Friday, October 7, 2011

Small scale molding and casting at UT


Vacuum pump and chamber on handy cart
More than a decade of disappointment with the quality of my casts is over, the day has finally arrived where I have assembled and tested a fantastic system for making itty-bitty casts with which I am extraordinarily happy.  After my exciting and implosive night back in June, the start of this fiscal year has allowed me to procure a new vacuum chamber, finally completing the apparatus for making beautiful bubble-free minute molds and casts. There are a number of publications detailing protocol and equipment for achieving these (e.g. Reser, 1981; Davies, 2010; Reser, 2011; and Cavin, 2011), our process most closely follows that refined by Reser (2011). Equipment includes a Welch Duo-seal 1402 vacuum pump, a Smooth-On branded five gallon vacuum chamber, a digital scale, and a 2.5 gallon pressure pot.

Test specimens were molded with Polytek Platsil 73-25 RTV silicone rubber, and cast using Smooth-On Smooth Cast 300 pigmented with Polytek Polycolor Brown (approx 1 drop per 100g). Specimens were clayed-up in standard fashion, including imprinting the specimen number into the sulfur-free clay. Silicone was de-aired in the vacuum chamber, and a very thin coat was painted onto the surface of the specimens. A light stream of air (~15psi) was passed across the silicone to spread and pop any air bubbles that might remain, and the remainder of the rubber was poured to cover the specimen completely.


Digital scale, pressure pot, and two in-progress molds
Upon demolding 24 hours later, the silicone molds contained no visible surficial air bubbles, even at 50x magnification. Casts were produced by painting in a thin coat of Smooth Cast 300, making sure that there were no trapped bubbles, quickly pouring the rest of the resin, and then the mold was pressurized to 40psi for 15 minutes. At this point, the resin was completely cured and demolded. Pressure cast examples showed no air bubbles at all in the final cast, while those that were merely painted and poured contained some bubbles at or just below the surface. Once the molds are made, the entire process of casting one specimen, including setup and cleanup, takes no longer than 30 minutes.

All of these specimens were scanned at UTCT, and we plan to CT scan these casts also. Our next step will be to compare the fidelity, ease of production and handling, and final costs associated with methods of digital imaging and printing vs. conventional casting.  A paper is soon to follow. Subsequent blog posts will detail the procurement and assembly of this casting system.
This is what I call one helluva bubble-free cast- TMM 41672-233 (cast) Click the image for full resolution.



References

From Kirk and Williams, 2011. TMM 41672-233 (actual specimen). Scale= 2mm
Cavin, J. 2011. Vacuum molding and casting for bubble-free fossil replicas. Proceedings of the 4th Fossil Preparation & Collections Symposium. Brigham Young University Geology Studies Volume 49(B):24-28

Davies, K., Cifelli, R., Davis, B. and Gordon, C. 2010. A simple microvertebrate molding and casting technique: A 20-year retrospective. Programs and abstracts from the Society of Vertebrate Paleontology meetings, October, 2010.

Kirk, E.C., and Williams, B. 2011. "New adapiform primate of Old World affinities from the Devil’s Graveyard Formation of Texas". Journal of Human Evolution

Reser, P. 1981. Precision casting of small fossils: An update. Curator, 24:157-180.

Reser, P. 2011. Cost effective assembly and operation of equipment to make excellent casts. Proceedings of the 4th Fossil Preparation & Collections Symposium. Brigham Young University Geology Studies Volume 49(B):6


1 comment:

Nithya said...

Thanks for sharing the useful information..

precision castings