Plastic Fantastic

PolyJet Master (clear and yellow) Silicon Mold and molded parts (clear and amber).

Abstract
Silicone rubber molding is a perfect solution to fill the gap between one-off rapid prototyping and prototype injection molding. Silicone molds produce urethane castings that are used for functional testing, product demonstration, and even low volume manufacturing. From rigid and tough to soft and flexible, quality parts are made in less than a week for only a few hundred dollars.

Silicone molds reproduce the tiniest of details, so the quality of the pattern is critical. With PolyJet™ technology, perfect patterns are created and ready for mold building immediately after they are cleaned. Investing hours or days in sanding, filling, and priming patterns is no longer needed. PolyJet expedites the silicone rubber molding process and reduces the cost.

Recommended Supplies 1. Silicone rubber (platinum based) 2. Cast urethane 3. Mold release 4. Modeling clay 5. Plywood or similar 6. Rod stock (1/16” - 1/4” diameter) 7. Disposable containers 8. X-Acto round carving cutter 9. X-Acto knife

Recommended Equipment 1. Eden333 or Eden260 2. Oven 3. Pressure pot 4. Vacuum chamber 5. Rubber mixer/dispenser 6. Urethane Mixer/dispenser

Optional Supplies and Equipment 1. Detergent based cleaning solution 2. 200-1000 grit sandpaper 3. Primer paint 4. Bead blaster 5. Table saw

Introduction
Silicone molds are a fast and affordable solution for functional prototypes and low volume production. Although urethanes are thermosets, not thermoplastics, the number of its applications is not limited. In fact, the wide variety of highly engineered urethanes that are on the market offer a vast array of mechanical, thermal, and electrical properties that can match those of injection molded plastics. In some cases, urethanes offer a superior alternative.

Rapid prototyping reignited the use of silicone molding. Prior to rapid prototyping, the creation of the pattern would take longer than building a rubber mold and casting parts. With rapid prototyping, silicone rubber molding is once again a competitive and attractive solution. PolyJet further reduces time, labor, and cost, making its use with urethane casting an ideal solution for prototype development and low volume production.

Silicone rubber molding offers lead times of three to seven days at just one-tenth (or less) of the cost and time of an aluminum tool, silicone rubber molding is an attractive alternative for many injection, blow, compression or rotationally molded plastic parts. Silicone rubber molding is unique because it combines low cost with reliable, highquality parts.

Silicone rubber molds are capable of reproducing extremely complex geometries and very fine detail. The flexibility in tool design and the silicone rubber material allow reproduction of complex forms with little impact on time or cost. This is not true with rigid machined tools. Similarly, fine detail, such as lettering, is reproduced with no additional expense or effort.

Due to cycle time, cost per piece, and tool life, silicone rubber molding is best suited for applications producing 1 to 100 parts. However, since tools are inexpensive and made quickly, multiple silicone rubber molds are a viable option for larger quantities of parts.

Perhaps the greatest limitation of silicone rubber molding is that it is an art. There are few sources of education on silicone rubber molding, so it has become a craft that is passed on from master to apprentice. The goal of this process guide is to overcome this obstacle by passing on the basics of the process.

PolyJet and Silicone Rubber Molding
Rapid prototyping slashed pattern development from weeks to days. Instead of hand fabrication and machining, CAD data is leveraged to directly produce patterns. Most rapid prototypes, however, require extensive labor to become suitable patterns. The one exception is PolyJet parts. PolyJet typically eliminates pattern benching, which expedites the process and makes it less costly.

Due to stair stepping, most rapid prototypes required extensive work-filling and sanding the surface-to make a useable pattern. As a labor-intensive process, benching drives up costs and delays tool production. Typically, the process takes one person 4 to 24 hours to complete. Building with 0.0006" (16 micron) layers, PolyJet eliminates the time and expense of benching.

The layer thickness and print resolution of the PolyJet process also produce superior details in the pattern. Since the silicone rubber reproduces these fine details, they become part of the final urethane casting.

PolyJet also eliminates the time and effort associated with priming patterns. Unlike other rapid prototyping materials, the PolyJet FullCure® 700 and FullCure® 800 series materials will not inhibit rubber curing. Therefore, there is no need to spray a primer coat on the pattern or to sand the primed pattern.

Combining PolyJet and silicone rubber molding allows plastic parts to be produced faster than ever, and costs are reasonable for all prototyping and low volume production applications.

Process Overview
Producing urethane casting from silicone rubber molds is a three-step process: build the pattern, make the mold and cast the parts.

The first step is to build the PolyJet pattern. With very few exceptions, the PolyJet pattern is built and cleaned just like a PolyJet prototype. This pattern forms the cavity in the silicone rubber where the urethane is cast.

