I’ve been getting lots of PM’s lately about in-mold clearcoats so I though that I’d put all the info in one spot. This way I wouldn’t have to type so much. 
What is an in-mold coating?
An in-mold coating (IMC) is a coating that is sprayed or painted into the mold prior to the layup. This coating either provides color, decoration, product branding, UV protection, or scuff protection of the fibers. This is also known as paint-in-the-mold fabrication (PITM)
Why use an in-mold coating?
The typical reason to use in-mold coatings is to save finishing labor and in some instances save weight. As most of you know, the time that it takes to sand, fill pinholes, sand, spray, sand, and polish is often greater than the time it takes to layup the part. In some instances in-mold coatings can be lighter because primers are not needed when painting. It also takes less skill to paint a mold than paint a part. The spraying environment doesn’t have to be pristine either. Any dust ends up being on the inside of the clearcoat (between clear and layup) rather than on the outside. If the mold surface is excellent the paint finish comes out excellent.
Why not just use a gel-coat?
Most clear gel-coats are not UV stable and will yellow over time. Gel-coats also tend to be heavier than paints/clearcoats because they must be sprayed to a certain thickness to cure properly. Many gel-coats are also prone to micro-cracking due to their brittleness.
What can be used for in-mold coatings (IMC)?
The options are actually quite vast. Rattle cans, high-end 2K automotive paints, and 2-part epoxy paints work well. I often used the lacquer based Dupli-color Paint Shop line for hobby based products. This paint requires zero mixing. Just pour it in the gun and go. I often left the clearcoat in the gun for up to a week if I was using it everyday (who likes to clean spray guns). The downside to this system is that it doesn’t have much solvent or UV resistance. For high-end products I used the PPG Deltron line. I used Deltron Concept for colored products and Deltron DC3000 for clear. I like high velocity systems, as the cure quickly, so the fabricator can move to the layup sooner.
I should note that not all resin/paint combinations work well together. Some resins will cause some clearcoats to turn hazy. Some combinations will leave the paint tacky for the life of the product. Comprehensive testing should be performed to insure that the IMC is compatible and still UV stable with your chosen layup resin.
How long should the IMC cure before the layup can performed?
I typically let the paint/clear significantly cure before performing the layup. I only use epoxy layup resins, due the requires of the parts that I produce, and have never had adhesion issues with the IMC. I do NOT scuff the surface before layup. I often sprayed the mold at the end of the day and performed the layup the need morning. With some lacquers I would do the layup 1 hour after spraying. If the IMC is tacky everything can still work fine. There is the risk that the IMC and the resin will commingle and foul the cure of the resin and/or paint. I’ve found that some combinations will have a tacky surface and poor release if the IMC isn’t allowed to cure enough. Most 2K paints can be quickly force cured with IR lamps or low temp ovens (140*F) to speed the process. High velocity products can be zapped with IR lamps and reach a near full cure in less than 30 minutes. I have limited experience with PE and VE layup resins. Their lower adhesive properties my cause interface issues between the IMC and resin. Again, testing should be performed to insure that the interface between the IMC and resin is adequate.
Challenges when using IMC’s
1) Fabric Print-thru - If the part experiences fabric print-thru then the whole IMC thing becomes unproductive. To reduce/eliminate fabric print-thru you want to use low shrink resins. Higher shrink resins like polyester can be impossible to eliminate print-thru. The curing and demolding process is also very important. The most common reason for fabric print-thru is demolding the part too soon. The part may look great right after demold but print-thru will often show-up during the next 24 to 48 hours. Ideally, you want the layup resin to fully cure before the part is demolded. For this reason I use a heat treatment to bring the resin to a near complete cure while the part was still in the mold. I would let the resin gel at 75F then slowly ramp the part up to temp to cure the resin. I would go up to 125F to 175*F depending on the resin and part requirements. Also, force curing the resin too quickly can promote more resin shrinkage. Slow, but near full, cure while in the mold is the name of the game.
2) Fisheyes on High Slip Mold Releases - The first time that you try and spray on top of a high slip release your eyes will opened and the cussing will ensue. This problem can be eliminated by first spraying PVA into the mold…but this defeats the whole advantage to IMC’s. Who wants to spray twice and have all those dust bits on the outer surface of their IMC? This leaves us with the challenge of spraying onto waxes and semi-permanents. I love the way semi-permanents release so I have favored them. If you like waxes you can use Dolphin wax to virtually eliminate the problem of fisheyes when spraying IMC’s. Dolphin wax allows for fabricators to do really fancy paint jobs right into the mold. The issues of paint pulling due to masking edges can be eliminated because the paint does need to set between coats. The masks can be pulled right away.
Spraying on semi-permanents and other waxes requires the following procedure. The first coat into the mold must be a very light mist coat. I do this by just barely opening the product release stop on the spray gun. This allows for a very controlled first application. This first mist coat must set before you can progress. If you are spraying a lacquer then this wait may only be a few minutes. With 2K paints the operator must wait longer or zap this mist coat for a few minutes with IR lamps or heat. Once the mist coat has set a slightly heavier coat can applied. Once this sets the applicator can apply a heavier coat. This takes practice with your specific paint and release. With experience you will learn how quickly you can progress to the next coat without causing fisheyes. 2K polyurethanes typically need more time, or force curing options, between coats than lacquers and epoxy based products. Once fisheyes have started to form it takes lots of light coats to stop them from forming again. The key to never let them start in the first place. Over-reducing the paint by 50% can help speed up the process with some paints/clears.
[SIZE=2]3) IMC Pre-release[/SIZE]
The early release of the IMC can be a problem with some processes. If you have a complex shape and require the use of spray adhesive to hold the fabric in position (infusion) then the fabric can pull the IMC off of the mold surface. I have not found a solution to this problem. Using a thicker layer of IMC can help is some scenarios.
The other pre-release issue is when using multi-piece molds that get joined (hollow structures for example). The seaming process can pull the IMC off the mold surface and the seaming resin will run between the IMC and mold surface near the seam. The key to solving this is to make sure that the IMC goes from the mold surface, around the mold edge, and onto the flange at least 1/8". I don’t like to paint the whole flange because it requires more clean-up time to remove the paint between cycles. I prefer to make reusable masks that are quick to use (see the first part of this video for an example). Joggle gasket seams can be really tricky to prevent pre-release due to the aggressive nature of prepping the seaming flap for bonding. I have opted to use bladders in the this scenario. This solution is not possible on large molds but large molds typically product parts that have thicker layups that resist a pre-release when using joggle gasket tools and seams.
Final Thoughts
Using in-mold coats can significantly reduce the finish time when making composite parts. Unfortunately, it may not work in all scenarios. Testing the compatibility between the mold release, IMC, and layup resin is an absolute must. Not all combinations play nice with each other. Finding products that work well together is worth the time and exploration. The process does take a little practice.
Lastly, if your layup processes are creating poor results (lots of pinholes, seam voids, fabric bridging) then an IMC will not improve your production. An IMC will not hide layup errors and the whole thing will need to be sanded and refinished. You will get the pleasure of painting your product twice. If your layup skills are consistently reproducible then an IMC can save you hours of labor on each part. 90% of what I product uses an IMC. Pull, trim, ship, done.