Air fitting/preasure to release molded part

[alexhiguera]

Hi, I know that air is used to force a molded part out of the mold itself. This is done by welding an air fitting to the outside of the mold. When an air hose is attached and preassurized the part will pop out.

Drill a hole in the metal mold, weld the fitting, prepare mold, release wax prep, and layup has cured, attach the hose and pop it out right?

But will the resin and fiber mat or cloth on the inside of the mold clog up the air fitting cavity on the inside where the layup is being laid even with mold release?

How to avoid this?

[Sealion]

Place a small piece of self adhesive tape over the air hole on the inside of the mould. Also, an air fitting is not necassary, a small hole drilled into the mould will do.

[alexhiguera]

Sealion,
You mean regular clear tape?
Will the resin and heat of curring melt this tape?

Why do you say "a small hole will do" ? To push the part out thru the other side with a piece of wood or fingers? Will it pop out this way?

[Corpus Skipper]

Masking tape works well, polyester won't stick to it. As far as the air hole, I'd just weld a bung big enough to screw in a standard quick connect fitting. Make sure the pressure is regulated to this fitting, I'd hate to hit it with 150 PSI and shoot the part across the shop (or into someone!) Personally I've never needed pressure to get parts out, A little water poured in through somewhere that has already separated usually does the trick. If you're having problems, try a new mold release, or more coats. Also, I use mold cleaner between parts, then rewax.

[rxcomposite]

Tape will do, if the hole is large stick in a ball of modeling clay.

Air pressure works for a quick release and only on small part. for very large part (hull) i inject water instead of air. This floats the part evenly.

[alexhiguera]

rx, this would be to release a flat fiberglass panel 5' x 7' with a 90 degree angle of 6 more inches on the long (7') side. Understand? looking at it from a side profile it would be 6" vertical X 5' horizontal the 7' would be back.

thickness of layup about 1/8"

air fitting or water hose fitting?

[rxcomposite]

You should not have any problem with that (flat panel with a 90 degree bend). A release fitting is not necessary. Insert a spatula on any of the part lines and you can easily lify the laminate.

As Corpus has said, check your release wax.

Air release fittings works best on complicated parts. Water injection on large part, especially hulls.

I think the problem you would encounter is warping. Especially that your panel is only 1/8" thick.

[alexhiguera]

rx,
warping?
For the mold I will use 1/16" sheet metal that would be reinforced with a 1"x1" square tubbing frame.
Will it still warp?

Or do you mean warping after the part is cured and taken of the mold?

Should I prep sand with 220 and 400 the sheet metal before mold release? the surface is very smooth and clean just as it came, without sanding.

Or should I skip and apply mold release like it is?

[rxcomposite]

Alex,

It is your part that will warp or curve.

If i remember correctly, there was an article written about large panel warping in Professional Boatbuilders magazine. It is very technical.

Briefly, When the resin starts to gel, it gives off heat in the form of exotherm. This is critical to achieve proper curing. Remove the heat and the resin undercures.

When you use a metal mold, the metal, especially when it has a large surface, absorbs the heat and dissipate it quikly. The result is a not so cured first layer. When the second layer is laminated over, the first layer acts a thermal barrier. Glass being an insulator, prevents heat transfer. Thus you have two layers of laminate that did not cure in the same condition. It is prone to warping (curling).

Best bet is to use a non heat absorbent mold such as marine plywood base reinforced with fiberglass laminate on both sides and further stiffened with wood (same material) to make it dimensionally stable.

I have heard of one boatbuilder who surfaced the bare plywood with aluminum foil using contact adhesive. He pulled out one or two part before reworking the mold. Me, i surface it with mylar based packing tape.

I use metal frames for the mold but attaches only on critical points, making in effect a floating point method or a cage. Glass and steel have different coefficient of thermal expansion. You will be inviting trouble when you glass the whole expanse of steel frame to the mold.

At high temperatures such as oven curing, the mold could twist the metal permanently like it was a piece of wire.
Rx

PS.
You can use metal mold on small parts such as 6"x12". Do not sand it as you want it to be shiny. just make sure it is oil free and absolutely clean when you wax it.

[alexhiguera]

rx, Achieving a smooth glass surface on the mold with plywood is dificult and takes a lot of fairing for when the actual part is laid on this fiberglass/plywood mold that you suggest, and the part pops out with a smooth finish.

What if the metal mold is sprayed in the back side with a foam insulator to retain heat for proper curing?

Or make a plywood mold that has been lined with very thin sheet metal with spray adhesive?
The smoothness of the sheet metal and the heat retention/insulation of the plywood for curing!

[rxcomposite]

Like i said, one boatbuilder lined the plywood with aluminum foil. Make sure the contact adhesive you will be using will not be dissolved by the resin.

If you are sold on metal, try heating the mold. Most metal mold are heated. that is the technique. Some use copper pipe with hot oil circulating.

You might have to experiment. try warm to the touch first. Avoid hot spots. you might have to use several infrared or bare incandascent lamp underneath.