I want to make a physical copy of a surface, such as the face of a coin, so that I can keep it a long time and compare it to the surface later. I want to figure out of the surface is eroding over time.
Maybe I could press modeling clay against it, pull it off, and pour plaster of Paris over it. But would the plaster set properly against clay? Are there oil based clays, the non drying kind, that I should use?
Is there a better approach? A special product just for this?
Thanks!
Epoxy putty.
Two part silicone is probably the most durable. For larger objects, you might need to reinforce it with mesh after applying.
brush on/pour on rubber will make a mold. get at an art supply store.
With something like a coin, the difference between proof quality and well-worn is probably in the millimeters. I doubt that a mold made of putty or silicone or rubber is going to have enough fidelity to accurately compare to the surface later.
Im sure you don’t mean millimeters, you probably mean microns.
PDMS (fancy two part silicone rubber) is capable of molding to structures 5 microns or so in size. I’ll see if I can dig up some citations
I think this one claims 2 nm Yikes!
http://rogers.matse.illinois.edu/files/2004/tubemoldingnanolett.pdf
Theres a bunch out there. Go with PDMS,
The problem with any homebrew solution such as clay is you’ll inevitably distort your negative mold in the course of peeling it off your positive original. So you need something purpose-designed for the task.
If budget is not a barrier or if you have contacts at the right engineering departments at a university/research center - then this will work well :
MikroCAD - Scan Surfaces and Small Parts with Submicron Precision
If the object in question is really something like a coin (as in the example), then wouldn’t the easiest solution just be to start with two samples? Store one away someplace protected and expose the second to the elements. Ideally, you’d want to actually measure the depth of the relief to compare the originals.
I just watched one of those true crime shows where the detective used the same substance to make a casting of the undamaged portion of an intentionally defaced gun barrel in order to compare ballistics. It worked. Sounds like the right stuff.
For the distortion concern, can you deface the original? Use something to put uniform scratches in the surface, possibly with a couple of different abrasive grades. Then you can easily observe minute wear as the scratches go away.
Well, it’s not a coin. It’s a big machine and there’s an area about the size of a dime that is corroding, a sort of a textured depression in an otherwise fairly flat surface, and I want to figure out how fast the corrosion is progressing. But the spot is hard to reach and only becomes accessible every few weeks or so.
LSLGuy’s comment is the kind of thing I was worrying about.
The MikroCAD thing is interesting. I wonder how it works when you can’t get the stand in there? Picture we’re talking about a spot on the side of a car engine, and we have to reach in there to access it.
You need a laser scanner like this http://www.david-3d.com/en/products/starter-kit-2
How do you plan on comparing it? A plaster cast will certainly absorb moisture and erode due to handling and measuring, and will provide only very poor fidelity on the scale of a coin or similar sized object.
For measuring simple profiles such as a blade edge or other wear surface a “go-no go” master gauge is sometimes made to the specified maximum material condition and gaps measured visually. For more complex surfaces generally the only way to scan the surface with a multiaxis coordinate measurement machine (colloq: “FaroArm”), laser scanner, or video measurement system.
Stranger
I think there are measuring devices that may work for you, or you can make one. Picture a flat, narrow piece of metal with a screw running through it. you lay the flat metal across the depression with the screw centered over the depression. Run the screw down until it barely touches the bottom. Next time do the same and note how much further you have to turn the screw to reach the bottom
Here is a pile of ‘depth gauges’
Well he was getting the texture measured so very accurately.
Surely you could just use a rule and feeler guage, to measure the bulk loss.
The texture is of very little concern compared to bulk loss…
unless the concern is contamination at low levels… (you need to decide if the first concern is mechanic failure (weakness or tolerance issues), flow rate or flow pattern or something (due to texure and surface roughness , or contamination
What if you put multiple colored layers of masking tape over it, like blue -> yellow -> white and see which color it’s down to after a few days/weeks?
He could take a lump of epoxy putty and smoosh it against the surface in question leaving enough for a handle, let it harden, then pull it straight off so it doesn’t get warped.
Yeah, given the actual task, I was going to suggest using a colored chalk or lubricant to see where the “rub” is. I like your idea, too.
The two part silicone idea, while great for copying ceramic molds or machine parts, and I have used them a lot, aren’t going to tell you what you want to know. It would be like dropping an ice cube back in the tray to see if it melted on the bottom.
Cerrosafe is a low melting point metal used to make castings of hard to measure features in machinrd parts. If you know any machinists they could help you measure by using a surface plate and height gage or CMM. Your depressions will show as raised areas in the castings. Its available in small amounts from gunsmithing suppliers such as Brownells or Midway.
I would give you links but Im on mybphone.