The second step is to make the silicone rubber tool. The pattern is combined with a
parting surface to form a parting block. Silicone rubber is poured over this combination and allowed to cure. The process is repeated to make the second half of the mold.

The third step is to cast liquid urethane into the cavity in the silicone rubber mold. After curing in the mold from 1/4 - 4 hours, the casting is removed and benched.

Process
1. Pattern Development
A. Pattern Design
Prior to creating the STL file for the pattern, incorporate any desired modifications in the CAD data. The most common changes are shrinkage compensations and removal of machined features.

Since the cast parts will have a small net shrinkage, typically one-tenth percent to one percent, many find it unnecessary to add shrinkage compensation to the pattern. However, if accuracy is critical, scale the CAD data to compensate for the combined shrinkage of the silicone rubber and urethane. Alternatively, the shrinkage compensation can be added to the STL file during build preparation.

Remove any features that will be machined into the casting. These features are those that complicate the tool design or require machined tolerances, such as a through hole with a centerline parallel to the tool's parting line. If left in the pattern, this hole would require either a hand-loaded insert or an additional mold part with a side pull (perpendicular to the core and cavity).

Note that the pattern does not require draft angles. Since the silicone rubber is flexible, the tool does not need draft for demolding of the casting.

Following the modifications, generate the STL file and build the pattern in a PolyJet Machine.

B. Pattern Building
For pattern construction, use standard part orientations and build parameters (Figure 1). In nearly every case, the pattern will be oriented to minimize build time and maximize feature resolution.

Figure 1: The PolyJet pattern is oriented for fastest build speed and best feature resolution.	Figure 2: Finished PolyJet pattern after support removal and bead blasting. Filling build lines, sanding and priming are unnecessary.

Following the build, remove the PolyJet pattern and thoroughly power wash it to remove all support material. Clear the pattern with a 10-20 minute soak in a sodium hydroxide solution and rewash in the WaterJet station. After support removal, dry the pattern and bead blast all surfaces with aluminum oxide blast media. Spray the master model with mold release and let dry.

When using the recommended silicones and following the support removal instructions above, inhibition (a reaction between the master model and the silicone) may occur with PolyJet master models. Here are few common issues that can aggravate this problem; incomplete support removal, using large amounts of super glues, excessive finger prints on the master model, mixing silicone in PVC containers, or mixing silicone at incorrect temperatures. In the rare event inhibition occurs a coat of clear paint will eliminate any undesired effects.

PolyJet patterns, unlike other rapid prototyping patterns, do not need to be sanded or primed (Figure 2). In most cases, the pattern is ready for tooling after bead blasting. However, both can be done to improve the quality of the casting.

When sanding the pattern, begin using 220 and 320-grit dry sandpaper and finish with 400-grit wet sandpaper. When priming the pattern, apply a fast curing, sandable, and lacquer-based primer. After priming, sand the part with 220 and 320-grit sandpaper and finish to desired surface finish with 400 to 1000-grit wet sandpaper. Remember, the smoother the surface of the master, the smoother the surface finish of the cast parts.

2. Tool Design
Designing the silicone rubber tool involves decisions on the parting line, the number of mold parts for the tool, how to address undercuts, how to address small features, and overall tool size.

For simplicity, this guide addresses a simple, two-part tool. Note that the A side of the tool refers to the half of the tool that forms the external, cosmetic side of the cast part. The B side forms the internal, non-cosmetic side of the casting. Side B will have the vents and gates since witness marks will not affect the part's cosmetic quality. When building a tool, either side may be built first. In this document, the B side is the first half of the tool that is made.

	The parting line will be at the periphery of the pattern. For the X-Y profile, look at the pattern in plan view (along the line of the mold pull) to find the parting line location. Then look at the pattern from all sides to determine the elevation of the parting line (Figure 3).
Figure 3: Determine the parting line, marked in red, prior to clay up and tool building.

Next, evaluate the pattern to determine if there are any undercuts. Small undercuts, less than a .250 inch (3.2 mm), can be ignored since the silicone rubber mold can be flexed for release. For larger undercuts, use hand-loaded inserts. These inserts are demolded with the urethane part, removed from the casting, and replaced in the tool prior to a second urethane pour. Inserts can be machined or built with PolyJet.

For features that will yield a thin wall of rubber, consider a machined plastic or a metal insert. Thin sections of rubber will shift when casting the urethane. Additionally, thin sections are more likely to tear when demolding the part.

In general, the mold will exceed the maximum dimension of the pattern by 1.0 inch (25.4 mm) on the top and bottom, and 1.0 to 2.0 inches (25.4 mm to 50.8 mm) on each side. These general guidelines will vary with the size of the part, the silicone rubber used, and the casting process.

