Guide to Creating Silicone Medical Devices

Anytime a castable material such as silicone comes into contact with a printed mold, we must consider the chemical interaction between the two materials. When casting platinum-catalyzed silicones in SLA molds, one particular challenge arises: the resin monomer can inhibit the silicone curing process, leaving an uncured gummy surface. However, high-quality platinum-catalyzed silicone parts can be produced by making sure SLA molds are fully cleaned and cured, and by applying protective coatings and mold releases.

There are many possible combinations of silicone, resin, protective coating, and mold release that you can try. We asked customers from a variety of industries to report on their successful workflows, and their responses are summarized below. We considered many of these insights when developing our own step-by-step process.

While the combination of materials clearly varies, our customers aligned on the following:

• Make sure your parts are fully washed and cured. The 3D printed mold must be washed thoroughly in clean IPA, such that surfaces are not tacky after the mold dries. The mold should also be cured completely to reduce the amount of unreacted monomer on the SLA parts. For this process, the Form Cure is a great option. Clear Resin parts will develop a yellow tint, which is a good indicator that your mold is fully cured.
• Try a protective coating. There are many acrylic or epoxy coatings available that are applied as an aerosol spray in a single step (e.g. Krylon Crystal Clear acrylic coating), or as a two-part brush- on material (e.g. XTC-3D from Smooth-On). Using very little material, both types of coatings will leave a glossy, glass-like finish on the internal surfaces of the mold. Multiple applications may be needed. Check the manufacturer’s instructions for drying or curing times.
• Use a mold release anytime silicone contacts another material. This will ease the de-moldingstep, and eliminate silicone residue inside the  mold, making it easier to reuse it for multiple castings. In many cases a generic or “general purpose” mold release from your silicone brand is sufficient (e.g. Stoner Dry film e408). A light mist of mold release is enough. Once again, check the manufacturer's instructions for recommended drying times.

Some silicone formulas, such as PDMS, are more susceptible to cure inhibition than others when they come into contact with SLA 3D printed tooling. We have listed further troubleshooting steps provided by Formlabs users, which build off of our general guidelines for RTV liquid silicones.

• Rinse with acetone. Spraying the 3D printed mold with an acetone squeeze bottle can help to remove residual solvent and any debris after the standard IPA washing step.
• Over-cure the mold. For tooling printed in Clear Resin, increase the curing time from 15-20 minutes to 60-120 minutes at 60 °C.
• Wait 24-48 hours after post-processing. Allow the mold to sit in ambient temperature and light before applying coatings and casting the silicone. It is helpful to expose the mold surfaces to open airflow during this step.
• Use an epoxy coating. A two-part epoxy sealant can act as an effective barrier between the silicone and the 3D printed mold.
• Use a higher concentration of catalyst. When using two-part silicones, we suggest adding a bit of extra catalyst component. If a small proportion of the catalyst is inhibited by the mold surface, the extra catalyst can help compensate for this and prevent formation of a tacky silicone surface.

While the outcome of each molding process may look similar, there are a few criteria that we recommend for selecting a mold type. These include target part geometry and the ability to accommodate undercuts and holes in the design, your scale of production, and the materials that you intend to cast. We name a few guidelines in the table below.

The Formlabs team has developed a step-by-step process for creating a silicone product prototype by combining different aspects of our customers’ successful workflows. One project presented below is a molded silicone keychain with an embedded Apple AirTag device. The other is a gasket designed by OXO. The design guidelines cover all critical steps from importing a reference geometry to adding alignment features for encapsulated hardware. We also used the novel silicone filling method employing an epoxy gun with a 1:1 volume mixing nozzle recommended by Dame Products.

Many of the steps and best practices shared are common across mold types. Each step of the tutorial indicates its relevance to injection filled molds, overmolds, or compression molds.

For information about eggshell molds, the Formlabs audiology white paper 3D Printing Custom Silicone Ear Molds provides a step-by-step guide.

From Formlabs:

• Formlabs BioMed Clear Resin or Clear Resin
• Compatible Formlabs SLA printer with up-to-date firmware
• PreForm software (most recent version)
• Compatible Build Platform
• Compatible Resin Tank
• Form Wash, Form Wash L, or Finish Kit
• Form Cure or Form Cure L

From Third-Parties: 

Molding Process: Injection Mold or Overmold

Molding Process: Compression Mold

Overmold

• Import the Reference If you are planning to overmold or encapsulate another object with silicone, import your reference object or substrate in the CAD software of your choice. In this example, we imported a digital replica of an Apple AirTag.

