Closed/matched mold tolerance for prepreg or RTM

[Slartibartfast_lives]

Hi,

Considering changing a steel tubular component that we already make to carbon fiber. We have experience with various methods of thermoset molding including VARTM and vacuum bagged prepreg for other components, but the part in question would require a solid carbon structure, i.e. dimensional surface on all sides and not hollow, which means matched molds and controlling compression of the fibers by controlling the loading of the mold. The part is relatively small, about 260mm in the long dimension, and each half of the mold would be about 330x230x75mm

My question regards the necessary accuracy in machining this type of closed mold. It's reasonable for our volume and process to seal the mold from the outside with mastic tape to be able to draw a vacuum for infusion or degassing, so a true matched and air-tight fit is not necessary. But, things generally need to line up well, as the part thickness and laminate compression will be controlled by the offset distance between the two mold halves.

If someone could share some real-world experience on what type of accuracy this process might require for this application, I would greatly appreciate it. And more secondarily, would these kind of molds be something exclusively done in the realm of thermally-compensated linear scale mold machines, or is a good condition standard VMC capable of this?

Thanks,

Brian

 

[Compositepro]

Good luck. Making tubular composite parts in a closed mold is a lot more challenging than it at first appears. You cannot compact circumferential fibers without causing waves or wrinkles, so these fibers must be wrapped tightly as they are applied and kept tight. Getting a tight wrap and maintaining it usually requires specialized equipment (rolling tables), and the use of prepreg with just the right tack.

Resin shrinkage during cure causes voids and surface pitting in closed molding processes. You will have a parting-line in the outer surface.

Most tubular parts are made using an inner mandrel and a shrink-tape wrap, which is then centerless sanded after cure.