Prepreg Compression Moulding
Very low cycle times since very high reactivity (snap cure) resins can be used
Tool faces apply heat very fast and dissipate heat generated by the curing resin - typically 2 minutes for thin (1-3mm thick)
Automated forming using diaphragms provides low labour cost and minimal waste
Lower consumables cost and labour compared to single sided (vacuum bag) prepreg processes since (no bagging or other consumables are required
Highest quality surface finish
Higher tooling cost compared to vacuum bag processing - the press provides very high pressure (up to 100 Bar) and hence the tooling must be metallic and use thick plate
Shape complexity is limited by the tool applying pressure horizontally and so all part faces should be no more than 60 degrees from horizontal
Process
The process was developed principally to reduce in mould time and hence is typically applied to high rate production, always using high cost matched face steel tooling.
Prepreg plies are laminated by hand or by automation, and transferred to a press with matched surface, pre-heated mould faces.
The tool cavity is closed to a specified part thickness, so to apply very high pressure and rapid heat transfer. The resin is cured and the part is demoulded without cooling. Curing may be completed in an oven.
Material Options
Hexcel’s HexPly® M77 rapid cure prepreg is cut to shape and stacked (together with adhesive) on a fully automated line in Neumarkt, Austria. BMW uses the preform to save weight and reinforce the metal shell of the BMW 7 Series. Photo Credit: Hexcel
Process Features, Benefits and Issues
Compression moulding rather than vacuum bag curing of prepregs allows heat to be applied very rapidly through both metallic tool faces. High reactivity resin prepreg can be processed in as little as three minutes for a 2mm thick part.
Very high pressure can be applied for both cored or monolithic parts, this can confer very high fibre fraction and can provide a very high surface finish. However if the diaphragm process is used the surface is not as smooth as the mould since the diaphragm film will have been compressed unevenly.
The application of very high pressure eliminates the requirement for a vacuum to remove air during cure, the pressure collapses and then helps dissolve air and volatiles into the matrix resin.
Process Guidelines
Simple shape parts with slight curvature and depth can be processes by placing a flat stack of prepreg plies in the cavity with draping occurring during press closing. More complex curvature and deeper shape parts require a controlled forming process prior to press closing. This is best achieved using a double diaphragm former (DDF.)
1. Lay up flat stack of prepreg plies
2. Trim edges if required
3. For automated - high rate processing
- place between diaphragm films in a forming press, apply vacuum. Press the combined stack to conform prepreg to the required shape - the diaphragms keep the prepreg under tension to restrain fibre wrinkling.
For low rate processing, the prepreg plies can be hand laminated onto the a tool and placed in the press.
5. Press using isothermal tool – to either a specified cavity gap or a specified pressure
6. Cure Typically 110C – 160C
7. Demould from hot tool
8. Place onto a curing jig for oven post-cure if required
9. Trim edges, holes, attach fasteners
Lay up prior to press loading between plastic sheet diaphragms Solvay Cytec
Langzauner Press DDF Moulding Equipment
Alfa Romeo Giulia Bonnet inner – Gurit
Nissan GTR Boot Lid: The decklids' outer panel, based on the aluminium lid's geometrywas limited to a very thin 1.1 mm. Just five plies of unidirectional carbon fibre reinforcement were used in a 0°/90/0°/90°/0° layup - Nissan Motor Co. Ltd
At 0.7 mm thickness, the aluminium inner panel's geometric limits posed an even greater challenge. Just two plies of 3K plain weave carbon fabric, plus local reinforcement, were enough to meet requirements - Nissan Motor Co. Ltd
In production, prepreg is first layed up, debulked and cut out. Next, the prepreg is preformed by heating for 1 minute until it reaches 60-70°C and immediately shaping it in an air-cylinder press under light pressure (0.3 MPa) using a two-sided tool made from polyurethane modeling board.
Preforms are then cooled to room temperature for a total preforming time of <5 minutes. An optional step, when aesthetics are critical, involves covering the preform with a silicone rubber sheet and pulling a vacuum to smooth out wrinkles prior to demolding.
If they won’t be molded right away, they are stored in containers to keep their shape. Normally, a few preforms accumulate, press-side, before being molded.
The molding cycle in a hydraulic compression press is 8 minutes at 140°C, using 8 MPa forming pressure.
Because both panels are molded to net shape, post-mold trimming is limited almost entirely to removal of a thin parting line flash. Thanks to high molding pressures, parts reportedly exit the mold with a very smooth surface and none of the pinholing seen in autoclave-cured parts. Only light sanding is done to ensure mold release residue is removed prior to painting.
Demolded, deflashed parts are bonded together with an epoxy structural adhesive.
Key Manufacturing and Quality Issues
Since the compaction process uses tool gap closing, vertical faces cannot be moulded without risk of severely distorting or tearing of the prepreg. The process is best suited to shallow, tray like parts without steeper faces than around 70 degrees from horizontal. For the same reason, resin content and void content will vary depending on the part face angle.
For parts without a foam or honeycomb core, the process simplicity of ‘squashing’ a stack of UD or woven prepreg plies in a press is complicated if variable thickness parts are required.
Whereas vacuum bag processing allows the sealing bag to compress variable thickness lay ups with even pressure wherever the plies are placed, compression moulding requires very accurate ply placement to avoid under or over compacting areas of the part.
Thickness variability is limited to flat or single curvature regions where no ply movement occurs on tool closure. However, generally only constant thickness parts are manufactured. Cored parts overcome this issue through core local crushing.
Embedding of metal inserts is best avoided owing to the difficulty in manipulating the prepreg plies around the fittings and the danger of a misplaced insert causing very costly tool face damage. However, insert position marks can be marked in the tool face for subsequent fitting positioning.
Solvay SolvaLiteTM 730
Hexcel HexPly® M77