Creating Production- Quality Parts From Silicone Tools

Although the use of silicone molding to produce urethane castings is common, there are few resources available that teach this art form.

Lauren Groth and Todd Grimm

Urethane Castings

Urethane castings created from silicone molds are commonly used in the product development process. They are leveraged to evaluate, validate and document a design and to promote the envisioned product prior to commercial release. These castings are an ideal solution for quick and cost-effective production of small quantities of plastic parts. For these reasons, the silicone molding and casting process has been used for decades to fill the gap between one-off prototyping and production tooling. And with advances in material properties, the application of this process has grown rapidly. Today, casting materials allow prototype parts to more closely mimic the functional properties of everything from TPE (Thermoplastic Elastomer) to ABS.

Although the use of silicone molding to produce urethane castings is common and widespread, there are few resources available that teach this art form. This is most evident when requirements move beyond basic, acceptable castings to production quality cast parts. Other than simple "how to" booklets, there is a lack of educational courses for the company that wants to upgrade its silicone molding and casting program. Since there is little information available, those that know how to produce top-notch castings rarely divulge the secrets of the process so that they maintain their competitive advantage.

Almost anyone can build simple silicone molds and cast parts. Basic moldmaking can be easily accomplished by purchasing silicone from a supplier and following their moldmaking and casting instructions. Like painting or sculpting, the supplies and basic how-to information are readily available. Unlike these arts, silicone molding has few resources that teach the skills that advance the budding artist from painting-by-numbers to a Monet or Michelangelo. Most find that the only way to obtain the high level skills in the art of silicone tooling and casting is to either hire someone that possesses them or invest years in hands-on practice and experimentation.

To address this knowledge gap, the key steps of chased silicone tooling and pressure casting are described. This process delivers high-end castings that satisfy demanding applications where tight dimensional accuracy, high cosmetic appeal and aggressive deadlines are required. With only a modest cash outlay, those with this process knowledge can produce top quality castings.

Cast Parts From Silicone Molds - The Basics

Without exception, there are five steps for casting plastic parts from silicone tools.

1. Produce the master pattern.
2. Build the mold box and pour the liquid silicone rubber over the pattern.
3. Extract the pattern from the silicone mold.
4. Cast thermoset material into the mold.
5. Extract the casting and finish to specification.

Both basic silicone moldmaking and the chased silicone tool process use these five steps. The difference in the results comes from the techniques and processes that each use to complete these steps. With the techniques used in chased silicone tooling and pressure casting, the result is production quality castings with advantages that include:

1. High dimensional accuracy.
2. Strong aesthetic appeal.
3. Higher tool yields.
4. Longer tool shelf life.
5. Repeatable and consistent casting quality.
6. Minimal secondary operations.
7. Lower material cost.

These castings are perfect for demanding applications, whether the castings are used for form, fit and function evaluations or product presentations where appearance is critical.

Chased Silicone Tooling and Pressure Casting Process

The chased silicone tooling and pressure casting process is designed to meticulously control every aspect that affects quality. While it requires some equipment (vacuum chamber and pressure pot), most of the benefits are a result of process knowledge and the skill of the craftsman.

1. Create the Master Pattern

This first step is crucial to the process. The quality of the pattern will directly affect the quality of the castings, total project time and the amount of labor required. Therefore, it is critical that the right process is selected for the creation of the master pattern. Although rapid prototyping is a source of patterns, it may not be the best choice. While rapid prototyping is often faster and less expensive than CNC machining, there are limitations when producing high-quality castings. To achieve a suitable surface finish, the rapid prototype requires significant effort on the part of skilled technicians. Sanding, filling and priming are necessary to produce a pattern that attempts to represent the design intent accurately. This manual labor, combined with the inherent inaccuracies in rapid prototyping, will typically deliver a pattern that does not match the surface finish and dimensional tolerances available from machining.

Another consideration, when casting quantity exceeds the life of one tool, is that most rapid prototyping materials are not robust enough to be extracted from the silicone mold without breakage. Should the pattern be damaged, it is either repaired or replaced. With an ABS CNC machined pattern, multiple tools are cast from the original master pattern. This reduces program expense, cuts delivery time, and ensures consistency and repeatability across all of the castings.

