What kind of fillers can I mix into Smooth-Cast resins?

Article courtesy of Smooth-On, Inc.

Virtually any type of powder filler can be used in Smooth-On urethane resins. Some examples of powder fillers:

You can attain different resin effects depending on the type and amount of filler that you add. For example, you can attain a realistic marble effect by adding URE-FIL 7 and pigment to our Smooth-Cast 325 resin (related FAQ: How can I make my castings look like marble?). You can make Smooth-Cast 320 plastic look just like wood by adding URE-FIL 5 filler. By adding metal powder to Smooth-Cast® 325 resin, your castings can have the look of bronze, copper, brass or other metals (related FAQ: How can I make my castings look like metal?).

Many fillers (like URE-FIL 3) cost a lot less than liquid plastics on a per lb./kg. basis. As a result, the more filler you add, the lower your cost per casting.

There are, however, factors that will affect how well a filler works for you and steps you can take.

  1. Light vs. Heavy fillers – Lightweight fillers will have a tendency to float in the resin and rise to the opening of your mold whereas heavy fillers will have a tendency to sink in the resin and fall to the mold surface. It is important to thoroughly mix the filler into resin to minimize this effect. Otherwise, castings may cure with areas of all resin and no filler and vice versa. This will create a weak or brittle casting.

  2. Small vs. Large Particle Size – Fillers of all sizes can be used to create various effects; however, keep in mind that the larger the particle size the weaker (brittle) the cured plastic will become. The reason for this is that there will be less urethane resin holding each individual particle together. Smaller particle sizes generally yield a more durable part.

  3. Moisture Contamination – Fillers, in general, tend to contain and/or absorb moisture. If filler does contain moisture, it will cause polyurethane resin to bubble or foam. Eliminating moisture content in fillers is accomplished by placing the filler flat on a metal plate or cookie baking sheet in an oven and "baking" it for a minimum of 2 hours at 150°F (60°C). After baking, allow filler to cool to room temperature and use promptly. If the filler is allowed to sit for extended periods, it may again absorb moisture.

  4. Pre-mix filler thoroughly with Part B of the resin before adding Part A. Adding filler to resin that has both components already mixed (Part A + Part B) will limit your ability to thoroughly mix the resin.

  5. How Much filler can I add to the urethane resin? This will depend on the particle size of the filler. Smaller particle size fillers like URE-FIL 9 have a large surface area and will absorb resin very quickly. The amount of filler that you add to your resin mix (A+B) is up to you.

Other Things You Need To Know

  • Increased viscosity – The viscosity of the resin/filler mix will increase in proportion to the amount of filler added. The thicker the resin and filler mix, the greater the chance for bubble entrapment.
  • Increased Working Time & Cure Time – pot life and demold time may be lengthened in proportion to the amount of filler added.
  • The finished look of the casting will change with the amount of filler added.

This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.