Vacuum Casting for Prototypes: How It Works and When to Use It
In Part 5, ‘SLS 3D Printing,’ we looked at each consideration for the process, step by step. In this edition of our knowledge series, we’ll be discussing the considerations needed for vacuum casting.
Vacuum casting (vac-casting), is a plastic moulding process used to manufacture high quality product prototypes. Vac-casting works by creating a silicone rubber mould using a product master pattern. After the silicone mould is cured in high temperatures, it is cut into sections using a zig-zag cutting technique; the cut is zig-zagged to ensure that when you put the sections together, the mould cavity aligns correctly, Plastic resin is poured in the silicone mould, after which the entire mould is put into a vacuum chamber to ensure that there are no trapped air bubbles.
Use
If you’re looking to prototype a complex design, a product that uses soft plastic, or you want to make multiple prototypes, vac-casting is your best bet. Compared to any 3D printing process, vac-cast prototypes have a higher quality and compared to CNC’d prototypes, vac-cast prototypes are less restricted as to what shapes can be made. Furthermore, the moulds can usually be reused up to 5 or 10 times and sometimes even up to 25 or even 50 times.
Quality
Vacuum cast prototypes exhibit physical qualities that are comparable to that of injection moulded products; they aren’t as good, but they’re closer to than CNC’d or 3D printed ones. This makes them great for functional user or basic mechanical testing and even for marketing or display purposes.
Because the moulds are reusable you’re able to repeat fine details such as patterns, dimensions & even textures. You will however notice that the quality does slightly decrease from the first pattern made to the last, until it decreases so much that a new silicone mould is required.
Time
Preparing vac-casting moulds can be a manually intensive and time consuming process depending on the complexity of the product and its’ size. Further, the size of the product directly affects the required curing time for which the pattern must sit in the mould. As a result, vac-cast prototypes have a longer lead time – generally looking at around 2 to 3 weeks to produce.
Materials & Cost
Depending on the resin used, or if you use metallic powder or pigment additives, different aesthetic or functional properties can be achieved, including:
- Heat resistance (from 100C up to 140C)
- UV resistance (only if the prototype is coated with a UV resistant paint after it is moulded)
- Flexibility / rigidity
- Softness / hardness
- Clear (can use transparent plastic resin)
- Colours (can tint/colour the resin before it is moulded – or the prototype can be painted once the resin is cured)
- Matte, satin, or gloss surface finish
Sometimes the material you would injection mould you final product from isn’t able to be used in the vac-casting process. If this is the case, the closet ‘like’ material is instead used; this happens with polypropylene products.
Due to the set-up costs, to make one single prototype, vac-casting will be more expensive than 3D printing or CNC’ing. This is because it requires you to make a prototype pattern that is sacrificed to make the mould itself which consequently means that the whole process takes more time and labour. However, because the moulds can be reused to make multiple patterns, your per prototype cost will reduce the more patterns you make; if you’re making multiple pattern copies your per prototype cost would be less than if you were using CNC’ing or 3D printing.
Vacuum Casting vs 3D Printing vs Injection Moulding
| Vacuum casting | 3D printing | Injection moulding | |
|---|---|---|---|
| Best quantity | 10 to 50 parts | 1 to 10 parts | 1,000+ parts |
| Finish | Close to production, can be textured and coloured | Visible layer lines or a matt finish, depending on method | Production finish |
| Material feel | Polyurethanes that mimic ABS, PP, rubber and clear plastics | Limited to printable resins and filaments | Final production material |
| Lead time | 1 to 3 weeks | A few days | Several weeks for tooling |
| Upfront cost | Low (silicone mould) | None | High (steel or aluminium tool) |
Vacuum Casting for Medical Device Prototypes
Medical products need to be tested by the people who will use them. That means patients, clinicians and carers handling a part that looks and feels like the real thing.
Vacuum casting gives us that before any tooling is cut. We can produce a small batch of housings, handles or soft-touch grips in materials close to the final plastic. Then they can go out for user testing, clinical feedback or investor demos.
Changing a silicone mould is cheap. Changing a production tool is not. Finding a problem at this stage, like a grip that’s hard to hold or a clip that’s too stiff, keeps it out of the production tool.
Vacuum Casting FAQs
How many parts can you get from one mould?
A silicone mould usually produces 10 to 20 parts before it wears out. For larger runs we make additional moulds from the same master pattern.
How long does vacuum casting take?
Most jobs take 1 to 3 weeks from approved design to finished parts. This includes making the master pattern, building the mould and casting.
Is vacuum casting cheaper than injection moulding?
It is for small quantities, because there’s no expensive tool to build. Once you need hundreds or thousands of parts, injection moulding becomes the cheaper option per part.
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