Injection Filled Mold, Overmold, or Compression Mold

• Design the Silicone Part Together, the silicone and any encapsulated items form the base volume. For overmolded parts, aim for a minimum silicone shell thickness of 2 mm. In our compression mold example, the object has a complete through-hole. We recommend defining a core as a separate object. The silicone part plus core can be thought of as your base volume. 
• Design the Mold Stock Create a block around your base volume that extends at least 1 cm beyond the outer shell of your part. 
• Split the Mold For a basic two-part mold, you will be splitting your base volume into two parts along a defined surface based on a parting line. Start by using draft analysis to define the parting line. Consider how the two halves of your mold will come apart, to ensure that the silicone part can be easily released from the mold. To avoid undercuts, the parting line might be a contour instead of a straight line. Next, extrude the parting line into a surface and use this surface to split your mold. For complex molds that require more than two parts, this process can be repeated multiple times to generate the number of mold parts required.
• Remove Base Volume From the Mold Stock Use boolean subtract in your CAD software to carve out the mold cavity from the solid block. In the case of our compression mold, the individual core component is added back with a boolean addition to one side of the mold, in effect creating two parting lines.
• Check Draft Angles It is a good idea to pause here and consider how your mold bodies will come apart after you inject the silicone. Our customers recommend using at least a 2° draft angle for silicone product prototypes. However, this aspect of silicone mold design is quite forgiving. Because the silicone rubber can generously deform during de-molding, many angles can be handled including 90° angles or slight undercuts. We recommend using the Draft Analysis tool in your CAD program at this step to analyze and edit any areas shown in red beyond the threshold draft angle.

​​​​​​​Injection Filled Mold, Overmold

• Design the Gate If you are relying on gravity to fill the recesses of the mold cavity, place the entrance of your gate high on the mold block and then, using a U-shape with a generous bend radius, connect the gate to the mold cavity at the lowest possible point. Filling at this location prevents air bubbles from forming. Boolean subtract the gate from the mold stock. Typically, the central axis of the fill gate is located on the parting line. Another configuration would be a short fill gate that directly connects to a point on your mold cavity; the main air vent will be located on the opposite side. 
• Define Air Vents Air is displaced when filling silicone into an enclosed space, so it is natural to place an air vent at the point of the mold that is last to fill. In our example, this is placed at the top of the mold stock. To place additional air vents, it is helpful to think about areas where air is likely to be trapped, and connect vents specifically to points such as sharp turns, corners, or areas where two liquid flow fronts connect. Typically air vents will be cylinders (~0.5-2 mm in diameter) that connect areas of your cavity to the top of the mold. Like the fill gate, it is helpful to align the axis of any air vent on the parting line. Once placed, boolean subtract these features from the mold stock.

Injection Filled Mold, Overmold, or Compression Mold

• Add Mating Features These features align and constrain separate parts of the mold stock to each other. Leave a ~ 0.1 mm offset gap between complementary mating features.

Overmold

• Add Alignment Pins for Encapsulated Devices If you are overmolding or encapsulating a part in silicone, the part must be fully constrained in the mold so it does not shift during silicone casting. Use as few alignment pins as necessary to constrain the part and aim for a pin diameter of ~1.25 mm. Pins should be pointing in the direction of draw, or perpendicular to the parting line of the mold. Note that alignment pins do leave small holes in the surface of the silicone part. If needed, these can be patched after de-molding using a small dab of liquid silicone which is then allowed to cure.

Injection Filled Mold, Overmold

• Add More Mold Features One additional mold feature is a fill trough. A trough is used for overflow of the silicone at both the fill gate and vent locations, and can be designed as a single continuous well that covers all gate and vent features. The trough also contains a reservoir of liquid silicone that can enter the mold during the curing process by freely seeping into the mold cavity to displace bubbles and voids, or to compensate for silicone that enters the interface between mold parts during cure. In this particular example, our trough is 10 mm deep.

• Add a Syringe Inlet We also recommend adding a syringe inlet feature that matches the geometry of your syringe. We made ours slightly oversized, with a 6° angle to fit a variety of syringe and mixing nozzle types.

• Add Pry Points Pry points can be added to assist with disassembly of the mold after curing. These are small rectangular cutouts along the parting line that allow a flathead screwdriver or another tool to be used to separate the mold parts. Our example cutouts extend 5 mm into the mold edge.

