Working with Akron RTV-RP Silicone Mold Rubber
Akron RTV-RP Silicone Mold Rubber is designed to work with all RP grown (printed) models and all Stereo Lithographic resin models. RTV-RP is a two part addition cure system exhibiting high tear strength and elongation. You will find it to be well suited to difficult applications where undercuts and thin sections can otherwise be a problem. The firmness of the finished mold will prevent deflection of thin sections during wax injection. Stored at room temperature, unmixed product will retain it's excellent properties for six months or more. Because settling can occur during extended storage, it is recommended that both the resin and catalyst be stirred in their respective containers before use. Always thoroughly clean the model to prevent cure inhibition.
To prevent settling, always thoroughly stir parts A and B in their individual containers before using. Weigh the resin (Part A) in a clean container of about four to five times the volume of the resin being weighed. This will allow room for expansion during deairing. Add the appropriate amount of catalyst (Part B). Mix thoroughly by hand or with a low speed mixer under 1000 rpm to prevent heat generation. Two 15 second cycles is sufficient when power mixing. Useful pot life is approximately 90 minutes.
Use a vacuum chamber capable of pulling 29 inches of mercury. Most investment units will handle this. Place mixture in vacuum chamber allowing the mixture to expand to about four times it's original volume, crest and recede. Continue under vacuum for 4 to 5 minutes. Although it may appear that there is still air in the mixture, do not continue to vacuum. Stop and allow to sit for 10 minutes, at which time you will note that the mixture is free from trapped air.
If you are using glass mold frames, it is recommended that you use teflon mold release spray on the glass surfaces to provide easier release. Pour your mixture carefully down the side of the mold frame, taking care not to introduce new bubbles. See that the mixture flows down the side of the frame and up into undercuts in the model without entrapping air. Further time under vacuum may be appropriate if you introduce air while pouring. Vacuum two to three minutes and allow to rest. With clear RTV, you can see and move stubborn bubbles with a straightened paper clip.
If you are making a mold from a carved or machined wax, you may use the 16 hour demold time at room temperature, or 60 minutes at 50ºC (122ºF). If your model is not sensitive to elevated temperatures, you can cure in 30 minutes at 88ºC (190ºF). A good low temperature cure method is to immerse the mold in a water tank maintained at the required temperature, or cure in an oven with controlled temperature.
Direct Alloy Casting
Akron RTV Silicone is rated for continuous exposure to temperatures up to 500ºF (260ºC). We do not recommend direct alloy casting as it is an unsafe procedure without proper training.
Alternate Catalyst (Softer Molds)
The standard Akron RTV kit includes a catalyst that will produce molds with all of the most important qualities for best reproduction of models or carved waxes. The standard mold is the same hardness as our Akron Jewelry Rubber Firm grade (40 durometer, Shor A). In the case where you have a very delicate wax model to copy, it may be desirable to produce a softer mold to prevent breakage during cutting. For this purpose, you may purchase a bottle of our CAT-5 catalyst. The resultant mold will have a durometer of 25º Shor A. You should recognize that a soft mold is not recommended where you have thin sections that will deflect during wax injection.
A 1.1 lb. kit will yield 5 to 6 molds from an average mold frame 3/4" x 2" x 3". Actual yield is dependent on model size and other factors within the user's control, such as not filling the mold frame to the top.
The approximate amount of mold rubber required to fill a specific mold frame is calculated as follows:
L= internal length of mold frame in inches
W= internal width of mold frame
T= thickness of mold frame
Lbs.= L x W x T x .04
Grams= L x W x T x .04 x 454
This will fill the mold frame but does not allow for the model. You will want to deduct an amount for the displacement of the model.
The above formula provides total mixed weight. To discover the proper portions of part A and part B, divide total weight by 1.1. This is the weight of part A (resin) required. Multiply this figure by .1 to discover the total weight required of part B (catalyst).
To clean up resin or catalyst before RTV cures use toluene, naphtha or a similar product such as mineral spirits. All are rather inexpensive and can be purchased at any paint or hardware store. There is no solvent for cured material, however, it will simply peel away.
Hardness: 40 shor A (durometer)
Pot Life: 1.5 hours
Initial Mixed Viscosity: 1000 poise
Tensile Strength: 800 psi
Linear Shrinkage: < 0.1%
Tear Strength: 125 ppli ASTM DIE B
Specific Gravity: 1.1 gm/cc