3. Tool Building
A. Build Parting Block
To create the parting line for the core and cavity, make a parting block from the pattern, plywood, and modeling clay.

Cut a rigid substrate to the length and width of the tool. Suitable materials include plywood, medium density fiberboard (MDF), and similar sheet material. On this surface, mount the pattern so that the bulk of the parting line is located where the pattern rests on the sheet stock. If there are any hand-loaded inserts, mount them on the pattern.

With modeling clay, define the remainder of the parting line. To prevent inhibition of the silicone rubber's curing, use a non-sulfur based modeling clay, such as "Kleen Clay". Apply the clay by hand to all features that are captured in the opposite side of the tool. Also, use the clay to build up from the mounting board to elevated areas of the parting line. Work and smooth the modeling clay with tools such as artist's spatulas, modeling picks, or tongue depressors.

	The clayed-up pattern defines the cavity for side B and the parting surface of the tool (Figure 4).
Figure 4: The pattern and the modeling clay (reddish brown material surrounding the PolyJet pattern) define the parting line for the tool. Vent and gate rods have been attached to the pattern

B. Build Tooling Frame
Build a four-sided box with inside dimensions equal to those of the B side of the tool. Any rigid, smooth material can be used. This includes MDF, finished plywood, or a Formica laminate.

Mount the parting block in the bottom of the box with the pattern facing inward. Ensure that the joints will not leak silicone rubber by filling the inside edges with modeling clay.

C. Add Vents and Gates
With the parting block in the tool frame, add vents and gating to the pattern (Figure 4). Venting allows air to exit the tool. Without venting, air pockets will prevent complete filling of the tool cavity. Vents are made from 1/16 inch (1.6 mm) metal or plastic dowels. Cut the vent dowels so that they are long enough to extend from the pattern surface out through the top of the tool. Attach the vents with cyanoacrylate (super glue) to all high points of the pattern, and any areas that are likely to trap air when casting the urethane into the tool.

Attach a gate to the part with cyanoacrylate. Make the gate from a 1/4 inch (6.4 mm) rod stock that is cut to a length. Allow it to extend from the part surface through the top of the tool. If casting a viscous urethane, use a larger diameter rod. After the tool has cured, remove the gate rod and attach a small paper cup over the gate channel. The cup will be the reservoir for the cast urethane.

Once assembled, coat all surfaces with mold release. Since mold release selection is dependent on the type of silicone rubber, refer to the manufacturer's recommendations.

D. Mix Silicone Rubber
Silicone rubber is a two-part material. Parts A and B are mixed and then poured into the tooling frame.

There are many silicone rubbers available from manufacturers like Dow Corning, Rhone Poulenc, and GE Bayer. Each offers different properties, and the rubber is selected for characteristics like flexibility and tool life. For most applications, a platinum-cured rubber is used. Some recommended platinum-cured rubbers include:

Manufacture MCP Shinetsu Wacker MG Chemicals

Product Description VTX 950 KE1310ST 4644 RTV630

Thoroughly mix the two parts of the rubber kit in a disposable one-gallon or five-gallon bucket. For large quantities, a paddle mixer is recommended. Optionally, a combination rubber mixing and dispensing machine can be purchased if producing large molds.

Following the mix, the rubber must be de-aired. If bubbles remain in the silicone rubber, they will rise and create voids in the surface of the tool. To de-air, place the silicone rubber into a vacuum chamber for three to four minutes. The rubber will expand to three times its original volume, so choose a properly sized chamber. The silicone rubber typically has a one-hour pot life-the amount of time that the material remains fluid-so there is sufficient time to mix, de-air, and pour the rubber.

E. Pour Tool Side B
Gradually pour the silicone rubber into the tooling frame created in step 3.A - 3.C. Do not pour material directly on the pattern.

Fill the tooling frame to the top (Figure 5) and allow the rubber to cure for the amount of time recommended by the manufacturer (typically 24 hours).

Figure 5: Fill the tooling frame with liquid silicone rubber. The vent and gate rods extend out of the tool.	Figure 6: Remove the parting block and modeling clay after the tool has cured

F. Remove and Clean Parting Block
Remove the parting plate from side B of the tool, but leave the pattern in the mold cavity. If the pattern is removed, it is nearly impossible to reinsert it into the tool. Next, remove all the clay from the pattern (Figure 6) and thoroughly clean it with alcohol or a mild detergent. Rinse and dry the pattern.

G. Build Tool Side A
With the pattern in side B, add locators for the two sides of the tool and attach the tool frame for side A.