• Add Fillets or Chamfers Add fillets or chamfers to angled mold features This is usually the final step after the mold geometry is locked. Adding fillets to soften the edges of the mold can also aid the 3D printing process. When printing a mold stock directly on the build platform, beveling any external corners eases part removal after printing and reduces the amount of resin flash that can accumulate around sharp corners.

Injection Filled Mold, Overmold, or Compression Mold

1. Select the Resin Many resins work, however Clear Resin or Biomed Clear Resin are great options for visualizing the flow of silicone while casting your part.
2. Print the Mold We recommend printing with a 50 microns layer height for a good balance between smooth surface texture and fast printing time. Since silicone picks up fine details on the surface, including layer lines, 25 microns layer height might be needed for some aesthetic applications. 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.
3. Wash and Post-Cure the Mold It is critical to thoroughly wash excess resin off of the mold stock with clean IPA because uncured resin can inhibit curing of silicone. The mold should also be cured completely to reduce the amount of unreacted monomer on the SLA parts. For Clear Resin, the recommended settings are 15 minutes at 60 °C. For BioMed Clear Resin, the recommended settings are 60 minutes at 60 °C. The preset time and temperature settings on Form Cure work well. The outer surface can be polished to a highly transparent finish. Adding mineral oil to the exterior surfaces is one option to help fill in surface scratches and provide an extremely clear window into the silicone filling process.

Injection Filled Mold, Overmold

1. Apply Coatings and Mold Release Apply coatings and mold release. 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. Consult our customer workflow table for recommendations. Follow the guidelines on your material packaging, and wait for the coatings to fully cure or dry before moving on to the next step. 
2. Insert Internal Parts/Hardware Alignment pins help to orient the part that will be encapsulated.
3. Close and Clamp the Mold Carefully clamp the mold starting in the direction of the draw, making sure that the mold stock and any encapsulated objects remain in alignment. In our example, we used a 3D printed replica of an Apple AirTag, fabricated in Rigid 10K Resin.

Injection Filled Mold, Overmold

If you are incorporating a pigment, mix the pigment into Part B before moving on to the steps below.

A frequent concern when casting silicone is trapping small air bubbles in the mold. This results in voids in the final casted part. A couple of supplemental methods are recommended to manage air bubbles, including degassing liquid silicone in a vacuum chamber prior to mold filling. As a general guideline, check the viscosity of your liquid RTV silicone formula. If it is over 18,000 cps (centipoise) degassing is recommended. If it is below this threshold, a degassing step may not be necessary. Another recommended technique is placing the filled mold in a pressure chamber during silicone curing in order to collapse and shrink any entrained bubbles. A great prototype can be produced without degassing the silicone or curing under pressure, but you can achieve a nearly perfect part using these supplemental methods. 

Vat Method

1. Degas parts A and B separately under vacuum, if needed.
2. Mix parts A and B together vigorously. Typically the two silicone components are mixed in equal volumes. Allow the mixture to degas under vacuum again, or to gently degas in air.

Mixing Nozzle Method

1. Degas parts A and B separately under vacuum, if needed.
2. Fill each side of an epoxy cartridge system with one part (A or B), using separate syringes to transfer the liquids. Pouring the silicone high above the cartridge in a thin stream will help to avoid remixing air into the silicone.
3. Degas parts A and B. Stand upright and allow the silicone to gently degas in air. Because silicone parts A and B remain separate, the filled cartridge can remain for days before you use it.

• Mix Parts A and B Thoroughly Mix together equal volumes of parts A and B of the silicone putty and knead by hand. The silicone rubber begins to cure after 90 seconds, so the parts should be kneaded for the minimum time necessary to generate a uniform color.

Injection FIlled Mold, Overmold

Vat Method

• Draw the degassed silicone mixture into a syringe. Next, invert the syringe so the nozzle faces up. Allow any air pockets to rise to the top. Depress the plunger to expel the air and extrude a small amount of your silicone to ensure no air bubbles remain in the syringe. Insert into the fill gate and slowly fill the mold until silicone exits from the air vents. If your model has an overflow trough feature, continue filling this area of the mold.

