I can easily read labels for materials and think that if I put one thing on top of the other then it will be good for external use, but time and time again I have been proven wrong. I have been working with making graphic designs that can go on magnets, both raised and flat designs and there are several issues that keep causing me to question the durability of the final product. At the store where I have been getting the magnets I am being told that people put stickers on the magnets and that's about it. The coating is PVC, or at least that's what we think it is.
Some inks, urethanes or epoxies bead up when I paint them across the surface. Others seem to bond well, but if I work on peeling them off I usually can. If I attach an object that is too thick to the magnet, then it is strong enough to bend the magnet away from the surface if it has a tendency to bend while being exposed to different temperatures and weather conditions. The conclusion here is that the item going on the magnet has to be either light and flexible to move with the magnet, or it has to be dimensionally stable itself in order to keep from changing the magnet and causing the entire piece to warp.
The testing is now back to basics, which is trying to make sure that I have a good surface on the magnet to print on and/or to attach an object to and to make sure the object is dimensionally stable. My most recent test were with vinyl sticker ink and a water based type of vinyl sticker ink by Nazdar. These are both compatible with screen printing, which is why I have gone with those first. I don't think these inks have any UV resistance in them, so I am thinking of them mostly as a bonding layer between the magnet and the next layer, which could be a print layer or an object. Epoxy and Urethane have been my materials of choice for making objects with and the potential of this one-part urethane I have been working with keeps me thinking that there is a place for that stuff in the process. Once the test pieces dry, then I need to put a layer of the urethane and the epoxy on to see if I can attach objects to the coated magnet sheet. If the objects will stay, then printing would not be an issue, so I have to test the bonding with an object that can be ripped off first.
My other test, the more interesting one, is to coat the back side of the magnet in the hope that I could coat the magnet with a clear layer that is easy to print on and then use epoxy or urethane as a print medium and use the magnet background darkish grey / black color of the magnet as the background color and print with light inks like silver or white.
The objects that I am going to attach will need to be more stable that some of the epoxy pieces I have been using. It seems that the urethane and the epoxy curl when exposed to wet conditions and heat tends to weaken the epoxy that would allow any substrate in the epoxy to bend internally. My solution is to use layers of carbon fiber, criss-crossed, to stiffen the back of the pieces that I am using. The expense is significant, but I don't think that if I just attach the piece to a sticker and stick it to the magnet that I will be able to sleep at night. Epoxy seems to be the best material for coating the carbon fiber with, but I can't help but think that if the epoxy gets warm then any tendency of the epoxy to bend will also allow the carbon fiber to bend. If I add too many layers then it will get more expensive and too difficult to cut out once the pieces are made.
My other project is to use a clear molded piece and then put the colored image behind the clear. I'll explain the reason later, but the big issue with clear is yellowing, especially with epoxy. This is where I think the use of the urethane will come in handy, but my first test with the urethane on the back of the epoxy was a fail. I have not put some epoxy behind the urethane and that looks promising, but without some good weathering I can't be sure that will work. The Urethane could be painted into the mold with thin layers, but this is much more time than simply pouring a little epoxy in the mold. I have heard of a clear epoxy that also has a UV resistance built in and I am going to have to get my hands on that if this urethane keeps acting up on me.
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