Features and benefits of vacuum casting

Vacuum casting is an established prototyping technology that has been around longer than 3D printing, so it might not seem as high-tech or glamorous as additive manufacturing. Nevertheless, vacuum casting in polyurethane resin remains a valuable option for producing high-quality prototypes. Here at Prototype Projects, we have in-house vacuum casting, five different 3D printing technologies, CNC machining, laser cutting and traditional modelmaking, so we can provide independent advice about which technology – or combination of technologies – is best for your project.

We have a separate page describing the vacuum casting process and our capabilities but this article focuses on the features and benefits of vacuum casting, a most versatile prototyping technology.

Wide choice of material grades

All of our vacuum castings are produced in polyurethane resins. This may sound limiting in terms of the material properties of the finished part but, in fact, the opposite is true. Polyurethane resins are available in a wide choice of grades to simulate materials ranging from glass-filled nylon through to soft elastomers – and everything in between. Polyurethane is also available in water clear grades.

Excellent material properties

The material properties you need will depend on your application. Polyurethane resin material properties vary considerably from one grade to the next but, in general, polyurethane is isotropic, non-porous, water resistant, heat resistant, UV-stable and some grades are fire retardant.

Almost unlimited colours

We can mix colours in-house or, if you need a specific shade, our supplier can colour-match a physical sample, RAL number or Pantone colour code. As mentioned above, clear grades are also available.

Any surface finish can be produced

Vacuum cast parts are typically produced for prototyping parts that will, ultimately, be injection moulded. Any surface finish required for an injection moulded part can also be produced on vacuum cast parts, including high gloss, satin, spark eroded and textured.

A cost-effective process

Compared with 3D printing or CNC machining, the vacuum casting process contains more steps. However, the silicone rubber moulds can typically be used 20 to 25 times (sometimes far more), so the cost per part can be very reasonable.

Excellent part-to-part consistency

The casting process is very tightly controlled, mould wear is almost non-existent and minimal hand-finishing is required. All of this means there is virtually no part-to-part variation, which can be highly beneficial where multiple parts are required for assembling with other parts and/or testing. If the mould reaches the end of its life and a new one is required, another master pattern will need to be 3D printed. Using the SLA process, the accuracy of the master pattern is excellent, so castings from moulds made from different masters should be indistinguishable.

Complex geometries can be cast

We usually 3D print the master pattern using one of our SLA machines, which enables the production of accurate, highly detailed parts with complex geometries. The mould is produced by pouring liquid silicone resin around the master pattern, resulting in a perfect mould-to-pattern fit and, therefore, accurate replication when the polyurethane parts are cast. Vacuum casting enables complex features to be produced, including snap fits, thin walls, living hinges, undercuts and similar features. There is also no need for draft angles, which can help to speed the prototyping process.

Choice of finishing options

Elsewhere we have said polyurethane resin is available in an almost unlimited range of shades for self-coloured parts, but additional finishes can also be applied. These include blackout/EMI/RFI coatings on internal surfaces, and vacuum metallisation. Painting suitability depends on the resin grade, so talk to us first if this is necessary.

Minimal shrinkage

Shrinkage depends on the part geometry and the process parameters but, generally, shrinkage (ie the difference between the master pattern and the cast part) is approximately 0.15 per cent. This is less than for injection moulded ABS or glass-filled Nylon 6/6, and an order of magnitude less than injection moulded Acetal, polypropylene or thermoplastic elastomers. This helps to ensure vacuum cast parts are as accurate as possible, which is important for visual prototypes and absolutely vital when the parts are for functional testing.

Wide range of sizes can be moulded

Our vacuum casting facilities give us the capability to cast very small parts up to a maximum weight of 1400 g. The maximum mould size is 450 x 470 x 400 mm. This is similar to the build envelope of our SLA machines, so we can vacuum cast almost anything that we can 3D print.

Multiple options for threaded holes

Sometimes customers prefer to assemble prototypes using screws rather than production technologies such as bonding or snap fits. Threaded fasteners enable the prototypes to be disassembled, inspected, modified and reassembled as required. Vacuum cast prototype parts can incorporate blind holes for subsequent tapping, plain bosses for drilling and tapping, or threaded inserts can be insert moulded or installed after casting. If in doubt about the best option, talk to us as early as possible.

Volume production

‘Volume’ is a relative term and the number of parts that can be cast from a mould depends on the part geometry, surface finish, grade of polyurethane resin and the skill of the operative. Typically, we expect a mould to produce 20 to 25 parts, though up to 100 can be moulded in some cases. If more parts are required, we can manufacture a new mould. A single mould is usually sufficient for prototyping requirements, but we also have customers for whom we vacuum cast low-volume production parts.

Suitable for many phases in product development

We have already shown that vacuum casting is an incredibly versatile process. As a result, it is used at virtually every stage in a product development project where multiple parts are needed. This includes concept development, visual and functional prototypes, handling trials, performance testing (eg airflow), assembly system concept development, assembly line trials and marketing – such as for exhibitions, presentations to backers, focus groups and promotional photography.

The ultimate prototyping technology?

Vacuum casting is not the fastest prototyping technology because of the multiple steps involved and the cure time for the silicone mould and individual castings. However, vacuum cast parts can typically have the same level of detail as high-quality SLA 3D printed parts yet but with a wider choice of material properties. In addition, if the prototype has particular functional requirements, CNC machined parts can be overmoulded by vacuum casting with polyurethane. The resultant parts benefit from the strength, stiffness and accuracy of CNC machined metal parts, as well as the complex surfaces and detailed design features associated with 3D printing. The piece part cost remains very reasonable and, although the timescales are longer than for 3D printing, it is far quicker and more cost-effective than injection moulding with soft tooling.

Talk to us

Whatever your prototyping requirements for vacuum casting, talk to us on 01763 249760. It won’t cost you anything, but it could save you time in the long run, as well as cutting costs and improving the quality of your prototype parts.