Mixing Nozzle Method

1. Assemble the epoxy gun. Insert the cartridges into the gun and screw on the mixing nozzle. It’s helpful to squeeze a little bit of liquid out of the nozzle tip at this stage to make sure the epoxy is properly mixed.
2. Fill the mold. Insert the epoxy gun nozzle into the fill gate and slowly fill the mold until silicone exits from the air vents. Again, fill up the trough feature if your mold includes one.

Use a Pressure Pot (optional)

• As an optional step, place the mold in the pressure pot and add your silicone manufacturer’s recommended air pressure to the pot (e.g. ~ 30 psi). This shrinks any bubbles trapped in the mold and forces the material in the trough down into the mold. Let the silicone cure under pressure according to the manufacturer's instructions.

Compression Mold

1. Stuff Silicone Into the Mold The silicone should be compacted into both sides of the mold, being sure to catch all details and crevices. Any excess material will simply spill outside the edges of the mold.
2. Squeeze Mold Together The mold should be squeezed perpendicular to the parting line. In compression molds, the mating surfaces don’t completely touch until pressure is applied. Close the clamp gently until it does not close further. The recommended Castaldo Quick-Sil silicone rubber cures completely in 20 minutes.

Injection Filled Mold, Overmold, Compression Mold

The cure time can range from 10 minutes to several hours depending on the silicone chemistry.

1. Define Mold Orientation With a gravity-filled configuration, like our example, the mold can remain oriented upright without an extra sealing step as the silicone cures. With an end-to-end filling configuration, we would experience a backflow of silicone if the filled mold was set on our workbench. After filling, we recommend sealing the gate with duct tape. The compression mold should remain clamped in a tabletop vise while curing.
2. De-Mold and Trim the Silicone Part It is normal to develop some flash within and around the parting line. Remove excess silicone from the exterior of the mold first. Pry open the mold by inserting a flat head screwdriver or another flat tool into the pry point and gently twisting to break the seal. Remove the silicone part, being mindful that some silicone may need to be gently trimmed away from any air vents. After removing the silicone part from the mold, use a sharp razor or flush cutters to trim the vent and gate features from the part. Residual bumps can be removed gently with wet fine grit sandpaper. Finally, you can wash the part with soap and water to remove sandpaper and mold release residues.
3. Prepare for the Next Casting Molds can be reused multiple times. You may need to clean out vents with a sharp tool and/or reapply mold release before filling with silicone again. It is common to accumulate incompletely cured silicone in the narrow surface between mold blocks. This can be wiped away with a clean cloth. 
4. Iterate Design It is normal to go through a couple of mold design iterations after trying the full end-to-end workflow. A few design changes you might need to consider include increasing the offset distance between mating features, moving alignment pins to ensure that encapsulated objects are fully constrained, or adding air vents in areas that tend to trap air bubbles during filling.

Mold material: Production molds are typically made from machined aluminum or steel.

Alignment pins for overmolding: The alignment pins for overmolding are oriented within the plane of the mold’s main parting line, rather than perpendicular to it. These are also designed to retract in and out of the mold, resulting in a silicone layer that does not have dimples and holes created by alignment pins.

Draft angle: Our customers recommend using at least two degrees draft angle for silicone products. While the prototype or short-run production molds presented in this report can handle deep undercuts, these types of silicone geometries must be carefully pried from the mold, and would be difficult to replicate by an automated injection system.

Injection molding configuration: Gravity-fill molds are highly reliable mold configurations for benchtop prototyping. For rapid mass production methods, inlet and outlet geometries are on opposite sides of the mold.

Silicone materials: While silicone chemistry used in production is quite similar to the RTV materials presented in this white paper, both liquid silicone rubbers (LSR) for injection moldingnand heat cured rubbers (HCR) for compression molding typically enter the mold at low temperature, and are then cured at elevated temperature.

From Formlabs

• Formlabs Silicone 40A Resin 
• Compatible Formlabs SLA printer with up-to-date firmware
• PreForm software (most recent version)
• Compatible Build Platform
• Compatible Resin Tank
• Form Wash, Form Wash L, or Finish Kit
• Form Cure or Form Cure L

From Third-Parties

• ​​​​​​​IPA (99% or higher): To make wash solvent blend for washing Silicone 40A parts
• N-Butyl Acetate (purchase in the US; purchase in EU): To make wash solvent blend for washing Silicone 40A parts
• Glass Beaker (at least 500 mL in size): To submerge Silicone 40A parts inbwater during the post-cure steps
• Febreze HD (optional): For masking of odor of printed Silicone 40A parts
• 1.5 Gallon Glass Jar (optional): For storing wash solvent and washing printed Silicone 40A parts if not using a Form Wash
• Ultrasonic Cutting Knife (optional): For removing support nubs on Silicone 40A parts

1.1. Design your parts with the Silicone 40A Design Guidelines in mind.

2.1. Import File

Check that your version of PreForm is up to date by going to Help, then Check For Updates in the upper left-hand corner. Import or open your part file by dragging them into PreForm, or by going to File and then Open in the upper right hand corner. 

