So I put my Speedmaster on today, having not worn it for about a month, and I noticed three, parallel scratches (or so I thought) at the top of the crystal. Figured I scratched it on something sharp, but when I took a loupe to it, it was actually on the inside, and not scratches, but fissures in the plastic. I would post a picture, but it's even hard to see with my loupe. Anyway, what would likely cause this? The was no trauma to the watch at all. I've seen crystals with these all over, and just assumed cycles of expansion and contraction via temperature change is the culprit. But this watch just sits in a drawer, in a climate-controlled room. Any ideas? One factor that may or may not be relevant: the crystal is after-market. Do plastic cystals vary in quality? Thanks for any insights.
Acrylic crystals are designed to flex with stress. The acrylic also hardens with age, and cracks can develop. These cracks are not likely to materialize on an acrylic crystal if the watch is not worn. There are degrees of "trauma", but inward pressure on the crystal would be one degree of trauma. Flexing of the hardened plastic would likely be the cause. You don't say how old the crystal is. But whether the crystal is new or old, cracks are caused, they don't just "happen".
plastics.... I am so sick of them.... but here's some of what I've learned in 20+ years of engineering the shit. Acrylic doesn't like long term stress, if the component is too tight a fit or held in a bent position it will crack over time as a means of unloading stress. As an acrylic crystal, or any plastic, ages its will degrade, reducing its ability to take the daily beating they take on our wrists. I've got samples of the stuff which has cracked sitting in my desk untouched.... time is the only enemy needed with some moldings, the stress of the original forming process can release over time in the form of cracks. Plastics don't like solvents, different types of plastic dislike differing types of solvents, PMMA, acrylic, really doesn't get along with alcohols, particularly ethanol... ethyl alcohol... which is a prime ingredient in Purel (70%) and other such hand cleaners. repeated short term exposure to the stuff will cause stress fractures in acrylic products. look.... old plastic... the shell of this shaver just buggered of due to age, its just powder in the bottom of the box. This is an extreme case but possible in many plastics under the right conditions. did I mention I'm sick of this stuff.
oh, and clear materials are often the most prone to cracking because its hard to make materials that are both tough and clear, in plastics clear often equals brittle. did I mention I have a shipment of vehicles in a shipping port half way round the world, right now, with cracks in the components due to molding stresses..... new parts, undamaged when shipped, properly installed, acrylic. damn plastics.