To locate side A with side B, cut channels into the rubber of side B. Using an X-Acto® round carving router, cut a channel around the periphery of the mold (Figure 7). For larger parts, a channel may also be cut around the contour of the cavity. The channel should be kept at least 1/4 inch (6.35 mm) from the sides of the mold and 1/8 inch (3.2 mm) from the cavity. To create a lock between sides A and B, cut the channels to a depth slightly more than the radius of the round carving router.

Figure 7: Using a round carving cutter, cut locator channels in the tool.	Figure 8: After the tool has cured, remove the pattern. The rubber will grip the pattern, so it may need to be pried from the tool.

Attach the four-sided box to the frame for side B and spray all surfaces with mold release. Repeat steps 3.D and 3.E. Allow the rubber to cure for 24 hours and then remove the pattern (Figure 8), vent rods, and gate rod. The tool is now ready for casting parts.

4. Part Molding
A. Urethanes
There are many cast urethanes available with a wide variety of mechanical and thermal properties. The urethanes will also have different pot lives. The selection of the urethane will depend on the desired properties for the cast part, the desired pot life, and the method of casting.

To enhance the material properties, or to change the color, fillers and pigments can beadded to the urethane. Refer to the material manufacturer's recommendations for more information.

Short pot life materials, generally less than one minute, are attractive since they allow many casting per day. However, cavities that are difficult to fill may not be well suited for short pot life urethanes. Additionally, these materials demand a material dispenser that mixes and feeds the urethane into the tool. There is not enough time to hand mix and pour the urethane before it begins to set.

Long pot life materials, generally two to five minutes, take longer to cure and therefore decrease the number of castings per day. However, these materials can be hand mixed and gravity cast, which eliminates the need for a dispensing unit.

B. Cast Part
To start the casting process, thoroughly mix the urethane's parts A and B, and de-air the urethane in a vacuum chamber.

Urethanes may be gravity cast, vacuum cast, pressure cast, or injected (Figure 9). The determining factors are the equipment that is available, the material's pot life, and the difficulty in filling the tool.

Figure 9: For short pot life urethanes, the material is simultaneously mixed and injected.	Figure 10: An alternative to gravity casting is to assist the pour by injecting the urethane with a large syringe.

Gravity casting is simply a pouring of urethane into the tool cavity without vacuum or pressure assist. In vacuum casting, the tool is filled and an ample supply of urethane is poured into the reservoir created by the gate cup. The tool is then placed in the chamber and a vacuum is drawn. The evacuation of air draws the urethane into the tool. A similar process is used for pressure casting, with the exception that the chamber is pressurized to force the urethane into the tool cavity.

When the cavity is difficult to fill, try injecting the urethane with a large syringe (Figure 10) or a caulking gun.

For long pot urethanes, allow the casting to cure for two to four hours. For short pot life urethanes allow the casting to cure for 15 to 30 minutes. After the casting has cured, demold the casting.

5. Part Finishing
A. Remove Cast Part
To demold the casting, separate the two halves of the silicone rubber mold. Due to the locking locators and the rubber's grip on the casting, separation will require some force. In most cases, tool separation will require a prying tool or mold spreader.

Once the mold is open, the casting will be in the B side of the tool. Carefully, yet forcefully, extract the casting from the tool (Figure 11) and remove any hand loaded inserts. The tool is now ready for a second casting.

Figure 11: After the urethane has cured, separate the tool and remove the cast part. Extraction may require a prying tool.	Figure 12: Trim the vents, gates and flash from the cast part with clippers, shears and X-Acto knives.

B. Post-Cure
Some urethane materials will require a thermal post-cure to reduce brittleness and improve mechanical and thermal properties. Typically, post-cure takes a few hours at 130° F (54.4° C). Refer to the urethane manufacturer's instructions for more information.

C. Trim Gate, Vents, and Flash
When fully cured, cut off the gate, vents, and flash from the casting (Figure 12). The gate and vents can be cut with shears. The flash is a thin membrane that is easily trimmed with an X-Acto knife.

The casting is now ready for secondary machining, paint or decoration.

Conclusion
Quality plastic parts in as little as three days; that is the advantage of combining PolyJet with silicone rubber molding. Plastic parts for functional testing or low volume production can be rapidly produced for any project.

PolyJet eliminates costly and time-consuming pattern benching. Pouring the silicone rubber tool and building a pattern on the same day expedites the process without sacrificing casting quality. And PolyJet patterns preserve the cast-in detail, accuracy, and surface finish that are made possible with urethane castings.

	With this process guide and a few basic tools, designers, engineers, and manufacturers can capitalize on the efficiency, capability, and affordability of silicone rubber molding.
Figure 13: PolyJet Master (clear and yellow) Silicon Mold and molded parts (clear and amber).