Making Molds and Taking Impressions

A discussion of materials used to make molds of existing mouldings in order to duplicate a unique original. December 26, 2006

How have you taken molds of curved details, say stickings on doors or moldings? We replicate a good bit and we are not always able to remove the item. I am accustomed to using calipers, taking pics and so forth. It would be quicker if I used a molder's clay or something more appropriate. Last week, I blueprinted an entrance door where the hundred or so year old mutton detail must be replicated. Couldn’t take the door. This happens about once a week. Any advice?

Forum Responses
(Architectural Woodworking Forum)
From contributor L:
You might try auto body filler, like Bondo. First put some saran wrap over the molding that you want to copy (for protection and to aid release). Then squeeze the Bondo over the piece you want to copy. It's pliable enough to follow the contour of your original when wet and it'll set hard quickly, so you can see if you got a good impression. Once released, I guess what you'd do next is bandsaw a sectional profile out of the hunk you cast. This is your full scale copy.

Probably be a good idea to try this in the shop and refine the technique before getting into trouble in the field. Beware of doing this on pieces that won't allow release of the Bondo mold once it sets up! (Any piece that can't be coped - because it closes in on itself - will likely be problematic when you try to release the mold.)

From contributor E:
I've seen a silicon based product that you pour on and let dry, then peel off. It is usually made so you can pour something into it and make a resin copy. I've seen it on TV's This Old House. Not sure what it is called, though.

From contributor T: supplies silcone mold materials. The site has some interesting on-line videos showing the molding process.

From contributor P:
The very best product for this is called ReproRubber. It is not cheap, but requires no release agent, will pick up the finest details and not get stuck. It cures in 15 minutes and stays flexible, so you can peel it off. The absolute best product. I use Bondo as well, but you have to be careful so as not to damage the original.

From contributor J:
Contributor L, you and I think alike. I use a lot of Bondo making templates, as I specialize in curved work. I typically use a Lexan plate scribed into close fitment, then fill in the gaps with Bondo, sanding smooth with edge sander/belt sander, and making the part blank with a pattern bit or a spiral cutter/rub collar. Also, I use blue masking tape on the location to be fitted, then spray with silicone lubricant to act as a release agent for the Bondo.

I have heard of a product used in the making of dentures. Unlike bondo, this stuff allegedly doesn't expand or contract. I'm hoping to eventually learn more and apply this product to my template making, as it sounds far superior to fiberglass/epoxy products. Anyone know what I'm talking about?

From contributor P:
I believe the product you are asking about is marketed to the industrial sector as "Tool Stone." It is very similar to what was commonly called dental plaster. This stuff is very hard and tough. You can get it in various grades that have compressive strengths of up to 30,000 psi. It is used in machine shops for supporting irregular parts, potting drill bushing in contoured and specialized tooling, supporting optical fixtures, etc. It mixes with water and is very safe. However, it will stick to anything, so a release agent must be used. It is also brittle. It may be way overkill for this application.

From contributor R:
Are you thinking about alginate? My dentist has used this to take molds off my teeth. Sets almost as fast as he puts it in.

From contributor L:
Alginate is great stuff and easy to use. It holds fine detail, but it deforms quickly over repeated uses. It also requires a backing mold of plaster to keep its shape, if I recall correctly.