For most hollow composite structures other than a simple tube & complex geometries ◾Cosmetic finishes
Bladder molding is used for parts that either has complex geometry, strict cosmetic requirements, tight outside tolerances or a combination of all three. Parts can be too complex for this method, in which case multiple tools are used to create separate components that then get bonded together in post-processing, e.g., bicycle frames.
What is the bladder molding process?
We start with sheets of pre-impregnated fibers called "pre-preg."
- The sheets are wrapped around an inflatable bladder and then placed inside a mold cavity
- Once the mold is closed, pressure is applied to the inside of the bladder causing the bladder to expand pushing the pre-preg outward against the inside of the mold cavity
- Heat is then applied to the mold to solidify or "cure" the composite in the shape of the inside of the mold cavity
After curing, the solid composite form is extracted from the mandrel as a tube. In the case of a tank or other closed end vessel, the mandrel could be left inside the composite form or dissolved out via various methods.