This video documents a proof-of-concept project for a novel carbon fiber manufacturing method called "trapped rubber molding," designed for creating tubular parts. The process is presented as an alternative to traditional bladder molding.
The core of the technique involves using a solid silicone rubber mandrel, which has a high coefficient of thermal expansion. When the mold is heated to cure the prepreg carbon fiber, the mandrel expands, creating passive internal pressure that compacts the laminate. The video details the entire workflow, from designing and CNC machining a two-part aluminum mold, to a specific hand-finishing technique for sharp edges, casting the silicone mandrel, laying up the carbon fiber, and the curing process. The initial part revealed a key learning: the mold required end caps to prevent the rubber from expanding outwards, thereby ensuring proper compaction pressure on the part. The project successfully validated the manufacturing technique for future production use.
A Step-by-Step Guide to Trapped Rubber Molding for Carbon Fiber Parts
00:00:02 The "Trapped Rubber Molding" Concept: The video introduces an alternative to bladder molding for tubular composite parts. This technique uses a solid silicone rubber mandrel that expands when heated during the curing cycle, creating the necessary internal pressure to compact the carbon fiber laminate.
00:01:18 Mold Design and Machining: The process starts in CAD with the design of a 30-degree tube joiner and a corresponding two-part aluminum mold. The mold is then precisely machined using a CNC mill.
00:02:54 Advanced Mold Finishing: A specific technique is used to ensure a sharp parting line on the final part. The internal mold cavity is hand-sanded first, and only then is the mold's top surface faced off. This prevents the rounded edges that typically result from hand-sanding.
00:05:16 Casting the Silicone Mandrel: To create the internal tool, a sheet wax layer is first applied to the mold cavity to create space for the carbon fiber. Silicone rubber is then vacuum-degassed to remove air bubbles and poured into the assembled mold to cast the solid mandrel.
00:09:22 Carbon Fiber Layup: The silicone mandrel is wrapped with a release film. Then, unidirectional prepreg carbon fiber plies are wrapped around it, followed by a final outer woven ply for a cosmetic finish.
00:12:52 Curing the Part: The mandrel with the carbon fiber layup is placed into one half of the mold, the mold is closed and bolted shut, and stick-on heaters are used to run a programmed heat cycle, curing the resin.
00:14:10 The First Result and Key Learning: The first part demolds successfully but suffers from poor compaction. The creator identifies the cause: the rubber mandrel expanded out of the open ends of the mold instead of pressing against the laminate.
00:14:22 The Solution: The key takeaway from this proof of concept was the need to add end caps to the mold. These caps contain the expanding rubber, forcing it to apply pressure inward and properly compact the carbon fiber layers.
00:15:28 A Successful Proof of Concept: Despite the flaw in the initial part, the project successfully validated the trapped rubber molding technique, providing crucial lessons that were later applied to manufacture high-quality production parts.