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Vacuum press

Category: Tools & equipmentCompiled from 4 source threads

Quotes are verbatim and attributed to their original authors.

A vacuum press consolidates the layup by drawing the air out from under a sealed membrane, so that atmospheric pressure pushes the ski down onto the mould. It is the simplest and cheapest press to build, and although the pressure it can apply is capped, builders consistently report it is enough to make good skis.

How much pressure

The ceiling is atmospheric pressure — you cannot pull harder than the air pushes.

"Max pressure you can get is atmospheric pressure, roughly 14.7psi at sea level on a standard day. For a pneumatic press, people press anywhere from 50-100psi… I've only seen one guy on here do hydraulic, seems like more hassle than air to me."
— thread #3938, "Vacuum bagging vs. Press Machine Assembly"

That is less than a bladder press, but as another builder put it, "0,95 atmosphere which is enough to press a board." Altitude costs you a little: at 4,500 ft builders report pulling roughly 20–23 inHg (about 10 psi).

The pump

Venturi pumps (run off a compressor) are cheap and simple but pull a limited vacuum; a dedicated electric rotary-vane pump pulls harder and runs continuously.

"I think you're better off buying a real vacuum pump. They're designed for continuous running and will last you forever… $50-$100 will get you a decent Gast pump on ebay."
— thread #210, "how to make a vacuum pump"

Popular DIY routes include the Joe Woodworker "Project V2" venturi kit run off an existing compressor, and converted refrigerator or A/C compressors. One builder cautions that venturi and diaphragm pumps only pull so far, and recommends "an oil based 'rough' pump, also known as a foreline pump… A decent one will pull down to 50 mtorr."

The bag and mould

You do not need a top mould — the membrane is the top half. Rather than wrestle the whole mould into a bag, most builders seal a sheet of vacuum film to the mould's perimeter.

"I've got a giant bag that fits the mold… but I wouldn't recommend that since it's a pain to wrestle the mold into the bag. A better way is to get vacuum film and seal it to the top of your mold… You don't need a top mold for a vacuum press."
— kelvin · thread #210, "how to make a vacuum pump"

A layer of breather fabric (or even cardboard) over the layup helps distribute the pressure and lets air escape to the port; a plastic sheet keeps the breather reusable.

Vacuum versus a bladder

The recurring debate is covered in the Ski press entry: vacuum consolidates and de-airs the layup well and is beginner-friendly, but with no way to raise pressure it is considered sub-optimal for a plain hand layup (resin ratio and void content are harder to control) unless a resin impregnator or prepreg is used. Heating is added the same way as any press — see Camber, rocker & the mould for how one-sided heat affects camber.

Source threads