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how to design parts and prototypes for the vacuum casting process

Vacuum Casting: Process Overview, Benefits & Limitations

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What is vacuum casting?

Vacuum casting, also known as urethane casting or polyurethane casting, is a casting process used to produce production-quality parts from a range of thermoplastics, rubbers and resins without the cost or lead time required for production tooling or “hard tooling.”

Where injection mould tools are made from metal (steel, aluminium etc), vacuum casting uses soft silicone moulds. The process is ideal for creating a small series of parts and is often used in the early stage of product development before committing to injection moulding.

In this introductory guide, we give you an overview of the vacuum casting process, materials used, considerations, and key benefits and limitations of this form of manufacturing.



vacuum casting services mould tooling for prototypes and low volume production


It is a cost-effective alternative to injection molding, which requires expensive tooling costs and long lead times. Vacuum casting produces accurate and durable parts quickly and at a fraction of the cost of traditional methods. The process is often used to produce prototypes and small production runs of parts.


How does vacuum casting work?

Overall, the process involves 3 main stages: building a master model, creating the silicone tool, and producing the parts.


Step 1: Building a master model

First, a high-quality master model with high dimensional accuracy is required to ensure. Typically, this master model is made via 3D printing or CNC machining, then sand, painted and primed to achieve the desired surface texture and quality. Every tiny detail will be replicated in the final vacuum casted part, so it’s essential to create a flawless master model.


Step 2: Creating the silicone mould tool

The master model is then suspended in a casting box in which liquid silicone is poured around it. The molten silicone is allowed to cure inside the casting box, which is maintained at 40*C, for about 8-16 hours. After it has solidified and curing is completed, the mould is cut open and the master model is taken out leaving a hollow space in the middle of the silicone mould, which fits the dimensions and specifications of the master model.


Step 3: Producing the parts

The ‘hollow’ mould is filled with polyurethane (PU) resin using a funnel for uniform distribution and to prevent any air bubbles from forming. The mould is then sealed left in a casting machine at around 70°C to cure. The shaped material is then cooled, removed from the mold, and post-processed as necessary. This process can, on average, be repeated 10 to 20 times. Going beyond these limits will cause the mould to gradually lose its shape, affecting dimensional accuracy.

Designing parts for vacuum casting

In vacuum casting, you can expect the general shrinkage rate — the difference between the master pattern and the cast part — to be approximately 0.15 to 0.25 per cent. This is less than the general shrinkage rate for injection moulded plastics.

While the process has many advantages, it also has its limitations. Simple design modifications such as adjusting rib thickness can help minimize sink effects and ensure better part quality. Check out how to design bosses, undercuts and more in our Vacuum Casting Design Guide.


What materials are used for vacuum casting?

Materials used in vacuum casting include thermoplastics, rubbers and resins – all of which emulates specific properties and characteristics.


  • Good impact resistance
  • Well-balanced physical properties
  • Durable and rigid
  • Lightweight
  • Good transparency
  • Good UV resistance
  • High impact resistance
  • Good flexibility
  • Good transparency
  • Good UV resistance
  • Flexible
  • Good shock absorption



What are the surface finishing options for vacuum casting?

Urethane casting is not just great for functional parts but is also one of the best manufacturing processes for cosmetic products. Many surface finish option can be achieved in the moulding process. These include polished, glossy, matte and textured finish. The appearance of vacuum casted parts can be taken to the next level through various post-processing techniques, including pad printing and silk screening.


What are the advantages of vacuum casting?


  • Excellent Surface Finish: Colours and surface finishes can be easily added, making the process ideal in an industry where aesthetics are valued.
  • Shorter Lead Times: It is much quicker to make silicone moulds (which takes days) rather than steel or aluminium moulds (that take weeks).
  • Lower Upfront Investment: Vacuum casting requires a much lower upfront tooling investment compared to injection moulding, allowing you to get up and running for less.
  • Large Components: Cast large parts in a single build, without the need to join them together. At HLH, we can easily cast 2m parts.


What are the disadvantages of vacuum casting?


  • High volume restrictions: Vacuum casting in its nature is meant for low volume production – typically within the range of 1 to 50 parts. As quantity increases beyond this limit, the process becomes more costly and less efficient.
  • Loose Tolerance: Despite engineers’ best efforts to predict and improve the shrinkage rate, it will still be around 0.15%. Therefore, urethane casting is not suitable for high precision parts.
  • Dependence on Master Model: Craftsmanship and attention to detail is important in vacuum casting. If there’s a flaw in the initial master model, it will reflect on every part follwoing.

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