose Cannon East

10309 Westmar Road Jacksonville, FL 32218

EMAIL: fuelmann@hotmail.com

SOURCES

RTV mold rubber and casting resin

Reynolds Advanced Materials 2131 S. Harwood St.

Dallas, TX 75215 1-800-421-4378

RTV mold rubber, casting resin, & synthetic modeling clay

Alumilite Corp. 315 E. North St.

Kalamazoo, MI 49007 1-800-447-9244

DID YOU ENJOY THIS CD?

Flagship Models produces an ever growing line of informative and helpful modeling CDs to assist the modeler building better models. Other CDs of interest to the ship modeler are: FMCD-01 Working With Photo Etched Parts FMCD-02 Photographing Models FMCD-03 Creating Realistic Water FMCD-04 Weathering Ship Models FMCD-05 Building Resin Ship Models FMCD-06 Building Realistic Ship Dioramas FMCD-08 Building Submarine Models FMCD-10 Building a Hybridized New Jersey