Monthly Tech-Tip from Tony Hansen SignUp

No tracking! No ads!That's why this page loads quickly!

1-9 | A | B | C | D | E | F | Frits | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Poly 74 Rtv Rubber


Poly 74 series rubbers consist of liquid Part A and Part B which, after mixing, cure at room temperature to flexible, high strength RTV mold rubbers. Poly 74 series rubbers make durable, easy releasing molds for casting gypsum plasters, and waxes without release agents, but are also excellent for casting cement, epoxy, polyester, urethane and acrylic with proper release agents.

Part A is a clear liquid with a viscosity of 5000 cps and a specific gravity of 1.02

Part B often darkens with age, but rubber properties do not change. All B's except 74-65 and 74-55 require stirring before each use. All Part B's have specific gravity of 1.0.

Physical properties of mixes:

Poly# 74-65 74-55 74-40* 74-30 74-20* 74-20*
Mix Ratio 4A:1B 4A:1B 2A:1B 1A:1B 1A:1B 1A:1B
Pour Time 10 min 15 min 20 min 30 min 30 min 30 min
Color Clear Clear Transluc Grey Clear-Blue Greyish
Blue Yellow Blue Amber Grey/Amber
Viscosity, cps 4,000 4000 3400 2000 2000 2000

Cu. Inches/pound all mixes: 27.5
Shrinkage during cure all mixes: nil

*Stir before use.

Model Preparation: Porous models such as plaster or wood should be sealed, preferably with wax or several coats of nitrocellulose lacquer (petroleum jelly, paint, PVA (soap on plaster) and other coatings work also well). If shellac is used as the sealer, it must be thoroughly coated with release agent as Poly 74 Series rubbers bond tenaciously to shellac. The sealed or non-porous model should be be coated with Pol-Ease Release Agent or wax which should be allowed to dry. Where there is any question about the compatibility between rubber and the prepared model surface, a test cure should be made on an identical surface to determine that complete curing and good release is obtained. Porous models must be vented from beneath to prevent trapped air from forming bubbles in the rubber.

Mixing and Curing: Unopened containers are usable at least one year from date of shipment. Both parts A and B tend to absorb atmospheric moisture, thus should be used up as soon as possible after the container is opened. Part A should be weighed into a clean metal, glass or plastic container, then the appropriate amount of Part B can be weighed into the same container. While mixing, avoid whipping air into the mix. A baker's rubber bowl scraper makes an excellent mixing paddle. While mixing, scrape down the sides and bottom of the mixing container several times to insure thorough mixing of both components. Cab-O-Sil can be added to the mixed liquid to thicken the rubber to a gel for application by brush or trowel.

Vacuum degassing will help to provide bubble-free molds but is usually not necessary. Allow to cure at room temperature (25C). Ultimate properties are reached in about seven days but molds may be used with care ofter 48 hours curing. Curing in a warm location, up to 60C (140F) will greatly accelerate the curing speed while low temperatures slow the cure. Avoid curing in areas where the temperature is below 15C (60F).

Part C for Softer Molds: Poly 74 Part C Softener can be added to the above product to soften the cured rubber and lower the viscosity of the mix. There will be a loss of strength, slower cure and increased tendency to shrink after repeated castings. To soften Poly 74-40 to Share A-30 hardness, add 50 parts C to 150 parts total mix of A and B by weight. To soften Poly 74-40 to Shore A-20, add 150 parts C to 150 parts of mixed A and B.

Using the Mold: No release agent is necessary when casting gypsum or molten wax in Poly molds. Sponging, dipping or spraying the mold with 1% detergent solution prior to pouring plaster reduces the surface air bubbles in the plaster and aids release. After the first few castings, plaster release becomes easier. Pol-Ease is an excellent release agent for most resins. Poly molds will last many years if stored undistorted on a flat surface in a cool, dry location out of direct sunlight.

Polytek Development Corp, Box 384, Lebanon, NJ 08833 908-534-5990
Also available from Axner Corp.

Related Information


Oxide Analysis Formula

By Tony Hansen

Tell Us How to Improve This Page

Or ask a question and we will alter this page to better answer it.

Email Address




CAPTCHA, All Rights Reserved
Privacy Policy