Fixing a Warped Hull
A recent question posed to me via email brought this topic up as something that would be worth talking about. I'll do so below, and I'll also focus on it during my next YouTube Live which is today at 11am EST.
Getting a warped hull, or at the very least warped parts, is an inevitable occurrence for the submarine modeler. The nature of the resins and plastics that we use means that the parts are prone to warping. In the body below, I'll go into a few different materials and my experiences with them, along with a few hints and tricks to get that boomerang-shaped part back to where it needs to be.
First off, if you ever get a model kit and are not planning to work on it right away, you need to make sure that you store it properly. The vast majority of major warping issues that I've seen are a result of improperly stored model kits. It is imperative that you align all hull components in place and secure them tightly together in their proper orientation prior to storing for an extended period of time. You can get away with using things as simple as rubber bands or masking tape (for heaven's sake, don't use packing tape unless you like scrubbing glue off the kit later!). I also find that shrink wrap is an excellent thing to use that will wrap tightly around the entire model, holding it securely until you're ready to work.
Storing in a hot place like an attic will compound the potential for irreversible warping, so be sure that if you are forced to store you kit in a hot place like that, you have it locked into place securely with no possibility of movement. Rubber bands will break over time, so bear that in mind if you're wanting to use them.
Plastic Model Kits
There are a number of excellent kits on the market that are perfect candidates for RC conversion. These include offerings from Trumpeter, Revell, Moebius and others. Most, if not all, of these kits are manufactured from a type of polystyrene plastic. Polystyrene is considered a thermoplastic because the polymers don't crystalize when cooled like most materials. Instead, the material maintains its shape through intermolecular attraction which declines as the temperature increases. There is a threshold temperature known as Glass Transition (GT) where the material becomes less springy and more rubbery. That is the magic temperature at which the polymer molecules can just begin to slide relative to one another.
There are two approaches you can try for fixing warps in plastic. The first is to simply clamp the parts and adhere them with adhesive. I prefer the liquid styrene cement, which actually fuses or welds the parts as opposed to adding an adhesive to create the bond. Once so fused, they're nearly impossible to separate. Most of your alignment issues can be addressed this way.
Most warping issues for these models come when you cut it to allow access for your RC components. These are usually long cuts along the mid-line of the boat. It is common for the cut parts to see warping along the length, creating gaps in the seam that are unsightly and hard to address via the first method. For these, the solution is to mechanically force the part straight, then heat the plastic up to its Glass Transition temperature for a short time, then cool it off.
Polystyrene begins GT between 175-195° F (79-91 C) depending on its molecular weight, plasticizers, pigments and fillers. In order to straighten warped polystyrene parts, you need to get the plastic up to that temperature, allow it to settle in the proper shape, and then cool it back down again.
There are a number of ways to accomplish this. The safest one for small parts is to use hot water. Simply heat up a bowl of water in the microwave to something around 200F, allow it to cool slightly and then dunk your part. Once the plastic has heated up, it should become more malleable and retain whatever shape it's put into. Dunk the part in cool water to lock in the new form and you're done! You can also use your oven, which allows for precise control of temperature in a larger format.
Most sub hulls are going to be much larger than your bowls or oven, so you're going to need to use something like a hair blow dryer or, ideally, a heat gun. This takes some practice, as it's easy to overheat the parts and get warping and distortion, or even burning if you're not careful. Never focus the heat gun on one section for too long. Keep it moving at all times and use broad strokes so that you're heating up a large area. You'll see the part relax into the proper shape. Once it does, let it cool thoroughly before releasing your clamps and checking alignment. Repeat as often as necessary for a great fit.
Fiberglass Model Kits
Most of the larger model kits you'll see will be constructed from a mix of fiberglass and resin parts. There are a few different things to think about, so we'll talk about each in turn.
First, let's discuss warped resin parts. This is quite common when resin parts are de-molded prematurely before the resin has completed its reaction. While a part may feel solid and cool, the chemical reaction takes much longer to completely finish. Until it does so, it will be prone to warping. The rule of thumb for quality, solid parts is to let them cure long, warm and slow. This process usually takes 72 hours.
Fortunately, the fix for these parts is very easy, and you follow the same procedure as you do for plastic parts, heating it until soft, setting it into the correct alignment, and then letting it cool. A good rule of thumb is to aim for the same 200F temperature, letting the part sit for 3 minutes per 1/8" of thickness.
Now, unlike plastics, resins are what is called "thermo-setting", where plastics are "thermo-forming". With these parts,as the heat goes up, it first softens and then hardens, never again to be re-softened. Once cast resins are completely set, further heating only serves to make them brittle and unworkable, and to lose color. This begins to happen at around 160F, so you really have just one or two chances to get the shape correct before the part becomes "locked in" forever.
Let's move on to the fiberglass hulls. First of all, let me explain exactly what "fiberglass" is. It's a composite material, meaning that it's comprised of two (or more) different materials mixed together. Typically, fiberglass consists of a matrix of glass fiber, saturated with a polymer resin. The glass fiber has good tensile strength, but is flexible (like cloth). The resin locks the glass fibers in place, to keep them rigid. The end result is a strong, relatively lightweight compound, that is resistant to moisture and chemicals.
There are two main types of resins used in creating the typical submarine hull, polyester and epoxy. Both have their advantages and disadvantages. Polyester resins are cheaper and you can adjust the curing time via the application of the catalyst. It is, however, more brittle than epoxy and is prone to shrinkage during curing. It is also not as resistant to water and chemicals as epoxy (not that it really matters in our applications). Epoxy resin does not shrink during curing and is not as brittle as polyester. It also has very little odor when you're working with it. You can't adjust the cure time, either. Epoxy parts are stronger, meaning you can get away with thinner parts with the same strength as a thicker polyester one.
Although "Thermo set" type plastics such as what we talked about above can, in many cases, be reshaped with a heat gun or at low temp in an oven, fiberglass is a totally different animal and will not respond to heating. If you end up with a badly warped hull, it's very possible that the part in question was pulled out of the mold before it was fully cured, and as the curing continued the part pulled out of shape.
If the hull is still very green and likely still curing, you may be lucky enough to have the ability to heat it in the sun, a large oven, or with a heat gun. Once fully cured, however, your options are limited.
Your first option is to mechanically force the parts into place and secure them there with bulkheads. This is a very common technique, and one that lends itself well to our application as we are typically installing bulkheads into our boats to rest our watertight cylinders in. These bulkheads can serve to pull in or push out the walls of the boats. Matching bulkheads for upper and lower hulls will ensure a tight seam for hull splits for access.
The next option for a warped hull is to create a release for internal stresses of the part when it's in the proper shape. This is accomplished by cutting the hull in one or several places. I have a video that documents what I had to do for my recent Upholder build that you can view here.
What happens in this situation is you create a gap in the hull that allows it to close or open up around it. Once the hull is in the proper shape, you adhere it in place, fiberglass up the back face, and fill the outer one for a smooth finish.
While seeing your boat's parts in a state of misalignment and with huge gaps that look terrible can be very disheartening, there are ways to correct the issue and get your boat looking ship-shape once again (within limits, of course). Patience and careful analysis of where the issue truly lies will make the repair a very straightforward, if labor-intensive, process.