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Injection Moulding with Continuous Local Reinforcements

Periodic Reporting for period 2 - IMCoLoR (Injection Moulding with Continuous Local Reinforcements)

Reporting period: 2018-08-01 to 2020-07-31

IMCOLOR’s main objective is the development of a novel process for the advantageous combination of injection molding (IM) and thermoplastic automated fiber placement with in-situ consolidation (TP-AFPisc) to make a partially continuous fiber reinforced air cycle machine part from PEEK. The TP-AFPisc is a change compared to the state of art, where composite structures typically build within process chains. It integrates the major steps of material deposition and consolidation without expensive autoclaves and represents an additive manufacturing process. PEEK IM is a challenge in combination with undercuts and composite inserts. Typically, these inserts are only half-sided overmolded in order to keep the insert in position during injection. However, there is no complete encapsulation of the inserts that could protect the exposed fibers or ensure a surface according to IM standards.

Objectives and conclusions:

• Integration of TP-AFPisc inserts in IM parts with firmly bonding between inserts and injected material. IM tools with exact fixations for inserts. Complete overmolding/encapsulation of inserts.
>> Victrex 150CA30 and TenCate TC1225 work well for that purpose. The part is prone to cracks once the 150CA30 melt meets itself in a solid state, even if it is heated to 200°C. One might prefer to combine a low melting with a high melting PAEK in every overmolding step.

• IM with an expendable core to enable undercuts.
>> Salt cores achieved a proof of concept. Voluminous cores are hard to manufacture in high-pressure die-casting. They must be split in several segments, which are bonded with epoxy. The core material is easy to remove with water. However, the core material was of bad quality as there were resources for only one try for the tool and casting. Therefore, the final part quality suffers, too, but the results are promising.

• Weight and cost reduction. Reduction of fuel consumption, CO2 and NOx emission.
>> Depends heavily on the test case of the final part. Tension or bending loads are optimal for the local composite inserts. However, with the lateral impact of a fan wheel being the governing load case, the design is unsuitable and requires many plies.

• Avoid Cr6+ surface treatments
>> Achieved by the choice of a durable, resistant thermoplastic that replaces metal for the most part.
The activities were conducted in cooperation between the partners: Development of TP-AFPisc tools (WP2), salt core technology (WP3,4), tape placement (WP1-3), automated sensor data analysis (WP2,3) and mechanical test scenarios (WP3) were in focus of TUM. The scientific partner TPRC developed the injection strategies (WP2-4), gave advice on material selection (WP1), simulated tooling concepts (WP2-4) and carried out the injections on a KraussMaffei machine (WP2-4). The industrial partner apppex has strong experience in fast production of prototype tools and took care of tool design, molds and the integration of inserts (WP2-4). The tier 1 supplier FACC concentrated on NDT part inspection (WP2,4). TUM evaluated the process with regard to industrialization (WP5) and was responsible for the project coordination (WP6).
The production route of the demonstrator part is shown in figure IMCOLOR PROCESS and the part/tool concept in figure DP SHOT 2 CONCEPT: First, a long pipe was produced by TP-AFPisc tape winding with an AFPT machine. The inserts were cut from the pipe. The mandrel was a piston and granted a high quality surface, which is necessary for demolding and constant insert dimensions. Then, the insert was overmolded for the first time and got a 150CA30 coating on its outside. After machining the runner system, the shot 1 part was put in the mold for shot 2 together with the salt core. The salt core consisted of four 90° segments that were bonded together and to the salt core carrier, which is a removable aluminum pipe. This had to be done in a separate mold. Then, the assembly of shot 1 including the insert, salt core and salt core carrier was placed in the mold for the final shot 2 injection. After washout of the salt core and trimming of the runner system, one got the final demonstrator part. In total there were 14 demonstrator parts.
Between the CFRP insert production and overmolding shot 1, there was a sub-step part called “model part”. It had a reduced number of plies (23 instead of 70) and no undercuts. It served to study the material combination for an optimal machine setup. The two shot overmolding strategy, insert fixture and material selection was proven. Machine data logs and micrographs revealed a good material quality. The model parts showed occasionally out-of-plane fiber wrinkling. Delamination between plies were observed in the wrinkle. The preheating temperature and high pressure loads during PEEK injection probably caused this fiber buckling. The insert wrinkling did not occur for the 70-plies version of the demonstrator part. There were 25 model parts. TUM managed overspeed tests with impactors of the Topic Manager. Containment of the fan wheel fragments was achieved as soon as a thin metal ring was used to prevent piercing loads, which act perpendicular to the continuous fiber.
Some parts serve as exhibition objects at the partners' sites. TUM presented a model part during JEC 2019 and supervised several student thesis that refer to the project.The consortium partners could use the results to gain experience with new materials and train personnel on complex processes.
The aim is to develop a synergy between IM and the TP-AFPisc process beyond the state of the art, which enables future lightweight design at high mechanical performance with automated, reproducible production techniques under ecological friendly conditions. IM is a wide spread technology for thermoplastic processing, but restricted to the use of short fiber material. It offers a large spectrum of geometric freedom and enables high production rates. In high-loaded structures, continuous fiber reinforcements are demanded because of their excellent thermomechanical properties. However, due to the casting process, it is not possible to use continuous fibers in IM. IMCOLOR closes this gap and enables a high efficient utilization of material. The expensive endless fiber are only used in the necessary sections of the part. The fiber architecture is precisely adapted to the user’s needs by engaging TP-AFPisc manufactured inserts. Effort in trimming of composite parts is minimized and scrap is reduced. No surface preparation is necessary for the inserts, because they are bonded to the polymer by autohealing during overinjection. The final part profits from the high loadable endless fiber reinforcement that is completely covered by IM compound. The IM process adds its benefits in terms of surface quality, tolerances and protection against environmental influences (e.g. impactors, corrosive media) to the sensitive continuous fibers.
IMCOLOR’s process demonstrators achieved a proof of concept and gave detailed insight in the challenges for industrialization. The high degree of automation and flexibility of machines enable the technology for automotive industry, too. Although IMCOLOR investigated a dedicated aerospace polymer, its conclusions can be applied to other engineering plastics.
Therefore, the project is an important step towards an emission-reduced mobility by applying advanced lightweight materials and contributes to maintain a healthy environment in the future.