Vacuum resin casting

I started experimenting with resin casting around 1992, and it took me a long time before I found a process that gave me consistently excellent castings. Inspiration came mostly from studying commercial resin parts; I never found a manual or article that describes the vacuum casting process in detail. There's an interesting cultural aspect to resin casting. It seems that vacuum casting is the European way of resin casting. In the USA pressure casting is the preferred method. I'm describing vacuum casting here.

My vacuum molding and casting process

I made a graphic of the steps of the process that I use. I keep improving the graphic; this is the eleventh version. There's one step that I haven't figured out completely, and that is post-curing at elevated temperature. Generally speaking, post-curing improves material properties of thermoset plastics. But I haven't noticed that effect with post-cured polyurethane castings. More experiments are planned.

Equipment

This is my no-brand Chinese single-stage rotary vane vacuum pump. It came with a vacuum gauge that's pretty useless for this purpose, since I'm most interested in the 0 to 5 mbar (abs) range. But gauges for this range are expensive, and I learned to listen to the pump to judge the vacuum level. Plus you can see the resin bubbling vigorously when the vacuum is deep enough. The black vertical cylinder is the oil mist filter. I tried several other solutions, but nothing would catch the fine oil mist generated by the pump. This filter does its job properly. I duct its exhaust into the ventilation system, since it could contain unhealthy components.

Here's my 9 liter vacuum desiccator by Kartell. The bottom half is made of polypropylene, the top half of polycarbonate. The dome used to be fully transparent, but it's coated with tiny drops of resin, because the resin bubbling can be violent. Plus I also used it for an industrial project, further dirtying the inside. I'm using thick-walled 12x8 mm PVC tube, otherwise the tube will collapse under the vacuum.

Various photos of my molding process

Here's a typical project, a set of small U-2C inlets. The master parts are seen at the rear, on their casting blocks. The parts are positioned vertically because of the openings. At the front, the castings with and without the gates can be seen.

The photo shows three parts to be copied, fitted with a casting block and a piece of 1 mm plastic card in between (gate). The soft silicone rubber accepts large undercuts, as you will see.

Here's a wheel, prepared to be copied. It's a pretty radical mold when it comes to undercuts, but for me this the best way to cast a good resin wheel. The spoke holes have been closed with Kristal Klear, which makes a very thin film when dry.

A view inside a typical mold, held open to show the interior. Note that I do not make cuts that reach the exterior of the mold, because they would form paths for air leakage during the vacuum process.

The resin has been cast and cured. The foaming of the resin under vacuum left some flash on the top face of the molds.

The result: five nice castings. Note the heavy undercuts on most of these parts. Releasing the parts can be a bit of a struggle, but the rubber takes the abuse well.

The parts come out perfect, consistently, without air bubbles. The castings blocks show a bit of variation in their volume. They need to have some thickness (strength), because they are the part that you grab while pulling the part out of the mold.

Usually I place my filled molds in a row after filling with resin (absent here), to hold the rather floppy molds to their original shape, while the resin cures. The mold second from the right shows clear signs of being at the end of its life. The silicone rubber slowly discolors to a whitish color. Another sign is that releasing the castings becomes more and more difficult; the release properties of the silicone rubber deteriorate.

There's a third indicicator of mold life. After twenty pours, my castings start showing a surface texture that the master definitely did not have. Here's an example, showing the 3rd casting (rear) and the 25th casting (front). The difference between the two castings can be seen clearly. The texture can be sanded off, but it's a lot of work and damages the details. I chose to use my moulds for a maximum of 20 castings.

Here's the result of a full day of casting - quite satisfying! My slow resin requires ~2 hours of curing, which determines the casting interval. But all castings were 100% perfect, not a single air bubble to be seen.

Vacuum pump oil replacement

After sixteen years of on and off use, I replaced the vacuum pump oil. I can't say exactly how much time the pump ran, but 5 hours total is my first estimate, 100 runs of 3 minutes each. That is less than a single overnight vacuum cure of a composite part. I read a warning that polyurethane resin vapors contaminate vacuum pump oil. But I don't have a way of checking that.

This close-up shows how the oil separated after 20 hours. The top part looked like regular oil to me, then a layer that looked like engine car oil sludge, oil that has mixed with coolant or water ('mayonaise') At the bottom I see some white parts. Weird!

Interestingly, four days later the division line had moved down, from 140 to 105 ml, suggesting that the oil-water mix was slowly separating. However the separation stopped roughly at this point.