Resin Casting: Tips and tricks to successful casting
Resin casting has a reputation for being either difficult, hazardous or both. While resin does have a bit of a learning curve, it’s nothing that the average hobbyist can’t over come. While the products referred to in this article are “house hold safe” i.e. your wife will not kick you out of the house for stinking the place up, please refer to the manufactures provided MSDS for full instructions for safe handling. My preferred brand of resin is Smooth-Cast from Smooth-On, product number 300. It is a two part Urethane Plastic with a 3 minute pot life and a 10 minute de-mold time. Pot life is the amount of time you have to mix the two part resin & pour into your mold. De-mold time is the wait time until the product can be pulled from the mold. Add the two together & you have about a 15 minute turnaround time per casting.Supplies:
- RTV mold – open faced
- casting resin
- Talc (baby powder)
- Mold release
- Clear, 8 oz plastic drinking cups
- Clear, 16 oz plastic drinking cup
- Stir stick
Step 1: Powder the mold with talc, aim for a smooth coat through out the deepest recesses of the mold. Shake free any access powder. The talc performs two functions. First, the talc helps pull the resin into the corners of the mold for better casting. Second, it reduces the impact of moisture on the resin.
Step 1a: Spray mold release into the mold. Using a mold release will prolong the life of the mold, preventing tearing of the RTV during demolding and restoring oils to the RTV leached out by the resin. This step is optional if you plan to only cast a few (fewer than a dozen) copies, or have mold with few undercuts to pull at the rubber.
Step 2: Pour equal amounts of part A and part B resin into the 8 oz plastic cups. If you are casting a sizable portion of resin, say a few ounces or more – you will be able to eye ball the volumes. The resin is forgiving enough to accept slight variances in the mixture. If you are casting small parts, you may want to look for a marked syringe or other measuring device to more exactly draw the resin.
Step 3: Pour the measured units of part A and part B into the 16 oz cup. A few quick stirs will blend the two components. Part A is colored a beer like yellow, while part B is a clear goo. The coloration makes it easy to see when the resin is properly mixed.
Step 4: Pour into mold- at this stage, the resin will pour like milk. Three minutes may not seem like much, but it’s plenty of time to mix and pour up to 16 ounces of resin. Fill the mold cavity to the lip of the mold. The resin will not be completely flush as surface tension either pulls it up the side of the mold (if the cavity is under filled) or pools over the top of the mold (if the cavity is over filled). In the case of an over filled cavity, use a plastic spackle scraper to level the resin.
Step 5: Over the next 10 minutes, the resin will first turn cloudy, then become a dull white/yellow color. The innermost portions of the mold will cure first as the resin works up the necessary heat to harden. Once the entire surface has changed to a solid state & the product is cool to the touch, it’s safe to de-mold. If your mold has any features that make demolding difficult, you may want to wait more than the listed 15 minutes. Other wise, you run the risk of twisting the still setting resin.
Final thoughts: Take care with the resin, wear a pair of rubber gloves- the last thing you want to do is splash resin on your skin. Standard dish washing gloves work well for me. Work on a surface that is either disposable or hard & smooth. I work on a counter top & spilled resin peals off with out any trouble.
Resin is not a product that can be kept around for long periods of time. Moisture from the atmosphere slowly degrades the product every time you open the jug. Once the manufacture’s seal is broken, expect to use the resin in the next month or two. Resin contaminated by moisture will foam when cast, giving an swiss cheese like effect.