Thermoplastic Overmolding: from HP-RTM to Prepreg Tapes, Laser Ablation and Part Referencing

 

An industry-leading integrated molding cell enables functional production of hybrid composite parts without adhesives in a single production cell, with a cycle time of just 2 minutes.

 

From HP-RTM to Prepreg Tapes

 

The original idea was to manufacture epoxy CFRP parts using C-RTM, a high-pressure resin transfer molding (HP-RTM) process also known as “gap impregnation” developed by the Institute for Plastics Processing (IKV) at RWTH Aachen University.

 

In the meantime, however, automated tape-based processes began to challenge liquid molding of non-crimp fabrics (NCFs), which reportedly reduce cutting waste by 30%.

 

Fast-curing liquid epoxies were also expanded into prepreg materials, making compression molding very attractive with potential cycle times of 1 to 2 minutes.

 

For the shell of the demonstrator, four unidirectional prepregs were evaluated. These were transformed into net-shape, 2D custom blanks using a STAXX tape placement cell from Broetje-Automation (Rastede, Germany).

 

 

Before being placed in the OPTO-Light forming unit, automated preforming equipment such as Broetje-Automation’s STAXX tape placement unit is used to transform the prepreg tows and prepreg tapes into a net-shape, two-dimensional custom blank (Image courtesy of RWTH Aachen University AZL)

 

 

The custom blanks are then preformed in preparation for placement in the OPTO-Light forming cell (Image courtesy of RWTH Aachen University AZL)

 

The molded CFRP shell is then overmolded with 30% short glass fiber reinforced polyamide 6 (GF/PA6) Durethan BKV 30 H2.0 901510 from LANXESS (Cologne, Germany).

 

A KraussMaffei CXW-200-380/180 SpinForm injection molding machine was selected as the basis for the OPTO-Light manufacturing cell and installed at the AZL. The machine uses a specially developed horizontal rotating platen technology to enable multi-component injection molding.

 

The mold, mounted on a horizontal rotating platen, has two different molding cavities for two different molding processes: compression molding of epoxy prepreg and overmolding of thermoplastics.

 

“No one has invented a mold like this before,” Schares said.

 

BMW and KraussMaffei spent several weeks implementing all the requirements for both processes, including tolerances based on different temperature zones, rotational accuracy and sealing of thermosetting resins, as well as standard details for injection molding molds.

 

Step 1: Place the carbon fiber/epoxy prepreg preform into the OPTO-Light molding unit

 

Step 2: The preform is cured into a CFRP shell in horizontal prepreg compression molding

 

Step 4: Laser ablation removes the top 10µm epoxy layer in preparation for overmolding

 

Step 5: The pre-treated shell is rotated on a horizontal rotating platen to be back-injected with short glass fiber reinforced PA6 compound

 

Step 6: The final part features precise overmolding along the laser preprocessed path

 

 Laser Ablation and Part Referencing

 

Epoxy CFRP shells produced by compression molding must be pre-treated before thermoplastic overmolding to ensure adequate bond strength between the different materials.

 

Compared to mechanical or chemical pre-treatment, laser ablation offers an environmentally friendly one-step process that allows precise ablation depth and path, ideal for joining ribs to parts along three-dimensional surfaces.

 

This ablation method requires local removal of the top 10µm thick epoxy layer to expose the carbon fibers. This cleans the surface and creates a microstructure that allows the overmolding compound to wet and penetrate the exposed fibers.

 

The multifunctional laser scanner emits a laser beam with a wavelength of 1.064nm in nanosecond pulses.

 

“You need high intensity, and pulses achieve this most effectively,” Schares explains. “We tried a continuous wave laser, but it introduced too much thermal stress into the composite laminate below the joining area, which reduced the fiber-epoxy bond. So it was not easy to find a beam source that was suitable for the process so that it could be used remotely in an industrial environment.”

 

The ablation process requires high positioning accuracy, as the overmolded ribs must match the pre-treated areas.

 

The subsequent placement of the overmolded glass-fiber PA6 composite is strictly defined by the mold tooling. Thus, a necessary part referencing method was developed by AZL.

 

“The offset between the pre-treated shape and the overmolded compound should be less than 300µm. Therefore, the accuracy of the center point of the laser scan field (the tool center point) must be within 150µm relative to the reference point. This was achieved with a cycle time of less than 2 minutes for the laser pre-treatment.

 

“Very important was the preliminary development work done by the Fraunhofer Institute for Production Technology (IPT) for the robot and the path generation of the laser beam, which was no small feat. ” Schares said.

 

The system has indeed demonstrated its capabilities – test results showed a shear strength of 27MPa between the overmolded GF/PA6 and the epoxy CFRP substrate.