Material for use in investment casting

[freerangequark]

I have a small machined part which will need to burn away during the investment casting process. Does anyone know if Delrin can be used for this purpose? If not, do you know a machinable plastic or wax appropriate for this use?

Thank you,
Glenn

 

[arunmrao]

Investment casting is also known as lost wax process. All the wax is melted and collected back in the process. There is not one single wax being used but a blend depending on local availability and costs.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[swall]

There are special machinable waxes made for this purpose. I also recall that styrene plastic can be used (not to be confused with polystyrene foam).

 

[TheTick]

Why couldn't the caster just use the same wax for the "machinable part"?

 

[Pud]

Delrin (i.e. Acetal) decompostion product(s) is mainly fomaldehyde. Great for a blocked sinus if you can survive the coughing fit. Not nice. Set alight to a small piece, blow out the (almost colourless) flame and have a waft of the fumes. You'll see what I mean.

As The Tick says, use the wax (or whatever the foundry recommend - they surely will have done it before!)

Cheers

Harry

 

[plasgears]

I had a jet engine part made by investment casting, and the pattern was largely wax, but the outlet guide vanes were modelled in plastic. I think they were nylon. It was successfully cast, and it became part of the early Airbus engines. It was the structure just before the combustor section.

 

[btrueblood]

Plasgears,

Sure it wasn't ABS? Was the preswirler part made by rapid prototyping?

 

[plasgears]

btrue,
The vane patterns had the translucent quality of nylon, and they had the flexible toughness of nylon. ABS has a more opaque appearance.

The vanes acted as an anti-swirler, the last stator of the compressor. There was some pressure rise in that half stage. The vanes appeared to be injection molded. The shape was 3 dimensional with variable section from root to tip. Surface finish was smooth like injection molding.

Assy of the pattern was by 'booking' the vanes together over 360 deg, and casting the wax around it. It was alternately coated with slurry and sand, and then set to dry in an moderate oven. The wax and plastic was then melted out in a high temp oven.

It was cast in a high temp stainless steel. Size was approx. 20" OD. In later versions, it had flange extensions fwd and aft, and weld projections aft for the combustor casing, inner and outer. The aerodynamicists worried about it, but it turned out to perform very well. It has the characteristic look of modern combustors today.

 

[sacem1]

The investment cast process uses mostly wax mixtures specifically natural bee wax and petroleum based parafin waxes in different combination according to the more detail (more bee wax) or more resistance (more parafin wax), afterwards they can be built into more complicated shapes using different piece forms or clustering several identical ones around a feeder made out of the same wax,
The whole resuting form or "tree" is initially covered with what is called Paris plaster which is a very fine meshed plaster applied either by dipping or by small brushes application, then it can be covered by a coarser fill in material which can be common plaster, sand or resin coated sand.
After the mold has been baked in an inverted position on an oven an all traces of wax have been removed the mold can be poured by simple investment casting (it is fixed on top of the molten metal carrier and inverted) or loaded into a centrifugal machine for casting with the aid of centrifugal force to obtain high quality, high density pieces with almost nill trapped gases.

If the need is of several pieces (up to hundreds) you can make a soft rubber mold which is made by pressing your original sample into layers of special rubber, then vulcanizing with applied heat the mold and afetr opening it you can load the rubber mold (with one or several cavities) into a centrifugal machine where the wax is pured in and after cooling you can get several samples with very high definition at a very resonable cost, and then you go into the process I described initially.

Hope that this helped

Regards
SACEM1