A Guide to using 3D printing for Silicone Moulding

Silicone is a great choice when you’re looking to produce detailed, fast, and affordable products. However the difficulties tend to arise when looking into manufacturing the moulds. 3D Printing is a great solution for this, and allows silicone mould making to be simplified.

Silicone Properties: Ideal for Mould Making

Silicone rubber (polysiloxane), belongs to the elastomer material family with similar elastic and low-hardness properties. So why this is the best choice above all the other rubber substances? Let’s have a brief look at its properties:

Elasticity

Silicones have a range of elasticity from 30% to 1200%, depending on the formulation. A right elasticity level is ideal for easily removing the cast once it’s ready, without damages like cracking or deformation on both the result and the mould. Additionally, this property enables the final shape to be nice and clean.

Stability

Silicone rubbers exceed on stability in comparison with other rubbers. While maintaining its properties constant in a range from -100°C up to 300°C, this elastomer won’t deform nor crack on both extreme cold and hot environments. Besides being one of the most reliable insulation materials, it can stretch with great force and won’t break easily. Regarding mould making, this is ideal for hot castings, baking foods, or freezing liquid to make ice.

Durability

It has decent tensile strength, tear-resistance and fatigue cycles for making reusable and good quality moulds, although not the best among other rubbers (At higher temperatures silicone is much more durable). But putting aside mechanical properties, silicone has a special resistance to microbiological growth, chemical reactivity, UV light, oxygen, and ozone, making it ideal for food safety, medical, and outdoor applications.

Watertight

Works great as a sealant, which is great to perfectly retain liquid casting materials while reducing leak and seam messes. On the other hand, silicone is gas-permeable, which along with its low toxicity, is another reason there are so many food and medical uses.

Cost

Silicone rubber is very costly in comparison to other rubbers. The exact price depends on which formula and the provider you buy from. However, always take this in mind before deciding to buy this material.

Silicone Mould Casting Materials

Now with a clearer view of its properties, is easy to see how versatile it can be. To illustrate this, the following are some common materials that can be cast:

• Polyurethane
• Epoxy resins
• Polyester
• Polyurethane foam
• Plaster
• Wax
• Low melting metals
• Concrete
• Edibles

Types of Silicone

To pick up the best choice for your application among a wide variety of silicone rubbers, let’s have a brief look at their classification:

• HCR (High Consistency Rubber): A putty-like substance, ideal for industrial compression moulding.
• LSR (Liquid Silicone Rubber): Used in liquid injection moulding.
• FSR (Fluorsilicone Rubber): The best choice for applications like engine seals and gaskets.
• RTV (Room Temperature Vulcanize): Cures at room temperature with the help of a catalyst, so it comes in two-part presentations (A and B). Among these there are two types:
  • Platinum Catalyzed: Long-term stability and very low shrinkage after curing, but with higher cost.
  • Tin Catalyzed: Lower-term stability, higher tendency to shrink, but more affordable.

For this article’s purposes, we will focus on RTVs.

Applications

It’s not hard to notice all around us the importance silicone has in many applications (like kitchenware, electronics, automotive, medical), but silicone-based mould making in and of itself has a broad spectrum of possibilities. Here we have some of the most relevant:

Prototyping: As stated in previous articles, tooling can be very expensive to implement and prototyping cycles are essential for testing before proceeding with full production investments. This is definitely the case for manufacturing techniques like injection moulding, where installing each moulding tool can take thousands of pounds and many weeks of development. For this, engineers must look for affordable and quick solutions to prototype the product, and one of the preferred techniques is silicone moulding. Since a silicone mould provides accurate results while remaining durable, this enables testers to produce small batches rather quickly.  

Consumer Goods: Many daily goods like soaps, candles and crayons.  

Foods: Thanks to silicone thermic and food safety properties, you can mould your cookies and muffins in the oven and your ice pops in the freezer.

Jewellery: Jewellers have the challenge of reproducing custom, small, and intricate shapes to use for lost wax casting processes, which is a one time only method. For casting a fast lot of wax patterns, silicone master moulds work exceptionally well.

Other: Figurines, art crafts, decorative.

Master Patterns and 3D Printing

One of the main concerns before creating a mould is how to get the original pattern, in other words, the master. Using traditional methods, you can either get it by carving it from wood or by sculpting it from materials like clay. In other cases, the original object itself can just be used as a pattern. Either case, it requires special talent and lots of time.

But why not 3D printing? Digital modelling offers a wide variety of tools to create, edit, scale, and store accurately any kind of design imaginable. Once the design is ready, just print the pattern and make some manual finishes to refine its surface. For the best printing resolutions, we recommend SLA printers like Formlabs.

Mould Making

Depending on what you’re casting, moulds can either be one or two piece moulds. But for either case, it’s important to have the following:

• The master pattern
• Liquid Silicone
• A non-porous material as a container to easily peel the silicone without damage once it is cured, smooth surfaces like MDF boards are ideal. Glass is an exception that must be avoided.
• Hot glue and tape for holding tight and properly sealing the container
• Mould release agent

One-Piece Moulds

One-piece moulds are the simpler ones to make. Resulting casts have the advantage of not having the parting line seam as seen in two-piece moulds. However one of its surfaces must be flat, just like in ice cube trays. Thanks to silicone’s elasticity and durability, you can turn the mould inside out like a glove to release the cast. But make sure your design doesn’t have deep undercuts, otherwise, you won’t be able to release the cast.

Two-Piece Moulds

Two-piece moulds have much more design freedom regarding flat surfaces and undercuts than one-piece moulds, but the price is the risk of having parting line leaks and seams. The process to make a two-part mould is more complex than its one-piece counterpart, so only take this route if it’s completely necessary.

What printers are best?

When looking into printers to be used for mould making, two names stand out due to their high quality and repeatability- Formlabs.

Precision features can be created on the moulds with ease, with minimal implications to overall cost and production times.

If you are interested in reading more about this, please see the white paper case studies as below

Formlabs White Paper- Custom Silicone Ear Moulds

Markforged Case Study – Thermoset Mould

Some Last Tips

• Every formulation has different curing times, make sure it is neither too low nor too hight for your intended use. Depending on the size of the mould, the curing time can take from a few minutes to a whole day.
• Since liquid silicone is very viscous, bubbles can be annoying. Having a vacuum chamber at your disposal would be ideal. If this is not the case, using vibration devices or gently knocking the recipient will do.
• Carefully pour the silicone from a corner to avoid trapping air.
• Take your time to properly seal your container, since liquid silicone can easily leak through any gap.
• Avoid abrasive sponges when washing the mould, using soap and rinsing it with water should be enough.
• Check the Materials Safety Data Sheet (MSDS) from the silicone manufacturer before using it for food moulds.

Solid Print3D is here to help you make the right decision with your next 3D Printer purchase. For more information, please call Solid Print3D at 01926 333 777 or email at info@solidprint3d.co.uk