Learn from the Vacuum Casting Expert

Find out all you need to know from our Vacuum Casting Expert.

Meet the expert…

Name

Tim Rowell

Job title

Rapid Prototyping Workshop Lead Technician

What do you do at Prototype Projects?

As the workshop team leader Tim is responsible for managing the workload of the rapid prototyping team.

As well as managing his team, Tim is hands on. He prepares the 3D printed parts, normally SLA’s, to be used as the Master Model in the Silicone Tooling process. These models are hand prepared and painted to give the desired surface finish that will be replicated when the PU (Polyurethane) Castings are poured.

Length of time at Prototype Projects

Since 1993 so more than 20 years of prototyping experience!

More about Tim

To find out more about Tim and rapid prototyping, take a look at our Meet the Rapid Prototyping Team article.

About vacuum casting

What is vacuum casting?

Vacuum casting is a process that uses silicone rubber tools. It is used to bridge the gap between 3D printed models and the production method, which is typically injection moulding.

Silicone rubber tools have a limited life and are normally capable of producing 20-25 parts, though this is dependent on the PU material being used.

How does it work?

A tool is produced from a 3D-printed master, usually using SLA – so there’s no need for expensive hard tooling. The master is then finished to the designer’s requirement which, for example, may be for a gloss or matt surface finish.

Once the 3D master has been produced it is then necessary to prepare the mould.

Firstly, a ‘split line’ is taped around the most central point of the mould. This split line is created to allow the tool to be split in two so that the part can be removed from the mould at the end of the process.

Secondly, so that the desired material can be poured inside and any excess air removed from the tool, a central feed and small vents – or risers – are added to the master. It is then fixed within an open box into which the liquid silicone rubber is poured and left to cure. Once cured, the master is cut out of the tool, leaving a core and cavity ready to be used.

Once the tool is completed and ready for use it is heated to between 60-70 Celsius then placed in the vacuum chamber. The PU resin is weighed (A & B components) and placed inside the upper chamber ready to be degassed and mixed. Once fully mixed, and the chamber has been evacuated, the PU resin is slowly released into the tool.

When sufficient mixed resin is flowing into the tool, the vacuum is released thus forcing the mixed resin to flow through the tool. Once the tool is filled with mixed resin it is placed in an oven to cure for up to an hour.

Once cured, the tool is removed from the oven and opened so the solid vacuum cast part can be removed. Finally, the part is finished by hand removing any flash and vent points.

The PU resin is in a green stage and will not reach full material properties for 7 days.

What are the vacuum cast tools made with?

The vacuum cast tools are made with RTV silicone rubber.

When would you use vacuum casting?

You would use vacuum casting when a prototype part is required that would be normally – in production – be made using the injection moulding process. You can also use vacuum casting for low volume production.

What sort of quality do you get from vacuum casting?

Vacuum casting produces parts with fine levels of detail and very good surface quality.

What are the benefits of vacuum casting?

Using vacuum casting as an aid to product development enables products to be realised in a shorter lead time and can be more cost effective for early prototype requirements.

Tim’s Top Tips

How could designers improve their CAD files to make their vacuum cast prototype more successful?

The wall thickness of a part is crucial, especially with vacuum casting. They should be the same thickness as the final part or product is intended to be.

Keeping undercuts to a minimum is very important in vacuum casting. Large undercuts can create issues when removing them from the mould. While this is manageable it can be problematic and give unsatisfactory results.

Having the correct tolerances for interlocking parts is vital. If, for example, one part of the part or product has a hole with a tolerance of 10, the part meant to fit inside the hole must have a tolerance of 9.9 so that it fits. It may seem obvious, but it’s quite common for both parts to have the same tolerance.

What are the most frequently asked questions and what are your answers?

1. How many parts can you get from one silicone tool?
2. It depends on the material being used but generally a tool can produce 20-25 parts. This number can be affected by the geometry of the part and the PU material being used.
3. What is the performance of the resin?
4. There are various blends of PU resins, Clear UV Stable, flexible and rigid. (Please contact us on 01763 249760 to discuss what specific properties you require.)

What are your three top tips for people designing for vacuum casting?

1. If you think you will be needing prototypes using the vacuum casting process, please talk to us as early as you can. We can give advice, such as we don’t necessarily need the draft angles that you would require for injection moulding.
2. Have realistic expectations of the prototype and remember that it is just that, a prototype. We may need concessions on the part design to compensate for the tooling process.
3. Come and look at what we can do at the bureau (we recently opened our new 3D Printing Suite) to see how we do it and ask us questions. It can be helpful to have a good understanding of the prototyping stage in the design cycle. If you don’t have time to visit us just yet, contact us on 01763 249760 for advice – we’re always here to help.

What would your great invention be?

An on / off switch for a toddler… with software upgrades as the child develops!

GOT A VACUUM CASTING REQUIREMENT?

Contact us on 01763 249760 and ask to speak to our vacuum casting expert or click here to request a quote.