2. Define the Parting Line

To reduce time, some promote the cut mold method. In this process, clear rubber encases the pattern in a single pour. Upon curing, the block of silicone is loosely cut along the intended parting line to free the pattern and produce the two mold halves. Chased silicone tooling parting lines are carefully planned and crafted before silicone is poured. This ensures:

a. Ease of part removal.
b. Robust tool parting surface without thin or feathered edges.
c. Integrity of cosmetic surfaces.
d. Preservation of challenging geometry like internal snaps and through holes.

Once the parting line has been selected, the pattern is mounted to a parting plate and the parting line is "laid-up." The lay-up combines the master pattern, solid supports and modeling clay to develop the surfaces that will be captured by the first half of the tool. Although lay-up requires more upfront time, it produces a superior result and an overall decrease in production time. By carefully establishing the parting line, the tool delivers an accurate and aesthetically pleasing casting with minimal clean-up labor.

3. Creating the Chased Silicone Tool

The major difference between chased silicone tools and non-chased molds is the use of a rigid frame (a chase) and high durometer, platinum-based silicone rubber. Once cast into the chase, the silicone rubber is locked in place, making it inseparable from the chase. This tooling practice minimizes flexing of the silicone during the injection and pressure casting process while stabilizing the part as the urethane solidifies. The result is dimensional accuracy and repeatability in the castings. In effect, this process delivers a soft "hard tool."

The chase is sized to encompass the parting plate while minimizing the amount of silicone rubber. Without the chase, excessive amounts of silicone are required to stabilize the mold. Reducing the amount of silicone to a typical thickness of 3/4" to one inch dramatically reduces the material cost for the tool.

After fabricating the chase halves, locking and location elements - such as key pins and bolt holes - are drilled. Then the parting plate and mounted, laid-up master pattern are attached to the first chase. An exact amount of silicone rubber is then carefully calculated, weighed, mixed and poured into the chase. Upon curing of the rubber, the parting plate and lay-up material are removed, while the master pattern remains secured in the tool half.

Before pouring the second half, all vents, gates and sprue holes are laid out and drilled in the second chase. If required, cores and slides are placed into the pattern. After attaching the first silicone tool half to the second chase, silicone is poured into the assembly. After curing, the master pattern is removed, the tool is cleaned, release is applied and the tool is ready to produce parts.

The integration of the chase with the 60A durometer silicone stabilizes the tool as resin is injected into the cavity and as it is subjected to pressure for part curing. This construction technique also allows for tool adjustment to compensate for resulting flash.

4. Casting Parts

Complete, void-free filling of the tool cavity is critical to the production of top quality castings. Voids diminish the mechanical properties and aesthetic qualities of the prototype. Although some voids can be repaired in a finishing operation and then hidden under a coat of paint, this adds cost and time to the project.

In the chased silicone tooling and pressure casting process, void-free castings are expected. To eliminate voids, the tool is positioned with the parting line in a 60- to 80-degree orientation. The thermoset resin is then injected into the tool at its lowest point. As the tool fills, the strategically placed vents allow air to escape so that the resin completely fills the cavity and reservoir. Immediately upon filling the tool, it is placed at an angle in a pressure pot and subjected to 60 to 80 pounds of pressure until the resin has set up.

Molded-in color and texture should be used whenever possible. With void-free, cast in color parts, painting of the casting is not necessary. By eliminating paint, the casting offers greater dimensional accuracy. Also, with color throughout the part, the casting retains its cosmetic appeal even if it is scratched or worn by a mating part.

5. Finishing

After extracting a casting from the tool, vent, sprue, gate and flash material are trimmed and residual mold release is cleaned from the part surface. The casting is then placed on a fixture where it is fully cured. If required, graphics and inserts are applied. The castings are now ready to be assembled into the final product and delivered for use.

The chased silicone tooling and pressure casting process for ultra-high quality castings is not for every project or every company. It is best applied to those projects that demand tight tolerance, crisp feature definition and a physical representation that looks exactly like the intended production unit. Yet, even mid-range requirements can benefit from elements of the chased silicone tooling process.

If an application demands high-end production quality parts, there are four options: develop the skills through practice, hire someone with the knowledge, purchase parts from one of the few high-end silicone tooling shops or seek out training.

Contact information:
Todd Grimm
T. A. Grimm & Associates, Inc.
3028 Beth Court, Edgewood, Kentucky 41017
Phone: (859) 331-5340      Fax: (859) 514-9721
tgrimm@tagrimm.com      www.tagrimm.com

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