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Vacuum casting (RTV moulding) of plastic parts

Highlights

Vacuum casting is ideal for producing small quantities of parts, especially if aesthetics are important. The master part, typically produced by Stereolithography or CNC machining is dressed and textured as required before establishing parting planes, feeds etc. The master is then mounted within a frame and liquid silicone cast around it. After curing the master is cut out to leave a cavity into which a variety of polyurethanes can be cast under vacuum.

The silicone has sufficient flexibility that undercuts can be released by flexing the tool, although deep draws may require additional tooling splits.

Tool life varies with resin cast, but is typically around 20 parts by which time the tool surface has started to harden and the tool flexibility deteriorates. Using multiple masters and producing multi cavity tools can produce greater quantities, although cavity life will remain around 20 parts.

The process is relatively labour intensive and typically requires between 2 and 5 days to produce first off castings. This is on top of the time to manufacture and dress the master part. These times are dependent upon part size as this affects material curing times.

In evaluating vacuum casting, especially if quantities require multiple cavities, it may be worth considering RIM or injection moulding. Alternatively if quantities are low it may prove faster and more cost effective to make multiple laser sintered parts. These alternatives are dependent upon part size, complexity and time available.

Process Features

Accuracy

Tolerances vary depending on material and supplier, however as a general rule using ± 0.25mm to 100mm and ± 0.25% thereafter will provide a 'safe' estimate. Note these are process tolerances and should be applied on top of any master part tolerance.

Minimum Feature Size: In a similar way to injection moulding, this is determined by the ability to get the polyurethane to 'flow' into small features. As a wall section, 1mm is about the minimum achievable, however if the area is small enough this can be improved upon.

Layer thickness

Not applicable to vacuum casting. (Although it may be applicable to the process used for making the master part.)

Machine sizes

Vary from circa 530 x 450 x 425 mm (maximum tool size) up to 2000 x 1000 x 700 mm allowing the single piece casting of a wide range of part sizes.

Post processing

Little post processing is usually required, the finish is achieved by dressing and texturing the master part before casting the tool. After casting the part there is only the feed and riser removal, although in some very colour critical conditions the parts are sometimes painted. (usually this is a colour pigment added to the resin)

Clear Parts using Vacuum casting

There are grades of PU that are clear and as such this process can be used to produce transparent components.

The two critical issues are the material selection and the surface finishing of the master pattern used to create the silicone mould.

This method of reproduction is very good at picking out small features and as such imperfections in the surface finishing of the master will be reproduced in the PU and detract dramatically from the clarity. Thus if clear parts are desired, the master pattern should be finished to a high gloss over the desired areas, and these areas communicated to the supplier in advance so that they are not compromised by the tool configuration.

There are grades of PU that are clear.

Flexible Parts using Vacuum casting

There are a number of grades of PU that are available offering a range of hardness's. 

This process was used by model makers way before the introduction of Rapid Prototyping techniques, usually from hand fabricated masters. Stereolithography brought easy access to the master parts and a high degree of confidence in reproduction directly from CAD.

As Stereolithography resins were frequently limited in the amount of functional testing they could survive, vacuum casting was the logical adjunct to give both better material properties and aesthetics.

The process steps can be summarised as

  • manufacture a master part and dress to achieve the required aesthetics
  • ensure the master part is clean and sealed (some materials will react with the silicone)
  • establish a parting line and apply coloured tape around the it
  • construct a casting frame and after attaching a feed and risers, suspend the master part in the casting frame
  • degass the silicone and pour into the casting frame, degassing the cast tool again to remove any entrained air and then leaving to cure
  • when fully cured, remove the casting frame and cut open the silicone to the coloured tape / parting line
  • the master model can now be removed, the tool reassembled, taped together and sent for casting

It is also possible to use the process to prototype overmouldings, either a flexible polyurethane on a rigid polyurethane or polyurethane onto a nylon (laser sintered) core.

To review your project requirements , please contact:  stefan@smartprototype.net 


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