2.2. Select Material

Select Silicone 40A by clicking the Printer Type box in the Job Info menu on the right-hand side. Select “Silicone” from the materials grid.

2.3. Orient

PreForm can auto-orient based on Formlabs best practices. To auto-orient, select your part and click Orientation on the left side of the screen. Then, click Auto-Orient Selected.

For best results, you may orient manually using the red, green, and blue actuators surrounding your part. Consider orientating so that areas where a smooth surface texture is most important are facing away from the build platform.

Print directly on the build platform whenever possible. Try to keep the bulk of the part’s weight as close to the build platform as possible and orient long parts parallel to the front edge of the build platform.

2.4. Add Supports

To auto-generate supports based on material properties, select your part and go to Supports on the left side of the screen and select Auto-Generate All.

For optimal results, we recommend manually editing auto-generated supports, or manually placing supports until the part is sufficiently supported (indicated by a green thumbs-up beside Supports in the Job Setup menu). PreForm will indicate in red what areas might require additional support.

If surface texture or post-processing time is a major concern, you may want to consider designing custom supports using CAD software. Guidelines for adding tearaway supports can be found here.

2.5. Layout

Set your part’s location on the build platform by clicking and dragging, using the red green and blue actuators surrounding your part, or by clicking Layout, then Layout All on the right side of the screen. It’s best to place parts in the center of the build platform. When printing multiple parts, space on the build platform can be optimized and material can be saved by overlapping rafts.

2.6. Send to the Printer

Send your job to the printer by clicking the orange Upload Print button on the bottom right. When the Print dialogue box opens, select the printer that you would like to use.

2.7. Set Up the Printer

Shake the Silicone 40A cartridge and then insert it. When using a new cartridge, double check that the silicone bite valve is opening successfully by squeezing it with a gloved finger. Insert a build platform and a compatible resin tank into the printer.

Begin printing by selecting your print job from the printer’s touch screen. Follow any prompts or dialogues shown.

3.1. Remove the Part

Remove the part from the build platform by wedging the part removal tool or a scraping tool under the part raft and rotating the tool. Parts can also be washed directly on the build platform in the Form Wash.

3.2. Wash

Place the part in the Form Wash filled with an 80/20 mixture of 99% isopropyl alcohol (IPA) and n-butyl acetate. Do not wash with IPA or n-butyl acetate alone. Wash using the recommended setting for Silicone 40A (20 minutes).

If the model is hollow or has internal channels, ensure liquid resin is thoroughly flushed out from these features. This can be done using a syringe filled with the mixture from the Silicone 40A wash. Allow parts to fully dry before post-curing.

3.3. Remove Supports

For rest results, remove supports after washing but before post-curing.

To remove supports, pull gently at the support structure. On thicker parts, just pulling may be sufficient. On thinner parts, consider using flush cutters, or sliding a sharp hobby knife along the part’s surface, being careful not to damage the part itself. An ultrasonic cutter can also be used here for greater precision.

3.4. Post-Cure

Post-cure your part to achieve optimal mechanical properties. Fully submerge the part in a beaker (or other UV-transparent container) of water, and place the beaker of water containing your part into the Form Cure. Use the recommended settings for Silicone 40A (45 min at 60 °C).

Mask the odor of Silicone 40A by adding Febreze HD to the water in the glass beaker before post-curing. For every 500 mL of water, use 12.5 g of Febreze HD. If not treated, this odor will dissipate over time.

3.5. Additional Post-Processing

A miniature, mounted Scotch Brite wheel can be used with a rotary tool to grind down support marks on parts either before or after curing.

Before applying any adhesives, ensure that the part has been thoroughly washed, is dry, and the surface is not tacky or dusty. Formlabs recommends using a primer such as DOWSIL 1200 series or a medical primer. After a primer has been applied, use a silicone adhesive such as DOWSIL 734. Do not use cyanoacrylate glues as they are brittle and leave a crust on the silicone.