Wet compression molding

Old process updated and automated to offer lower cycle time and cost in BMW 7 Series plus potential for void-free, 65% fiber volume composite parts.

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Wet compression molding is used to produce "Carbon Core" passenger cell components for the BMW 7 Series, offering lower cycle time and cost for parts with less geometry complexity.
SOURCE: BMW Group.

Wet compression molding is not a new process. Molded Fiberglass Companies (MFG, Ashtabula, OH, US) claims it has used the process for volume composites production since 1948. However, for the last few decades, compression molding has mostly processed sheet molding compound (SMC) and thermoplastic materials such as glass mat thermoplastic (GMT). However, liquid compression molding (LCM), also known as wet molding and wet compression molding, does not use prefabricated sheets of resin/ thermoplastic polymer combined with discontinuous fibers, but instead begins with dry continuous reinforcements and liquid resin, like many other composite processes, including wet layup, resin infusion or resin transfer molding (RTM). But wet compression molding is gaining attention thanks to BMW's use of it in the new 7 series sedan's carbon fiber reinforced plastic (CFRP) passenger compartment, referred to as the "carbon core".

Wet molding manufacturing at BMW
In the 2015 issue of Presses and More Composites, equipment and technology supplier Dieffenbacher (Eppingen, Germany) describes wet compression molding as a more economical alternative to high-pressure RTM (HP-RTM) thanks to elimination of resin injection. Instead of placing a preform into an RTM mold and then injecting and curing the mixed resin in the closed mold, wet molding applies the mixed resin onto the surface of the carbon reinforcements outside of the press. This allows parallel processing — application of resin while other parts are curing in the press. The resin, usually epoxy, can also be more reactive because no latency is required to allow filling a heated RTM mold before cure. Dieffenbacher reports that both save time, cutting cycle time to roughly 180 seconds and also allowing multiple components to be pressed simultaneously.

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The new BMW 7 series include 27 CFRP components manufactured on 5 Dieffenbacher wet molding lines and 2 hybrid process lines. Hybrid processing cures and bonds carbon reinforcements impregnated with wet resin to sheet steel in a compression molding press to form hybrid components, like the 7 Series B-pillar.
SOURCE: text from Dieffenbacher and BMW, image BMW Group.

Dieffenbacher describes the wet molding process as characterized by a high degree of automation, including fully automated tool change systems so that changeover from one product to another is possible within roughly 10 minutes. BMW's wet molding line uses Dieffenbacher's CompressLite press series designed for easy access in an automated environment, high precision and energy efficiency.

The process chain reportedly begins with ultrasonically welded stacks of carbon reinforcements. These are transported to a camera table by a robot with a needle gripping system. The camera checks the position of the stacks and reports on the accuracy of the layers. The stacks are corrected if required and then placed on a shuttle table with an integrated weighing system. The table may have double or quadruple stack cavities. Two resin robots apply epoxy resin to the stacks, a process which is weight-controlled to ensure reproducibility and quality of the final parts. The resin cabin's ventilation system can handle 5,000 m3/hr for venting of fumes during this open process.

Next, the shuttle advances to the feeding position and a robot places the stacks into the compression press. The press closes and cures the stacks at 150ºC. After the press opens, a robot extracts the cured parts and places them into a cooling press. Once cooled, the parts are removed from the cooling press and placed on a discharge converyor for quality inspection before being queued for vehicle assembly.


Wet molding manufacturing at BMW.
SOURCE: BMW Group.

Dieffenbacher describes wet compression molding as suitable for parts with high loading requirements but low 3D shape complexity such as vehicle drive tunnels or roof reinforcements. Though the process has received publicity with the 7 Series, it apparently was also used in the BMW i3 and i8 models.

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According to charts in a presentation given by SGL Group in Jan 2015,
17 "wet-pressed parts" (Nasspress-Teile) were used in the BMW i3 (right) and 21 parts in the i8 (left) were made with wet compression molding. SOURCE: "Multi- Material Approach for More Robust RTM Processing", SGL Group.

Cost
According to the 2015 SPE ACCE paper "EVALUATION OF A NEW 'INLINEPREPREG' PROCESS APPROACH TO ESTABLISHED PROCESSES FOR THE MANUFACTURING OF STRUCTURAL COMPONENTS OUT OF CARBON FIBRE REINFORCED PLASTICS" presented by Fraunhofer Institute for Chemical Technology ICT (Pfinztal, Germany) and Institute for Vehicle System Technology FAST, Karlsruhe Institute for Technology (Karlsruhe, Germany), wet compression molding offers lower material cost and higher production capacity than autoclave or inline prepreg. The paper calculates a part cost of 70€, raw material cost of 49 €, daily production rate of 294 parts (>73,000 parts/yr) and staffing of four workers and one foreman per line. That line was calculated to include an automated cutting table, fiber storage units, a handling unit, a resin application unit, a press, a tool and finishing station.

Resins
Dow Automotive Systems (Horgen, Switzerland) announced at JEC Europe 2014 (Paris, France) that its low-viscosity, fast-cure VORAFORCE epoxy resin held particular promise for the wet compression molding process. Working with KraussMaffei (Munich, Germany) and Cannon (Peschiera, Itlay), Dow developed a process that applies resin in 15-20 seconds and cures in 30 seconds, for a total manufacturing time of less than 60 seconds. However, part complexity was still limited, with the process most suitable for relatively flat or lightly contoured parts.

Hexion (Columbus, OH, US) has also developed epoxy resins for liquid compression molding, offering a total cycle time of 75-135 seconds with additional reduction possible by replacing part of the in-mold cure with a postcure.

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Huntsman's DFCM process using Araldite epoxy resins offers cycle times as low as 1.30 minutes. SOURCE: Huntsman Advanced Materials.

Dynamic Flud Compression Molding (DFCM)
Huntsman Advanced Materials (Basel, Switzerland), however, has not only developed epoxy resins for wet compression molding, but claims to have advanced the process to achieve production cycles of less than 90 seconds for parts with complex geometry (medium draw or >2.5D). Termed Dynamic Fluid Compression Molding (DFCM), it is described as a low pressure process (typically 30 bar) offering highly structural parts made with fast-cure Araldite epoxy resin, and often removes the need for a preform. DFCM reportedly can achieve fiber volume of up to 65% in a low-waste process that produces void-free parts for high mechanical performance. It also claims lower equipment investment vs. HP-RTM and elimination of racetracking and fiber wash issues.


Thanks to Arkema's (Colombes, France and King of Prussia, PA, US) Elium resins, wet compression molding may also be extended to thermoplastics. The liquid acrylic thermoplastic resins reportedly can be used in wet compression molding or cold press molding at temperatures ranging from  20°C to 80°C to make recyclable composite parts in a very efficient way. Arkema claims a variety of reinforcements can be used, as well as functional fillers to impart additional properties to the composite part. This material and process combination also touts 30-50% weight reduction vs. steel parts with the same resistance and lower part manufacturing costs vs. other thermoplastic technologies.