New epoxy or polyester molds do often stick whereas older molds don't. The cause is that a new mold has a reactive surface. In a cured resin, about 94-96% of the molecules are reacted; the remaining molecules remain reactive. At a new mold, these reactive molecules would like very much to stick to the molding part.
Here we need the film release agent PVA because wax release agents do not form an impervious barrier between the mold and the part.
Epoxy is a very good adhesive, so it sticks to a mold easily. A wax release agent does not form an impervious barrier between the mold and the part; it is very thin and fragile. Therefore, wax release agents often require several layers, but even then it can sometimes cause sticking. A foolproof release agent is PVA. How to use PVA easily, see here.
Sometimes a product still sticks in the mold when PVA is used; the PVA film may be damaged.
The mold or part must be flexible to allow separation. If both are rigid, separation is difficult or even impossible, even when PVA is used. This is important in the manufacturing of the mold. The plug, the model, is always stiff. If the mold is made stiff too, then separation is often impossible. The plug, which takes a lot of work to build, will be destroyed easily.
PVA (polyvinyl alcohol) is a reliable release agent but it may be hard to apply it without leaving brush strokes and particles on the mold. This will be visible on the part too. Wax release agents, such as Formula Five® Release wax, do not suffer from these problems but have insufficient release properties, especially at new molds. I have experimented as long as until I had a good solution for applying PVA without leaving any texture or dust on the surface.
Some types of PVA require a special priming wax to avoid fisheyes. Polish the priming wax after it has been dried on for 5 to 10 minutes. Repeat it one or two times for new molds to give a good base coating. Don't use mold release wax, such as Formula Five®, because it causes fisheyes with PVA.
Vacuum bagging can lead to overbleeding where the surface finish is pitted and dry. But even worse is that the laminate contains too little resin, making it less strong. The reason is that by the force of the vacuum, too much resin is sucked out of the laminate. Here, I want to see what can be done to avoid overbleeding.
Lamination process A
As common, these layers are used with the vacuum bagging process:
Lamination process B
Solution to avoid overbleeding: using an impervious release film, without holes. Excess resin is not removed. The vacuum is just used for pressure.
Lamination process C
Using an additional impervious film and a breather layer. The vacuum presses the excess resin from the laminate into the bleeder, but will not suck out resin from the laminate. Ideally, the bleeder cloth and the perforated release film can be dimensioned accordingly to bleed off the right amount of resin.