Silicone is a popular material to fabricate soft end-use parts as well as produce soft molds for casting rigid materials. Silicone “molding and casting” terminology is broadly used to reference any multi-step method that employs silicone rubbers. Here we differentiate between two processes: “silicone mold making” and “silicone part production.”

In silicone mold making, sometimes also called silicone molding, castable silicone is used as a mold material for replicating a rigid master model. Silicone rubber is poured around a 3D printed master, and then the cavity in the silicone mold is filled with another castable material. This is often a rigid material that begins as a liquid and then cures upon cooling or a chemical reaction, such as thermoplastics, resins, cement, wax, or plaster, and in some cases expanding materials such as polymer foams.

In silicone part production, a 3D printed mold contains castable silicone for the production of soft, rubbery end products. There are a variety of ways to produce silicone parts, including:

• Compression molding of silicone putty
• Injection filled mold for liquid silicone
• Overmolding for complete or partial silicone encapsulation of hardware
• Eggshell mold for custom silicone goods

Traditional molds for silicone part production are CNC machined from blocks of aluminum or steel.. With 3D printing, users can fabricate tools rapidly and at a low cost, including prototyping, small-scale production runs, and custom part production.

Clear Resin is the most widely used due to its ability to polish to optical clarity and offer visibility into mold cavities and alignment pins during casting. However, Formlabs has a wide range of materials for various applications.

When selecting a mold type, take the following criteria into account: target part geometry, the ability to accommodate undercuts and holes in the design, scale of production, and casting materials that you intend to cast.

Different mold types necessitate differing design considerations. See the white paper Silicone Part Production With 3D Printed Tools for more information.

Formlabs recommends printing in 25–50 µm layer height for a balance between a smooth surface texture and fast printing time. More aesthetic applications may require a 25 µm layer height.

Orient your parts on the build plate so that critical interior surfaces are free of support marks. Placing the mold stock flat on the build platform can work in some cases.

Thoroughly wash excess resin off of the mold stock with clean IPA as uncured resin can inhibit curing of silicone. Completely cure the mold stock to reduce the amount of unreacted monomer on the SLA parts. Refer to the individual material articles for specific wash and cure instructions.

Add mineral oil to the exterior surfaces to fill in surface scratches and provide an extremely clear window into the silicone filling process.

Consider using mold release anytime two different materials come into contact with each other. This includes embedded hardware that you plan to insert and remove from an overmolded silicone sheath. Follow the guidelines on your material packaging, and wait for the coatings to fully cure or dry before moving on to the next step.

Formlabs also recommends the additional best practices:

• Rinse with acetone to remove residual solvent and debris.
• Over-curing the mold for 60–120 minutes.
• Wait 24–48 hours after post-processing to allow the mold to sit in ambient temperature and light before applying coatings and casting the silicone.
• Use an epoxy coating for an effective barrier between the silicone and the 3D printed mold.
• Use a higher concentration of catalyst component.

3D printing is a powerful solution to fabricate silicone molds and silicone parts rapidly and at a low cost. It is particularly recommended for:

• Prototypes
• End-use products
• Models and props
• Part reproduction
• Short-run production of silicon devices for consumer, medical, or R&D applications

Even though 3D printed molds can offer advantages when used appropriately, there are still limitations. 3D printed molds do not have the same performance abilities as machined metallic molds. Some limitations include:

• Molds typically last 10–20 uses before signs of wear appear.
• Mold performance depends heavily on curing accuracy and silicone type.
• Adhesion of the part to the mold can cause deterioration of the mold during extraction.

• Silicone Part Production With 3D Printed Tools white paper
• Creating Silicone Parts using 3D Printed Molds webinar