How does Vacuum Casting work?

Following on from our Vacuum Casting video, we have pulled out the key points that Justin Pringle, MD of Prototype Projects, discussed: how does vacuum casting work?

What is Vacuum Casting?

Vacuum Casting is used for producing small numbers of accurate polyurethane prototype parts with characteristics and quality comparable to an injection moulded product. 

The parts are made by using a polyurethane resin from a silicone tool. 

Why use vacuum casting? 

Vacuum Casting is ideal when you want to make parts in the prototype stage of the design cycle. The process is generally used for prototyping but it can be for low volume production.  

Our PU resins mimic plastics with various properties such as clear, rubber, hard plastics as well as flexible plastic like polypropylene. 

How does it work? 

The vacuum casting process begins the creation of a master model. This is usually a 3D-printed part made using the SLA process. The part is finished in our model shop by rubbing the raw SLA down to remove any build lines and to apply the desired paint finish to the surface of the part. This surface finish will be replicated in the silicone tool which will then be replicated in the polyurethane part.  

Once the SLA master has been made and finished, it needs to be encased in silicone rubber. The master is suspended in a box before vents and a feed are added (these allow the polyurethane resin to flow through the tool). Once the silicone has been poured in, it is cured overnight. 

The tool is then a solid block of rubber which needs to be cut in half so it can be used for casting. The tool has serrated edges to ensure it stays together, ready for the next part. This creates the core and the cavity; the ‘A’ surface and the ‘B’ surface. 

When the tool is ready, the first part can be cast. 

The tool is placed inside the bottom half of the vacuum casting chamber. A separation plate sits between that and the top half of the chamber. In the top half, the polyurethane resin is mixed to create a thermic reaction and begin the curing process. 

The resin is degassed to remove any air from it and the chamber is vacuumed. When the right vacuum has been reached and the chamber is at the correct temperature, the resin is then poured through a funnel into the tool. The air is forced from the top half of the chamber so it can flow through into the bottom half of the chamber which pushes the resin into the tool. 

The tool is removed from the chamber and placed in an oven at around 70oC for 40-50 minutes. 

Once the part has been cured off and cooled, the tool is separated so the part can be removed and tidied by removing flash and vent traces. It is important that the part is then left to rest to cure; it can take seven days to fully cure and obtain full mechanical properties. 

A flexible process

Vacuum Casting is a very flexible process as so many different results can be achieved:  

• Textures can be applied to the master so it is replicated on all of the parts
• A gloss paint can be added which results in a gloss feel. Likewise, a matt paint will result in a matt feel 
• Inserts can be added and moulded into the tool, for example, a screw fixture. 
• Technologies can be combined, for example, CNC, laser etching and then vacuum casting 
• Clear resins can be pigmented. Colour matching is possible and can be handled both in-house or via our pigment colour matching supplier if a more accurate colour match is required 
• Separate areas of the master tool can be adjusted so that some parts are clear gloss and other parts are frosted, for example
• As the materials used can be combined through the casting process, the final part can incorporate both hard and soft areas
• Very fine levels of detail are easily achieved
• Undercuts are also possible
  

There is also no need to worry about draft angles or wall thicknesses as there is no shrinkage in vacuum casting. 

Ultimately, vacuum casting is a good alternative to injection moulding. 

How fast is the process? 

It is possible to achieve a quick turnaround of parts with vacuum casting. The SLA tool is made in four-five days. One–to–two sets are then cast each day. 

Tool life is around 20-25 parts, depending on the material and complexity of the part, although it can be as low as five, and as high as 100.

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